arXiv subjects

Snehalata Sahu

Publications and source records attributed to Snehalata Sahu.

At least 37 records · Page 2Linked to original sources

arXiv2023

Unveiling the white dwarf in J191213.72-441045.1 through ultraviolet observations

J191213.72-441045.1 is a binary system composed of a white dwarf and an M-dwarf in a 4.03-hour orbit. It shows emission in radio, optical, and X-ray, all modulated at the white dwarf spin period of 5.3 min, as well as various orbital sideband frequencies. Like in the prototype of the class of radio-pulsing white dwarfs, AR Scorpii, the observed pulsed emission seems to be driven by the binary interaction. In this work, we present an analysis of far-ultraviolet spectra obtained with the Cosmic Origins Spectrograph at the Hubble Space Telescope, in which we directly detect the white dwarf in J191213.72-441045.1. We find that the white dwarf has an effective temperature of 11485+/-90 K and mass of 0.59+/-0.05 solar masses. We place a tentative upper limit on the magnetic field of ~50 MG. If the white dwarf is in thermal equilibrium, its physical parameters would imply that crystallisation has not started in the core of the white dwarf. Alternatively, the effective temperature could have been affected by compressional heating, indicating a past phase of accretion. The relatively low upper limit to the magnetic field and potential lack of crystallisation that could generate a strong field pose challenges to pulsar-like models for the system and give preference to propeller models with a low magnetic field. We also develop a geometric model of the binary interaction which explains many salient features of the system.

astro-ph.SR↗

arXiv2023

An HST COS ultra-violet spectroscopic survey of 311 DA white dwarfs.I. Fundamental parameters and comparative studies

White dwarf studies carry significant implications across multiple fields of astrophysics, including exoplanets, supernova explosions, and cosmological investigations. Thus, accurate determinations of their fundamental parameters (Teff and log g) are of utmost importance. While optical surveys have provided measurements for many white dwarfs, there is a lack of studies utilising ultraviolet (UV) data, particularly focusing on the warmer ones that predominantly emit in the UV range. Here, we present the medium-resolution far-UV spectroscopic survey of 311 DA white dwarfs obtained with Cosmic Origins Spectrograph (COS) onboard Hubble Space Telescope confirming 49 photometric Gaia candidates. We used 3D extinction maps, parallaxes, and hydrogen atmosphere models to fit the spectra of the stars that lie in the range 12 000 < Teff < 33 000 K, and 7 <= log g < 9.2. To assess the impact of input physics, we employed two mass-radius relations in the fitting and compared the results with previous studies. The comparisons suggest the COS Teff are systematically lower by 3 per cent on average than Balmer line fits while they differ by only 1.5 per cent from optical photometric studies. The mass distributions indicate that the COS masses are smaller by approximately 0.05 Msol and 0.02 Msol than Balmer lines and photometric masses, respectively. Performing several tests, we find that the discrepancies are either arising due to issues with the COS calibration, broadening theories for hydrogen lines, or interstellar reddening which needs further examination. Based on comparative analysis, we identify 30 binary candidates drawing attention for follow-up studies to confirm their nature.

astro-ph.SR↗

arXiv2023

Study of blue metal-poor stars using UVIT/AstroSat

Blue metal-poor stars are main-sequence stars that are bluer and brighter than typical turn-off stars in metal-poor globular clusters. They are thought to have either evolved through post-mass transfer mechanisms as field blue straggler stars or have accreted from Milky Way dwarf satellite galaxies. It has been found that a considerable fraction of blue metal poor stars are binaries, possibly with a compact companion. We observed 27 blue metal poor stars using UV imaging telescope of AstroSat in two far-UV filters, F148W and F169M. In this work, we explain the possible formation channels of two stars, BMP17 and BMP37. We fit BMP17 with a single-component spectral energy distribution whereas BMP37 with a binary-component spectral energy distribution. As both of them are known SB1s, we suggest that the WD companion of BMP17 may have cooled down so that it is out of UV imaging telescope detection limit. On the other hand, we discover a normal mass white dwarf as the hot companion of BMP37, indicating mass transfer as the possible formation channel

astro-ph.SR↗

arXiv2023

Globular Cluster UVIT Legacy Survey (GlobULeS) $-$ II. Evolutionary status of hot stars in M3 and M13

We present a far-ultraviolet (FUV) study of hot stellar populations in the second parameter pair globular clusters (GCs) M3 and M13, as a part of the Globular cluster UVIT Legacy Survey program (GlobULeS). We use observations made with F148W and F169M filters of the Ultraviolet Imaging Telescope (UVIT) onboard {\em AstroSat} along with ground-based data (UBVRI filters), {\em Hubble Space Telescope (HST)} GC catalogue, and {\em Gaia} EDR3 catalogue. Based on the FUV-optical colour-magnitude diagrams, we classify the sources into the horizontal branch (HB) stars, post-HB stars, and hot white dwarfs (WDs) in both the GCs. The comparison of synthetic and observed colours of the observed HB stars suggests that the mass-loss at the red giant branch (RGB) and He spread in both clusters have a simultaneous effect on the different HB distributions detected in M3 and M13, such that, HB stars of M13 require a larger spread in He (${\rm 0.247-0.310}$) than those of M3 (${\rm Y= 0.252-0.266}$). The evolutionary status of HB stars, post-HB stars, and WDs are studied using SED fit parameters and theoretical evolutionary tracks on the H-R diagram. We found that the observed post-HB stars have evolved from zero-age HB (ZAHB) stars of the mass range $0.48-0.55$ \Msun\ in M3 and M13. We detect 24 WD candidates in each cluster having ${\rm \log(L_{bol}/L_\odot)}$ in the range $-0.8$ to $+0.6$ and ${\rm \log(T_{eff}/K)}$ in the range of 4.2 to 5.0. Placing the WDs on the H-R diagram and comparing them with models suggest that M13 has a population of low-mass WDs, probably originating from binary evolution.

astro-ph.SR↗

arXiv2022

GlobULeS-IV. UVIT/AstroSat detection of extremely low mass white dwarf companions to blue straggler stars in NGC 362

We report the discovery of extremely low-mass white dwarfs (ELM WDs) as a companion of blue straggler stars (BSSs) in the Galactic globular cluster NGC 362 using images from AstroSats Ultra Violet Imaging Telescope (UVIT). Spectral Energy Distributions (SEDs) for 26 FUV bright member BSSs are created using data from the UVIT, UVOT, Gaia EDR3, and the 2.2 m ESO/MPI telescope. A single SED is fitted to 14 BSSs, whereas double-SED fits revealed ELM WDs as binary companions in 12 of the 26 BSSs studied. The effective temperature, radius, luminosity and mass of the 12 ELM WDs are found to have a range (Teff = 9750-18000 K, R = 0.1-0.4 Rsun, L = 0.4-3.3 Lsun, and M =0.16-0.20 Msun). These suggest that 12 BSSs are post-mass-transfer systems formed through Case A/B mass transfer pathway. To the best of our knowledge, this is the first finding of ELM WDs as companions to BSS in globular clusters. This cluster is known to have a binary BSS sequence and the 12 binary and 14 single BSSs (as classified by the SEDs) follow the mass transfer and collisional sequence of BSS in the colour-magnitude diagram. The cooling ages of 9 of the ELM WDs are found to be younger than 500 Myr. Though the binary BSSs may have formed during the core-collapse (approx 200 Myr) or as part of the dynamical evolution of the cluster, they provide new insights on the dynamics of this cluster.

astro-ph.SR↗

arXiv2022

Globular Cluster UVIT legacy Survey (GlobUleS) III. Omega Centauri in Far-Ultraviolet

We present the first comprehensive study of the most massive globular cluster Omega Centauri in the far-ultraviolet (FUV) extending from the center to ~ 28% of the tidal radius using the Ultraviolet Imaging Telescope aboard AstroSat. A comparison of the FUV-optical color-magnitude diagrams with available canonical models reveals that the horizontal branch (HB) stars bluer than the knee (hHBs) and the white dwarfs (WDs) are fainter in the FUV by ~ 0.5 mag than model predictions. They are also fainter than their counterparts in M13, another massive cluster. We simulated HB with at least five subpopulations, including three He-rich populations with a substantial He enrichment of Y up to 0.43 dex, to reproduce the observed FUV distribution. We find the He-rich younger subpopulations to be radially more segregated than the He-normal older ones, suggesting an in-situ enrichment from older generations. The Omega Cen hHBs span the same effective temperature range as their M13 counterparts, but some have smaller radii and lower luminosities. This may suggest that a fraction of Omega Cen hHBs are less massive than those of M13, similar to the result derived from earlier spectroscopic studies of outer extreme HB stars. The WDs in Omega Cen and M13 have similar luminosity-radius-effective temperature parameters, and 0.44 - 0.46 M$_\odot$ He-core WD model tracks evolving from progenitors with Y = 0.4 dex are found to fit the majority of these. This study provides constraints on the formation models of Omega Cen based on the estimated range in age, [Fe/H] and Y (in particular), for the HB stars.

astro-ph.SR↗

arXiv2022

Globular Clusters UVIT Legacy Survey (GlobULeS) I. FUV-optical Color-Magnitude Diagrams for Eight Globular Clusters

We present the first results of eight Globular Clusters (GCs) from the AstroSat/UVIT Legacy Survey program GlobULeS based on the observations carried out in two FUV filters (F148W and F169M). The FUV-optical and FUV-FUV color-magnitude diagrams (CMDs) of GCs with the proper motion membership were constructed by combining the UVIT data with HST UV Globular Cluster Survey (HUGS) data for inner regions and Gaia Early Data Release (EDR3) for regions outside the HST's field. We detect sources as faint as F148W $\sim$ 23.5~mag which are classified based on their locations in CMDs by overlaying stellar evolutionary models. The CMDs of 8 GCs are combined with the previous UVIT studies of 3 GCs to create stacked FUV-optical CMDs to highlight the features/peculiarities found in the different evolutionary sequences. The FUV (F148W) detected stellar populations of 11 GCs comprises 2,816 Horizontal Branch (HB) stars (190 Extreme HB candidates), 46 post-HB (pHB), 221 Blue Straggler Stars (BSS), and 107 White Dwarf (WD) candidates. We note that the blue HB color extension obtained from F148W$-$G color and the number of FUV detected EHB candidates are strongly correlated with the maximum internal Helium (He) variation within each GC, suggesting that the FUV-optical plane is the most sensitive to He abundance variations in the HB. We discuss the potential science cases that will be addressed using these catalogues including HB morphologies, BSSs, pHB, and, WD stars.

astro-ph.GA↗

arXiv2021

AstroSat study of the globular cluster NGC 2298: probable evolutionary scenarios of hot HB stars

We present the far-UV (FUV) photometry of images acquired with UVIT on AstroSat to probe the horizontal branch (HB) population of the Galactic globular cluster NGC 2298. UV-optical color-magnitude diagrams (CMDs) are constructed for member stars in combination with HST UV Globular Cluster Survey (HUGS) data for the central region and Gaia and ground-based photometric data for the outer region. Blue HB (BHB) sequence with a spread and four hot HB stars are detected in all FUV-optical CMDs and are compared with theoretical updated BaSTI isochrones and synthetic HB models with a range in helium abundance, suggesting that the hot HB stars are helium enhanced when compared to the BHB. The estimated effective temperature, radius, and luminosity of HB stars, using best SED fits, were compared with various HB models. BHB stars span a temperature range from 7,500-12,250 K. The three hot HB stars have 35,000-40,000 K, whereas one star has around 100,000K. We suggest the following evolutionary scenarios: two stars are likely to be the progeny of extreme HB (EHB) stars formed through an early hot-flasher scenario; one is likely to be an EHB star with probable helium enrichment, the hottest HB star is about to enter the WD cooling phase, could have evolved from BHB phase. Nevertheless, these are interesting spectroscopic targets to understand the late stages of evolution.

astro-ph.SR↗

arXiv2021

Study of dynamical status of the globular cluster NGC 1851 using Ultraviolet Imaging Telescope

We present the study of dynamical status of the globular cluster NGC 1851. A combination of multiwavelength space and ground-based data sets are used for the present analysis. In order to select the genuine cluster members, we used the astro-photometric data available from HST and GAIA-DR2 catalogs. The BSS radial distribution of the cluster is plotted from the center of the cluster to the outskirts. The radial distribution of BSS shows a central peak, followed by a dip at the intermediate radii (rmin ~ 90'') and a rising trend in the outskirts. We also estimated A+rh parameter as 0.391 +/- 0.006 to validate the findings of the radial distribution study. On the basis of the minima in the BSS radial distribution and the value of A+rh parameter, we conclude that NGC 1851 belongs to Family II classification and is an intermediate dynamical state cluster.

astro-ph.GA↗

arXiv2020

UV photometry of spotted stars in the horizontal branch of the globular cluster NGC 2808 using AstroSat

A recent study of hot (20,000 to 30,000 K) extreme horizontal branch (EHB) stars in globular clusters (GCs) has led to the discovery of their variability. It is suggested that this variability is driven by the projected rotation of magnetic spots on the stellar surfaces and is expected to have higher amplitudes at shorter wavelengths. Here, we present the analysis of such hot stars in the massive GC NGC 2808 using the Ultraviolet Imaging Telescope (UVIT), aboard AstroSat. We use the UVIT data in combination with the Hubble Space Telescope UV Globular Cluster Survey (HUGS) data for the central region (within $\sim$ $2.7'$ $\times$ $2.7'$) and ground-based optical photometry for the outer parts of the cluster. We generate the far-UV (FUV) - optical colour-magnitude diagrams (CMDs) and in these we find a population of hot EHB stars fainter than the zero-age horizontal branch (ZAHB) model. A comparison of our FUV magnitudes of the already reported variable EHB stars (vEHBs) shows that the longest period vEHBs are the faintest, along with a tentative correlation between rotation period and UV magnitude of spotted stars. In order to firmly establish any correlation, further study is essential.

astro-ph.SR↗

arXiv2020

The First Extensive Exploration of UV-bright Stars in the Globular Cluster NGC 2808

In this study, we identified and characterized the hot and luminous UV-bright stars in the globular cluster NGC 2808. We combined data from the Ultra Violet Imaging Telescope (UVIT) on-board the Indian space satellite, AstroSat, with the Hubble Space Telescope UV Globular Cluster Survey (HUGS) data for the central region (within $\sim$ $\ang[angle-symbol-over-decimal]{;2.7;} \times \ang[angle-symbol-over-decimal]{;2.7;}$) and Gaia and ground-based optical photometry for the outer parts of the cluster. We constructed the UV and UV-optical color-magnitude diagrams, compared the horizontal branch (HB) members with the theoretical zero-age HB and terminal-age HB models and identified 34 UV-bright stars. The spectral energy distributions of the UV-bright stars were fitted with theoretical models to estimate their effective temperatures (12500 K - 100,000 K), radii (0.13 to 2.2 $R_{\odot}$), and luminosities ($\sim 40$ to $3000$ $L_{\odot}$) for the first time. These stars were then placed on the H-R diagram, along with theoretical post-HB (pHB) evolutionary tracks to assess their evolutionary status. The models suggest that most of these stars are in the AGB-manqué phase and all, except three, have evolutionary masses $<$ 0.53 $M_{\odot}$. We also calculated the theoretically expected number of hot post-(early)-AGB (p(e)AGB) stars in this cluster and found the range to match our observations. Seven UV-bright stars located in the outer region of the cluster, identified from the AstroSat/UVIT images, are ideal candidates for detailed follow-up spectroscopic studies.

astro-ph.SR↗

arXiv2020

Study of UV bright stellar populations in the globular cluster NGC 1261 using AstroSat

We present the UV photometry of the globular cluster NGC 1261 using images acquired with the Ultraviolet Imaging Telescope (UVIT) on-board ASTROSAT. We performed PSF photometry on four near-UV (NUV) and two far-UV (FUV) images and constructed UV colour-magnitude diagrams (CMDs), in combination with HST, Gaia, and ground-based optical photometry for member stars. We detected the full horizontal branch (HB) in NUV, blue HB in the FUV and identified two extreme HB (EHB) stars. HB stars have a tight sequence in UV-optical CMDs well-fitted with isochrones generated (12.6 Gyr age, [Fe/H] = -1.27 metallicity) using updated BaSTI-IAC models. Effective temperatures (Teff), luminosities and radii of bright HB stars were estimated using spectral energy distribution. As we detect the complete sample of UV bright HB stars, the hot end of the HB distribution is found to terminate at the G-jump (Teff ~ 11500 K). The two EHB stars, fitted well with single spectra, have Teff= 31,000 K and a mass = 0.495Msun and follow the same Teff-Radius relation of the blue HB stars. We constrain the formation pathways of these EHB stars to extreme mass loss in the RGB phase (either due to rotation or enhanced Helium), OR early hot-flash scenario.

astro-ph.SR↗

arXiv2020

UOCS. V. UV study of the old open cluster NGC 188 using AstroSat

We present the UV photometry of the old open cluster NGC188 obtained using images acquired with Ultraviolet Imaging Telescope (UVIT) on board the ASTROSAT satellite, in two far-UV (FUV) and one near-UV (NUV) filters. UVIT data is utilised in combination with optical photometric data to construct the optical and UV colour-magnitude diagrams (CMDs). In the FUV images, we detect only hot and bright blue straggler stars (BSSs), one hot subdwarf, and one white dwarf (WD) candidate. In the NUV images, we detect members up to a faintness limit of ~22 mag including 21 BSSs, 2 yellow straggler stars (YSSs), and one WD candidate. This study presents the first NUV-optical CMDs, and are overlaid with updated BaSTI-IAC isochrones and WD cooling sequence, which are found to fit well to the observed CMDs. We use spectral energy distribution (SED) fitting to estimate the effective temperatures, radii, and luminosities of the UV-bright stars. We find the cluster to have an HB population with three stars (Teff = 4750 - 21000 K). We also detect two yellow straggler stars, with one of them with UV excess connected to its binarity and X-ray emission.

astro-ph.SR↗

arXiv2020

Peculiarities in the horizontal branch stars of globular cluster NGC 1851: Discovery of a Blue straggler companion to an EHB star

We present a study of Far Ultraviolet (FUV) bright horizontal branch (HB) stars to understand the peculiarities seen in the HB sequence of the Globular Cluster NGC 1851, using ground and space based multi-wavelength data. Optical and UV color magnitude diagrams are used to classify HB stars and their membership from HST and Gaia DR2 data. The Spectral energy distributions (SEDs) of the hot HB stars located from the core to tidal radii are constructed. The SEDs reveal that the HB stars near the G-jump show a decrease in the FUV flux when atmospheric models of cluster metallicity are used for fitting, but a better fit is found with higher metallicity models, as expected due to atmospheric diffusion. We report on four particularly interesting extreme HB (EHB) stars, two each in inner and outer regions. We detect a sub-luminous EHB and a "blue-Hook" candidates with temperatures Teff ~ 25,000 K and 31,000 K, respectively. We found an EHB star (Teff ~ 17,000 K) with the radius lies between the BHB and normal EHB stars. The most peculiar of our EHB stars (Teff ~ 28,000 K) is found to be a photometric binary to a Blue Straggler star (BSS) (Teff ~ 7,000 K), which is an important target for spectroscopic study. This discovery of the candidate EHB+BSS binary system, could help to explain the mass loss in the RGB phase, leading to the formation of EHB stars.

astro-ph.SR↗

arXiv2020

UVIT/ASTROSAT studies of Blue Straggler stars and post-mass transfer systems in star clusters: Detection of one more blue lurker in M67

The blue straggler stars (BSSs) are main-sequence (MS) stars, which have evaded stellar evolution by acquiring mass while on the MS. The detection of extremely low mass (ELM) white dwarf (WD) companions to two BSSs and one yellow straggler star (YSS) from our earlier study using UVIT/ASTROSAT, as well as WD companions to main-sequence stars (known as blue lurkers) suggest a good fraction of post-mass transfer binaries in M67. Using deeper UVIT observations, here we report the detection of another blue lurker in M67, with an ELM WD companion. The post-mass transfer systems with the presence of ELM WDs, including BSSs, are formed from Case A/B mass transfer and are unlikely to show any difference in surface abundances. We find a correlation between the temperature of the WD and the $v\ sin i$ of the BSSs. We also find that the progenitors of the massive WDs are likely to belong to the hot and luminous group of BSSs in M67. The only detected BSS+WD system by UVIT in the globular cluster NGC 5466, has a normal WD and suggests that open cluster like environment might be present in the outskirts of low density globular clusters.

astro-ph.SR↗

arXiv2019

Radial Distribution of Blue Stragglers in NGC 5466 using AstroSat

We present the results obtained from Near-UV observations of the cluster NGC 5466 taken with UVIT onboard AstroSat to study the radial distribution of Blue Straggler Stars (BSSs), covering the cluster region up to a radius of 14$'$. Our study confirms that the BSSs are more centrally concentrated than Horizontal Branch (HB) stars in the cluster. We do not find a statistically significant rising trend in the radial distribution of BSSs for the outer regions, as most of the previously catalogued BSSs that are located in the cluster outskirts were found to be non-members, quasars or galaxies. This study highlights the importance of UV imaging combined with membership information to probe the radial distribution of BSSs.

astro-ph.SR↗

arXiv2019

Detection of a White Dwarf companion to a Blue Straggler Star in the outskirts of globular cluster NGC 5466 with the Ultraviolet Imaging Telescope (UVIT)

We report the discovery of a hot white dwarf (WD) companion to a blue straggler star (BSS) in the globular cluster (GC) NGC 5466, based on observations from the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat. The Spectral Energy Distribution (SED) of the Far-UV detected BSS NH 84 was constructed by combining the flux measurements from 4 filters of UVIT, with GALEX, GAIA and other ground-based observations. The SED of NH 84 reveals the presence of a hot companion to the BSS. The temperature and radius of the BSS (T$_{\mathrm{eff}} = 8000^{+1000}_{-250}$ K, R/R$_\odot = 1.44 \pm 0.05$) derived from Gemini spectra and SED fitting using Kurucz atmospheric models are consistent with each other. The temperature and radius of the hotter companion of NH 84 (T$_{\mathrm{eff}} = 32,000 \pm 2000$ K, R/R$_\odot = 0.021 \pm 0.007$) derived by fitting Koester WD models to the SED suggest that it is likely to be a hot WD. The radial velocity derived from the spectra along with the proper motion from GAIA DR2 confirms NH 84 to be a kinematic member of the cluster. This is the second detection of a BSS-WD candidate in a GC, and the first in the outskirts of a low density GC. The location of this BSS in NGC 5466 along with its dynamical age supports the mass-transfer pathway for BSS formation in low density environments.

astro-ph.SR↗

arXiv2018

UVIT-HST-GAIA view of NGC 288: A census of hot stellar population and their properties from UV

A complete census of Blue Horizontal Branch (BHB) and Blue Straggler Star (BSS) population within the 10$'$ radius from the center of the Globular Cluster, NGC 288 is presented, based on the images from the Ultraviolet Imaging Telescope (UVIT). The UV and UV$-$optical Colour-Magnitude Diagrams (CMDs) are constructed by combining the UVIT, HST-ACS and ground data and compared with the BaSTI isochrones generated for UVIT filters. We used stellar proper motions data from GAIA DR2 to select the cluster members. Our estimations of the temperature distribution of 110 BHB stars reveal two peaks with the main peak at $T_{eff}\sim$ 10,300 K with the distribution extending up to $T_{eff}\sim$ 18,000 K. We identify the well known photometric gaps including the G-jump in the BHB distribution which are located between the peaks. We detect a plateau in the FUV magnitude for stars hotter than $T_{eff}\sim$ 11,500 K (G-jump), which could be due to the effect of atomic diffusion. We detect 2 Extreme HB (EHB) candidates with temperatures ranging from 29,000 to 32,000 K. The radial distribution of 68 BSSs suggests that the bright BSSs are more centrally concentrated than the faint BSS and the BHB distribution. We find that the BSSs have a mass range of 0.86 - 1.25 M$_{\odot}$ and an age range of 2 - 10 Gyr with a peak at 1 M$_{\odot}$ and 4 Gyr respectively. This study showcases the importance of combining UVIT with HST, ground, and GAIA data in deriving HB and BSS properties.

astro-ph.SR↗