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تسجيل مستخدم جديدSingle and Composite Hot Subdwarf Stars in the Light of 2MASS All-Sky Photometry
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Utilizing the Two Micron All Sky Survey (2MASS) All-Sky Data Release Catalog, we have retrieved useful near-IR J, H, and Ks magnitudes for more than 800 hot subdwarfs (sdO and sdB stars) drawn from the Catalogue of Spectroscopically Identified Hot Subdwarfs (Kilkenny, Heber, & Drilling 1988, 1992). This sample size greatly exceeds previous studies of hot subdwarfs. We find that, of the hot subdwarfs in Kilkenny et al., ~40% in a magnitude limited sample have colors that are consistent with the presence of an unresolved late type (FGK) companion. Binary stars are over-represented in a magnitude limited sample. In an approximately volume limited sample the fraction of composite-color binaries is ~25%.
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Utilizing the Two Micron All Sky Survey (2MASS) Second Incremental Data Release Catalog, we have retrieved near-IR magnitudes for several hundred hot subdwarfs (sdO and sdB stars) drawn from the Catalogue of Spectroscopically Identified Hot Subdwarfs
(Kilkenny, Heber, & Drilling 1988, 1992). This sample size greatly exceeds that of previous studies of hot subdwarfs. Examining 2MASS photometry alone or in combination with visual photometry (Johnson BV or Stromgren uvby) available in the literature, we show that it is possible to identify hot subdwarf stars that exhibit atypically red IR colors that can be attributed to the presence of an unresolved late type companion. Utilizing this large sample, we attempt for the first time to define an approximately volume limited sample of hot subdwarfs. We discuss the considerations, biases, and difficulties in defining such a sample. We find that, of the hot subdwarfs in Kilkenny et al., about 40% in a magnitude limited sample have colors that are consistent with the presence of an unresolved late type companion. Binary stars are over-represented in a magnitude limited sample. In an approximately volume limited sample the fraction of composite-color binaries is about 30%.
We have conducted a survey of candidate hot subdwarf stars in the southern sky searching for fast transits, eclipses, and sinusoidal like variability in the Evryscope light curves. The survey aims to detect transit signals from Neptune size planets t
o gas-giants, and eclipses from M-dwarfs and brown dwarfs. The other variability signals are primarily expected to be from compact binaries and reflection effect binaries. Due to the small size of hot subdwarfs, transit and eclipse signals are expected to last only twenty minutes, but with large signal depths (up to completely eclipsing if the orientation is edge on). With its 2-minute cadence and continuous observing Evryscope is well placed to recover these fast transits and eclipses. The very large field of view (8150 sq. deg.) is critical to obtain enough hot subdwarf targets, despite their rarity. We identified 11,000 potential hot subdwarfs from the 9.3M Evryscope light curves for sources brighter than mg = 15. With our machine learning spectral classifier, we flagged high-confidence targets and estimate the total hot subdwarfs in the survey to be 1400. The light curve search detected three planet transit candidates, shown to have stellar companions from followup analysis. We discovered several new compact binaries (including two with unseen degenerate companions, and several others with potentially rare secondaries), two eclipsing binaries with M-dwarf companions, as well as new reflection effect binaries and others with sinusoidal like variability. The hot subdwarf discoveries identified here are spectroscopically confirmed and we verified the Evryscope discovery light curve with TESS light curves when available. Four of the discoveries are in the process of being published in separate followup papers, and we discuss the followup potential of several of the other discoveries.
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
clarify the question of how common magnetic fields are in subdwarf stars. We consider a sample of about 40 hot subdwarf stars. About 30 of them have been observed with the FORS1 and FORS2 instruments of the ESO VLT. Here we present new FORS1 field measurements for 17 stars, 14 of which have never been observed for magnetic fields before. We also critically review the measurements already published in the literature, and in particular we try to explain why previous papers based on the same FORS1 data have reported contradictory results. All new and re-reduced measurements obtained with FORS1 are shown to be consistent with non-detection of magnetic fields. We explain previous spurious field detections from data obtained with FORS1 as due to a non-optimal method of wavelength calibration. Field detections in other surveys are found to be uncertain or doubtful, and certainly in need of confirmation. There is presently no strong evidence for the occurrence of a magnetic field in any sdB or sdO star, with typical longitudinal field uncertainties of the order of 2-400 G. It appears that globally simple fields of more than about 1 or 2 kG in strength occur in at most a few percent of hot subdwarfs, and may be completely absent at this strength. Further high-precision surveys, both with high-resolution spectropolarimeters and with instruments similar to FORS1 on large telescopes, would be very valuable.
We discuss whether the hypothesis that all (or most) subdwarfs are in close binaries is supported by the frequently reported observations of photometrically or spectroscopically composite character of many hot subdwarf stars. By way of a possible cou
nter-argument, we focus on resolved companions (optical pairs) of hot subdwarf stars. On a statistical basis, many of these are physically associated with the hot subdwarfs, i.e. are common proper motion pairs. These resolved pairs make a several percent contribution to the catalog of hot subdwarf stars per decade in projected separation. If they are just the relatively wide members of a binary population similar to the local G-dwarf binary population (Duquennoy & Mayor 1991), which has a very wide distribution of orbital separations, then many of the unresolved but composite hot subdwarf binaries may not be close in the astrophysical sense. In that case, binary channels for hot subdwarf formation may be less important than thought, or must involve companions (white dwarfs) that do not result in a composite spectrum system.
We give a brief review over the observational evidence for close substellar companions to hot subdwarf stars. The formation of these core helium-burning objects requires huge mass loss of their red giant progenitors. It has been suggested that beside
s stellar companions substellar objects in close orbits may be able to trigger this mass loss. Such objects can be easily detected around hot subdwarf stars by medium or high resolution spectroscopy with an RV accuracy at the km/s-level. Eclipsing systems of HW Vir type stick out of transit surveys because of their characteristic light curves. The best evidence that substellar objects in close orbits around sdBs exist and that they are able to trigger the required mass loss is provided by the eclipsing system SDSS J0820+0008, which was found in the course of the MUCHFUSS project. Furthermore, several candidate systems have been discovered.
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