Department of Physics

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Author: search.filters.author.Vaidya, KausharSubject: search.filters.subject.UVIT/AstroSat

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    Study of Blue Straggler Stars in Open Cluster NGC 2506 using UVIT/AstroSat
    (COSPAR Scientific Assembly, 2022-07) Vaidya, Kaushar
    Blue 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.
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    Studies of Blue Stragglers in star clusters with AstroSat
    (COSPAR Scientific Assembly, 2022-07) Vaidya, Kaushar
    We 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.
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    A UVIT/AstroSat Study of Blue Straggler Stars of NGC 7789
    (COSPAR Scientific Assembly, 2022-07) Vaidya, Kaushar
    We 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.
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    Field blue straggler stars: discovery of white dwarf companions to blue metal-poor stars using UVIT/AstroSat
    (OUP, 2023-08) Vaidya, Kaushar
    Blue metal-poor (BMP) stars are the main-sequence stars that appear bluer and more luminous than normal turn-off stars of metal-poor globular clusters. They 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 potential binaries. We observed 27 BMP stars using UVIT/AstroSat in two FUV filters, F148W and F169M. We report the discovery of white dwarf (WD) companions of 12 BMP stars for the first time. The WD companions have estimated temperatures Teff ∼10 500–18 250 K, and masses 0.17–0.8 M⊙. Based on [Fe/H] and space velocity, we group the 12 BMP/FBSS stars as the thick disc (5) and halo (5), whereas two stars appear to be in-between. All the 5 thick disc BMP/FBSS have extremely low-mass (M < 0.2 M⊙) WDs as companions, whereas the 5 halo BMP/FBSS have low (0.2 M⊙ < M < 0.4 M⊙), normal (0.4 M⊙ < M < 0.6 M⊙), and high mass (M > 0.6 M⊙) WD companions. Our analysis suggests that at least ∼44 of BMP stars are FBSS, and these stars hold the key to understand the details of mass transfer, binary properties, and chemical enrichment among the FBSS.
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    UOCS – VII. Blue straggler populations of open cluster NGC 7789 with UVIT/AstroSat
    (OUP, 2022-01) Vaidya, Kaushar
    NGC 7789 is a ∼1.6 Gyr old, populous open cluster located at ∼2000 pc. We characterize the blue straggler stars (BSS) of this cluster using the Ultraviolet (UV) data from the UVIT/AstroSat. We present spectral energy distributions (SED) of 15 BSS candidates constructed using multiwavelength data ranging from UV to IR wavelengths. In 8 BSS candidates, a single temperature SED is found to be satisfactory. We discover hot companions in 5 BSS candidates. The hot companions with Teff ∼ 11750–15500 K, R ∼ 0.069–0.242 R⊙, and L ∼ 0.25–1.55 L⊙, are most likely extremely low mass (ELM) white dwarfs (WDs) with masses smaller than ∼0.18 M⊙, and thereby confirmed post mass transfer systems. We discuss the implication of this finding in the context of BSS formation mechanisms. Two additional BSS show excess in one or more UV filters, and may have a hot companion, however we are unable to characterize them. We suggest that at least 5 of the 15 BSS candidates (33 per cent) studied in this cluster have formed via the mass-transfer mechanism.