text:
# A multi-epoch, multi-wavelength study of the classical FUor V1515 Cyg
approaching quiescence

Zs. M. Szabó Konkoly Observatory, Research Centre for Astronomy and Earth
Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-Thege Miklós út
15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence, Budapest,
Konkoly Thege Miklós út 15-17., H-1121, Hungary  Max-Planck-Institut für
Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany  Scottish
Universities Physics Alliance (SUPA), School of Physics and Astronomy,
University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK  Eötvös Loránd
University, Department of Astronomy, Pázmány Péter sétány 1/A, 1117 Budapest,
Hungary  Á. Kóspál Konkoly Observatory, Research Centre for Astronomy and
Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-Thege Miklós út
15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence, Budapest,
Konkoly Thege Miklós út 15-17., H-1121, Hungary  Max Planck Institute for
Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany  ELTE Eötvös Loránd
University, Institute of Physics, Pázmány Péter sétány 1/A, H-1117 Budapest,
Hungary  P. Ábrahám Konkoly Observatory, Research Centre for Astronomy and
Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-Thege Miklós út
15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence, Budapest,
Konkoly Thege Miklós út 15-17., H-1121, Hungary  ELTE Eötvös Loránd
University, Institute of Physics, Pázmány Péter sétány 1/A, H-1117 Budapest,
Hungary  S. Park Konkoly Observatory, Research Centre for Astronomy and Earth
Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-Thege Miklós út
15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence, Budapest,
Konkoly Thege Miklós út 15-17., H-1121, Hungary  M. Siwak Konkoly Observatory,
Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research
Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary  CSFK,
MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út 15-17., H-1121,
Hungary  J. D. Green Space Telescope Science Institute, 3700 San Martin Dr.,
Baltimore, MD 21218, USA  A. Pál Konkoly Observatory, Research Centre for
Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-
Thege Miklós út 15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence,
Budapest, Konkoly Thege Miklós út 15-17., H-1121, Hungary  Eötvös Loránd
University, Department of Astronomy, Pázmány Péter sétány 1/A, 1117 Budapest,
Hungary  ELTE Eötvös Loránd University, Institute of Physics, Pázmány Péter
sétány 1/A, H-1117 Budapest, Hungary  MIT Kavli Institute for Astrophysics and
Space Research, 70 Vassar Street, Cambridge, MA 02109, USA  J. A. Acosta-
Pulido Instituto de Astrofísica de Canarias, Avenida Vía Láctea, Tenerife,
Spain  Departamento de Astrofísica, Universidad de La Laguna, Tenerife, Spain
J.-E. Lee School of Space Research, Kyung Hee University, 1732, Deogyeong-
daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Republic of Korea  M.
Ibrahimov  Institute of Astronomy, Russian Academy of Sciences, 48
Pyatnitskaya st., 119017, Moscow, Russia  K. Grankin  Crimean Astrophysical
Observatory, p/o Nauchny, 298409, Republic of Crimea  B. Kovács  Konkoly
Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd
Research Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
Zs. Bora  Konkoly Observatory, Research Centre for Astronomy and Earth
Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-Thege Miklós út
15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence, Budapest,
Konkoly Thege Miklós út 15-17., H-1121, Hungary  A. Bódi  Konkoly Observatory,
Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research
Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary  CSFK,
MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út 15-17., H-1121,
Hungary  MTA CSFK Lendület Near-Field Cosmology Research Group, 1121,
Budapest, Konkoly Thege Miklós út 15-17, Hungary  B. Cseh  Konkoly
Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd
Research Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
CSFK, MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út 15-17.,
H-1121, Hungary  MTA-ELTE Lendület ”Momentum” Milky Way Research Group,
Hungary  G. Csörnyei  Konkoly Observatory, Research Centre for Astronomy and
Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-Thege Miklós út
15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence, Budapest,
Konkoly Thege Miklós út 15-17., H-1121, Hungary  Marek Dróżdż  Mount Suhora
Astronomical Observatory, Cracow Pedagogical University, ul. Podchorazych 2,
30-084 Kraków, Poland  O. Hanyecz  Konkoly Observatory, Research Centre for
Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-
Thege Miklós út 15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence,
Budapest, Konkoly Thege Miklós út 15-17., H-1121, Hungary  B. Ignácz  Konkoly
Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd
Research Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
CSFK, MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út 15-17.,
H-1121, Hungary  Cs. Kalup  Konkoly Observatory, Research Centre for Astronomy
and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-Thege
Miklós út 15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence,
Budapest, Konkoly Thege Miklós út 15-17., H-1121, Hungary  R. Könyves-Tóth
Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös
Loránd Research Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest,
Hungary  CSFK, MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út
15-17., H-1121, Hungary  Department of Optics & Quantum Electronics,
University of Szeged, Dóm tér 9, Szeged, 6720, Hungary  M. Krezinger Konkoly
Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd
Research Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
CSFK, MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út 15-17.,
H-1121, Hungary  Eötvös Loránd University, Department of Astronomy, Pázmány
Péter sétány 1/A, 1117 Budapest, Hungary  L. Kriskovics  Konkoly Observatory,
Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research
Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary  CSFK,
MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út 15-17., H-1121,
Hungary  ELTE Eötvös Loránd University, Institute of Physics, Pázmány Péter
sétány 1/A, H-1117 Budapest, Hungary  Waldemar Ogłoza Mount Suhora
Astronomical Observatory, Cracow Pedagogical University, ul. Podchorazych 2,
30-084 Kraków, Poland  A. Ordasi  Konkoly Observatory, Research Centre for
Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-
Thege Miklós út 15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence,
Budapest, Konkoly Thege Miklós út 15-17., H-1121, Hungary  K. Sárneczky
Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös
Loránd Research Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest,
Hungary  CSFK, MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út
15-17., H-1121, Hungary  B. Seli Konkoly Observatory, Research Centre for
Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-
Thege Miklós út 15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence,
Budapest, Konkoly Thege Miklós út 15-17., H-1121, Hungary  Eötvös Loránd
University, Department of Astronomy, Pázmány Péter sétány 1/A, 1117 Budapest,
Hungary  R. Szakáts Konkoly Observatory, Research Centre for Astronomy and
Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-Thege Miklós út
15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence, Budapest,
Konkoly Thege Miklós út 15-17., H-1121, Hungary  Á. Sódor  Konkoly
Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd
Research Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
CSFK, MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út 15-17.,
H-1121, Hungary  MTA CSFK Lendület Near-Field Cosmology Research Group, 1121,
Budapest, Konkoly Thege Miklós út 15-17, Hungary  A. Szing  Konkoly
Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd
Research Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
CSFK, MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út 15-17.,
H-1121, Hungary  K. Vida Konkoly Observatory, Research Centre for Astronomy
and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-Thege
Miklós út 15-17, 1121 Budapest, Hungary  CSFK, MTA Centre of Excellence,
Budapest, Konkoly Thege Miklós út 15-17., H-1121, Hungary  J. Vinkó Konkoly
Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd
Research Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
CSFK, MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út 15-17.,
H-1121, Hungary  ELTE Eötvös Loránd University, Institute of Physics, Pázmány
Péter sétány 1/A, H-1117 Budapest, Hungary

###### Abstract

Historically, FU Orionis-type stars are low-mass, pre-main sequence stars. The
members of this class experience powerful accretion outbursts and remain in an
enhanced accretion state for decades or centuries. V1515 Cyg, a classical
FUor, started brightening in the 1940s and reached its peak brightness in the
late 1970s. Following a sudden decrease in brightness it stayed in a minimum
state for a few months, then started a brightening for several years. We
present results of our ground-based photometric monitoring complemented with
optical/NIR spectroscopic monitoring. Our light curves show a long-term fading
with strong variability on weekly and monthly time scales. The optical spectra
show P Cygni profiles and broad blue-shifted absorption lines, common
properties of FUors. However, V1515 Cyg lacks the P Cygni profile in the Ca II
8498 Å line, a part of the Ca infrared triplet (IRT), formed by an outflowing
wind, suggesting that the absorbing gas in the wind is optically thin. The
newly obtained near-infrared spectrum shows the strengthening of the CO
bandhead and the FeH molecular band, indicating that the disk has become
cooler since the last spectroscopic observation in 2015. The current
luminosity of the accretion disk dropped from the peak value of 138
$L_{\odot}$ to about 45 $L_{\odot}$, suggesting that the long-term fading is
also partly caused by the dropping of the accretion rate.

FU Orionis stars — Young stellar objects — Stars: Individual: V1515 Cyg —
Circumstellar disks — Multi-color photometry — Spectroscopy

††journal: ApJ††facilities: NOT, BOAO, SOFIA, TESS††software: FITSH package
(Pál, 2012), Vartools (Hartman & Bakos, 2016), molecfit (Smette et al., 2015;
Kausch et al., 2015), IRAF (Tody, 1986, 1993)

##  1 Introduction

A rather small, but spectacular class of the low-mass pre-main sequence stars
are the FU Orionis type objects (briefly FUors). The group was defined by
Herbig (1977) based on the properties of the first few outbursting sources
observed in the optical, today known as classical FUors. However, these and
the other optically visible sources represent only part of a more complex
phenomenon that is conspicuous also at infrared wavelengths. FUors are
characterized by their enormous inner disks brightness increase, which is
caused by the enhanced accretion from the disk onto the protostar. Enhanced
accretion is most likely caused by disk instabilities, and this stage can last
for several decades, or likely even centuries (Paczynski, 1976; Lin &
Papaloizou, 1985; Kenyon et al., 1988; Kenyon & Hartmann, 1991; Bell et al.,
1995; Turner et al., 1997; Audard et al., 2014; Kadam et al., 2020). FUors
show very similar optical spectra with the properties of F or G supergiants
with broad absorption lines, P Cygni profile of H$\alpha$, wind components,
and strong Li I 6707 Å absorption in the optical regime.

This paper is the second in a series started by Szabó et al. (2021), with the
aim of characterising past and current evolutionary stages of the so called
classical FUors by means of archival and new photometric as well as
spectroscopic observations. The target of the present study, V1515 Cyg, was
the third discovered FUor. Unlike the first two classical FUors (FU Orionis
and V1057 Cyg), V1515 Cyg did not reach the photometric maximum brightness
within a year, but this process lasted significantly longer: it started in the
1940’s and it reached the peak (13.7 mag in the $B$-band) at the end of the
1970’s (Landolt, 1977). In 1980 a significant, yet only temporary fading
occurred, which was the sharpest decline yet observed during a FUor outburst
(Kolotilov & Petrov, 1981; Kolotilov, 1983; Kenyon et al., 1991). V1515 Cyg
faded 1.5 mag in the $B$-band within a few months, but after reaching the
minimum at the end of 1980, the source started to brighten again (Clarke et
al., 2005). The fading observed in V1515 Cyg after reaching the light curve
maximum is commonly observed among the classical FUors: both FU Ori and V1057
Cyg showed similar dips after reaching their maximum (Kolotilov & Kenyon,
1997a; Kenyon et al., 1991; Kenyon & Hartmann, 1991).

Unlike V1057 Cyg, where the pre-outburst spectrum showed properties of a
classical T Tauri-type star (CTTS; Herbig 1977; Herbig et al. 2003), no
spectroscopic observations were carried out prior to the brightening of V1515
Cyg. The spectra obtained during the first fading event in the 1980’s by
Kolotilov (1983) showed absorption lines typical for high-luminosity G2–G5
star and despite the fading, the spectrum of V1515 Cyg has not fundamentally
changed since the observations obtained around 1974–1975 by Herbig (1977). All
features common for FUors like broad H$\alpha$ line having P Cyg profile,
strong Li I 6707 Å as well as strong Na D lines, were present. The presence of
blueshifted absorption in the H$\alpha$ and H$\beta$ lines are common
signatures for the outflowing wind in these objects. In the case of V1515 Cyg
the blueshifted features were interpreted as an expanding envelope by
Kolotilov (1983). Using photometric data, Kenyon et al. (1991) proposed to
explain the 1980 fading by a sudden dust grain condensation event in the
outflowing wind. They proposed that the slow recovery from the minimum was
most likely caused by the gradual dispersal (expansion) of a newly formed dust
cloud.

Several years ago, we started a photometric monitoring program of a few
classical FUors at the Piszkéstető Mountain Station of Konkoly Observatory in
Hungary, including V1515 Cyg. After combination with other public-domain
photometric data of this source, we realized that about 15 years ago the
source started a long-term fading trend. Given that the last detailed
photometric analysis of V1515 Cyg was presented by Clarke et al. (2005), while
the last optical and infrared spectral analysis were performed by Kenyon et
al. (1991), Agra-Amboage & Garcia (2014) and Connelley & Reipurth (2018), we
decided to increase the cadence of our photometric monitoring and obtain new
spectroscopic observations to properly characterise this fading process.
Although the FUor phenomenon can occur throughout the YSO evolution (see e.g.,
Hartmann & Kenyon, 1996; Fischer et al., 2022), it is still an open question
how an eruption ends, and how the properties of a system undergoing an
eruption are evolving. Considering the long-term fading trend, V1515 Cyg could
easily be the first FUor going back to the quiescent stage, therefore, it is
crucial to gather as much data as possible to better understand an important
phenomenon in the evolution of young stars.

In Section 2 we briefly summarize our observations and the data reduction. We
analyse the data in Section 3 and we discuss obtained results in Section 4.
Lastly, we summarize our findings in Section 5.

##  2 Observations and data reduction

###  2.1 Ground-based optical and near-infrared photometry

The bulk of photometric observations was obtained at Piszkéstető Observatory
between 2005 and 2021 in $B$, $V$, $R_{\rm C}$, $I_{\rm C}$, $g^{\prime}$,
$r^{\prime}$, and $i^{\prime}$ filters. Three telescopes, each having somewhat
different optical system, were used. Between 2005 October and 2011 May we
observed the star with the 1 m Ritchey-Chrétien-Coudé (RCC) telescope,
equipped with a 1300$\times$1340 pixel Roper Scientific VersArray: 1300B CCD
camera (pixel scale: 0$\farcs$306). In August 2011 – December 2019 we observed
the star with the 60/90 cm Schmidt telescope (pixel scale: 1$\farcs$027) and
from May 2020 we use the Astro Systeme Austria AZ800 alt-azimuth direct drive
80-cm Ritchey-Chrétien (RC80) telescope (pixel scale 0$\farcs$55).

In 2006 we also observed V1515 Cyg at Mt. Maidanak observatory by means of the
100 and the 60 cm Zeiss telescopes, both equipped with the same IMG1001E
1k$\times$1k CCD camera and $BVR_{C}I_{C}$ filters set: the larger reflector
was used during 9 nights between 17 – 27 June, while the smaller during 35
nights between 4 July – 28 August. Due to different focal lengths, the larger
telescope offered 6.35, while the smaller 11.74 arcmin field of view, with the
pixel scale of 0.372 and 0.688 arcsec pix-1, respectively. Typically 4 images
in $B$, 2 – 3 images in $V$, and single images in $R_{C}I_{C}$ filters per
night were obtained. After standard reduction steps on bias, dark and flat
field, we extracted aperture photometry of V1515 Cyg using three comparison
stars for the 100 cm, and five comparison stars for the 60 cm telescope. The
results from separate images obtained during the same night were averaged.
This resulted in typical uncertainty of 0.04 – 0.05 mag in $B$, 0.01 – 0.02
mag in $V$, and 0.01 mag in $R_{C}I_{C}$ filters.

In July – September, 2021, we observed the star at the Mt. Suhora Observatory
of the Cracow Pedagogical University (Poland). We used the 60 cm Carl-Zeiss
telescope equipped with an Apogee AltaU47 camera, giving 1$\farcs$116 pixel
scale and $19\farcm 0\times 19\farcm 0$ field of view. Johnson $BV$ and Sloan
$g^{\prime}r^{\prime}i^{\prime}$ filters were used.

Occasionally we also used other telescopes: on 2006 July 19 and 2012 October
14 we obtained $B$, $V$, and $R_{\rm J}$ data with the IAC80 telescope of the
Instituto de Astrofísica de Canarias located at Teide Observatory (Canary
Islands, Spain) (0$\farcs$537 pixel scale). The data reduction for the RCC,
Schmidt, RC80, Mt. Suhora and IAC80 observations was carried out in the same
way as described in detail by Szabó et al. (2021). We show our photometric
results in Table 4 in Appendix A and in Figs. 1 and 2. The typical uncertainty
of our measurements is 0.02–0.03 mag in $B$ and 0.01 mag in all other filters.

We obtained near-infrared images in the $J$, $H$, and $K_{\rm s}$ bands at six
epochs between 2006 July 15 and 2012 October 13 using the 1.52 m Telescopio
Carlos Sanchez (TCS) at the Teide Observatory. Lastly, we used the NOTCam
instrument on the NOT on 2021 August 9. Because of the brightness of our
target in the infrared, we used a 5 mm diameter pupil mask intended for very
bright objects to diminish the telescope aperture, which gave about 10%
transmission. The detailed data reduction was carried out the same way as for
V1057 Cyg using TCS as described in Szabó et al. (2021). In the same paper we
also provided further details about the above instruments. The typical
photometric uncertainties are of 0.01–0.03 mag. We present the $JHK_{s}$
photometry in Table 4 in Appendix A. We also observed V1515 Cyg with the
$2.56$ m Nordic Optical Telescope (NOT) located at the Roque de los Muchachos
Observatory (Canary Islands; Plan ID 63-401, PI: Zs. M. Szabó) We used the
NOTCam instrument in wide-field imaging mode on 2021 April 25. The instrument
consists of a 1024$\times$1024 pixel HgCdTe Rockwell Science Center ‘HAWAII’.
The field of view was $4^{\prime}\times 4^{\prime}$ with a pixel scale of
0$\farcs$234.

We supplemented our ground-based optical photometry with publicly available
data from the ASAS-SN (Shappee et al., 2014; Kochanek et al., 2017) and ZTF
surveys (Masci et al., 2018), which were reduced by dedicated pipelines. We
also used the DASCH database (Grindlay et al., 2012) for our work. The first
ASAS-SN data were obtained on 2015 February 24, while the latest on 2022
January 4, whereas the ZTF observations range from 2018 March 28, to 2021
November 3. The DASCH data ranges between 1944 and 1989, and seasonal averages
were used for plotting purposes in Fig. 1. We also use the 1982 – 2003 Mt.
Maidanak $UBVR$ data set, already analysed in detail by Clarke et al. (2005).

###  2.2 Space-based optical and infrared photometry

We used the Transiting Exoplanet Survey Satellite (TESS, Ricker et al., 2015),
observed V1515 Cyg between 2019 July 18 and September 11 (Sectors 14 and 15),
and then between 2021 July 20 and August 20 (Sector 41). The photometry was
extracted from the full-field images using 1.5 pix aperture radius in the same
way as described in Szabó et al. (2021). In the final step, the instrumental
TESS magnitudes were aligned to our Schmidt- (direct) and RC80-telescope
(transformed) measurements in the $I_{C}$ band, and the results are shown in
Fig. 3. The seven breaks visible in the light curve in 2019, and the single
break in 2021, are caused either by the regular data transfers to the ground
or by scattered Earth and Moon light, preventing accurate sky subtraction –
these affected data were discarded from the presented plots and from further
analyses.

On 2018 August 29, V1515 Cyg was observed with the Stratospheric Observatory
for Infrared Astronomy (SOFIA; Young et al. 2012) using the Faint Object
infraRed CAmera for the SOFIA Telescope (FORCAST; Herter et al. 2013). We
obtained mid-infrared imaging using the F056, F077, F088, F111, F242, F315,
and F348 filters with effective wavelengths of 5.6, 7.7, 8.8, 11.1, 24.2,
31.5, and 34.8$\,\mu$m, respectively (Plan ID 06_0062, PI: J. D. Green). Total
on-source integration times were between and 45 s and 87 s depending on the
filter. The images were processed using the SOFIA pipeline and retrieved as
Level 2 data products from the SOFIA Science Archive as ingested into the IRSA
database. We obtained aperture photometry on the images with a series of
apertures with increasing radii and selected an aperture for the final
photometry where this curve of growth flattened. The resulting fluxes are:
$F_{5.6}=0.486\pm 0.071$ Jy, $F_{7.7}=0.502\pm 0.065$ Jy, $F_{8.8}=0.531\pm
0.069$ Jy, $F_{11.1}=0.981\pm 0.069$ Jy, $F_{24.2}=2.42\pm 0.13$ Jy,
$F_{31.5}=2.06\pm 0.16$ Jy, $F_{34.8}=2.21\pm 0.18$ Jy. We also used data from
the Wide-field Infrared Survey Explorer (WISE, Wright et al. 2010). In Tab.
LABEL:tab:phot_wise in Appendix A we summarize the saturation corrected WISE
data for V1515 Cyg shown in Fig. 1.

###  2.3 Spectroscopy

We obtained a new optical spectrum of V1515 Cyg with the high-resolution
FIbre-fed Echelle Spectrograph (FIES) instrument on the NOT on 2021 May 29. We
used a fibre with an entrance aperture of $2\farcs 5$ which provided a
spectral resolution $R$=25 000 between 370 – 900 nm. The final spectrum is
composed of three individual spectra, each with 2400 s exposure time. We
reduced the spectra using the FIEStool software. We also observed V1515 Cyg
with the Bohyunsan Optical Echelle Spectrograph (BOES; Kim et al., 2002)
installed on the 1.8 m telescope at the Bohyunsan Optical Astronomy
Observatory (BOAO). BOES provides $R$=30 000 (using a 300 $\mu$m fiber) in the
wavelength range $\sim$4000–9000 Å. We binned the spectra with 2$\times$2
pixels to increase the S/N. We took four spectra of V1515 Cyg between 2015 and
2018, for the exact dates, see the spectroscopic log of observations in Tab.
1. We reduced these spectra in a standard way within IRAF: after bias and
flatfield corrections, the ThAr lamp spectrum was used for wavelength
calibration, and continuum fitting was performed using the continuum task.
Finally, heliocentric velocity correction was applied by the rvcorrect task
and the published radial velocity of V1515 Cyg ($-$15 km s-1; Hartmann et al.,
2004). Unlike the usual observing method (and our 2021 NOT spectrum) with
several exposures, the BOES spectra were taken with one, long exposure. This
made the spectrum sensitive for cosmic rays with no means to remove them using
the usual procedures.

As no telluric standard stars were observed either for FIES or BOES, we
performed the telluric correction using the molecfit software (Smette et al.,
2015; Kausch et al., 2015) by fitting the telluric absorption bands of O2 and
H2O. This generally provided good correction except for the deepest lines
where the detected signal was close to zero.

Table 1: Log of Spectroscopic Observations  Telescope | Instrument | Spectral | Observation | Exp. time
---|---|---|---|---
|  | Resolution | Date [UT] | [sec]
BOAO | BOES | 30,000 | 2015 Dec 26 | 3600
$\cdots$ | $\cdots$ | $\cdots$ | 2017 May 29 | 3600
$\cdots$ | $\cdots$ | $\cdots$ | 2018 Oct 07 | 600
$\cdots$ | $\cdots$ | $\cdots$ | 2018 Dec 19 | 3600
NOT | FIES | 25,000 | 2021 May 29 | 2400 $\times$ 3
NOT | NOTCam | 2500 | 2021 Aug 09 | 400 $\times$ 4

On 2021 August 9, we used the NOTCam on the NOT to obtain a new near-infrared
spectrum of V1515 Cyg and Iot Cyg (telluric standard, A5 V) in the $JHK_{s}$
bands (1.25–2.2 $\mu$m). We used the low-resolution camera mode ($R$=2500)
with ABBA dither positions, and exposure times ranged from 60 to 100 seconds
(Tab. 1). For each image, flat-fielding, bad pixel removal, sky subtraction,
aperture tracing, and wavelength calibration steps were performed within IRAF.
A Xenon lamp spectrum was used for the wavelength calibration. The Hydrogen
absorption lines in Iot Cyg were removed by Gaussian fitting and then the
spectrum of V1515 Cyg was divided by the normalized spectrum of Iot Cyg for
telluric correction. Flux calibration was performed by applying the accretion
disk model (Sec. 4.2). It is based on the NOT $JHK_{s}$ photometry taken on
the same night, but the photometry was taken in broadband filters, hence gives
fluxes only for the effective wavelengths of these filters. We therefore use
the SED model (desribed in Sec. 3.7) fitted to the photometric points to
interpolate the fluxes for any given wavelength needed for the flux
calibration of the spectrum.

##  3 Results and Analysis

###  3.1 Light Curves

In order to study the brightness variations of V1515 Cyg in the long-term, we
constructed light curves with data gathered from the literature (Herbig, 1977;
Landolt, 1977; Kolotilov, 1983; Kenyon et al., 1991; Kenyon & Hartmann, 1991;
Herbst & Shevchenko, 1999; Clarke et al., 2005; Grindlay et al., 2012) and we
complemented the archival data with results from our monitoring. We present
the long-term photometric light curves of V1515 Cyg in Fig. 1. In order to
stay consistent between numerous data sets and to simplify further analyses,
we transformed our RC80, Mt. Suhora, and ZTF Sloan $r^{\prime}i^{\prime}$
magnitudes into Johnson–Cousins $R_{\rm C}I_{\rm C}$ system by constructing
the SED for each day observations and interpolate for the effective
wavelengths of the $R_{C}$ and $I_{C}$ filters using a spline function in the
log-log space. The resulting interpolated fluxes were transformed to
magnitudes with the zero points of the $R_{C}I_{C}$ filters. We stress that
this approach gave more consistent results than the transformation formulas
between Johnson and Sloan systems given by Jordi et al. (2006). Any remaining
offsets between overlapping data sets in optical bands were then removed by
constant shifts, as listed in Tab. 6 in the Appendix. We give a brief summary
of the different data sets used for Fig. 1 in Tab. 2.

As mentioned in Section 1, the outburst of V1515 Cyg was unlike those any
other classical FUors: it brightened for $\sim$30 years, before reaching the
peak brightness in the late 1970’s (Landolt, 1975, 1977), then in 1980 it
started a quick fading event, reaching the minimum brightness, and re-brighten
again for several years (Kolotilov & Petrov, 1981; Kolotilov, 1983; Kenyon et
al., 1991; Clarke et al., 2005). Between 1984–2004 the light curve showed a
plateau shape (Clarke et al., 2005). However, a significant long-term fading
of V1515 Cyg started around 2005–2006, with a sudden drop in the brightness
($\sim$1 mag in the $BVR$ filters), a trend that is currently still ongoing
with the mean rate of 0.085 mag yr-1 in the $V$-band. We note, that both the
plateau and the long-term fading are visible in the optical $BVR$, the near-
infrared $JHK$, and the WISE light curves also confirm the fading trend at 3.4
and 4.6 $\mu$m.

The small amplitude light changes were thoroughly analysed by Clarke et al.
(2005) between the 1980’s and early 2000’s. However, our careful look into the
same Mt. Maidanak data set revealed a feature unnoticed earlier: between 1984
– 2003 the small-scale variations visible on top of the major light curve
plateau predominantly manifest themselves in the form of peaks. At first
glance, they resemble accretion bursts typical for magnetospheric accretion in
CTTS, however, the $UBVR$ light curves show very similar variability
amplitudes. In combination with the characteristic time scales of these
brightness fluctuations ($\sim 10-30$ d) this strongly suggests that all these
variations most likely originated in the inner protoplanetary disk.

The INTEGRAL $V$-band data are sparse, thus the peak-like events could not be
seen clearly. Instead, these data revealed at least one well-defined dip at
$JD\approx 2454700$ (2008 August) – perhaps a signature of gradual disk
envelope cavity closure by increased dust condensation as the outburst is
weakening. The dips are in fact more numerous in the data obtained in 2010s
and 2020s.

Table 2: Summary of the photometric data used for Fig. 1 Date | Filters | Ref.
---|---|---
1974 – 1991 | $JHK$ | Molinari et al. (1993)
1975, 1976 | $V$ | Herbig (1977)
1976 | $UBV$ | Landolt (1977)
1978–1983 | $UBV$ | Kolotilov (1983)
1983–1995 | $V$ | Herbst & Shevchenko (1999)
1983–2005 | $UBVR$ | Clarke et al. (2005)
1982–1991 | $V$ | Kenyon et al. (1991)
1985–1991 | $JHK$ | Kenyon et al. (1991)
1985 – 1998 | $K$ | Kolotilov (1990)
2005–2011 | $BVRI$ | This work
2006, 2012 | $JHK_{s}$ | This work
2011-2019 | $BVRI$ | This work
2019–2021 | $BVr^{\prime}i^{\prime}$ | This work
2021 | $JHK_{s}$ | This work
Figure 1: Long-term optical and near-infrared light curves of V1515 Cyg. The
vertical solid black line marks the fading event occured in 1980. We
complemented our work with data from the literature prior to 2006 and it is
summarized in Tab. 2. Figure 2: Optical light curves of V1515 Cyg between 2006
and 2022. Most of the $BVR_{C}I_{C}$ data were obtained at the Piszkéstető
Observatory, while the rest of the data taken in the $V$\- and $g$-band are
from the ASAS-SN database. The vertical dashed dark grey lines mark the BOAO
and NOT optical/IR observations between 2012 and 2021 as well as the 2015
spectrum of V1515 Cyg by Connelley & Reipurth (2018). Yellow dash-dotted lines
indicated the TESS Sector 14, 15 and 41 observations of the source.

Figure 3: Light curves of V1515 Cyg gathered by TESS in 2019 and 2021, aligned
to our $I_{C}$ light curves.

###  3.2 Frequency Analysis

In order to search for periodic and quasi-periodic oscillations (QPOs) in the
ground-based data, we utilised the $V$\- and $g$-band light curves, providing
both the best temporal coverage and showing variability present in other
optical bands. In the first step, all outlier points were removed. As the
ASAS-SN survey gradually switched to the $g$-band, we re-scaled the amplitude
of these variations to match those observed in the $V$-band by the
multiplicative factor of 0.95. We determined this value during the time
period, when simultaneous $V$\- and $g$-band data were obtained, then, we
aligned the scaled $g$-band light curve to the $V$-band by a constant shift.

In the first step we searched for long-term quasi-periodic variations in the
1982–2021 dataset superimposed on the major light curve plateau. For this
purpose we subtracted the major long-term trend from the light curve by
fitting a 2nd order polynomial. Although the Lomb-Scargle (Zechmeister &
Kürster, 2009) diagram computed from the entire light curve reveals a forest
of peaks, none of them represent stable period nor quasi-period. For this
reason, in the second approach we decided to split the light curve based on
the dominant variability morphology and analyse each part separately, i.e. the
first part obtained before 2005 ($JD=2453500$), dominated by peaks, and the
second part obtained after, which is dominated by dips. No significant quasi-
periodicity was found, except for the three consecutive dip-like 350–370 d
variations visible directly in the light curve between $JD=2458000-2459000$
(2017–2020).

In the third step we decided to analyse each well-sampled observing season
separately. Prior to this analysis, any trends visible during certain seasons
were removed by using low order (1–2) polynomials to enhance the shorter
changes. We obtained that in the 1987 data set there is a significant 13.91 d
QPO with false alarm probability of 4$\times$10-5. This is similar to the
13.89 d period found by Clarke et al. (2005) in the same data. In addition, we
found a significant 13.06 d peak with false alarm probability of 10-3 in the
2003 data set. During the other seasons, the light curve usually exhibited
only two consecutive oscillations of a similar pattern, quickly disappearing
and giving way to light changes with different patterns.

In order to study the small-scale variability unavailable from the ground, we
utilised the TESS light curves obtained in 2019 and 2021. We performed
frequency $f$ analysis using the procedure of Rucinski et al. (2008), where
the amplitude $a_{f}$ errors are estimated by means of the bootstrap sampling
technique. Prior to the analysis, the general downward trends in the
brightness visible during both in 2019 and 2021 (Fig. 3a,b) were removed by
linear fits. Subsequently, the residual magnitudes were transformed to
normalized flux units. We found that in both seasons the amplitude-frequency
spectrum exhibits a Brownian random-walk nature (Fig. 4a,b), in which the
amplitudes scale with the frequency as $a_{f}\sim f^{-1}$ (Press, 1978). Only
in the 2021 data set there is a single peak at $f=0.439$ c d-1, which appears
to represent a significant 2.28 d QPO.

###  3.3 Wavelet Analysis

It is known that Fourier analysis cannot trace temporal changes nor localize
in time finite oscillatory packages such as those present in our data. As it
was for the first time proven by Rucinski et al. (2008, 2010), the wavelet
analysis can be successfully applied to extract such information from the
evenly-sampled space-based light curves of vigorously accreting CTTS and
Herbig Ae stars. In particular, the Morlet wavelet allows us to separate
individual oscillatory packages and directly characterise their temporal
changes. Although the variability observed in the above mentioned classes of
young stars is caused by the changing visibility, number, and the lifetime of
the numerous hot spots resulting from the (most often) unstable or moderately
stable magnetospheric accretion (Blinova et al., 2016), or in some particular
cases the pulsed accretion (Tofflemire et al., 2017; Kospál et al., 2018;
Tofflemire et al., 2019; Fiorellino et al., 2022), it was already
theoretically proven that the hot spot behavior actually reflects the physical
conditions in the inner disks of CTTS (see Blinova et al. 2016 and references
within). In the case of the FUors, where the disk radiation is dominating over
the stellar one, and the magnetosphere is likely totally crushed by the
enhanced disk-plasma pressure so that hot spots typical for CTTS cannot form,
effects brought by these inner disk instabilities can be directly observed in
the visual light only in the objects with residual traces of the envelope
(Siwak et al., 2013; Green et al., 2013a; Baek et al., 2015; Siwak et al.,
2018, 2020).

We obtained that the Morlet-wavelet spectrum of the 2019 TESS data does reveal
only a single, weak signature of QPO drifting from $\sim$6 to $\sim$5.5 d
between $BTJD-2458000=690-720$ (Fig. 4c). In accord with the visual
examination of the light curve, we note random changes, usually showing only
one cycle or two self-similar oscillatory cycles. The shortest variations of
this kind have a period of 1.5 d, but most of them are longer than 2 d. We
have to stress that this conclusion is unfortunately disturbed by the seven
interruptions in the TESS data acquisition, strongly affecting the short
periodic part of the spectrum, as indicated by the solid white lines in Fig.
4c,d. Moreover, there is a non-zero chance that any of these affected regions
may propagate towards the longer periods inside the open cone(s), which can be
imaginatively extended as indicated by the white continuous lines.

The shortest significant variations in the 2021 TESS wavelet spectrum are of
0.8–0.9 d and are mostly localised in the middle of the data set. The spectrum
does also reveal that the 2.28 d peak obtained in Sec. 3.2 is unstable, and is
seen only between $BTJD-2459000\approx 425-430$, i.e. it lasted for barely two
oscillatory cycles (Fig. 4d), it re-appeared around $BTJD-2459000\approx 441$,
but it is not well defined in the light curve itself. However, together with
the former case it can produce an excess of power in the frequency spectrum,
which leads to the spurious detection.

Similarly to Szabó et al. (2021), we also applied the weighted wavelet
Z-transform (WWZ, Foster 1996), designed for analysis of unevenly sampled time
series, and available within the Vartools package (Hartman & Bakos, 2016). In
general, the analysis did not show any oscillatory packages other than those
directly inferred from the best-sampled Mt. Maidanak light curves themselves,
i.e. typically consisting of two self-similar oscillations (see in Sec. 3.2).
We show only the results for the 1987 and 2003 seasons, although instead of
the WWZ, we decided to present the ordinary Morlet wavelet spectra. We stress
that the spectra were computed from the original data interpolated into
evenly-spaced time grid with 1 day step, which is equal to the median spacing
of the Mt. Maidanak original data (Fig. 4e,f). The signatures of the 13.1 d
QPO are evidently best defined for the 2003 data set, but we stress that the
detection efficiency is a function of the data sampling, as shown in the
associated bottom panels. For that reason, the 13.9 d QPO from the 1987 season
is less securely defined, as the maximum amplitude appears before the first
dashed line, denoting edge effects, and then the amplitude and the period
appear to change.

Figure 4: Upper panels: Fourier spectra (black line) calculated from the de-
trended and transformed to normalised flux units 2019 (Sectors 14 &15) and
2021 (Sector 41) TESS light curves. The stochastic nature ($a_{f}\sim f^{-1}$)
of these oscillations is indicated by the two parallel green dashed lines. The
amplitude errors are represented by blue dots. Middle panels: respective
Morlet wavelet spectra for TESS. Bottom panels: Morlet wavelet spectra showing
the 13.1-13.9 d QPO observed in 1987 and 2003. The original Johnson $V$ data
are shown below. In all wavelet spectra, the major edge effects lie outside of
the white dashed lines. The secondary effects caused by interruptions in TESS
data acquisition are marked by white solid lines.

###  3.4 The Curious Case of the 2021 Phenomenon

Figure 5: Upper panel: detrended 2021 data indicated in different filters.
TESS coverage is indicated by two parallel dashed lines. Bottom panel: color-
magnitude diagrams for the detrended light curve. Evolutionary paths obtained
from accretion disk modelling (Sec. 4.2) are indicated by lines of different
colors.

The data coverage from ground-based telescopes was exceptional in 2021. The
early 2021 season data showed brightening trend by 0.2 – 0.4 mag, which
stabilised for about 1 – 1.5 month, then (in June 2021) it faded quickly and
stayed nearly-constant for about one month, exactly during the TESS
observations in July – August. In early September, the brightness started to
rise and stabilized on the level expected from the long-term fading trend.

Taking advantage of the excellent coverage, we analysed this phenomenon in
detail as follows. First we detrended the long-term fading trend by linear fit
to the high brightness level, as indicated in the upper panel of Fig. 5. One
can notice that brightness evolution in different filters occurred in slightly
different way. This is better emphasized on color-magnitude diagrams, as shown
in the bottom panels of Fig. 5. It is evident that the brightness drop
followed the extinction path. Then, in the light curve minimum the colors
became bluer, which suggests scattering by small grains, like in UXor stars
(see e.g., Ábrahám et al., 2018). Finally during the rising branch, all color
indices remained almost unchanged, i.e. the brightness went to the upper level
almost on a grey path.

We performed the same analysis for the 2019 and 2020 observing seasons, but
conclusive results could not be reached due to the coarser sampling.
Therefore, we only focused on the 2021 data set, since this is the most useful
one to draw conclusions thanks to the good data sampling.
We discuss in detail the possible physical interpretation of the dimming
events of V1515 Cyg in comparison with the other classical FUors and the
classical T Tauri star (CTTS) RW Aur in Sec. 4.1.

###  3.5 Spectroscopy

We monitored V1515 Cyg from 2012 to 2021 with optical and near-infrared
spectrographs and obtained six spectra. We detected P Cygni profiles, broad
blue-shifted absorption lines, and metallic lines in absorption. For the
identification of the lines the NIST Atomic Spectra
Database111https://physics.nist.gov/PhysRefData/ASD/lines_form.html was used.

####  3.5.1 Optical Spectroscopy

Classical FUors share several common spectroscopic features in their optical
spectra, such as the P Cygni profile of H$\alpha$, strongly blue-shifted
absorption lines like the Li I 6707 Å absorption, and the fact that their
spectral type is wavelength-dependent (Hartmann & Kenyon, 1996; Audard et al.,
2014). Some of these spectroscopic characteristics are also seen in our
observations, presented in the upper panel of Fig. 6. Apart from 2018 December
spectrum, all the other spectra show remarkably similar profiles within the
measurement uncertainties. The 2018 December spectrum in Fig. 6 is different
in the way that the absorption profile was not as deep as in all the other
epochs. We note that for plotting purposes in this section, we used our
NOT/FIES spectrum obtained in 2021 May.

We observed several P Cygni profiles, specifically in H$\alpha$, H$\beta$, and
two lines of the Ca infrared triplet (IRT), Ca II 8542, and Ca II 8662 Å
lines, but interestingly not in the Ca II 8498 Å line. Fig. 7 shows observed P
Cygni profiles in the 2021 spectrum. In the case of the H$\alpha$ line, the
previously published optical spectrum of V1515 Cyg presented by Herbig (1977)
and Kenyon et al. (1991) showed a pure absorption profile, but we observed
H$\alpha$ with a clear P Cygni profile in all our observations taken between
2015 and 2021 (middle panel in Fig. 6), including a clear emission component.
H$\alpha$ and H$\beta$ show higher velocity of both blue-shifted absorption
($\sim-$300 km s-1) and red-shifted emission ($\sim$100 km s-1) components
compared to the Ca II lines ($\sim-$200 km s-1 and $\sim$50 km s-1).

Wind signatures in the form of broad blue-shifted absorption lines were also
observed in the Na I D 5889 and 5895 Å, Mg I 5167 and 5184 Å, and K I doublet
lines (7664 and 7698 Å) (Fig. 8). The highest velocity component extended up
to about 250 km s-1, similar to the blue-shifted absorption component of
H$\alpha$ and H$\beta$. Moreover, some lines show multiple velocity
components, namely the Na I D 5889, H$\alpha$ 6562, K I 7664, and Ca II 8542 Å
lines (Fig. 9). These lines share at least three velocity components among
around $-$141, $-$65, $-$44, and $-$16 km s-1, implying that the lines might
form in the same region. Here we note that jet tracer lines, such as [S II],
[N II], and [O III] were not detected in our multi-epoch spectra as opposed to
V1057 Cyg (Szabó et al., 2021).

We also observed numerous metallic absorption lines during our observations.
These lines include various Fe I lines between 5446 and 7511 Å, Ca I 6122–6471
Å, Ti I 5965, 7344 Å, Ni I 6108, 6767 Å, and the Li I 6707 Å line (see some of
these lines in Fig. 16 in Section 4.3). In order to estimate the rotational
velocity ($v$sin$i$), we convolved the stellar spectrum with a Gaussian
profile to account for the rotational broadening of the lines and compared
their spectra to V1515 Cyg. For this analysis, we adopted the standard stellar
spectra from F8 to G8 supergiants used in Park et al. (2020), which are
observed with the same observing setup as V1515 Cyg. First, we averaged four
spectral lines (Fe I 6141.7, Fe I 6411.6, Ca I 6439.1, and Fe I 7511.0 Å) by
normalizing each peak intensity for V1515 Cyg and standard stars. Then, the
stellar spectrum was convolved with a Gaussian to account for various
rotational broadening from 0 to 30 km s-1 with a 1 km s-1 step. The best fit
was found by ${\chi}^{2}$ minimization, and the best fit rotational velocity
is 15 km s-1 found using G5 Ib-II type stellar template (Fig. 10). The
obtained rotational velocity is similar to the previously found $\sim$20 km
s-1 (Hartmann & Kenyon, 1985) and consistent with low disk inclination
(Gramajo et al., 2014; Hillenbrand et al., 2019; Milliner et al., 2019).
Furthermore, the best fit spectral type (G5 Ib-II) is also consistent with G
supergiants (Kenyon et al., 1991).

Figure 6: Time evolution of some characteristic lines in the optical spectra
of V1515 Cyg. To show each line profile clearly, we smoothed them with a 30
pixel moving window. The Li I 6707 Å line observed in 2015 December 26 is
affected by an artifact, which is indicated with a dashed line in the third
panel. The artifacts on the other lines are also presented with dashed lines.
Figure 7: Comparison of the profiles of lines with P Cygni shape in V1515 Cyg
in 2021 May.

Figure 8: Broad blue-shifted absorption lines of V1515 Cyg in 2021 May: Na I D
doublet (5889 and 5895 Å), Mg I 5167and 5184 Å, and K I (7664 and 7698 Å)
doublet.  Figure 9: Multiple velocity components observed in V1515 Cyg in 2021
May: Na D 5889, H$\alpha$ 6562, K I 7664, and Ca II 8542 Å. The dashed line
indicates zero velocity, and dotted lines mark blue-shifted velocity
components of about –141, –65, –44, and –16 km s-1. The spectra are smoothed
by 5 pixels to show line profiles clearly.  Figure 10: Averaged metallic
absorption profiles of V1515 Cyg in 2021 May (black) and standard star (red)
convolved with a rotational velocity of 15 km s-1.

####  3.5.2 Near-infrared Spectroscopy

Our recent NIR spectrum taken in 2021 shows several absorption lines:
Pa$\beta$, Al I, Mg I, FeH molecular bands, and strong CO absorption bandhead
features are present, similarly like in the previous spectra of V1515 Cyg
taken by Connelley & Reipurth (2018). In Fig. 11 we compare their 2015 June
spectrum and our spectrum from 2021 August: in the first panel we show the
full $JHK$ spectra, the next three panels show the $J$, $H$, and $K$-band
spectrum respectively. Although the NIR photometric observations are
insufficient since 2012, V1515 Cyg has almost the same brightness in 2012 and
2020 observations. There is no significant flux change of the $JHK_{s}$
spectrum in 2015 and 2020. However, our observation shows interesting changes
in some spectral features: the FeH ($>$ 1.62 $\mu$m) and CO ($>$ 2.29 $\mu$m)
molecular bands became stronger. Both the CO rovibrational and the FeH
molecular bands are stronger at lower temperatures (Wallace & Hinkle, 2001;
Bieging et al., 2002; Hargreaves et al., 2010), implying that V1515 Cyg became
cooler than observed in the previous observation in 2015 by Connelley &
Reipurth (2018).

Figure 11: The NIR $J$, $H$, and $K$ spectrum of V1515 Cyg. Gray and red
colors present the spectra observed with IRTF (Connelley & Reipurth, 2018) and
NOTCam, respectively. First panel shows the full $JHK$ spectrum, second the
$J$, third the $H$, then the $K$-band spectrum.

The Pa$\beta$ equivalent widths (EWs) became stronger in our recent NOTCam
(1.33 $\pm$ 0.53 Å) observation compared to the IRTF (1.03 $\pm$ 0.09 Å)
observations of Connelley & Reipurth (2018). However the uncertainties are
higher for the new observations, therefore, this change may not be
significant. The usual scaling relations between the fluxes of the hydrogen
emission lines were established for T Tauri stars (see e.g., Alcalá et al.,
2017). These relations work because the accretion in all of these stars happen
according to the magnetospheric accretion scenario (see e.g., Hartmann et al.,
2016). This scenario cannot be applied to FUors, as indicated by their lack of
emission lines. The absorption lines are associated with an active accretion
disk, and their formation is explained by temperature inversion (hot midplane,
colder disk atmosphere) (see e.g., Hartmann & Kenyon, 1996). In V1515 Cyg we
see the Pa$\beta$ line in absorption, as most of the lines observed in the
spectrum of FUors in general. Therefore, the hydrogen lines cannot be used for
the mass accretion calculation unlike in T Tauri stars.

###  3.6 Accretion Disk Modeling

In this section we model the inner part of the system with a steady, optically
thick and geometrically thin viscous accretion disk. We already applied this
method for a number of young eruptive stars (Kóspál et al., 2016, 2017;
Ábrahám et al., 2018; Kun et al., 2019; Szegedi-Elek et al., 2020; Szabó et
al., 2021) to separate the effects of variable accretion, extinction and study
their long term evolution quantitatively. The mass-accretion rate in our model
is constant in the radial direction (Eq. 1 in Kóspál et al., 2016). We neglect
any contribution from the star itself, assuming that all optical and near-
infrared emission in the outburst originates from the hot accretion disk. We
calculated synthetic SEDs of the disk by integrating the blackbody emission of
concentric annuli between the stellar radius and $R_{\rm out}$. The inner
radius of the disk mainly influences the emission in the optical bands. In
practice, we usually cannot discriminate between a small stellar radius with
higher line-of-sight extinction and a larger radius with lower extinction,
using only broad-band optical photometry. In the subsequent modeling we
prescribe that the average of our $A_{V}$ values obtained between 2012 and
2021 must comply with the $A_{V}$=3.5$\pm$0.4 mag proposed by Connelley &
Reipurth (2018) based on an infrared spectrum taken in 2015. This constraint
sets $R_{\rm in}$ = 1.4 R⊙. The outer radius of the accretion disk can be
estimated by fitting the mid-IR emission of the disk. We fixed $R_{\rm
out}=0.45$ au, which matches the $L$\- and $M$-band observations of V1515 Cyg
from 1989 (Kenyon & Hartmann, 1991). The inclination of the disk is also an
important input parameter of our accretion disk model. Most studies agree that
V1515 Cyg is seen close to pole-on (see e.g., Gramajo et al., 2014; Milliner
et al., 2019). In the following we adopt $i$=10∘.

Our models have only two free parameters: the product of the accretion rate
and the stellar mass $M\dot{M}$, as well as the line-of-sight extinction
$A_{V}$. Disk SEDs were calculated for a large range of $M\dot{M}$ values, and
at each step the fluxes were reddened using a grid of $A_{V}$ values assuming
the standard extinction law from Cardelli et al. (1989) with $R_{V}=3.1$.
Finding the best $M\dot{M}$ – $A_{V}$ combination was performed with
${\chi}^{2}$ minimization, by taking into account all quasi-simultaneous flux
values between 0.4 and 2.5$\,\mu$m. The formal uncertainties of the data
points were set to a homogeneous 5% of the measured flux value, which also
accounted for possible differences among photometric systems. The model fits
reproduced the measurements well, with low reduced $\chi^{2}$ values. The
resulting temporal evolution of the accretion rate and extinction values,
together with the optical $V$-band and the near-infrared $K$-band light curve,
are plotted in Fig. 12, where the $K$-band light curve is shifted by $+$4.2
mag for plotting purposes. We discuss these results further in the next
subsection and in the Sec. 4.

Figure 12: Optical $V$-band and near-infrared $K$-band light curves of V1515
Cyg (upper panel), temporal evolution of the accretion rate (middle panel) and
extinction toward the star (bottom panel) derived from our accretion disk
modeling described in Sect. 4.2.

###  3.7 Spectral Energy Distribution

Fig. 13 displays the SED of V1515 Cyg at several representative phases of its
outburst. The optical and infrared points up to ${\lambda}{\sim}5$ $\mu$m were
taken from Fig. 1, while photometry at longer wavelengths are used from
different space-borne missions, such as the IRAS (Neugebauer et al., 1984;
Joint Iras Science, 1994), MSX (Egan et al., 2003), Spitzer222Data were taken
from ”A Spitzer Legacy Survey of the Cygnus-X Complex” catalogue, available at
the IRSA database https://irsa.ipac.caltech.edu/data/SPITZER/Cygnus-X/ (Hora
et al., 2007), Herschel (Green et al., 2013b) and the WISE (Wright et al.,
2010), and we also used data from the airborne SOFIA (Young et al., 2012)
missions. The data points from the Herschel and the AllWISE catalogs were
obtained within a year, thus we combined the two data sets into one SED. In
the far-infrared domain (${\lambda}>25$ $\mu$m) the IRAS and ISO measurements
were apparently contaminated by some extended emission in their large beams,
thus were discarded from the plot. Contamination was confirmed by archival 70
$\mu$m and 160 $\mu$m images by the Herschel Space Telescope, which show that
V1515 Cyg is located at the edge of a dust cloud, on a structured far-infrared
background.

There are several properties of the SED of V1515 Cyg, which are discussed
below in Sec. 4.2

Figure 13:  Spectral energy distribution of V1515 Cyg at different
representative epochs. The data points are from Fig. 1, as well as from space-
based (IRAS, Spitzer, WISE, Herschel) and airborne (SOFIA) missions, as
indicated in the legend. Solid curves show the results of our accretion disk
models for the individual epochs.

##  4 Discussion

###  4.1 Dimming events in V1515 Cyg

In Sec. 3.4, we described the interesting dimming event of V1515 Cyg observed
in 2021. In the following we investigate the possible physical
cause/interpretation of such dimming events. Kenyon & Hartmann (1991) and
Kolotilov & Kenyon (1997b) assumed that the occultation events were due to
dust formation in an expanding shell in the wind. However, Clarke et al.
(2005) considered that in the case of V1515 Cyg the occultation events are
interactions between the wind and the surrounding infalling envelope. They
proposed that, for instance, V1057 Cyg has a strong wind and was able to clear
the circumstellar material in response to the outburst. The wind in V1515 Cyg
has always been weaker, therefore it has never been able to completely clear
the line of sight of dusty material, which can explain the variations since
the beginning of the outburst. The dust lifted by the wind could cause this
variability. In contrast, FU Ori was able to clear a large cavity, hence its
light curve is free from this kind of variability (Clarke et al., 2005). This
could also explain why there is no correlation between the photometric and the
spectroscopic variation suggested by Clarke et al. (2005). We tried to draw an
analogy with the T Tauri star, RW Aur, where multiple, similar dimming events
were observed with similarly good data sampling. Koutoulaki et al. (2019)
suggested that the deep minima are most likely caused by a layer of dust
obscuring the inner 0.05 – 0.1 au of the system, while the accretion rate did
not change significantly (see also other studies, e.g., Petrov et al., 2015;
Facchini et al., 2016, and references therein). However, there are many
obvious differences between the two systems. Firstly, that although V1515 Cyg
is approaching the quiescence phase, it is still in outburst, hence exhibiting
a more enhanced accretion rate compared to RW Aur. Therefore, the optical
photometry of V1515 Cyg is still dominated by the accretion disk (and the star
is negligible), as is usual in FUors in outburst. Moreover, in V1515 Cyg we
see the continuous decrease of the accretion rate despite the dimming events.
Another difference is the that RW Aur is a binary, while V1515 Cyg seems to be
a single star.

In order to understand the observed dimming event in 2021, additional,
similarly well sampled multi-filter light curves, ideally complemented by
spectroscopic monitoring, are needed to look for similarities or differences
between these two kinds of dimming events.

###  4.2 Accretion Disk Modeling and SED

Fig. 12 shows the temporal evolution of the accretion rate and the extinction
together with the optical $V$-band and the near-infrared $K$-band light curves
of V1515 Cyg. The long-term evolution of the accretion rate until 2010 is
approximately consistent with an exponential decay, with an e-folding time of
20600 days ($\sim$56 years). The dashed line in Fig. 12 was fitted to the
$V$-band data after 2003 in order to determine the e-folding time of the long-
term optical fading, corresponding to approximately 12 years.

The peak and current $M\dot{M}$ values are 1.29$\times$10-5 and
4.17$\times$10-6 M${}_{\odot}^{2}$yr-1, respectively. Assuming a stellar mass
of 0.3 M⊙, the real accretion is higher by a factor of about 3. The $A_{V}$
curve is rather constant: a weak gradual decrease is hinted before 2000, which
might be responsible for the brightening of the system during this period, in
spite of the decreasing accretion rates. The current luminosity of the
accretion disk is about 45 $L_{\odot}$, dropped from a peak value of 138
$L_{\odot}$.

The SED of the source has several properties which we discuss further below.
The SED is dominated by a peak at optical-near-NIR wavelengths. The clear
excess above a blackbody-like extrapolation of this peak to infrared
wavelengths implies the presence of a significant amount of circumstellar
matter (the total circumstellar mass, based on IRAM observations, is 0.04 M⊙
(Fehér et al., 2017)). The spectral range around 10 $\mu$m and 18 $\mu$m
exhibits broad emission features, which is attributed to submicrometer size
silicate grains (see also e.g., Green et al., 2006). The strength of the short
wavelength peak monotonically decreased with time, reflecting the evolution of
the hot inner accretion disk modeled in Sec. 3.6. A temporal descrease of the
mid-infrared flux is also suggested, although the data coverage is less
complete than in the optical.

At far-infrared and submillimeter wavelengths the Herschel observations may
represent the emission of the circumstellar disk, with no clear sign of a
massive cold circumstellar envelope. There is no information on the temporal
evolution of the system during the outburst at these wavelengths. The apparent
consistency between the 450 $\mu$m JCMT/SCUBA (Sandell & Weintraub, 2001a)
single dish measurement from 1998 (Sandell & Weintraub, 2001b) and the
500$\,\mu$m Herschel/SPIRE observations from 2011 suggests that the flux
change at these long wavelengths is not significant.

Due to the dispersion of the data points, the color evolution can be
consistent with evolution along either constant accretion or constant
extinction path, in general the color evolution is consistent with the
evolution along constant accretion or constant extinction path, obtained from
our accretion disk model in Sec. 3.6 and Sec. 4.2. Considering the small
variability amplitude and the photometric uncertainty, it is not possible to
decide which is the dominant mechanism. As the global long-term color
evolution is accounted by the modeling, we do not present here the global
color-magnitude diagrams but refer the reader to Sec. 3.6 and Sec. 4.2 for the
general results.

###  4.3 Comparison between V1515 Cyg and V1057 Cyg

In this section we give a comparison between V1515 Cyg and V1057 Cyg. First we
compare their light curves, then their spectroscopic properties using our
previous study by Szabó et al. (2021) with the caveat that the spectra for the
two sources were taken in different stages of their outburst evolution. Both
sources are considered classical FUors (Herbig, 1977; Herbig et al., 2003;
Connelley & Reipurth, 2018), and we were able to observe similar properties in
the spectra of both targets, comparing them in the following.

_Similarities._ A similar aspect of both sources can be found in their light
curves: both objects underwent a sharp dip event after reaching the peak
brightness. In the case of V1515 Cyg this happened right after reaching the
light maximum in 1980, recovering and then showing a plateau stage until the
early 2000’s (Fig. 1). However, the light curve of V1057 Cyg is different,
this source also experienced a similar, sharp dip, around 1995 after an
exponential fading trend. Similarly, just like in V1515 Cyg, V1057 Cyg also
recovered from this event and showed a plateau stage afterwards (called
’second plateau’ Szabó et al., 2021). Interestingly, FU Ori also showed a
similar event, followed by a long-term plateau stage (see e.g., Hartmann et
al., 2016).

In order to compare two FUors homogeneously, we used the latest observation of
our NOT/FIES optical spectra for both. Our targets show four Balmer lines from
H$\alpha$ to H$\delta$ in our observations with broad blue-shifted absorption
profiles. Among these four lines, relatively clear (higher S/N) H$\alpha$ and
H$\beta$ are presented in Fig. 14. H$\alpha$ and H$\beta$ show P Cygni
profiles with broad blue-shifted absorption component and red-shifted emission
profile in both targets. The two FUors show P Cygni profiles of the H$\alpha$,
Ca II 8542, and 8662 Å lines as shown in Fig. 14. In addition, both FUors show
blue-shifted absorption line profiles formed by an outflowing wind in several
neutral and singly ionized metallic lines (Fig. 15). However, V1057 Cyg shows
higher velocity components of shell features than V1515 Cyg (e.g., Ba II 4934
Å and Fig. 11 in Szabó et al., 2021).

_Differences._ Even though we see similarities in their light curves, perhaps
the most striking difference between the two classical FUors is the
brightening phase presented in their light curves. The outburst of V1057 Cyg
was an abrupt one, happening within less than a year (see Fig. 1 in Szabó et
al., 2021). The outburst of V1515 Cyg was different from the beginning
compared to the other classical FUors, taking about four decades to reach the
maximum brightness. This difference suggests that overall the outburst
mechanisms are different.

Interestingly, in the optical spectrum of V1515 Cyg there is a lack of P Cygni
profile in the Ca II 8498 Å line compared to V1057 Cyg. In our previous study
on V1057 Cyg, we found that all three lines of the Ca II IRT showed clear P
Cygni profiles, whereas in the case of V1515 Cyg, we found P Cygni profiles
only for the Ca II 8542 and Ca II 8662 Å lines. The strength/depth of the
absorption is also weaker than those observed in V1057 Cyg (Herbig et al.,
2003; Szabó et al., 2021). Several works investigated the relation between the
Ca II IRT lines in T Taur stars and found that if the lines are optically thin
the ratio of the three lines is 1:9:5, and in the optically thick case it is
1:1:1 (see e.g., Herbig & Soderblom, 1980; Hamann & Persson, 1989, 1990;
Azevedo et al., 2006). This means that the absorption in the 8498 Å line would
be 9 times weaker than in the 8542 Å line. The fact, that the absorption
component is not detected (i.e., remains below our detection threshold) in the
8498 Å line suggests that the gas in the outflowing wind causing the
absorption component is probably optically thin.

Since the strength of the blue-shifted absorption component in a P Cygni
profile is formed by an outflowing wind in the system, in the case of V1515
Cyg, the weaker strength could be an indication of the weaker outflowing wind
in the structure. Compared to V1057 Cyg where the Ca IRT lines clearly varied
with time, in V1515 Cyg, we did not observe strong temporal variation of the
lines either. The strength of the P Cygni profiles varies over time, however,
they stay within the same velocity range. In addition, no shell lines nor
forbidden emission lines were observed in V1515 Cyg as opposed to V1057 Cyg
(Szabó et al., 2021). Fig. 16 shows the comparison of atomic metallic lines in
both FUors. Unlike V1057 Cyg, a number of atomic metallic lines are observed
in the spectrum of V1515 Cyg that are not present in V1057 Cyg.

In the multi-epoch spectrum of V1515 Cyg, there is no evidence of jet tracer
lines, such as [S II], [N II], and [O III], as opposed to V1057 Cyg (Szabó et
al., 2021). The absence of these lines further supports the idea of a weaker,
uncollimated optically thin wind in this system.

Figure 14: P Cygni profiles of H$\beta$ 4861, H$\alpha$ 6562, Ca II 8542, and
Ca II 8662 Å. Figure 15: Broad blue-shifted absorption lines: Ba II 4934, M I
5183, Na D 5889/5895, K I 7664, and K I 7698 Å.  Figure 16: Atomic absorption
lines of Fe I 6141, Ca I 6439, Li I 6707, and Fe I 7511 Å. Figure 17: The
comparison of the NIR spectra of V1515 Cyg and V1057 Cyg. Light gray and blue
lines indicate V1057 Cyg observed with IRTF and NOTCam, respectively. Dark
gray and red lines present V1515 Cyg observed with IRTF and NOTCam,
respectively.  Figure 18: CO bandhead of V1515 Cyg (upper spectra) and V1057
Cyg (lower spectra). Dashed lines indicate the integration range for EW.

Fig. 17 shows the comparison of the NIR spectra between V1515 Cyg (dark gray
and blue) and V1057 Cyg (light gray and red). Overall, the flux of V1515 Cyg
is lower than the one of V1057 Cyg, and the flux change between 2015 and
2020/21 is also smaller in V1515 Cyg. Most of the observed lines are the same
as those of V1057 Cyg in the NIR spectra. To compare spectral changes in V1515
Cyg itself and the two FUors, we measured the EW of the CO overtone bandhead
features (2.293 – 2.317 $\mu$m) with the same method described in detail in
Szabó et al. (2021). Fig. 18 shows the CO bandhead of V1515 Cyg (upper
spectra) and V1057 Cyg (lower spectra), and the measured EW is listed in Table
3. In the case of V1057 Cyg, the CO 2–0 bandhead feature became weaker with
the decreasing brightness, and it is interpreted as the decreasing mass
accretion rate (Szabó et al., 2021). On the other hand, in the case of V1515
Cyg, the bandhead features almost remain the same strength (Fig. 18),
similarly to the monitoring results of the optical spectra (Fig. 6). In our
observation, the EW of the CO increases, which is mostly affected by the lower
temperature rovibrational transitions ($\sim$2.31 $\mu$m) in Fig. 18. This
result is consistent with the presence of the stronger FeH in our observation
(Fig. 11). We estimated the gas temperature by comparing the FeH feature at
1.62 $\mu$m in V1515 Cyg and those of standard stars (Rayner et al., 2009)
between M4 and L5 spectral types (Fig. 19). Both the target and the stellar
spectrum were normalized. We found the best fit by eye due to the blended
molecular nature of this feature. The previous spectrum taken in 2015 by
Connelley & Reipurth (2018) was well-matched with an M6 – M7 dwarf spectrum
(Teff$\sim$2700 K (GJ 406); Kuznetsov et al., 2019), while the 2021 spectrum
is well-matched with a later type dwarf of M9 – M9.5 (Teff$\sim$2500 K (LHS
2924); Gagné et al., 2015). Together with the decreasing temperature from the
FeH, and the increased EW of the CO bandhead in the 2021 observation (see Sec
3.5.2), we conclude that the disk of V1515 Cyg became cooler since 2015,
further suggesting that the source is approaching a quiescence phase.

An accurate comparison is still difficult to make between the two sources. The
two system have very different light curves, indicating that overall different
energetics were present in the early stages of the outburst (see Fig. 1 in
Szabó et al., 2021). While a long term fading trend is present in both cases,
the decline is currently much faster and drastic in V1515 Cyg. As shown, the
current mass accretion rate of V1515 Cyg is much lower (4.17$\times$10-6
M${}_{\odot}^{2}$yr-1) than in V1057 Cyg (10-4 M${}_{\odot}^{2}$yr-1), which
can justify the lower mass ejection rate and the optically thin outflow in the
former system. Overall, the weaker wind in V1515 Cyg further suggests that the
outburst mechanisms are quite different in the two sources. It is important to
mention that different geometries can also play a role in differences between
systems, however the inclination of the two sources are similar as both
systems are seen close to face-on (see e.g., Gramajo et al., 2014; Milliner et
al., 2019).

Table 3: EW of CO overtone bandhead

Target | Observation Date | EW | Reference
---|---|---|---
| [UT] | [Å] |
V1515 Cyg | 2015 Junea | 16.25 $\pm$ 0.42 | This work
V1515 Cyg | 2021 August | 23.02 $\pm$ 0.66 | This work
V1057 Cyg | 2015 Junea | 22.75 $\pm$ 0.36 | Szabó et al. (2021)
V1057 Cyg | 2020 August | 27.03 $\pm$ 0.45 | Szabó et al. (2021)

aafootnotetext: Spectrum from Connelley & Reipurth (2018)

Figure 19: FeH features around 1.62 $\mu$m. Black and red lines present V1515
Cyg and standard stellar spectrum (Rayner et al., 2009). The IRTF and NOTCam
spectra are well-matched with M6 and M9 dwarfs, respectively.

##  5 Conclusions

In this paper we performed a multi-epoch, multi-wavelength study of the
classical FU Orionis-type star V1515 Cyg and we arrived to the following
conclusions:

  * •

Our long-term photometric monitoring of V1515 Cyg supplemented with data
obtained at Mt. Maidanak and data from publicly available archives such as
INTEGRAL, ASAS-SN, and ZTF show a long-term fading trend which started around
2006. The fading is not monotonous: after the maximum brightness (1984–2005),
the inner disk apparently experienced several obscuration-like events. The
long-term evolution of the source’s optical brightness after 2003 is
approximately consistent with an exponential decay, with an e-folding time of
approximately 12 years.

  * •

The Mt. Maidanak photometry enabled us to investigate the light curve changes
occurring in the timescale of 5–100 d during the maximum. Comparisons of these
multi colour light curves suggest that the majority of these light curve
changes did originate from the inner disk. We confirmed the 13.9 d QPO found
in the 1987 data set by Clarke et al. (2005) and we found a similar peak of
13.06 d QPO in the 2003 data set. Although observed in two seasons only, these
values are surprisingly similar to the persistent 14 d modulation of spectral
wind features found in FU Orionis (Herbig et al., 2003; Powell et al., 2012).
Time-series spectroscopy can decide about the possible similarities in V1515
Cyg in the future.

  * •

Although TESS enabled investigation of short-periodic, down to $\sim$20 min
oscillations, the 2019 and 2021 light curves do not show rapidly occurring
light variations on such time scales. The frequency spectra exhibit the
random-walk character, being of the same type as the one found in our previous
study of V1057 Cyg (Szabó et al., 2021). It is plausible that the dimming
event studied in great detail during the 2021 season could be similar to what
was observed in the CTTS RW Aur, however, there are many differences between
the two system. Additional, similarly well sampled seasons are required to
investigate and compare this phenomena further in the future.

  * •

Our accretion disk modelling reveals that the current luminosity of the
accretion disk dropped from the peak value of 138 $L_{\odot}$ to about 45
$L_{\odot}$. The peak and current $M\dot{M}$ values are 1.29$\times$10-5 and
4.17$\times$10-6 M${}_{\odot}^{2}$yr-1, respectively. This indicates that the
long-term fading is also partly caused by the dropping of the accretion rate.
The optical and near-infrared peak present in the SED indicates the presence
of significant circumstellar matter surrounding the source. The long-term
evolution of the accretion rate until 2010 is approximately consistent with an
exponential decay, with an e-folding time of $\sim$56 years.

  * •

We performed optical spectroscopic monitoring of the source between 2015–2021
and detected several absorption lines, which show small variations over time.
Several high-velocity wind features were also observed in the form of
absorption components of P Cygni profiles which similarly show small
variations. The source lacks the P Cygni profile in the Ca II 8498 Å line,
part of the Ca infrared triplet (IRT), compared to V1057 Cyg and other
classical FUors (Szabó et al., 2021). The strength of the blue-shifted
absorption component in this P Cygni profile is formed by an outflowing wind.
The weaker strength in this case could be an indication of the weaker,
optically thin outflowing wind in the structure.

  * •

During our spectroscopic monitoring despite the pure emission of the Ca II
8498 Å line, we did not observe any other emission, nor forbidden emission
lines in the spectra of V1515 Cyg. The absence of certain forbidden emission
lines indicate that there are no jets/outflows present while these were
detected in V1057 Cyg (Szabó et al., 2021).

  * •

For our study, we obtained a new NIR spectrum of V1515 Cyg in 2021, and
compared it with the one obtained in 2015 by Connelley & Reipurth (2018). In
the new one, we found that both the CO overtone bandhead features and the FeH
molecular bands became stronger since the previous observation. Together with
the decreasing temperature from the FeH, and the increased EW of the CO
bandhead in the 2021 observation, we conclude that the disk of V1515 Cyg
became cooler by about 200 K since 2015, further suggesting that the source is
approaching the quiescent phase.

  * •

Despite the long-term fading of V1515 Cyg, our results from the spectroscopic
monitoring and analysis still resemble the properties those of a
classical/bona fide FUor.

The continuation of the photometric monitoring supplemented with additional
spectroscopic snapshot observations can be crucial for our understanding of
the late evolution of FUor outbursts and how they eventually end. This study
is the second part of a series with the aim of revisiting the first few
classical FUors after the famous 1936 eruption of FU Orionis, the prototype
source. Our study highlights the importance of scrutinizing these objects
which ultimately can help us to better understand the accretion processes in
the early stages of Sun-like star formation.

## Acknowledgements

We kindly thank Michael Connelley for handing us over the 2015 IRTF spectra of
V1515 Cyg in order to carry out a more accurate analysis. This project has
received funding from the European Research Council (ERC) under the European
Union’s Horizon 2020 research and innovation programme under grant agreement
No 716155 (SACCRED). M. Ibrahimov and K. Grankin acknowledge an active
participation of their colleagues: Drs. S.Yu. Melnikov, S.D. Yakubov, O.V.
Ezhkova, S.A. Artemenko and some others - to obtain UBVRcIc photoelectric and
BVRI CCD obsrevations of V1515 Cyg at Maidanak Astronomical Observatory (UBAI,
Tashkent, Uzbekistan). This article is partly based on observations made with
the 1.5 m TCS and IAC-80 telescopes operated on the island of Tenerife by the
Instituto de Astrofisica de Canarias in the Spanish Observatorio del Teide.
B.K. was supported by the National Research, Development and Innovation Fund
of Hungary, financed under the 2020-1.2.1-GYAK-2020-00007 funding scheme. The
work was also supported by the Hungarian NKFIH grant KH-30526 and K-31508. Our
study is supported by the project ”Transient Astrophysical Objects” GINOP
2.3.2-15-2016-00033 of the National Research, Development and Innovation
Office (NKFIH), Hungary, funded by the European Union. This project has been
supported by the Lendület grant LP2012-31 of the Hungarian Academy of
Sciences. J-E. Lee was supported by the Basic Science Research Program through
the National Research Foundation of Korea (grant no. NRF-2018R1A2B6003423).
R.K.T. has been supported by the NKFIH/OTKA FK-134432 grant. K.CS. has been
supported by the ÚNKP-21-2 New National Excellence Program of the Ministry for
Innovation and Technology from the source of the National Research,
Development and Innovation Fund. This research was supported by the KKP-137523
‘SeismoLab’ Élvonal grant of the Hungarian Research, Development and
Innovation Office (NKFIH). A.B. is supported by the Lendület LP2014-17. K.V.
is supported by the Bolyai János Research Scholarship of the Hungarian Academy
of Sciences. K.V. acknowledges the support of the Hungarian National Research,
Development and Innovation Office grant OTKA KH-30526 and K-3150. L.K.
acknowledges the financial support of the Hungarian National Research,
Development and Innovation Office grant NKFIH PD-134784 and K-131508. L.K. is
a Bolyai János Research Fellow. A.P. acknowledges the financial support of the
Hungarian National Research, Development and Innovation Office (NKFIH) grant
K-138962. Á.S. is supported by the Élvonal grant KKP-137523.

This paper includes data collected by the TESS mission. Funding for the TESS
mission is provided by the NASA Explorer Program.. Based on observations made
with the Nordic Optical Telescope, operated by the Nordic Optical Telescope
Scientific Association at the Observatorio del Roque de los Muchachos, La
Palma, Spain, of the Instituto de Astrofisica de Canarias. The data presented
here were obtained (in part) with ALFOSC, which is provided by the Instituto
de Astrofisica de Andalucia (IAA) under a joint agreement with the University
of Copenhagen and NOTSA. The DASCH project at Harvard is partially supported
by NSF grants AST-0407380, AST-0909073, and AST-1313370. Based on observations
obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory
as part of the Zwicky Transient Facility project. ZTF is supported by the
National Science Foundation under Grant No. AST-1440341 and a collaboration
including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein
Center at Stockholm University, the University of Maryland, the University of
Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los
Alamos National Laboratories, the TANGO Consortium of Taiwan, the University
of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories.
Operations are conducted by COO, IPAC, and UW. Based on observations obtained
with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the
Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is
supported by the National Science Foundation under Grant No. AST-2034437 and a
collaboration including Caltech, IPAC, the Weizmann Institute for Science, the
Oskar Klein Center at Stockholm University, the University of Maryland,
Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium
of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin,
Lawrence Livermore National Laboratories, and IN2P3, France. Operations are
conducted by COO, IPAC, and UW. This paper also uses observations made at the
Mount Suhora Astronomical Observatory (MSO) in Poland.

##  Appendix A Photometry of V1515 Cyg

Table 4 contains our original optical and near-infrared photometry of V1515
Cyg before the shifts discussed in Sec. 2, while Table LABEL:tab:phot_wise
contains the saturation corrected WISE data that we use in Fig. 1. Table 6
contains shifts applied for the different data sets in the optical bands
described in Sec. 3.1

Table 4: Optical and near-IR photometry of V1515 Cyg.

Date | MJD | $B$ | $V$ | $g^{\prime}$ | $R_{\rm C}$ | $I_{\rm C}$ | $R_{\rm J}$ | $I_{\rm J}$ | $r^{\prime}$ | $i^{\prime}$ | $J$ | $H$ | $K_{\rm s}$ | Instrument
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---
2005-11-18 | 53692.83 | … | $13.11$ | … | $12.17$ | $11.13$ | … | … | … | … | … | … | … | RCC
2006-07-12 | 53928.15 | … | … | … | … | … | … | … | … | … | $9.04$ | $8.18$ | $7.55$ | TCS
2006-07-19 | 53935.01 | $14.85$ | $12.97$ | … | $12.13$ | … | … | … | … | … | … | … | … | IAC80
2011-08-05 | 55778.97 | $15.20$ | $13.35$ | … | $12.45$ | $11.39$ | … | … | … | … | … | … | … | Schmidt
2020-05-18 | 58988.06 | $15.91$ | $14.03$ | … | … | … | … | … | $13.42$ | $12.63$ | … | … | … | RC80
2021-04-25 | 59329.20 | … | … | … | … | … | … | … | … | … | $9.72$ | $8.52$ | $7.92$ | NOT
2022-01-23 | 59603.20 | $15.90$ | $14.04$ | $14.99$ | … | … | … | … | $13.43$ | $12.63$ | … | … | … | RC80

Note. — The table is published in its entirety in the machine-readable format.
A portion is shown here for guidance regarding its form and content.

Table 5: Saturation corrected WISE data of V1515 Cyg Date | MJD | W1 mag | W2 mag | Survey Phase
---|---|---|---|---
2010-05-09 | 55334.3 | 6.94$\pm$0.03 | 6.07$\pm$0.01 | 4–Band Cryo
2010-11-14 | 55514.6 | 7.18$\pm$0.05 | 6.21$\pm$0.02 | 4–Band Cryo
2014-05-20 | 56797.8 | 7.04$\pm$0.02 | 6.27$\pm$0.01 | Post–Cryo
2014-11-17 | 56978.0 | 7.03$\pm$0.03 | 6.23$\pm$0.01 | Post–Cryo
2015-05-19 | 57161.1 | 7.10$\pm$0.02 | 6.29$\pm$0.01 | Reactivation
2015-11-11 | 57337.7 | 7.18$\pm$0.02 | 6.39$\pm$0.01 | Reactivation
2016-05-17 | 57525.2 | 7.11$\pm$0.02 | 6.39$\pm$0.02 | Reactivation
2016-11-04 | 57696.3 | 7.19$\pm$0.02 | 6.44$\pm$0.01 | Reactivation
2017-05-18 | 57891.1 | 7.17$\pm$0.02 | 6.41$\pm$0.01 | Reactivation
2017-11-02 | 58059.7 | 7.21$\pm$0.02 | 6.46$\pm$0.02 | Reactivation
2018-05-18 | 58256.6 | 7.26$\pm$0.02 | 6.49$\pm$0.01 | Reactivation
2018-10-29 | 58420.2 | 7.25$\pm$0.03 | 6.45$\pm$0.02 | Reactivation
2019-05-17 | 58620.7 | 7.23$\pm$0.02 | 6.46$\pm$0.01 | Reactivation
2019-10-28 | 58784.7 | 7.23$\pm$0.02 | 6.43$\pm$0.01 | Reactivation
2020-05-18 | 58987.8 | 7.35$\pm$0.02 | 6.52$\pm$0.01 | Reactivation
2020-10-29 | 59151.3 | 7.26$\pm$0.02 | 6.49$\pm$0.02 | Reactivation
2021-05-18 | 59352.6 | 7.26$\pm$0.02 | 6.52$\pm$0.02 | Reactivation
2021-10-28 | 59516.1 | 7.27$\pm$0.02 | 6.51$\pm$0.02 | Reactivation

Table 6: Shifts applied to match overlapping optical data sets. telescope/filter | Schmidt | RC80 | Mt. Suhora | ZTF | ASAS-SN | INTEGRAL | IAC | DASCH
---|---|---|---|---|---|---|---|---
$B$ | 0.00 | 0.00 | +0.03 | +0.266 | – | – | -0.15 | -0.28
$V$ | 0.00 | 0.00 | +0.05 | +0.115 | -0.03 | -0.53 | +0.02 | –
$R_{C}$ | 0.00 | 0.00 | +0.07 | +0.16 | – | – | – | –
$I_{C}$ | 0.00 | 0.00 | +0.115 | – | – | – | – | –
$g$ | – | 0.00 | +0.03 | +0.09 | +0.02 | – | – | –
$r$ | – | 0.00 | +0.07 | +0.15 | – | – | – | –
$i$ | – | 0.00 | +0.11 | – | – | – | – | –

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authors:
Zs. M. Szab\'o, \'A. K\'osp\'al, P. \'Abrah\'am, S. Park, M. Siwak, J.
  D. Green, A. P\'al, J. A. Acosta-Pulido, J.-E. Lee, M. Ibrahimov, K. Grankin,
  B. Kov\'acs, Zs. Bora, A. B\'odi, B. Cseh, G. Cs\"ornyei, Marek Dr\'ozdz, O.
  Hanyecz, B. Ign\'acz, Cs. Kalup, R. K\"onyves-T\'oth, M. Krezinger, L.
  Kriskovics, Waldemar Ogloza, A. Ordasi, K. S\'arneczky, B. Seli, R.
  Szak\'ats, A. Szing, K. Vida, J. Vink\'o

submitter: Zs\'ofia Marianna Szab\'o


text:
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