UOCS. IV. Discovery of diverse hot companions to blue stragglers in the old open cluster King 2
Vikrant V. Jadhav, Sindhu Pandey, Annapurni Subramaniam, Ram Sagar
JJ. Astrophys. Astr. (0000) :
UOCS. IV. Discovery of diverse hot companions to blue stragglers in theold open cluster King 2
Vikrant V. Jadhav , Sindhu Pandey , Annapurni Subramaniam & Ram Sagar Indian Institute of Astrophysics, Sarjapur Road, Koramangala, Bangalore, India. Joint Astronomy Programme and Department of Physics, Indian Institute of Science, Bangalore, India. Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital, India * Corresponding author. E-mail: vikrant.jadhav[at]iiap.res.inMS received 31 August 2020; accepted 31 August 2020
Abstract.
King 2, one of the oldest clusters in the Milky Way, with an age of ∼ ∼ ASTROSAT . With membership information derived from
Gaia
EDR3, the cluster is found to have 39 blue straggler stars (BSSs). We created multi-wavelength spectra-energydistributions (SED) of all the BSSs. Out of 10 UV detected BSSs, 6 bright ones fitted with double component SEDsand were found to have hotter companions with properties similar to extreme horizontal branch (EHB) / subdwarfB (sdB) stars, with a range in luminosity and temperature, suggesting a diversity among the hot companions. Wesuggest that at least 15% of BSSs in this cluster are formed via mass-transfer pathway. When we compared theirproperties to EHBs and hotter companions to BSS in open and globular clusters, we suggest that EHB / sdBs likecompanions can form in binaries of open clusters as young as 6 Gyr. Keywords.
Open star clusters (1160) — Blue straggler stars (168) — Extreme horizontal branch stars (513) — Bsubdwarf stars (129) — Ultraviolet astronomy (1736) — Spectral energy distribution (2129) — Binary stars (154)
1. Introduction
The evolution of binary systems strongly depends onthe initial orbital parameters and its further evolution,where any change in their orbits can lead to a widelydi ff erent evolution. If one of the stars evolves and fillsits Roche lobe, the system will undergo mass transfer.Details such as duration and rate of mass transfer willdepend on the orbits and masses of the binary stars. Ifsuch a binary is present in a star cluster, and the sec-ondary of the binary has mass similar to the main se-quence turno ff (MSTO) mass, then the secondary willbecome brighter than the MSTO and appear as a bluestraggler star (BSS). Otherwise, the secondary will befainter than the MSTO and can be classified as a bluelurker. The blue lurkers are identified by their highstellar rotation (Liener et al. et al. + MS, contact bina-ries (Rucinski 1998), common envelop, MS + horizontalbranch (HB; Subramaniam et al. + extremeHB (EHB; Singh et al. + subdwarf-B (sdB;Han et al. + WD (Jadhav et al. + WD (Marsh et al. / mergerby reducing the orbital separation (Kozai 1962).We are carrying out a long term project of charac-terising products of binary stars such as BSSs in openclusters (OCs). Ultraviolet imaging of binary systemsreveals the presence of hotter companions in the bi-nary system, given that the hotter companion is lumi-nous in UV. Old OCs such as NGC 188 and NGC 2682are rich with BSSs, binary stars and contain many suchoptically sub-luminous UV-bright companions (Subra-maniam et al. et al. etal. et al. et al. et al. (2020) provides asummary of the BSSs and post mass transfer systemsin star clusters.King 2 is one of the oldest clusters in the MilkyWay, with an age of ∼ ∼ UOCS: UVIT Open Cluster Study © Indian Academy of Sciences 1 a r X i v : . [ a s t r o - ph . S R ] F e b J. Astrophys. Astr. (0000) :
Table 1 . Age, distance, reddening (E(B − V)) and metallicityof King 2 estimated by various investigators are listed.
Age Distance E(B − V) Metallicity Ref.(Gyr) (pc) (mag)6.02 5750 0.31 -0.42 [1]6 5690 ±
65 0.31 ± ∼ et al. (2002), [2] Aparicio et al. (1990),[3] Tadross (2001), [4] Kaluzny (1989)due to its considerable distance and unknown member-ship information. For identifying and characterising hotBSSs and their possible companions, we obtained Ul-tra Violet Imaging Telescope (UVIT) / ASTROSAT ob-servations of rich OC King 2 ( α = . ◦ δ =+ . ◦ l = . ◦ b = − . ◦
7) under ASTROSATproposal A02 170. Kaluzny (1989) presented the firstoptical colour-magnitude diagram (CMD) study of thisdistant cluster using BV CCD photometric data. Thisyielded a range of plausible ages and distances for dif-ferent assumed reddenings and metallicities. The galac-tocentric distance of the cluster was estimated to be ∼
14 Kpc. Aparicio et al. (1990) (A90 hereafter) did acomprehensive study on the cluster using
UBVR pho-tometry and derived an age of 6 Gyr and a distanceof 5.7 kpc for solar metallicity. They also indicatedthe presence of a good fraction of binaries in the MS.Tadross (2001) estimated a value of [Fe / H] = − .
32 us-ing the ( U − B ) colour excess from the literature data,while Warren & Cole (2009; WC09 hereafter) deriveda value of [Fe / H] = − . ± .
09 using spectroscopicdata. These metallicity estimates are significantly sub-solar and inconsistent with the finding of A90. WC09found a distance of 6.5 kpc and a slightly younger age, ∼ − V) = =
13 kpc,where its metallicity falls close to the trend of the galac-tic abundance gradients derived in Friel et al. (2002).There has been no proper motion study available forthis cluster till
Gaia
DR2 (Gaia Collaboration et al. et al. (2018) provided a mem-bership catalogue of King 2 with 128 members with
Gaia
DR2, and Jadhav et al. (2021) provided kinematicmembership of 1072 stars (and 340 probable members)using kinematic data taken from
Gaia
EDR3.Above discussed optical photometric studies indi-cate a good number of post-MS hot stars in King 2. Infact, Ahumada et al. (2007) have identified 30 BSScandidates based on the location of these stars in thecluster. We present the UVIT and the archival data used BP G RP [mag]1415161718192021 G [ m a g ] CMD of King 2
Gaia Members (1412)BSS (39)UV bright BSS (6)Other UVIT detected BSS (4)
Figure 1 . CMD of King 2 cluster candidates using
Gaia
EDR3 data. All BSS members are shown as blue circles.The UV bright BSS (see section. 3.) are shown as redsquares, and other UVIT detected BSS are shown as red X’s.The
Gaia members are shown as black dots along with thePARSEC isochrone of log age = / H] = -0.4, DM = − V) = in this study in the next section, followed by analyses,results and discussion.
2. UVIT and archival data
We observed King 2 with UVIT, which is one of thepayloads on the first Indian multiwavelength space ob-servatory
ASTROSAT , launched on 28 September 2015.The observation was carried out by UVIT on 17 De-cember 2016, simultaneously in two filters. The tele-scope has three channels with a set of filters in them:Far-UV (FUV; 130 - 180 nm), near-UV (NUV; 200- 300 nm) and visible (VIS; 350 - 550 nm), wherethe VIS channel is intended to correct the drift of thespacecraft (see Kumar et al. et al. ≈
23 mag) andone NUV (N219M, limiting magnitude ≈
22 mag) fil-ter for exposure time of ∼ . (cid:48)(cid:48)
33 and 1 . (cid:48)(cid:48) ccd - lab (Postma & Leahy 2017) and PSF photometry wasperformed using daophot package of iraf (Tody 1993).More details of the reduction process are presented inJadhav et al. (2021). We have detected ten member . Astrophys. Astr. (0000) : stars in either F148W and / or N219M filter.We obtained archival optical ( UBVR ) photome-try data from A90 catalogue (Calar Alto observatory;CAHA) and cross-matched with UVIT data using top - cat (Taylor 2005). The cluster was observed with GALEX under All-sky Imaging (AIS) survey in NUVfilter (exp. time ∼
100 sec). All the detected mem-ber stars were further cross-matched with photomet-ric data from UV to IR wavelength bands obtainedfrom
GALEX (Bianchi et al. et al.
Gaia
EDR3 (Gaia Collabo-ration et al. et al. et al. et al.
3. SED fitting and Colour Magnitude diagrams
The data were corrected for reddening (E(B − V) = ± etal. (2005) and calibrated with the cluster distance of5750 ±
100 pc (we have overestimated the error to coverdistance estimates from Dias et al. (2002) and A90).We have adopted the metallicity of [Fe / H] = − et al. et al. g ∈ (3 . , .
0) were fitted to optical and IR points(above 3000 Å) using VOSA (Bayo et al. Binary SED Fitting . Inpreliminary double component fits, the hottercomponents were found to be compact objects,hence they were fitted with log g = http://svo2.cab.inta-csic.es/theory/vosa/index.php https://github.com/jikrant3/Binary_SED_FittingBinary SED Fitting is a python code which uses χ minimisa-tion technique to fit two component SEDs.
4. Very small errors in PAN-STARRS PS1,
Gaia
EDR3 and A90 photometry led to ignoring rel-atively high error UV data-points, hence theywere replaced with mean errors for better resid-ual across all wavelengths. A few data pointswere removed to achieve better fits and lower χ (see Fig. 2 (a)). The error values given in Table 3are original unmodified errors. The detailed SEDfitting method is explained in section 3.3 of Jad-hav et al. (2019).5. The best fit parameters for single stars or coolercomponents are taken from the VOSA fits. Thehotter component parameters are taken from theleast χ model in the two component fitting.6. The errors in cooler component parameters arefairly low and are taken as the grid values. To de-rive errors in the hotter component parameters,we used a statistical approach. We first gener-ated 100 iterations of observed SEDs with addedGaussian noise in each data point. These 100SEDs were then fitted with double components.However, not all the double fits converged, hencewe only kept hotter components with 6000 K < T e f f < K . Logarithmic distributions ofthe parameters from the noisy & converging it-erations were then fitted with Gaussian distribu-tions. FWHM of these Gaussian distributions aredefined as the upper and lower limits of the fittingparameters (temperature, radius and luminosity).Fig. 1 shows the CMD of 1412 cluster candidatesidentified from Gaia
EDR3 with probability of over50% (Jadhav et al. G < . G BP − G RP < . age = GALEX . We fitted Kurucz modelSEDs to all BSSs and found excess UV flux in 15 BSS(BSS 1, 2, 3, 4, 5, 7, 8, 9, 10, 19, 26, 28, 29, 33 and 36).Among these, BSS1, 2, 3, 4, 5 and 7 have multiple UVdata points from UVIT or
GALEX or both. Only thesesix were fitted with double component SEDs, becausea hot component fit can be reliable if the number of UVdata points is more than one. Hereafter, these six BSSwill be referred to as ‘UV bright BSSs’ and others willbe referred to as ‘UV faint BSSs’. The four BSS de-tected in UVIT but not fitted with hotter component areshown as red X’s in the CMD.We have shown an example of a double componentSED fit of BSS1 in Fig. 2 (a). The BSS1-A componentis a BSS with 7750 K, while the BSS1-B component
J. Astrophys. Astr. (0000) :
Double FitsName Comp. log g Te ff R L Scaling Factor N fit χ r [K] [ R (cid:12) ] [ L (cid:12) ] ( χ r , single )BSS1 A 4.5 7750 ±
125 2.44 ± ± + − + . − . + . − . ±
125 3.72 ± ± + − + . − . + . − . ±
125 3.56 ± ± + − + . − . + . − . ±
125 2.16 ± ± + − + . − . + . − . ±
125 2.29 ± ± + − + . − . + . − . ±
125 2.90 ± ± + − + . − . + . − . g Te ff e Te ff R e R L e L Scaling Factor N fit χ r [K] [K] [ R (cid:12) ] [ R (cid:12) ] [ L (cid:12) ] [ L (cid:12) ]BSS6 4 6500 125 2.32 0.04 8.74 0.41 8.15E-23 11 3.4BSS8 3 7000 125 4.19 0.07 38.17 2.05 2.66E-22 11 3.2BSS9 3.5 7500 125 2.81 0.05 22.58 1.14 1.20E-22 11 4.2BSS10 3 6250 125 2.91 0.05 11.67 0.54 1.28E-22 11 3.9BSS11 4 6750 125 5.28 0.09 52.55 2.64 4.22E-22 11 1.6BSS12 3.5 7000 125 1.97 0.03 8.66 0.43 5.85E-23 12 47.9BSS13 3.5 6750 125 2.32 0.04 10.17 0.51 8.16E-23 11 3.2BSS14 3 6750 125 2.38 0.04 10.63 0.48 8.54E-23 11 2.5BSS15 3 6500 125 2.44 0.04 9.71 0.49 9.02E-23 11 3.6BSS16 3 6750 125 1.99 0.03 7.42 0.37 6.00E-23 11 3.1BSS17 3.5 7000 125 2.32 0.04 11.60 0.55 8.12E-23 11 4.3BSS18 4 6500 125 2.22 0.04 8.08 0.34 7.48E-23 15 12.3BSS19 4.5 7250 125 2.89 0.05 20.92 1.17 1.26E-22 8 13.3BSS20 5 6500 125 2.23 0.04 8.05 0.32 7.53E-23 11 3.4BSS21 5 6500 125 3.08 0.05 15.32 0.61 1.43E-22 11 4.9BSS22 3.5 6250 125 3.28 0.06 14.88 0.62 1.63E-22 11 1.5BSS23 3.5 6500 125 2.84 0.05 13.00 0.59 1.22E-22 11 3.7BSS24 3 7250 125 2.12 0.04 11.24 0.59 6.77E-23 11 2.1BSS25 5 6250 125 2.64 0.05 9.73 0.39 1.06E-22 15 11.7BSS26 3 7250 125 2.28 0.04 13.06 0.67 7.89E-23 15 11.3BSS27 4.5 6250 125 2.17 0.04 6.47 0.26 7.10E-23 11 4.3BSS28 4 7250 125 2.23 0.04 12.57 0.68 7.54E-23 15 9.8BSS29 4 6500 130 2.27 0.04 8.40 0.41 7.81E-23 5 125.1BSS30 4.5 6250 125 2.45 0.04 8.35 0.34 9.08E-23 11 5.3BSS31 5 8250 127 1.40 0.02 8.09 0.35 2.98E-23 15 9.0BSS32 4 6250 125 2.57 0.04 9.27 0.39 9.98E-23 15 19.5BSS33 4 6500 125 2.04 0.04 6.79 0.28 6.29E-23 15 10.9BSS34 4 6500 125 3.80 0.07 23.42 1.01 2.19E-22 11 4.2BSS35 3 6250 125 2.41 0.04 8.07 0.38 8.75E-23 11 4.4BSS36 3.5 6500 125 2.37 0.04 9.13 0.44 8.51E-23 11 3.3BSS37 5 6500 125 2.25 0.04 8.23 0.34 7.65E-23 11 3.6BSS38 5 6500 125 2.68 0.05 11.70 0.48 1.09E-22 11 3.4BSS39 3.5 6500 125 3.31 0.06 17.67 0.80 1.65E-22 11 3.9 Table 2 . Fitting parameters of the best fit of the double and single component fits of BSSs with the hotter component. Scalingfactor is the value by which the model has to be multiplied to fit the data, N f it is the number of data points fitted and χ r is thereduced χ for the composite fit. The χ r values of single fits of the cooler components are given in brackets. Note: the log gvalues are imprecise due to the insensitivity of the SED to log g.. Astrophys. Astr. (0000) : F l u x ( e r g s c m Å ) (a) BSS1
A 7750.0 K, logg 4.5B 22000.0 K, logg 5ModelNo FitObs R e s i d u a l (b) Å )012 i (c) = 6.9 r = 0.53 l o g L ( L ) (d) IsochroneAB (best fit) r (e) R ( R ) Figure 2 . Two-component SED of BSS1. (a) Composite SED (green curve) is shown along with the observed flux(as black error-bars). The unfitted point (in this case: CAHA.R) is shown as orange dot. The cooler (BSS, orangedot-dashed curve) and hotter (blue dashed curve) component are also shown with their T e f f and log g . The model, Bcomponent and residuals of noisy iterations are also shown as light coloured lines. (b) The fractional residual is shownfor single component fit (orange dot-dashed curve) and composite fit (green solid curve). The fractional observationalerrors are also indicated on X-axis. (c) The χ i of each data point. (d) H–R diagram of the two components along with theisochrone for reference. The density distribution of the noisy B component fits is plotted in blue. (e) T e f f – χ distribu-tion for the noisy B component fits coloured according to their radii. The dashed lines are the quoted limits of the temperature. has T e f f of 22000 K. The reduction in residual afterincluding the hotter component is visible in Fig. 2 (b).The χ i for individual points is shown in Fig. 2 (c) with χ r of 0.53. Although, we note that the χ r need not be ∼
1, for a non-linear model fitting (Andrae et al. χ both to determinethe goodness of fit. Fig. 2 (d) shows the Hertzsprung–Russell (H–R) diagram of A and B components. Thedensity distribution of noisy & converging iterations isalso shown to get an idea of degeneracy in tempera-ture and luminosity. Fig. 2 (e) panel shows the best fitand the noisy & converging iterations in T e f f – χ phase-plane. The double component fits of BSS2, 3, 4, 5 and7, and single-component fits of BSS10 and BSS15 areshown in Fig. 4. The fitting parameters are mentionedin Table 2.The H–R diagram of the BSSs detected in King2 is shown in Fig. 3. We have shown the UV faintBSSs as blue dots, UV bright BSSs are represented asblue diamonds, and the hotter components of UV brightBSSs as filler circles. We have taken the parameters ofthe hotter companions detected along with the BSSs inNGC 188 from Subramaniam et al. (2016) and NGC2682 from Sindhu et al. (2019), Jadhav et al. (2019)and Sindhu et al. (in prep). They are plotted in the fig-ure as orange cross and triangles respectively. The pa- rameters of the EHB stars in NGC 1851 are taken fromSingh et al. (2020) and are shown as orange stars. ThePARSEC isochrone of log age = et al. (thick grey curves, Tremblay& Bergeron 2009) and BaSTI zero age HB (ZAHB;dashed black curve; Hidalgo et al.
4. Results and DiscussionBSS and their companions in literature:
The BSSs have T e f f range of 5750 to 8500 K andradii of 1.4 to 5.21 R (cid:12) . By comparison to isochrones,they have mass in the range of 1.2 to 1.9 M (cid:12) , the bright-est BSS being 3 mag brighter than the MSTO. Majorityof BSSs in King 2 have T e f f similar to the older NGC188 (6100–6800 K; Gosnell et al. et al. in prep.),which is expected due to its slightly younger age. TheBSSs in NGC 188 (Geller & Mathieu 2011; Gosnell et al. et al. stev.oapd.inaf.it/cgi-bin/cmd basti-iac.oa-abruzzo.inaf.it/isocs.html J. Astrophys. Astr. (0000) : l o g L [ L ]
12 34 5712 34 57
LogAge = 9.7ZAMSZAHBWD (0.5 M )WD (0.2 M ) UV faint BSSUV bright BSSNGC 2682NGC 1851NGC 188 Figure 3 . H–R diagram of locations of components ofbinaries in King 2, NGC 2682 (Sindhu et al. et al. et al. in prep.), NGC 188 (Subramaniam et al. et al. (Sindhu et al. / HB according to their luminosity andtemperature.BSS2-A, BSS3-A and BSS7-A lie above / on theZAHB in Fig. 3. There is a degeneracy in this regionof the H–R diagram where one could find both massiveBSS as well as ZAHB stars. Stars in these two evo-lutionary phases will have di ff erent masses (HB mass < MSTO; BSS mass > MSTO), that could be used tolift the degeneracy. Bond & Perry (1971) measured themasses of stars in this region of the NGC 2682 CMDand determined that they are indeed high mass BSSs.One star is found in this region of the NGC 188 CMDand it is classified as a BHB (Rani et al. 2020), this staris significantly brighter than the rest of the BSSs. Inthe case of King 2, the BSSs show a continuous distri-bution up to the brightest BSS, hence BSS2-A / BSS3-Aare most likely normal BSSs. However, their mass esti-mations (via log g measurements or asteroseismology)are required before confirming their evolutionary sta-tus. What are the hotter companions?
The hotter companions in UV bright BSSs haveT e f f of 14000 to 26000 K (spectral type B) and radii of0.09 to 0.27 R (cid:12) . Fig. 3 shows the density distributionsof the best 100 fits for the hotter companions. Fig. 3also shows the location of various companions to BSSin NGC 188 and NGC 2682, and EHB stars in NGC1851, one of which has a BSS as its companion (Singh et al. e f f , radii and luminosity. Theseare core-helium burning stars with T e f f in the rangeof 20,000 - 40,000 K and are compact (0.15-0.35 R (cid:12) ;Heber 2016; Sahoo et al. etal. et al. (2001) found . Astrophys. Astr. (0000) : a good fraction of the sdB stars in detached, but shortperiod binary systems. sdB stars are thought to provideimportant clues to common envelope evolution in tightbinaries.BSS2-B lies on the blue end of ZAHB and verysimilar to the EHBs in the outskirts of GC NGC 1851.Similarly, hot and bright post-AGB / HB candidate wasfound as a companion to a BSS in NGC 188 (Subrama-niam et al. et al.
Formation pathways of BSSs and EHBs / sdBs: BSS formation mechanism involves mass gainwhile the EHB / sdB formation involves the strippingof the envelope of a post-MS star. The detection ofEHB / sdBs confirms ‘binary mass transfer’ as the for-mation mechanism for BSS1 to BSS5 and BSS7. As thecooler companions are BSSs that are supposed to havegained mass, we can infer that the detected EHBs / sdBshave transferred mass to the BSSs companions. There-fore, the BSS + EHB / sdB systems in King 2 illustratestars on both sides of the mass exchange. We see arange in their temperature and luminosity, suggesting adiversity among the hotter companions.The lifespan of sdB stars is expected to be between100 to 200 Myr (Bloemen et al. etal. / unstable mass transfer and become a WD + WDsystem. Alternatively, it can merge through a common-envelop phase and become a massive WD. The exactevolution will depend on then orbital parameters, masstransfer e ffi ciency and mass loss.King 2 is one of the oldest OC, lies in the outskirtsof the galactic disk. It is metal-poor compared to theGalactic disc OCs. The environment is quite similar tooutskirts of GCs, which are also metal-poor, old andof comparable density. While most of the BSSs in GCslie in core and are formed via mergers (Chatterjee et al. et al. et al. ∼
5. Conclusions and Summary • The old OC King 2 has a large population (39)of BSSs, spreading up to 3 mag brighter than theMSTO. We constructed SEDs of all the BSS us-ing UV to IR data. The BSSs have T e f f in therange of 5750–8250 K, luminosity in the rangeof 5.6–57.5 L (cid:12) and mass in the range of 1.2–1.9M (cid:12) . • Six of the UV bright BSS showed excess UV fluxand were successfully fitted with double compo-nent SEDs. The hotter components have T e f f of14000–26000 K and R / R (cid:12) of 0.09–0.27, suggest-ing a range of properties. Two of the hotter com-panions to the BSS are likely EHB stars, whilefour are likely sdB stars. • EHB / sdB companions imply that these 6 (out of39) BSSs have formed via binary mass transfer.The SED fits show that sdB stars can be createdin old OCs such as King 2 (similar to old OCNGC 188 and GC NGC 1851).Spectroscopic time series and radial velocity varia-tions can uncover the binary nature as well as proper-ties of these systems and help in estimating the log g and mass of these stars. The mass and orbital estima-tions will expand our knowledge of BSS, EHB and sdBformation scenarios. Acknowledgements
We thank the anonymous referee for their valuablecomments and inputs. RS would like to thank the Na-tional Academy of Sciences, India (NASI), Prayagrajfor the award the NASI Honorary Scientist; Alexan-der von Humboldt Foundation, Germany for the awardof long-term group research linkage program and Di-rector, IIA for hosting and making available facilitiesof the institute. UVIT project is a result of a collabo-ration between Indian Institute of Astrophysics (IIA),Bengaluru, The Inter-University Centre for Astronomyand Astrophysic (IUCAA), Pune, Tata Institute of Fun-damental Research (TIFR), Mumbai, several centres ofIndian Space Research Organisation (ISRO), and Cana-dian Space Agency (CSA). This work has made use
J. Astrophys. Astr. (0000) : of data from the European Space Agency (ESA) mis-sion
Gaia ( ),processed by the Gaia
Data Processing and Analy-sis Consortium (DPAC, ). This publica-tion makes use of VOSA, developed under the SpanishVirtual Observatory project supported by the SpanishMINECO through grant AyA2017-84089.
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J. Astrophys. Astr. (0000) : F l u x ( e r g s c m Å ) (a) BSS2
A 8250.0 K, logg 3.5B 24000.0 K, logg 5ModelObs R e s i d u a l (b) Å )020 i (c) = 51.1 r = 3.93 l o g L ( L ) (d) IsochroneAB (best fit) r (e) R ( R ) F l u x ( e r g s c m Å ) (a) BSS3
A 7250.0 K, logg 4.5B 24000.0 K, logg 5ModelNo FitObs R e s i d u a l (b) Å )012 i (c) = 12.7 r = 0.98 l o g L ( L ) (d) IsochroneAB (best fit) r (e) R ( R ) F l u x ( e r g s c m Å ) (a) BSS4
A 8000.0 K, logg 5.0B 26000.0 K, logg 5ModelNo FitObs R e s i d u a l (b) Å )010 i (c) = 32.6 r = 2.33 l o g L ( L ) (d) IsochroneAB (best fit) r (e) R ( R ) Figure 4 . The descriptions of double component fits are same as Fig. 2. . Astrophys. Astr. (0000) : F l u x ( e r g s c m Å ) (a) BSS5
A 6500.0 K, logg 3.5B 14000.0 K, logg 5ModelNo FitObs R e s i d u a l (b) Å )0.00.51.0 i (c) = 3.7 r = 0.41 l o g L ( L ) (d) IsochroneAB (best fit) r (e) R ( R ) F l u x ( e r g s c m Å ) (a) BSS7
A 8500.0 K, logg 3.5B 19000.0 K, logg 5.0ModelNo FitObs0.00.5 R e s i d u a l (b)2000 5000 10000 20000Wavelength ( Å )012 i (c) = 7.1 r = 0.64 l o g L ( L ) (d) IsochroneAB (best fit) 3.63.84.04.24.44.6 log T (K)123 r (e) 0.10.20.30.4 R ( R ) Figure 4 (Continued...) . The single component fits of BSS10 and BSS15 are shown as an example with model fit (bluecurve), fitted data points (red points) with 1 σ and 3 σ errors as solid and dashed lines. The theoretical spectra (in grey)is added for reference. The observed (reddening a ff ected) SED is shown in grey below the corrected data-points. The titlementions the T e f f , log g , metallicity and A V of the model fit. P a g e f J . A s t r oph y s . A s t r . ( ) : Table 3 . Coordinates, Gaia EDR3 source IDs, flux and flux errors of the stars in all used filters. All flux are given in [erg s − cm − Å − ]. GALEX, Gaia (EDR3), Pan-Starrs(PS1) and 2MASS photometry is taken from archives. CAHA photometry is taken from A90. Name R.A (J2016) Dec. (J2016)
Gaia
EDR3 source id UVIT.F148W ± err UVIT.N219M ± err GALEX.NUV ± err CAHA.U ± err CAHA.B ± errBSS1 12.735411 58.185675 424416887106018688 1.87e-16 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± +
00 6.75e-16 ± + ± ± ± ± ± ± ± ± ± . A s t r oph y s . A s t r . ( ) : P a g e f Table 3 (Continued...).
Name CAHA.V ± err CAHA.R ± err GAIA3.Gbp ± err GAIA3.G ± err GAIA3.Grp ± err PS1.r ± err PS1.g ± err PS1.i ± errBSS1 1.04e-15 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± +
00 6.64e-16 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± +
00 4.42e-16 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± +
00 5.13e-16 ± +
00 7.09e-16 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±±
00 7.09e-16 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±± ± ± ± P a g e f J . A s t r oph y s . A s t r . ( ) : Table 3 (Continued...).
Name PS1.z ± err PS1.y ± err 2MASS.J ± err 2MASS.H ± err 2MASS.Ks ± errBSS1 5.25e-16 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± + ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± +
00 1.57e-17 ± + ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±±