Department of Physics
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Item VizieR Online Data Catalog: Positions, plx, & phot. from Gaia for Berkeley 39 (Chand+, 2024)(2025-09) Vaidya, KausharIn order to characterize the BSS and YSS populations, we use multiwavelength data, ranging from UV to infrared (IR). UV data: The fluxes of sources in UV wavelengths are obtained from the Ultraviolet and Optical Telescope (Swift/UVOT) and Galaxy Evolution Explorer (GALEX). The Swift spacecraft is equipped with UVOT, a 30cm telescope sensitive in the optical/UV band in the wavelength range of 170-650nm. We utilize near-UV (NUV) data (Siegel+2019, J/AJ/158/35) from UVOT in this work. Berkeley 39 was observed by Swift/UVOT in three UV filters, UVW2 (180-260nm), UVM2 (200-280nm), and UVW1 (220-400nm), in 2011. In addition to the Swift/UVOT data, we also use data from GALEX, a 50cm aperture space-based telescope which observed Berkeley 39 in its NUV band (1770-2730Å). Optical data: Gaia data (DR3; Gaia Collab.+2022, I/355) provides proper motions, parallaxes, and three-band photometry (G, GBP, GRP) for over a billion sources observed between 25 July 2014 and 28 May 2017. The wavelength of filter G is centered at 673nm, GBP at 532nm, and GRP at 797nm. The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) is a ground-based telescope which observed from 2010 to 2015 for its first data release (Chambers+2019, II/349). It utilizes five filters, g, r, i, z, and y, with effective wavelengths 481, 617, 752, 866, and 962nm respectively.Item Vizier online data catalog: tidal tails in the open cluster ngc 752(VizieR, 2021) Vaidya, KausharAgarwal et al. (2021MNRAS.502.2582A) using Gaia DR2 (2018A&A...616A...1G, Cat. I/345) data with their novel machine learning membership determination algorithm found hints of tidal tails in NGC 752. Also, Hu et al. (2021ApJ...912....5H, Cat. J/ApJ/912/5) also identify the elongated morphology of the peripheral regions of NGC 752 from the Gaia DR2 data using the members identified by Cantat-Gaudin et al. (2018A&A...618A..93C, Cat. J/A+A/618/A93). As the first step, we do a membership selection in NGC 752 from the Gaia EDR3 (2021A&A...649A...1G, Cat. I/350) data set is carried out in a region of radius 5 degrees around its centre. This sample is termed All sources. We use ML-MOC (Agarwal et al. 2021MNRAS.502.2582A) to identify cluster members using the proper motion and parallax information. It identifies cluster members in the PM-ω parameter space, independent of the spatial density of the cluster, thereby allowing for the identification of faint extended spatial structures such as tidal tailsItem Blue Straggler Stars: setting up a dynamical clock for open clusters(The PoPuPS, 2024) Vaidya, KausharBlue straggler stars are late bloomers that are bluer and brighter than the main sequence turnoff stars on a color-magnitude diagram of a cluster. Being a massive population compared to their other cluster siblings, their sedimentation level has been used to probe the dynamical evolution of globular clusters. In a series of studies, we explored the dynamical evolution of old open clusters, for the first time, making use of this same trait of the BSS that they settle quicker in the cluster core than any other cluster population. In these works, we have compared the theoretical estimation of dynamical ages, Nrelax, with the observed signatures of blue straggler stars segregation either by placing open clusters in Family I/II/III classification of dynamical ages defined in globular clusters or by estimating the area enclosed between the cumulative radial distributions of BSS and another reference population in 23 open clusters.Item Uocs xiv: study of the open cluster ngc 2627 using uvit/astrosat(IOP, 2024-07) Vaidya, KausharWe study the intermediate-age open cluster NGC 2627, located at a distance of ∼2 kpc, using UVIT/AstroSat and other archival data. Using a machine learning-based algorithm, ML-MOC, on the Gaia DR3 data, we identify 422 cluster members, including four blue straggler stars (BSSs), one yellow straggler star (YSS), one blue lurker (BL), one red clump (RC) star, and two binary candidates with detection in both UVIT/F148W and UVIT/F169M filters. We characterise them using multiwavelength spectral energy distributions (SEDs). Out of the above nine sources, one BSS, the BL, and one binary candidate have a source nearby; hence, we did not fit their SEDs. Of the remaining six sources, we successfully fit two with single-component SEDs and four with binary-component SEDs. The binary-component SED-based parameters indicate that the hot companions of BSSs, the YSS, the RC star, and the binary candidate are extremely low-mass white dwarfs, confirming that at least four out of nine stars (44%) are formed via the mass transfer channel. We fit King's profile function to the high-probability (p > 0.8) cluster members and estimate the cluster core radius (rC) to be 3.84' and the tidal radius (rt) to be 36.85'. We find that the equal-mass binaries are most concentrated towards the cluster center, followed by the single massive stars, and single low-mass stars. The BSS population of the cluster is also found to be located within a radius r ∼ 10 × rC from the cluster center, suggesting the dynamical evolution of the cluster.Item Dynamical evolution of four old galactic open clusters traced by their constituent stars with gaia DR3(IOP, 2024-10) Vaidya, KausharWe investigate the evolutionary stages of four open clusters—Berkeley 39, Collinder 261, NGC 6819, and NGC 7789—of ages ranging from 1.6 to 6 Gyr. These clusters have previously been classified into dynamically young and intermediate age groups based on the segregation level of BSS with respect to red-giant-branch stars and main-sequence stars, respectively. We identify members of these four clusters using the ML-MOC algorithm on Gaia DR3 data. To examine the relative segregation of cluster members of different evolutionary stages, we utilize cumulative radial distributions, proper motion distributions, and spatial distributions in galactocentric coordinates. Our analysis shows that Berkeley 39 and NGC 6819 exhibit moderate signs of population-wise segregation from evolved to less-evolved members. NGC 7789 shows signs of mass segregation only in the cumulative radial distributions. On the other hand, Collinder 261 exhibits high segregation of BSS in the cumulative radial distribution, while other populations show the same level of segregation.Item Characterization of Blue and Yellow Straggler Stars of Berkeley 39(IOP, 2024) Vaidya, KausharWe characterize blue straggler stars (BSSs) and yellow straggler stars (YSSs) of the open cluster (OC) Berkeley 39 using multiwavelength observations including the Swift/Ultraviolet and Optical Telescope (UVOT). Our analysis also makes use of ultraviolet (UV) data from Galaxy Evolution Explorer, optical data from Gaia DR3 and Panoramic Survey Telescope and Rapid Response System, and infrared data from Two Micron All Sky Survey, Spitzer/IRAC, and Wide-field Infrared Survey Explorer. Berkeley 39 is a ∼6 Gyr old Galactic OC located at a distance of ∼4200 pc. We identify 729 sources as cluster members utilizing a machine-learning algorithm, ML-MOC, on Gaia DR3 data. Of these, 17 sources are classified as BSS candidates and four as YSS candidates. We construct multiwavelength spectral energy distributions (SEDs) of 16 BSS and two YSS candidates, within the Swift/UVOT field, to analyze their properties. Out of these, eight BSS candidates and both the YSS candidates are successfully fitted with single-component SEDs. Five BSS candidates show marginal excess in the near-UV (NUV; fractional residual <0.3 in all but one UVOT filter), whereas three BSS candidates show moderate to significant excess in the NUV (fractional residual >0.3 in at least two UVOT filters). We present the properties of the BSS and YSS candidates, estimated based on the SED fits.Item Study of Blue Straggler Stars in Open Cluster NGC 2506 using UVIT/AstroSat(COSPAR Scientific Assembly, 2022-07) Vaidya, KausharBlue straggler stars (BSS) are one of the exotic stellar populations that prolong their main-sequence lifetimes by acquiring extra mass through either mass transfer or complete merger in a binary or multiple stellar system, or through direct stellar collisions in dense stellar environments. UV-based studies enable detection of hot companions of BSS from the excess flux in the UV wavelengths hence shedding light on the formation mechanisms of BSS. NGC 2506 is an intermediate age (∼ 2.2 Gyr) open cluster located at a distance of ∼ 3110 pc. According to our membership analysis of the cluster based on the Gaia EDR3 data, this cluster has 9 BSS candidates. We use the UVIT/AstroSat data in combination with other multi-wavelength data to characterize these BSS candidates. The spectral energy distributions (SEDs) of these BSS showed excess UV flux in 3 BSS candidates. Based on the properties of the hot companions derived from the SEDs, these hot companions are low-mass (LM) or extremely low-mass (ELM) white dwarfs (WD). The detection of LM/ELM WDs in a binary system strongly indicates the formation of BSS via Case-A/Case-B mass transfer mechanism. We, therefore suggest that at least 3 out of 7 (42%) BSS studied have formed via the Case-A/Case-B mass-transfer mechanism.Item Studies of Blue Stragglers in star clusters with AstroSat(COSPAR Scientific Assembly, 2022-07) Vaidya, KausharWe present an overview of Blue Stragglers (BSs) studies in star clusters using UVIT observations onboard AstroSat. The formation of BSs has puzzled scientists for several decades since their discovery. Two main scenarios proposed for their formation are stellar collisions leading to mergers in high-density environments and mass transfer between an evolved primary donor to the secondary star, mostly dominating in low-density environments. These stars being bluer and brighter than the main-sequence (MS) turn-off stars in the optical color-magnitude diagrams should have evolved long ago given their respective cluster ages according to the standard stellar evolution theory. BSs, being massive and hotter than the dominant cluster population of MS and red-giant stars mainly emit in UV. Thus, UV observations are advantageous in identifying these stars by reducing the crowding. Most of the studies related to the BSs are based on the NUV and optical data from HST, Gaia, and ground-based observations. However, dedicated FUV surveys of star clusters are needed to identify the hot companions to BSs and understand their nature. Taking the advantage of multiple filters in the two UV channels of UVIT, we have combined them with the multi-wavelength archival data to sample the spectral energy distributions of BSs and unravel their properties. We discuss the exciting results from the UV analysis of BSs in open and globular clusters.Item A UVIT/AstroSat Study of Blue Straggler Stars of NGC 7789(COSPAR Scientific Assembly, 2022-07) Vaidya, KausharWe present a study of blue straggler stars (BSS) of open cluster NGC 7789 using the Ultraviolet (UV) data from the UVIT/AstroSat. NGC 7789 is a ∼1.6 Gyr old, populous open cluster located at ∼2000 pc. There are 13 BSS candidates of this cluster which are members according to their radial velocities as well as Gaia DR2 proper motions and parallaxes. We identify 4 additional BSS candidates in this cluster using Gaia DR2 proper motions and parallaxes. We construct spectral energy distributions (SED) of these BSS candidates using multi-wavelength data ranging from UV to IR wavelengths. We discover five of the BSS with a hot companion with T _{eff} ∼11750--15500 K, R ∼0.069--0.242 R _{sun}, and L ∼0.25--1.55 L _{sun}. Two additional BSS show excess in one or more UV filters, and may have a hot companion, however we are unable to characterize them. All the five hot companions to the BSS are most likely extremely low mass (ELM) white dwarfs (WDs) with masses smaller than ∼0.18 M _{sun}, and thereby confirmed post mass transfer systems. We suggest that at least 33% of the BSS candidates studied in this cluster have formed via Case-A/Case-B mass transfer mechanism.Item Discovery of white dwarf companions of blue metal-poor stars using UVIT/AstroSat(American Physical Society, 2023) Vaidya, KausharBlue metal-poor (BMP) stars are main-sequence stars that appear bluer and more luminous than normal turn-off stars of metal-poor globular clusters. These stars are believed to be either field blue straggler stars (FBSS) formed via post mass transfer mechanism or accreted from dwarf satellite galaxies of the Milky Way. A significant fraction of BMP stars are discovered to be binaries, typically with long periods and circular orbits, likely to have a compact companion. UVIT/AstroSat has observed 27 BMP stars in two FUV filters, F148W and F169M. We report the first ever discovery of white dwarfs companions in 14 stars. Out of these 14 stars, 10 are known to be either single-lined spectroscopic binaries or binary candidates with the periods of 0.9 to 840 days and eccentricity 0 to 0.5. 3 BMP stars are known to be radial velocity constant stars. We suspect that orbits of these stars may be either inclined or distance between binaries may be too large to observe the variation in radial velocities. For the remaining 1 star, no previous information on binarity is available. The hot companions with estimated temperatures, Teff ∼ 10500 − 40000 K, are white dwarfs (WDs) of extremely low-mass (∼ 0.20 M_sun), low-mass (∼ 0.20 – 0.40 M_sun), normal mass (∼ 0.40 – 0.60 M_sun), and high-mass (∼ 0.8M_sun). Our analysis suggests that both mass transfer and merger in hierarchial triple system channels play a major role in the formation of BMP stars and thus atleast 14 BMP stars in our sample are confirmed FBSS.
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