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تسجيل مستخدم جديدHot Subdwarf Stars Among the Objects Rejected from the PG Catalog: a First Assessment Using GALEX Photometry
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Richard A. Wade
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The hot subdwarf (sd) stars in the Palomar Green (PG) catalog of ultraviolet excess (UVX) objects play a key role in investigations of the frequency and types of binary companions and the distribution of orbital periods. These are important for establishing whether and by which channels the sd stars arise from interactions in close binary systems. It has been suggested that the list of PG sd stars is biased by the exclusion of many stars in binaries, whose spectra show the Ca II K line in absorption. A total of 1125 objects that were photometrically selected as candidates were ultimately rejected from the final PG catalog using this K-line criterion. We study 88 of these PG-Rejects (PGRs), to assess whether there are significant numbers of unrecognized sd stars in binaries among the PGR objects. The presence of a sd should cause a large UVX. We assemble GALEX, Johnson V, and 2MASS photometry and compare the colors of these PGR objects with those of known sd stars, cool single stars, and hot+cool binaries. Sixteen PGRs were detected in both the far- and near- ultraviolet GALEX passbands. Eleven of these, plus the 72 cases with only an upper limit in the far-ultraviolet band, are interpreted as single cool stars. Of the remaining five stars, three are consistent with being sd stars paired with a cool main sequence companion, while two may be single stars or composite systems of another type. We discuss the implications of these findings for the 1125 PGR objects as a whole. (slightly abridged)
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The Palomar-Green (PG) survey for UV-excess objects selected objects with U-B<-0.46 for spectroscopic follow-up. The color selection was done from photographic photometry, with typical error sigma_(U-B)=0.38. Spectroscopic detection of the Ca II K li
ne in color-selected candidates was thought to indicate that a metal-weak cool star (sdF-sdG) had entered the list owing to photometric errors. About 1100 such K-line stars were rejected on this basis from the final published PG catalogue, as not having genuine UV excesses. However, another possibility is that some of these objects are composite (binary) stars, consisting of a hot subdwarf (sdB or sdO) and a cool companion (F-G-K). Recent interest in binary-star formation channels for sdB stars in particular has brought renewed attention to the issue of completeness of lists of known hot subdwarfs, especially ones with cool companions. We have studied the nature of the rejected PG candidate stars, by assembling available information for a subset of 173 stars between r magnitudes 14.0 and 16.0 that have photometric data from both Data Release 2 of the Sloan Digital Sky Survey (SDSS) and the Two-Micron All Sky Survey (2MASS). Two of the stars have SDSS spectra. Both from their location in (u-g,g-r) and (g-r,r-K_s) two-color diagrams and from detailed fitting of single-star models to the spectral energy distributions, we conclude that the vast majority of these stars can be interpreted as metal-poor F and G subdwarfs, consistent with the original interpretation by the authors of the PG survey. We discuss the seven outliers individually; these may plausibly be binary systems that include a hot subdwarf star as a member, or they are hot stars that entered the list of rejected stars by accident.
Detection of magnetic fields has been reported in several sdO and sdB stars. Recent literature has cast doubts on the reliability of most of these detections. We revisit data previously published in the literature, and we present new observations to
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amed the Zwicky Transient Facility and exhibits a period of $P_{rm orb}=39.3401(1)$ min, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of $T_{rm eff}=42,400pm300$ K and a surface gravity of $log(g)=5.77pm0.05$. A high-signal-to noise GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass $M_{rm sdOB}=0.337pm0.015$ M$_odot$ and a white dwarf accretor with a mass $M_{rm WD}=0.545pm0.020$ M$_odot$. Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a $2.5-2.8$ M$_odot$ star lost its envelope when crossing the Hertzsprung Gap. To match its current $P_{rm orb}$, $T_{rm eff}$, $log(g)$, and masses, we estimate a post-common envelope period of $P_{rm orb}approx150$ min, and find the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of $approx0.1$ M$_odot$ and will merge with its carbon/oxygen white dwarf companion after $approx17$ Myr and presumably explode as a thermonuclear supernova or form an R CrB star.
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