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Journal ArticleDOI

A Hot Companion to a Blue Straggler in NGC 188 as Revealed by the Ultra-Violet Imaging Telescope (UVIT) on ASTROSAT

TL;DR: In this article, the spectral energy distribution (SED) of the star WOCS-5885 was constructed using the ASTROSAT observatory, and the resulting SED spans a wavelength range of 0.15 μm to 7.8 μm.
Abstract: We present early results from the Ultra-Violet Imaging Telescope (UVIT) on board the ASTROSAT observatory. We report the discovery of a hot companion associated with one of the blue straggler stars (BSSs) in the old open cluster, NGC 188. Using fluxes measured in four filters in UVIT's far-UV (FUV) channel, and two filters in the near-UV (NUV) channel, we have constructed the spectral energy distribution (SED) of the star WOCS-5885, after combining with flux measurements from GALEX, Ultraviolet Imaging Telescope, Ultraviolet Optical Telescope, SPITZER, WISE, and several ground-based facilities. The resulting SED spans a wavelength range of 0.15 μm to 7.8 μm. This object is found to be one of the brightest FUV sources in the cluster. An analysis of the SED reveals the presence of two components. The cooler component is found to have a temperature of 6000 ± 150 K, confirming that it is a BSS. Assuming it to be a main-sequence star, we estimate its mass to be ~1.1–1.2 M ⊙. The hotter component, with an estimated temperature of 17,000 ± 500 K, has a radius of ~ 0.6 R ⊙ and L ~30 L ⊙. Bigger and more luminous than a white dwarf, yet cooler than a sub-dwarf, we speculate that it is a post-AGB/HB star that has recently transferred its mass to the BSS, which is known to be a rapid rotator. This binary system, which is the first BSS with a post-AGB/HB companion identified in an open cluster, is an ideal laboratory to study the process of BSS formation via mass transfer.
Citations
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Journal ArticleDOI
TL;DR: The results of in-orbit calibrations of the UVIT are presented in this article, along with the results of the in-space calibration of the UAV in the presence of the ASTROSAT payload.
Abstract: The Ultra-Violet Imaging Telescope (UVIT) is one of the payloads in ASTROSAT, the first Indian Space Observatory. The UVIT instrument has two 375 mm telescopes: one for the far-ultraviolet (FUV) channel (1300–1800 A), and the other for the near-ultraviolet (NUV) channel (2000–3000 A) and the visible (VIS) channel (3200–5500 A). UVIT is primarily designed for simultaneous imaging in the two ultraviolet channels with spatial resolution better than 18, along with provisions for slit-less spectroscopy in the NUV and FUV channels. The results of in-orbit calibrations of UVIT are presented in this paper.

142 citations

Journal ArticleDOI
TL;DR: The results of in-orbit calibrations of the UVIT are presented in this article, along with the results of the in-space calibration of the UAV in the first Indian Space Observatory (ASTROSAT).
Abstract: The Ultra-Violet Imaging Telescope (UVIT) is one of the payloads in ASTROSAT, the first Indian Space Observatory. The UVIT instrument has two 375mm telescopes: one for the far-ultraviolet (FUV) channel (1300--1800A), and the other for the near-ultraviolet (NUV) channel (2000--3000A) and the visible (VIS) channel (3200--5500A). UVIT is primarily designed for simultaneous imaging in the two ultraviolet channels with spatial resolution better than 1.8 arcsec, along with provision for slit-less spectroscopy in the NUV and FUV channels.The results of in-orbit calibrations of UVIT are presented in this paper.

135 citations

Journal ArticleDOI
TL;DR: In this paper, a detailed study of the ultraviolet (UV) bright stars in the old open star cluster, M67, is presented based on the far-UV observations using the Ultra Violet Imaging Telescope ({\it UVIT}) on {\it ASTROSAT}.
Abstract: A detailed study of the ultraviolet (UV) bright stars in the old open star cluster, M67 is presented based on the far-UV observations using the {\it Ultra Violet Imaging Telescope} ({\it UVIT}) on {\it ASTROSAT}. The UV and UV-optical colour-magnitude diagrams (CMDs) along with overlaid isochrones are presented for the member stars, which include blue straggler stars (BSSs), triple systems, white dwarfs (WDs) and spectroscopic binaries (SB). The CMDs suggest the presence of excess UV flux in many members, which could be extrinsic or intrinsic to them. We construct multi-wavelength spectral energy distribution (SED) using photometric data from the {\it UVIT, Gaia} DR2, 2MASS and WISE surveys along with optical photometry. We fitted model SEDs to 7 WDs and find 4 of them have mass $>$ 0.5 \(M_\odot\) and cooling age of less than 200 Myr, thus demanding BSS progenitors. SED fits to 23 stars detect extremely low mass (ELM) WD companions to WOCS2007, WOCS6006 and WOCS2002, and a low mass WD to WOCS3001, which suggest these to be post mass transfer (MT) systems. 12 sources with possible WD companion need further confirmation. 9 sources have X-ray and excess UV flux, possibly arising out of stellar activity. This study demonstrates that UV observations are key to detect and characterise the ELM WDs in non-degenerate systems, which are ideal test beds to explore the formation pathways of these peculiar WDs. The increasing detection of post-MT systems among BSSs and main-sequence stars suggests a strong MT pathway and stellar interactions in M67.

30 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present constraints on the formation histories and mass transfer efficiencies for two blue straggler-white dwarf binaries in open cluster NGC 188 resulting from measuring white dwarf cooling temperatures and surface gravities with HST COS far-ultraviolet spectroscopy.
Abstract: Recent studies show that the majority of blue straggler stars in old open clusters are formed through mass transfer from an evolved star onto a main sequence companion, resulting in a blue straggler and white dwarf in a binary system. We present constraints on the formation histories and mass transfer efficiencies for two blue straggler-white dwarf binaries in open cluster NGC 188 resulting from measuring white dwarf cooling temperatures and surface gravities with HST COS far-ultraviolet spectroscopy. We determine that one system, WOCS 4540, formed through Case C mass transfer resulting in a CO-core white dwarf with a temperature of $17200^{+100}_{-80}$ K and a $\log g$ of $7.76^{+0.03}_{-0.01}$. These fitted values correspond to a mass of $0.50^{+0.05}_{-0.01}$ $M_{\odot}$ and an age of $94^{+7}_{-3}$ Myr. The other system, WOCS 5379, formed through Case B mass transfer resulting in a He-core white dwarf with a temperature of $15400^{+280}_{-260}$ K and a $\log g$ of $7.45^{+0.06}_{-0.06}$, corresponding to a mass of $0.40^{+0.04}_{-0.04}$ $M_{\odot}$ and an age of $230^{+22}_{-23}$ Myr. We determine possible progenitor binary systems with a grid of accretion models using MESA, and investigate whether these systems would lead to stable or unstable mass transfer. WOCS 4540 likely resulted from stable mass transfer during periastron passage in an eccentric binary system, while WOCS 5379 challenges our current understanding of the expected regimes for stable mass transfer from red giant branch stars. Both systems are examples of the benefit of using a detailed analysis to fine-tune our physical understanding of binary evolutionary processes.

23 citations

Journal ArticleDOI
TL;DR: A complete census of the 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 UVIT.
Abstract: 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.

17 citations

References
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Journal ArticleDOI
TL;DR: In this article, the average extinction law over the 3.5 micron to 0.125 wavelength range was derived for both diffuse and dense regions of the interstellar medium. And the validity of the law over a large wavelength interval suggests that the processes which modify the sizes and compositions of grains are stochastic in nature.
Abstract: The parameterized extinction data of Fitzpatrick and Massa (1986, 1988) for the ultraviolet and various sources for the optical and near-infrared are used to derive a meaningful average extinction law over the 3.5 micron to 0.125 wavelength range which is applicable to both diffuse and dense regions of the interstellar medium. The law depends on only one parameter R(V) = A(V)/E(B-V). An analytic formula is given for the mean extinction law which can be used to calculate color excesses or to deredden observations. The validity of the law over a large wavelength interval suggests that the processes which modify the sizes and compositions of grains are stochastic in nature and very efficient.

11,704 citations

Journal ArticleDOI
TL;DR: In this article, the main general features of extinction from the IR through the UV are reviewed, along with the nature of observed spatial variations, and several strategies for dereddening are discussed along with estimates of the uncertainties inherent in each method.
Abstract: This paper addresses the issue of how best to correct astronomical data for the wavelength‐dependent effects of Galactic interstellar extinction. The main general features of extinction from the IR through the UV are reviewed, along with the nature of observed spatial variations. The enormous range of extinction properties found in the Galaxy, particularly in the UV spectral region, is illustrated. Fortunately, there are some tight constraints on the wavelength dependence of extinction and some general correlations between extinction curve shape and interstellar environment. These relationships provide some guidance for correcting data for the effects of extinction. Several strategies for dereddening are discussed along with estimates of the uncertainties inherent in each method. In the Appendix, a new derivation of the wavelength dependence of an average Galactic extinction curve from the IR through the UV is presented, along with a new estimate of how this extinction law varies with the paramet...

2,867 citations

Journal ArticleDOI
TL;DR: In this article, a large set of theoretical isochrones are presented, whose distinctive features mostly reside on the greatly improved treatment of the thermally-pulsing asymptotic giant branch (TP-AGB) phase.
Abstract: We present a large set of theoretical isochrones, whose distinctive features mostly reside on the greatly-improved treatment of the thermally-pulsing asymptotic giant branch (TP-AGB) phase. Essentially, we have coupled the TP-AGB tracks described in Paper I, at their stages of pre-flash quiescent H-shell burning, with the evolutionary tracks for the previous evolutionary phases from Girardi et al. (2000, AA the bell-shaped sequences in the Hertzsprung-Russell (HR) diagram for stars with hot-bottom burning; the changes of pulsation mode between fundamental and first overtone; the sudden changes of mean mass-loss rates as the surface chemistry changes from M- to C-type; etc. Theoretical isochrones are then converted to about 20 different photometric systems - including traditional ground-based systems, and those of recent major wide-field surveys such as SDSS, OGLE, DENIS, 2MASS, UKIDSS, etc., - by means of synthetic photometry applied to an updated library of stellar spectra, suitably extended to include C-type stars. Finally, we correct the predicted photometry for the effect of circumstellar dust during the mass-losing stages of the AGB evolution, which allows us to improve the results for the optical-to-infrared systems, and to simulate mid- and far-IR systems such as those of Spitzer and AKARI. We illustrate the most striking properties of these isochrones by means of basic comparisons with observational data for the Milky Way disc and the Magellanic Clouds. Access to the data is provided both via a web repository of static tables (http://stev.oapd.inaf.it/ dustyAGB07 and CDS), and via an interactive web interface (http://stev.oapd. inaf. it/cmd), which provides tables for any intermediate value of age and metallicity, for several photometric systems, and for different choices of dust properties.

1,740 citations

Journal ArticleDOI
TL;DR: In this paper, the extinction ratio between two of the filters, e.g., AJ/AK, is required to calculate A[λ]/AK from those measured ratios, which shows a flattening across the 3-8 μm wavelength range, roughly consistent with the extinction measurements derived by Lutz and coworkers for the sight line toward the Galactic center.
Abstract: We determine and tabulate A[λ]/AK, the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25 μm ≤ λ ≤ 8.0 μm along two lines of sight: l = 42° and 284°. The first is a relatively quiescent and unremarkable region; the second contains the giant H II region RCW 49, as well as a "field" region unrelated to the cluster and nebulosity. Areas near these Galactic longitudes were imaged at J, H, and K bands by 2MASS and at 3-8 μm by Spitzer for the GLIMPSE Legacy program. We measure the mean values of the color excess ratios (A[λ] - AK)/(AJ - AK) directly from the color distributions of observed stars. The extinction ratio between two of the filters, e.g., AJ/AK, is required to calculate A[λ]/AK from those measured ratios. We use the apparent JHK magnitudes of giant stars along our two sight lines and fit the reddening as a function of magnitude (distance) to determine AJ kpc-1, AK kpc-1, and AJ/AK. Our values of A[λ]/AK show a flattening across the 3-8 μm wavelength range, roughly consistent with the extinction measurements derived by Lutz and coworkers for the sight line toward the Galactic center.

914 citations

Journal ArticleDOI
TL;DR: In this article, the extinction ratio between two filters, e.g. A(J)/A(K), is calculated from the color distributions of observed stars, and the mean values of the color excess ratios (A(λ)-A(k))/(A(J)-A (K)) are calculated from those measured ratios.
Abstract: We determine and tabulate A(lambda)/A(K), the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25um

829 citations

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