Browsing by Author "Vaidya, Kaushar"

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The American Astronomical Society logo. The Institute of Physics logo. An X-Ray and Near-Infrared Study of Young Stars in the Carina Nebula
(IOP, 2007) Vaidya, Kaushar
We present a multiwavelength study of the central region of the Carina Nebula, including Trumpler 16 and part of Trumpler 14. Our analysis of the Chandra X-Ray Observatory archival data led to the identification of 454 X-ray sources. These sources were then cross-identified with optical photometric and spectroscopic information available from the literature and with newly obtained near-infrared (JHKs) imaging observations. A total of 38 known OB stars are found to be X-ray emitters. All the O stars and early-B stars follow the nominal relation between the X-ray and bolometric luminosities, LX ~ 10-7Lbol. A few mid- to late-B stars are found to be associated with X-ray emission, likely attributable to T Tauri companions. We discovered 16 OB star candidates that suffer a large extinction in the optical wave bands. Some 300 sources have the X-ray and infrared characteristics of late-type pre-main-sequence stars. Our sample represents the most comprehensive census of the young stellar population in the Carina Nebula so far and should be useful for the study of the star formation history of this massive starburst region. We also report the finding of a compact (2' × 4') group of 10 relatively bright X-ray sources, all of which are detected in the near-infrared wavelengths and are highly reddened. The group is spatially coincident with the dark V-shaped dust lane bisecting the Carina Nebula and may be part of an embedded association. The distribution of the young stellar groups surrounding the H II region associated with Trumpler 16 is consistent with the collect-and-collapse scenario of triggered star formation.
The blue straggler population of the old open cluster Berkeley 17
(The European Southern Observatory, 2019) Vaidya, Kaushar
Blue straggler stars (BSSs) are observed in Galactic globular clusters and old open clusters. The radial distribution of BSSs has been used to diagnose the dynamical evolution of globular clusters. For the first time, with a reliable sample of BSSs identified with Gaia DR2, we conduct such an analysis for an open cluster.
Blue straggler populations of seven open clusters with Gaia DR2
(Oxford, 2020-06) Vaidya, Kaushar
Blue straggler stars (BSS) are well studied in globular clusters but their systematic study with secure membership determination is lacking in open clusters. We use Gaia DR2 data to determine accurate stellar membership for four intermediate-age open clusters, namely Melotte 66, NGC 2158, NGC 2506, and NGC 6819, and three old open clusters, namely, Berkeley 39, NGC 188, and NGC 6791, to subsequently study their BSS populations. The BSS radial distributions of five clusters, namely Melotte 66, NGC 188, NGC 2158, NGC 2506, and NGC 6791, show bimodal distributions, placing them with Family II globular clusters that are of intermediate dynamical ages. The location of minima, rmin, in the bimodal BSS radial distributions, varies from 1.5rc to 4.0rc, where rc is the core radius of the clusters. We find a positive correlation between rmin and Nrelax, the ratio of cluster age to the current central relaxation time of the cluster. We further report that this correlation is consistent in its slope, within the errors, with the slope of the globular cluster correlation between the same quantities, but with a slightly higher intercept. This is the first example in open clusters that shows BSS radial distributions as efficient probes of dynamical age. The BSS radial distributions of the remaining two clusters, Berkeley 39 and NGC 6819, are flat. The estimated Nrelax values of these two clusters, however, indicate that they are dynamically evolved. Berkeley 39 especially has its entire BSS population completely segregated to the inner regions of the cluster.
Blue Straggler Stars: setting up a dynamical clock for open clusters
(The PoPuPS, 2024) Vaidya, Kaushar
Blue 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.
Characterization of Blue and Yellow Straggler Stars of Berkeley 39
(IOP, 2024) Vaidya, Kaushar
We 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.
Characterization of hot populations of Melotte 66 open cluster using Swift/UVOT
(OUP, 2022-10) Vaidya, Kaushar
Ultraviolet (UV) wavelength observations have made a significant contribution to our understanding of hot stellar populations of star clusters. Multiwavelength spectral energy distributions (SEDs) of stars, including ultraviolet observations, have proven to be an excellent tool for discovering unresolved hot companions in exotic stars such as blue straggler stars (BSS), thereby providing helpful clues to constrain their formation mechanisms. Melotte 66 is a 3.4 Gyr old open cluster located at a distance of 4810 pc. We identify the cluster members by applying the ML-MOC algorithm on Gaia EDR3 data. Based on our membership identification, we find 1162 members, including 14 BSS candidates, 2 yellow straggler candidates (YSS), and one subdwarf B candidate (sdB). We generated SEDs for 11 BSS candidates and the sdB candidate using Swift/UVOT data combined with other archival data in the optical and IR wavelengths. We discover a hot companion of one BSS candidate, BSS3, with temperature of 38 000 K, luminosity of 2.99 L⊙, and radius of 0.04 R⊙. This hot companion is a likely low-mass WD with an estimated mass of 0.24–0.44 M⊙. We report one BSS candidate, BSS6, as an Algol-type eclipsing binary with a period of 0.8006 d, based on the Gaia DR3 variability classification. We suggest that BSS3 is formed via either the Case A or Case B mass-transfer channel, whereas BSS6 is formed via the Case A mass transfer.
Constraining convection parameters from the light curve shapes of pulsating white dwarf stars: the cases of EC 14012-1446 and WD 1524-0030
(IOP, 2008) Vaidya, Kaushar
Montgomery [1] developed a method to probe convection in pulsating white dwarf stars which allows the recovery of the thermal response time of the convection zone by fitting observed nonsinusoidal light curves. He applied this method to two objects; the Whole Earth Telescope (WET) observed the pulsating DB white dwarf GD 358 for just this purpose. Given this WET run's success, it is time to extend Montgomery's method to pulsating DA white dwarf (ZZ Ceti) stars. We present observations of two ZZ Ceti stars, WD 1524—0030 and EC 14012—1446, both observed from multiple sites. EC 14012—1446 seems better suited thAN WD1524—0030 for a future WET run because it has more pulsation modes excited and because it pulsation spectrum appears to be more stable in time. We call for participation in this effort to take place in April 2008.
Contact Binary Variables as X-ray Sources
(ARXIV, 2005-08) Vaidya, Kaushar
We present cross-identification of archived X-ray point sources with W UMa variable stars found in the All-Sky Automated Survey (ASAS). In a surveyed sky area of 300 square degrees of ASAS, 36 W UMa stars have been found associated with X-ray emission. We compute the distances of these W UMa systems and hence their X-ray luminosities. Our data support the "supersaturation" phenomenon seen in these fast rotators, namely that the faster a W UMa star rotates, the weaker its X-ray luminosity.
The cool Galactic R Coronae Borealis variable DY Persei
(EDP Sciences, 2005-04) Vaidya, Kaushar
Results of first CCD photometry during the recent deep light decline, and high-resolution spectroscopy, are presented for DY Persei. The spectra show variable blueshifted features in the sodium D lines. The C i lines are strong whereas neutron-capture elements are not enhanced. The isotopic 13CN(2, 0) lines relative to 12CN are of similar strength with those for the carbon star U Hya. All these confirm the RCB nature of DY Per and the existence of ejected clouds. At least two clouds are revealed at -197.3 and -143.0 km s-1. A star was detected about 04 to the west and 25 to the north from DY Per. This anonymous companion, with observed colour indices and , may be a foreground star. 
Determination of dynamical ages of open clusters through the A+ parameter – I
(OUP, 2021-12) Vaidya, Kaushar
The sedimentation level of blue straggler stars (BSS) has been shown to be a great tool to investigate the dynamical states of globular clusters (GCs). The area enclosed between the cumulative radial distributions of BSS and a reference population up to the half-mass radius of the clusters, ⁠, is known to be a measure of the sedimentation of BSS in GCs. In this work, we calculate for 11 open clusters (OCs) using a combined list of main-sequence turn-off stars, sub-giant branch stars, and red giant branch stars as reference population. The BSS, the reference populations, and the cluster members are identified using the proper motions and parallaxes from the Gaia DR2 data. In a subset of clusters, the BSS are confirmed cluster members on the basis of radial velocity information available in the literature. Using the Pearson and Spearman rank correlation coefficients, we find weak correlations between the estimated values of and other markers of dynamical ages of the clusters, i.e. the number of central relaxations a cluster has experienced since its formation, and the structural parameters of the clusters. Based on statistical tests, we find that these correlations are similar to the corresponding correlations among the less evolved GCs, albeit within large errors
Determination of dynamical ages of open clusters through the A+ parameter – II
(OUP, 2023-09) Vaidya, Kaushar
Blue straggler stars (BSS), one of the most massive members of star clusters, have been used for over a decade to investigate mass segregation and estimate the dynamical ages of globular clusters (GCs) and open clusters (OCs). This work is an extension of our previous study, in which we investigated a correlation between theoretically estimated dynamical ages and the observed values, which represent the sedimentation level of BSS with respect to the reference population. Here, we use the ML-MOC algorithm on Gaia EDR3 data to extend this analysis to 23 OCs. Using cluster properties and identified members, we estimate their dynamical and physical parameters. In order to estimate the values, we use the main sequence and main sequence turnoff stars as the reference population. OCs are observed to exhibit a wide range of degrees of dynamical evolution, ranging from dynamically young to late stages of intermediate dynamical age. Hence, we classify OCs into three distinct dynamical stages based on their relationship to and Nrelax. NGC 2682 and King 2 are discovered to be the most evolved OCs, like Family III GCs, while Berkeley 18 is the least evolved OC. Melotte 66 and Berkeley 31 are peculiar OCs because none of their dynamical and physical parameters correlate with their BSS segregation levels.
Discovery of double BSS sequences in the old Galactic open cluster Berkeley 17
(OUP, 2022-10) Vaidya, Kaushar
Blue straggler stars (BSS) are peculiar objects which normally appear as a single broad sequence along the extension of the main sequence. Only four globular clusters (GCs) have been observed to have two distinct and parallel BSS sequences. For the first time for any open cluster (OC), we report double BSS sequences in Berkeley 17. Using the machine-learning based membership algorithm ML-MOC on Gaia EDR3 data, we identify 627 cluster members, including 21 BSS candidates out to 15 arcmin from the cluster centre. Both the BSS sequences are almost equally populated and parallel to one another in Gaia as well as in Pan-STARRS colour–magnitude diagram (CMD). We statistically confirm their presence and report that both BSS sequences are highly segregated compared to the reference population out to ∼5.5 arcmin and not segregated thereafter. The lower densities of OCs make BSS formation impossible via the collisional channel. Therefore, mass transfer seems to be the only viable channel for forming candidates of both sequences. The gap between the red and blue BSS sequences, on the other hand, is significant and presents a great opportunity to understand the connection between BSS formation and internal as well as external dynamics of the parent clusters.
Discovery of white dwarf companions of blue metal-poor stars using UVIT/AstroSat
(American Physical Society, 2023) Vaidya, Kaushar
Blue 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.
Disintegration of the Aged Open Cluster Berkeley 17
(IOP, 2017) Vaidya, Kaushar
We present the analysis of the morphological shape of Berkeley 17, the oldest known open cluster (∼10 Gyr), using the probabilistic star counting of Pan-STARRS point sources, and confirm its core-tail shape, plus an antitail, previously detected with the 2MASS data. The stellar population, as diagnosed by the color–magnitude diagram and theoretical isochrones, shows many massive members in the clusters core, whereas there is a paucity of such members in both of the tails. This manifests mass segregation in this aged star cluster with the low-mass members being stripped away from the system. It has been claimed that Berkeley 17 is associated with an excessive number of blue straggler candidates. A comparison of nearby reference fields indicates that about half of these may be field contamination.
Dynamical evolution of four old galactic open clusters traced by their constituent stars with gaia DR3
(IOP, 2024-10) Vaidya, Kaushar
We 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.
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.
A Gaia EDR3 search for tidal tails in disintegrating open clusters
(OUP, 2022-10) Vaidya, Kaushar
We carry out a search for tidal tails in a sample of open clusters with known relatively elongated morphology. We identify the member stars of these clusters from the precise astrometric and deep photometric data from Gaia Early Data Release 3 using the robust membership determination algorithm, ML-MOC. We identify 46 open clusters having a stellar corona beyond the tidal radius, 20 of which exhibit extended tails aligned with the cluster orbit direction in Galactocentric coordinates. Notably, we find NGC 6940 (at a distance of ∼1 kpc) is the furthest open cluster, exhibiting tidal tails that are ∼50 pc from its centre, while also identifying ∼40 pc long tidal tails for the nearby Pleiades. Using the minimum spanning tree length for the most massive stars relative to all cluster members, we obtain the mass segregation ratio (⁠⁠) profiles as a function of the number of massive stars in each cluster. From these profiles, we can classify the open clusters into four classes based on the degree of mass segregation experienced by the clusters. We find that clusters in the most mass segregated classes are the oldest on average and have the flattest mass function slope. Of the 46 open clusters studied in this work, 41 exhibit some degree of mass segregation. Furthermore, we estimate the initial masses (M⁠) of these open clusters, finding that some of them, having M⁠, could be the dissolving remnants of young massive clusters.
GRB 050502a : r band observation at Lulin observatory
(GRB Coordinates Network, 2005) Vaidya, Kaushar
RB 050502a : r band observation at Lulin observatory. - NASA/ADS Now on home page ads icon ads Enable full ADS view NASA/ADS GRB 050502a : r band observation at Lulin observatory. Sanchawala, K. ; Wu, WL ; Huang, KY ; Ip, WH ; Qiu, Y. ; Zhou, W. ; Lou, YQ ; Urata, Y. Abstract Publication: GRB Coordinates Network Pub Date: 2005 Bibcode: 2005GCN..3335....1S full text sources Publisher | data products SIMBAD (1) © The SAO/NASA Astrophysics Data System adshelp[at]cfa.harvard.edu The ADS is operated by the Smithsonian Astrophysical Observatory under NASA Cooperative Agreement NNX16AC86A NASA logo Smithsonian logo Resources About ADS ADS Help What's New Careers@ADS Social @adsabs ADS Blog Project Switch to full ADS Is ADS down? (or is it just me...) Smithsonian Institution Smithsonian Privacy Notice Smithsonian Terms of Use Smithsonian Astrophysical Observatory NASA
Hot Stellar Populations of Berkeley 39 using Swift/UVOT
(ARXIV, 2023-08) Vaidya, Kaushar
Open clusters are excellent tools to probe the history of the Galactic disk and properties of star formation. In this work, we present a study of an old age open cluster Berkley 39 using the observations from UVOT instrument of the Neil Gehrels Swift observatory. Making use of a machine learning algorithm, ML-MOC, we have identified a total of 861 stars as cluster members out of which 17 are blue straggler stars. In this work, we present a characterisation of 2 blue straggler stars. To estimate the fundamental parameters of blue straggler stars and their companions (if any), we constructed spectral energy distributions using UV data from swift/UVOT and GALEX, optical data from Gaia DR3, and infrared (IR) data from 2MASS, Spitzer/IRAC, and WISE. We find excess flux in UV in one blue straggler star, implying the possibility of a hot companion.
A hubble space telescope view of the interstellar environments of young stellar objects in the large magellanic cloud
(IOP, 2009-12) Vaidya, Kaushar
We have used archival Hubble Space Telecope (HST) Hα images to study the immediate environments of massive and intermediate-mass young stellar object (YSO) candidates in the Large Magellanic Cloud (LMC). The sample of YSO candidates, taken from Gruendl & Chu, was selected based on Spitzer IRAC and MIPS observations of the entire LMC and complementary ground-based optical and near-infrared observations. We found HST Hα images for 99 YSO candidates in the LMC, of which 82 appear to be genuine YSOs. More than 95% of the YSOs are found to be associated with molecular clouds. YSOs are seen in three different kinds of environments in the Hα images: in dark clouds, inside or on the tip of bright-rimmed dust pillars, and in small H ii regions. Comparisons of spectral energy distributions for YSOs in these three different kinds of environments suggest that YSOs in dark clouds are the youngest, YSOs with small H ii regions are the most evolved, and YSOs in bright-rimmed dust pillars span a range of intermediate evolutionary stages. This rough evolutionary sequence is substantiated by the presence of silicate absorption features in the Spitzer Infrared Spectrograph spectra of some YSOs in dark clouds and in bright-rimmed dust pillars, but not those of YSOs in small H ii regions. We present a discussion on triggered star formation for YSOs in bright-rimmed dust pillars or in dark clouds adjacent to H ii regions. As many as 50% of the YSOs are resolved into multiple sources in high-resolution HST images. This illustrates the importance of using high-resolution images to probe the true nature and physical properties of YSOs in the LMC.