11institutetext:  Helmholtz-Institut für Strahlen- und Kernphysik, Universität
Bonn, Nussallee 14-16, D-53115 Bonn, Germany
11email<EMAIL_ADDRESS>22institutetext: Inter-University Centre
for Astronomy and Astrophysics (IUCAA), Post Bag 4, Ganeshkhind, Pune 411007,
India 33institutetext: Indian Institute of Astrophysics, Koramangala II Block,
Bangalore-560034, India
33email<EMAIL_ADDRESS><EMAIL_ADDRESS>
# UVIT Open Cluster Study. X.

Rich Collection of Post Mass Transfer Systems in NGC 6791

Vikrant V. Jadhav  0000-0002-8672-3300 1122 Annapurni Subramaniam
0000-0003-4612-620X 33 Ram Sagar  0000-0003-4973-4745 33

(Received January 13, 2023; accepted June 27, 2023)

###### Abstract

Aims. NGC 6791 is one of the richest old open clusters in the Milky Way. Its
position above the Galactic plane and the number density makes it an
interesting middle ground between Galactic open and globular clusters. We aim
to detect the UV bright population of NGC 6791 using AstroSat/UVIT images in
near-UV and far-UV filters and characterise the known post mass transfer
systems such as blue straggler stars (BSSs).

Methods.  We identified 20 members with large UV flux (out of 91 cluster
members among 1180 detections), suggestive of binarity, interactions or
stellar activity using multi-wavelength spectral energy distribution analysis.

Results. We characterised 62 isolated cluster members, including five hot
subdwarfs (sdA/sdB). Additionally, we detected ten sdA/sdB/extremely low mass
white dwarf (ELM) type candidates hidden alongside other cluster members.
Additionally, we report the discovery of four candidate blue lurkers, which
are main sequence stars with mass accretion history.

Conclusions. We report that this cluster has a variety of stellar
(pre-)remnants, such as sdBs, sdAs, and ELM white dwarfs, that are by-products
of binary evolution. The above are likely to be post mass transfer binaries
found throughout the evolutionary phases from the main sequence to the post
horizontal branch. Therefore, this dynamically old open cluster is unique,
making it an ideal testbed for dynamical studies.

###### Key Words.:

(Galaxy:) Open clusters and associations: individual: NGC 6791 – (stars:)
binaries: general – Ultraviolet: stars – Catalogues

##  1 Introduction

Open clusters are integral to studying stellar populations due to their
relative homogeneity (in metallicity and age) and robust cluster membership
due to precise astrometry. UV imaging has proved very useful in identifying
optically subluminous stars such as white dwarfs (WDs) and hot subdwarfs (Sahu
et al., 2019; Jadhav et al., 2021; Rao et al., 2022). We are currently
conducting UVIT Open Cluster Study (UOCS) to understand the UV bright
population, focusing on post mass transfer systems. M67 has proven to be home
to multiple extremely low mass (ELM) WDs, which are products of mass transfer
(Sindhu et al. 2019; Jadhav et al. 2019; Subramaniam et al. 2020; Pandey et
al. 2021; Vernekar et al. 2023, in press). Similar mass transfer binaries have
been found in NGC 7789 (Vaidya et al., 2022) and NGC 2506 (Panthi et al.,
2022). Here, we extend this study to the open cluster NGC 6791
($\alpha_{2000}$ = 19h 20m 53s; $\delta_{2000}$ = +37∘46′18″; l =
69$.\\!\\!^{\circ}$959; b = +10$.\\!\\!^{\circ}$904) which is $\sim$ 8.5 Gyr
old and also one of the metal-rich cluster ([Fe/H] $\sim$ 0.4) known in the
Milky Way (Bossini et al., 2019). It is located at a distance of $\sim$ 4.1
Kpc and is well studied massive ($\sim 5000$ M⊙) open star cluster.

Using ground-based and Kepler photometry and multi-epoch spectroscopy data,
Brogaard et al. (2018) identified the binary star V106 as a blue straggler
star (BSS) member of the NGC 6791. They derived the primary mass as 1.67 M⊙,
more massive than the cluster turn-off mass and the secondary star as a
bloated (proto) ELM helium WD. A detailed study also reveals that V106 is
potentially a prototype progenitor of old field giants masquerading as young.
Villanova et al. (2018) presented and discussed detailed abundances of 17
evolved stars of NGC 6791 using high-resolution spectra obtained with the
UltraViolet and Visible Echelle Spectrograph at the European Southern
Observatory Very Large Telescope and High Resolution Echelle Spectrometer at
the Keck telescope. They obtained a mean [Fe/H] = $+0.313\pm 0.005$, in good
agreement with recent estimates. Tofflemire et al. (2014) provided the epoch
radial velocity (RV) and related results for 280 stars, including main
sequence (MS), red giant branch (RGB) and horizontal branch (HB) stars. Jadhav
& Subramaniam (2021) identified 47 potential BSSs in NGC 6791 using Gaia DR2
data. Kamann et al. (2019) combined Gaia data with the archival line of sight
velocities and studied the internal dynamics of the NGC 6791 in three
dimensions. Martinez-Medina et al. (2018) performed an orbital analysis within
a Galactic model (including spiral arms and a bar) and found that it is
plausible that NGC 6791 formed in the inner thin disc or the bulge and was
later displaced by radial migration to its current orbit. The birthplace and
journeys of NGC 6791 are imprinted in its chemical composition, mass-loss and
flat stellar mass function, supporting its origin in the inner thin disc or
the bulge.

NGC 6791 has one of the largest populations of BSSs among all known open
clusters (Jadhav & Subramaniam, 2021). The BSSs result from binary
interactions such as collisions (Hills & Day, 1976) or mergers (Perets &
Fabrycky, 2009). However, knowing which formation pathway had been followed is
difficult due to the incomplete parameterisation of such systems. The mass of
the BSS, stellar rotation, abundance peculiarities and characteristics of the
donor remnant can be used to identify the formation pathway.

In this work, we analysed the UV bright population of NGC 6791 using multi-
wavelength spectral energy distributions (SEDs). §2 gives the details of the
data and analytical methods, §3 presents the results and their implications.

Table 1: Exposure times in UVIT filters and detected sources. Filter | Exp. Time [s] | Detections | Members
---|---|---|---
F172M | 3017.251 | 55 | 6
N245M | 1259.539 | 734 | 35
N263M | 1156.669 | 1180 | 81
N279N | 364.296 | 165 | 8

##  2 Data and analysis

###  2.1 UVIT data and Gaia membership

We observed NGC 6791 in August 2017 with the Ultra-Violet Imaging Telescope
(UVIT) onboard AstroSat observatory (proposal ID: A03_008). The UVIT performs
observations in far-UV (130–180 nm), near-UV (NUV; 200–300 nm) and VIS
(350–550 nm) channels. More details of the calibration and instrumentation are
present in Tandon et al. (2017, 2020); Kumar et al. (2012). The exposure
times, source detections and other observational details are given in Table 1.
The UVIT data reduction and astrometry were performed using ccdlab (Postma &
Leahy, 2017, 2020). The point spread function photometry of the UVIT images
was done with iraf (Tody, 1993). Preliminary analysis and cross-matching were
done with topcat (Taylor, 2005).

Figure 1: (a) Spatial distribution of Gaia members. (b) Spatial distribution
of UV detected members (blue circles) and UV detected sources which are not
present in Gaia EDR3 catalogue (grey dots). (c) Magnitude vs error plots for
UVIT photometry. (d) Gaia CMD showing the Cantat-Gaudin et al. members (as
grey), UV detected members (as orange) and members with UV excess (as blue).
(e) The UV-optical CMD of cluster members. (f) The HR diagram created using
SED fitting results. The figure shows 42 sources without UV excess (hollow
green circles) and ten sources with UV excess (green filled circles) modelled
using single-component SED. The ten sources with double-component fitting are
shown as hollow red squares and red-filled squares for A and B components,
respectively. Their corresponding IDs are also shown. The isochrone of
log($age$) = 9.93 (grey curve), 0.5 M⊙ WD cooling curve (black dashed curve)
and 0.2–0.4 M⊙ WD cooling curves (black dotted curves) are shown wherever
needed in (d)–(f).

We have used Gaia DR2 (Gaia Collaboration et al., 2016, 2018) based membership
catalogue by Cantat-Gaudin et al. (2020). It is one of the latest membership
catalogues available, and the inclusion of Gaia DR3 data does not
significantly improve the membership for the bright stars targeted in this
study. The catalogue contains 1654 members with probability $\geq 0.6$. Fig. 1
(a) shows the spatial distribution of the Gaia members. Fig. 1 (b) shows the
spatial distribution of UVIT-detected sources. The magnitude vs error
distribution for UVIT filters is shown in Fig. 1 (c). Among the Gaia members,
91 are detected in at least one UV filter. The numbers of UV-detected members
in individual filters are given in Table 1.

###  2.2 Colour magnitude diagrams

Fig. 1 (d) shows the Gaia colour-magnitude diagrams (CMDs) of NGC 6791. The
isochrones shown are generated using PARSEC v3.7 (Bressan et al., 2012) with
log($age$) = 9.93, [M/H]=0.4, distance modulus = 13.085 mag and Av = 0.302 mag
(Bossini et al., 2019). Also shown are the WD cooling curves (Bédard et al.
2020 and references therein) with a mass of 0.5 M⊙, which is the expected mass
of WDs based on the cluster turn-off and WD initial-final mass relation
(Cummings et al., 2018). Fig. 1 (d) shows that UVIT has detected stars in the
MS, RGB, BSS and hot subdwarf (sdA, sdB) phases. The sources with UV excess
(see §2.3) are present on the MS and RGB. The UV-optical CMD in Fig. 1 (e)
shows the UV-detected members in UVIT/N263M filter.

###  2.3 Spectral energy distributions

Figure 2: SEDs of stars with fractional UV excess of more than 0.5 in at least
one filter. The top panel for each star shows the observed flux (black error
bars), data omitted while fitting SED (grey error bars), fitted Kurucz+UVBLUE
model (red dashed curve), fitted Koester model (blue dot-dashed curve;
wherever applicable) and total model flux (green curve; wherever applicable).
The bottom panel shows the residual flux after the single component fit (red
dashed curve) and double component fitting (green curve). The panel for star75
shows extra details such as the high-resolution spectral models (Kurucz: red,
UVBLUE: orange, Koester: blue) and the used filter transmission curves (filled
curves at the bottom).

We visually checked the neighbourhoods ($\sim$3″) of all 91 UV-detected
sources to remove 29 sources with bright neighbours. We then constructed the
SEDs of 62 isolated sources using AstroSat/UVIT, Gaia EDR3 (Gaia Collaboration
et al., 2021), swift/UVOT (Siegel et al., 2019), Pan-STARRS (Chambers et al.,
2016), 2MASS (Skrutskie et al., 2006) and WISE (Wright et al., 2010). The
virtual observatory cross-matching was done using vosa (Bayo et al., 2008) and
Vizier (Ochsenbein et al., 2000). The sources were extinction corrected using
known extinction–wavelength relations (Fitzpatrick, 1999; Indebetouw et al.,
2005).

We used vosa111http://svo2.cab.inta-csic.es/theory/vosa/index.php to fit
Kurucz models (Castelli & Kurucz, 2003) to the SEDs using $\chi^{2}$
minimisation technique. The metallicity of the Kurucz model was fixed to +0.5
(closest to the cluster metallicity of +0.4). Also, we fixed the distance to
4139$\pm$100 pc and Av to 0.302$\pm$0.005. The free parameters were
temperature (3500 to 50000 K) and log$g$ (2 to 5). However, note that the SED-
derived log$g$ parameter is imprecise. After fitting the SEDs, we calculated
the fractional residual as follows:

$Fractional\ residual=\frac{F_{obs}-F_{model}}{F_{obs}}$ (1)

The fractional residual is shown in the bottom sub-panels of the SEDs (Fig.
2).

Twenty-nine sources showed more than 0.5 fractional residual in at least one
UV filter. In the UOCS series of papers, we have been using Kurucz models to
fit the SEDs, as they cover a wide range of wavelengths. In this study, we
checked whether the detected UV excess changes are due to underestimating the
Wein tail flux in the Kurucz models. Hence, we created a hybrid Kurucz+UVBLUE
model which uses specialised UVBLUE models (Rodríguez-Merino et al., 2005) in
850–4700 Å region and Kurucz models for the remaining red part of the
spectrum. The UVBLUE models were convolved with the UVIT and UVOT filters
(where the complete filter falls within UVBLUE’s range) to get synthetic
photometry for a temperature range of 3000 to 50000 K, log$g$ range of 0 to 5
and metallicity range of $-2$ to 0.5. The filter transmission profiles were
taken from SVO Filter Profile Service222http://svo2.cab.inta-
csic.es/theory/fps/ (Rodrigo et al., 2012; Rodrigo & Solano, 2020). We could
fit nine stars as single stars with this hybrid model using the python code
Binary_SED_Fitting333https://github.com/jikrant3/Binary_SED_Fitting (Jadhav et
al., 2021). This leaves 20 sources with UV excess using the Kurucz+UVBLUE
models.

The UV excess has been linked to the presence of hot compact objects such as
WDs (Jadhav et al., 2019; Rao et al., 2022; Panthi et al., 2022) or hot
subdwarfs (Jadhav et al., 2021). One can use double-component SED fitting to
deconvolve the two stars. As vosa cannot use hybrid models, we used
Binary_SED_Fitting to perform double component fits using the hybrid model for
the primary star (Kurucz+UVBLUE) and Koester model (Tremblay & Bergeron, 2009;
Koester, 2010) for the possible hotter companion (see Jadhav 2022 for more
details). Ten sources could be fitted with satisfactory double components,
while the remaining ten could not be fitted with available data.

The results of single and double-component SED fitting are given in Table 2.
An extended table, which includes photometry and fitting parameters, is
available as supplementary material and on CDS. Fig. 2 shows the SED fits for
single and double-component fitting for the 20 sources with UV excess. The
observed data points and associated error bars are shown in black colour. The
line plots connecting estimated flux from synthetic photometry of the various
models are shown in different colours (red: Kurucz+UVBLUE model, blue: Koester
model, green: total flux of Kurucz+UVBLUE and Koester models). In the double
SEDs, the cooler component (Kurucz or Kurucz+UVBLUE model) is denoted as ‘A’,
while the hotter component (Koester model) is denoted as ‘B’.

In the double-component fitting, the parameters for the cooler model SED are
improved as the effects of any potential hotter companion or unusual activity
are reduced. However, we have found that the parameter estimates derived from
only 1–2 UV data points are only partially reliable. The luminosity values are
relatively accurate, but the temperature may be underestimated ($\equiv$
overestimated radius; Jadhav et al. in prep). Hence, we recommend not using
the temperature/radius estimates of the hotter model SEDs.

##  3 Results

Table 2: The list of target stars along with the comments from Tofflemire et al. (2014); Sanjayan et al. (2022a) and this work. BSS: Blue straggler stars, ECL: Eclipsing Binary, HB: horizontal branch, Puls: pulsator, RC: red clump, Rot. var: rotational variable, RR: rapid rotator. An extended table with UVIT photometry for all 91 UV detected members and SED fitting parameters of 62 isolated sources is given as supplementary material and available at the CDS. Name |  $\alpha_{J2016}$ [∘] |  $\delta_{J2016}$ [∘] | Comment | Name |  $\alpha_{J2016}$ [∘] |  $\delta_{J2016}$ [∘] | Comment
---|---|---|---|---|---|---|---
No UV excess |  | star56 | 290.257766 | 37.763117 | MS
star0 | 290.182556 | 37.706893 |  | star57 | 290.178641 | 37.803567 | BSS
star1 | 290.330379 | 37.728380 |  | star58 | 290.377713 | 37.966663 | BSS
star2 | 290.272239 | 37.785515 |  | star59 | 290.196283 | 37.748383 |
star3 | 290.278053 | 37.806657 |  | star61 | 290.273850 | 37.804970 |
star5 | 290.180772 | 37.783681 |  | star62 | 290.302801 | 37.830190 | BSS
star6 | 290.143487 | 37.695005 |  | star64 | 290.411506 | 37.724085 | MS
star7 | 290.263954 | 37.783218 | sdB, Puls | star65 | 290.217391 | 37.764303 |
star8 | 290.174187 | 37.706013 | MS | star66 | 290.215274 | 37.718092 | BSS
star9 | 290.229360 | 37.822990 |  | star68 | 290.211291 | 37.792004 |
star10 | 290.167968 | 37.897422 | sdA | star69 | 290.248584 | 37.774346 | BSS, RR, Variable
star11 | 290.237421 | 37.755679 | BSS | star71 | 290.186347 | 37.854764 | MS
star12 | 290.332132 | 37.751261 | MS | star72 | 290.303946 | 37.594908 | MS
star16 | 290.200673 | 37.773090 |  | star74 | 290.209131 | 37.772973 | BSS
star17 | 290.293412 | 37.795104 | BSS, SB1, RR | star76 | 290.145732 | 37.764532 | BSS, SB1, RR, Variable
star18 | 290.191630 | 37.798388 |  | star77 | 290.147393 | 37.575227 | BSS
star20 | 290.217878 | 37.698901 | MS | star79 | 290.188213 | 37.825343 | sdB, Puls
star21 | 290.242940 | 37.785566 | BSS, SB1 | star82 | 290.231777 | 37.812650 |
star22 | 290.224330 | 37.778367 |  | star83 | 290.264080 | 37.772164 |
star23 | 290.254358 | 37.792965 |  | star84 | 290.253508 | 37.769471 | BSS
star24 | 290.191568 | 37.614156 | BSS | star85 | 290.357116 | 37.781353 | MS
star26 | 290.259442 | 37.707294 | MS | star86 | 290.253143 | 37.730550 |
star28 | 290.245497 | 37.937647 | BSS | star88 | 290.358896 | 37.727381 | MS
star29 | 290.201645 | 37.792628 |  | star89 | 290.211412 | 37.793906 |
star30 | 290.194181 | 37.799349 | BSS | star90 | 290.249771 | 37.767570 | BSS
star31 | 290.221496 | 37.753805 |  | UV excess - Single fit
star32 | 290.284253 | 37.786966 |  | star4 | 290.196492 | 37.901509 | MS
star34 | 290.236587 | 37.787253 | MS | star13 | 290.239528 | 37.795310 | MS
star35 | 290.199415 | 37.636157 | MS | star19 | 290.195735 | 37.830140 | MS
star36 | 290.216948 | 37.877327 |  | star25 | 290.184428 | 37.819743 | MS
star37 | 290.276987 | 37.691396 |  | star46 | 290.378518 | 37.767541 | MS
star38 | 290.473241 | 37.834930 | MS | star67 | 290.252538 | 37.780422 | BSS
star39 | 290.193870 | 37.813223 |  | star70 | 290.271927 | 37.729174 | MS
star40 | 290.209807 | 37.760943 | BSS | star73 | 290.174902 | 37.798373 | RC
star42 | 290.224628 | 37.707220 | BSS, SB1 | star78 | 290.148421 | 37.824601 | MS
star43 | 290.218488 | 37.790879 |  | star81 | 290.444898 | 37.712358 | MS
star44 | 290.207915 | 37.694110 |  | UV excess - Double fit
star45 | 290.379465 | 37.706632 | MS | star14 | 290.218634 | 37.837626 | RC
star47 | 290.294602 | 37.758718 | BSS, $\gamma$ Dor Puls | star15 | 290.157917 | 37.819014 | MS
star48 | 290.188867 | 37.805342 | sdB | star27 | 290.276235 | 37.749896 | RC
star49 | 290.303713 | 37.764194 | sdB, Puls | star33 | 290.151458 | 37.665758 | MS, ECL
star50 | 290.191585 | 37.846677 | RGB | star41 | 290.211737 | 37.781781 | BSS, Rot. var.
star51 | 290.285072 | 37.748605 |  | star60 | 290.283629 | 37.797057 | post-HB
star52 | 290.174360 | 37.665663 | MS | star63 | 290.335191 | 37.825374 | MS
star53 | 290.218961 | 37.797860 | BSS, Rot. var. | star75 | 290.125237 | 37.838649 | RC
star54 | 290.489114 | 37.714080 |  | star80 | 290.171933 | 37.776700 | MS
star55 | 290.177668 | 37.781130 | BSS, RR | star87 | 290.227669 | 37.769334 | BSS

Fig. 1 (d) shows that most of the BSSs are detected in NUV along with some MS
and RGBs. We found UV excess in 20 of these sources. Among them, 12 are MS
stars, three are BSSs, and five are giants. Fig. 1 (e) demonstrates the excess
flux in these sources as the MS stars and RGBs are brighter than the
isochrone, and in fact, all the sources detected in the UV and located on the
MS have some fraction of excess flux in the NUV. The remaining 42 isolated
UVIT-detected stars (21 BSSs, 15 MS, one RGB, four sdBs and one sdA) did not
show significant UV excess.

The SED fitting gives the temperatures, luminosities, and radii of the
sources. The temperature range for BSSs is 5250–7000 K while for MS stars it
is 5250–6000 K. Fig. 1 (f) shows the Hertzsprung–Russell (HR) diagram of the
SED fitting results. We have also included the parameters of the potential
hotter companions. If they are real, their luminosities will be of the order
of 0.1 L⊙. Their SED-based temperatures are 11000–25000 K, and the radii are
0.02–0.11 R⊙. They could be (i) sub-luminous sdAs/sdBs or (ii) ELM WDs or
(iii) bloated proto-WDs (e.g. Brogaard et al. 2018) or (iv) their actual
temperature is significantly larger, and thus these radii are overestimated
which could make them typical WDs. The grey markers in the background show the
HR diagram positions of field ELMs and field sdO/sdBs for comparison. The
field ELMs are a sub-sample of sources in Brown et al. (2016). The field
sdO/sdBs are a sub-sample of hot subdwarfs given in Geier (2020). Both samples
were fitted with single-component Kurucz SEDs using vosa.

In the course of the SED fitting, we also tried making only Kurucz and Koester
model binary SEDs. That resulted in 29 sources with UV excess, among which ten
gave satisfactory double fits. Among these, six were common with the hybrid
and Koester fit, while four are currently classified as single fits with UV
excess (star4, star19, star70 and star73). Four current binary systems
(star15, star63, star80 and star87) could not be fit satisfactorily with
Kurucz and Koester models using vosa. The total number of satisfactory binary
fits is the same if we include UVBLUE models. However, there are an equal
number of unsatisfactory fits when the UVBLUE models are included or excluded.
We explored if there is any specific dependency, and in the case of UVBLUE
models, the NUV flux for stars cooler than 6000 K is more than the Kurucz
models, whereas the difference is marginal in the case of hotter stars. In
general, the estimated temperatures of hotter companions were found to be
higher if the UVBLUE models were incorporated. Overall, the number of sources
with successful binary fits is the same in either approach; however, the
overall number of sources with UV excess is reduced if UVBLUE is included.

###  3.1 Comments on individual systems

We did not create SEDs of UV-detected stars with neighbours within 5″. Among
them, there is a notable contact binary, V5 (P = 0.31265938 d; Sanjayan et al.
2022a; Mochejska et al. 2003). There are also a few variables (long period,
non-periodic, rotational variables): KIC 2437079 (Sanjayan et al., 2022a), V65
(P $\sim$ 11.3 d), V66 (P $\sim$ 49 or 99 d), and V19 (Mochejska et al.,
2003). These stars are not discussed further due to close neighbours.

Hot subdwarfs: star7 (B5, KIC 2437937), star49 (B4, KIC 2438324), and star79
(B3, KIC 2569576) are known pulsators (Sanjayan et al., 2022a). star48 (B6)
and star79 are RV variables and likely binary systems (Sanjayan et al.,
2022b). star49 is also an RV variable with P = 0.398495 d and a low mass MS
companion (Sanjayan et al., 2022b)444Mochejska et al. (2003) stated P =
0.796993 d $\sim 2\times 0.398495$ d. The SED temperatures (using Kurucz
models) for the three hot subdwarfs are slightly underestimated compared to
Sanjayan et al. (2022b) (using Tlusty models), likely due to differences in
the models.

Blue straggler stars: star41 (KIC 2437238) showed rotational variability
(Sanjayan et al., 2022a) and significant UV excess hinting at a $\gtrapprox
0.25$ M⊙ WD as its companion. The absence of eclipses and RV variability
suggests a low inclination orbit in the potential binary system. The single-
lined spectroscopic binary (SB1) and $\gamma$Dor type star47 (V106, KIC
2438249, WOCS 54008) showed no excess UV flux, which is expected as Brogaard
et al. (2018) derived the temperatures of binary components as 7110 + 6875 K
and their masses as 1.62 + 0.176 M⊙. The low temperatures are not enough to
create significant UV flux (note that the SED estimated radius of the BSS is
overestimated as we modelled both components as a single source). The system
is an eclipsing W Uma binary system with P = 1.4464 d (Tofflemire et al.,
2014). The absence of UV excess and the low temperature of the companion
suggests that significant time has passed since the mass transfer event. The
system is a prime candidate for a detailed asteroseismic study to constrain
the internal structure and mass transfer history in BSSs. star53 (KIC 2437338)
also showed rotational variability (Sanjayan et al., 2022a) but no UV excess
or RV variability. star61 showed UV excess, but we could not fit a hotter
companion. star69 (KIC 2437745) is a rapid rotator and photometric variable (P
= 1.43) (Tofflemire et al., 2014; Sanjayan et al., 2022a). However, it did not
show UV excess. star76 (WOCS 46008, KIC 2436421) is SB1 and a rapid rotator
(Tofflemire et al., 2014). Sanjayan et al. (2022a) detected variability in its
Kepler light curves with period of 1.44 days. However, we did not detect any
UV excess and could not confirm whether the RV variability is periodic (with
1.44 d) due to poor sampling. star87 shows UV flux compatible with an ELM WD.
star90 is an RV constant star with no UV excess. Tofflemire et al. (2014)
suggested that it is a 1.9 M⊙ merger product formed 1 Gyr ago in line with the
absence of any hotter companion.

Red clump stars: There are 19 red clump stars identified using asteroseismic
measurements (Stello et al., 2011) in NGC 6791. UVIT detected four of them,
all having significant UV excess (only three are successfully fitted with a
double SED). Among them, star27 (KIC 2438051), star73 (KIC 2436732) and star75
(KIC 2569055) are known pulsating red clump stars. These red clump stars did
not show detectable RV variability or eclipses.

Main sequence star: star33 (Gaia DR3 2051286413614206208) has been classified
as eclipsing binary with a period of 0.3664 d (Gaia Collaboration et al.,
2022; Mowlavi et al., 2023). The Gaia light curves and the double-component
fitting both suggest a close hotter star as its companion. However, more RV
variation analysis is needed to confirm the companion and any mass transfer
history. The HR diagram positions of companions to star15, star63 and star80
indicate that they are likely WDs with mass $\gtrapprox 0.3$ M⊙.

Post horizontal branch star: star60 (WOCS 9007) is a proper motion member of
the cluster. However, its RV is different from the cluster by $\sim 25$ km s-1
with a parallax of 0.2901$\pm$0.0115 mas yr-1 (cluster parallax is 0.241 mas
yr-1). It showed significant UV excess indicative of a young ELM companion.
However, if the star is not a cluster member, then the distance measurement
and the derived classification may be incorrect.

##  4 Discussion

The five single hot subdwarfs detected in UV all require mass transfer for
their formation (Heber, 2009). None of the hot subdwarfs showed UV/IR excess
flux, which indicates that their subdwarf component is the dominant flux
emitter in the UV–IR range. Furthermore, the hotter companion candidates in
three red clumps, four MS stars, two BSS and one post-HB star are also likely
low luminosity sdAs/sdBs or ELMs according to their HR diagram position. If
these are indeed hot subdwarfs or ELMs, then it means that all these are post
mass transfer systems. Among the double SED-fitted MS stars, star33 is a known
binary with a close companion. The MS stars which have accreted mass are known
as blue lurkers (Leiner et al., 2019; Jadhav, 2022). These are the MS
equivalents of the BSSs. As such, they would be similar to BSSs in terms of
formation scenarios (mergers or mass transfer) and detection techniques (high
rotation, chemical peculiarities, post mass transfer companions). Thus, star33
is a prime candidate for being a blue lurker. At the same time, the other
three MS stars are also blue lurker candidates subject to confirmation of the
companion’s nature. If confirmed, this will make NGC 6791 one of the few
clusters known to host blue lurkers. However, further observational evidence
of rapid rotation and chemical alteration is necessary to confirm their status
as blue lurkers. We note that this cluster has several short-period binaries,
suggestive of possible Case-A/Case-B mode of mass transfers, that can lead to
binaries with very low mass stellar remnants (for example, star47). The UV
images are able to detect relatively hotter systems, and there could be many
more that are beyond the detection limit of UVIT. We note that such systems
are found across the evolutionary stages from MS to post-HB.

The only detected RGB star showed UV excess when fitted with Kurucz models,
which could later be explained by the updated Wein tail of UVBLUE models. The
other five giants (four red clumps and one post-HB) showed significant UV
excess flux. NUV excess in RGBs has been known to be caused by rotation (Dixon
et al., 2020). However, a hot compact companion can also give rise to UV
excess. UV spectroscopy is required to confirm the exact source of the NUV
flux from these RGBs.

Given the distance of NGC 6791, only the brightest WDs will be visible even in
UV (all single subdwarfs detected presently are brighter than $\approx 6$ L⊙).
After cross-matching with Hubble Space Telescope proper motion catalogue of
NGC 6791 (Libralato et al., 2022), we could identify one WD candidate
(290.20235, 37.76336 at 21.83 mag in F814W) within the small footprint covered
by the Hubble. However, there were insufficient data points to parameterise it
using its SED. There could be more such member WDs present, which could be
discovered after deep multi-epoch wide-field imaging, such as the Legacy
Survey of Space and Time (Ivezić et al. 2019).

The colour of BSSs in the CMD has been linked with their origin. Ferraro et
al. (2009) claimed that bluer BSSs in globular cluster M 30 are formed via
collisional mergers, while redder BSSs are formed via mass transfer. Similar
double BSS sequences have been seen in other globular clusters (NGC 362:
Dalessandro et al. 2013; Dattatrey et al. 2023, NGC 2173: Li et al. 2018).
Similarly, BSSs’ UV excess has been linked to binarity (Gosnell et al., 2015;
Sindhu et al., 2019; Pandey et al., 2021). In NGC 6791, the three BSSs with UV
excess (indication of binarity) are generally redder than the rest of the BSSs
of similar luminosity. This could be similar to the colour-dependent formation
scenarios proposed by Ferraro et al. (2009). However, a bigger sample is
needed to reach a statistically significant conclusion.

NGC 6791 has single sdBs and one sdA, along with possible low-luminous sdBs
and ELMs as companions to stars in different evolutionary phases. All of these
point to a very significant role that the binaries play in this dynamically
old system, resulting in the formation of various types of stars due to
differences in the masses of the binary members as well as their orbital
properties. Therefore, This old open cluster is unique in possessing several
features arising out of dynamics and binary systems.

##  5 Summary & Conclusion

UVIT detected 91 members of NGC 6791 in either far-UV or NUV filters,
including MS, BSSs, hot subdwarfs, RGB and red clump stars. We provide SED-
based parameters for 62 of the isolated sources. Twenty of these sources
showed UV excess flux, which is an indicator of a hot compact binary
companion. We estimated approximate parameters for ten potential compact
binary companions using SED analysis.

The total number of satisfactory binary fits is the same if we use a hybrid of
UVBLUE+Kurucz models, but the number of sources with UV excess is more without
UVBLUE. In general, we estimated higher temperatures for hotter companions
when the UVBLUE models were included.

NGC 6791 contains a significant population of single hot subdwarfs (four sdB
and one sdA) and optically sub-luminous compact objects (ten hot
subdwarf/ELMs). This is unique in open clusters and likely due to the dense
and rich nature of NGC 6791. There are four blue lurker candidates in the
cluster, which need to be confirmed using a combination of abundance studies,
UV spectroscopy, rotational velocity, and RV/flux time series analysis. We
note the presence of candidate post mass transfer systems across the
evolutionary phases in this cluster, from MS, RGB, HB to post-HB, suggestive
of a rich population of interacting binaries.

The cluster appears to show properties of a low-density globular cluster,
along with some features seen in open clusters. This cluster is, therefore, a
potential test bed, ideal for performing numerical simulations to understand
the underlying processes regarding dynamics, binarity and stellar evolution.

###### Acknowledgements.

We thank the anonymous referee for constructive comments, which helped improve
the methodology and discussion. VJ thanks the Alexander von Humboldt
Foundation for their support. AS thanks for the support of the SERB power
fellowship. RS thanks the National Academy of Sciences, India (NASI),
Prayagraj, for the award of a NASI honorary Scientist position; the Alexander
von Humboldt Foundation, Germany, for the award of Group linkage long-term
research program between IIA, Bengaluru and European Southern Observatory,
Munich, Germany, and the Director, IIA for providing institutional,
infrastructural support during this work. UVIT project is a result of the
collaboration between IIA, Bengaluru, IUCAA, Pune, TIFR, Mumbai, several
centres of ISRO, and CSA. This publication uses vosa, developed under the
Spanish Virtual Observatory project.

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