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Register a new userHow Many Hot Subdwarf Stars Were Rejected from the PG Survey?
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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 line 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.
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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)
We have completed a survey of twenty-two ultraviolet-selected hot subdwarfs using the Fiber-fed Extended Range Optical Spectrograph (FEROS) and the 2.2-m telescope at La Silla. The sample includes apparently single objects as well as hot subdwarfs paired with a bright, unresolved companion. The sample was extracted from our GALEX catalogue of hot subdwarf stars. We identified three new short-period systems (P=3.5 hours to 5 days) and determined the orbital parameters of a long-period (P=62.66 d) sdO plus G III system. This particular system should evolve into a close double degenerate system following a second common envelope phase. We also conducted a chemical abundance study of the subdwarfs: Some objects show nitrogen and argon abundance excess with respect to oxygen. We present key results of this programme.
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 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 besides 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.
We present photometric and spectroscopic analyses of gravity (g-mode) long-period pulsating hot subdwarf B (sdB) stars. We perform a detailed asteroseismic and spectroscopic analysis of five pulsating sdB stars observed with {it TESS} aiming at the global comparison of the observations with the model predictions based on our stellar evolution computations coupled with the adiabatic pulsation computations. We apply standard seismic tools for mode identification, including asymptotic period spacings and rotational frequency multiplets. We calculate the mean period spacing for $l = 1$ and $l = 2$ modes and estimate the errors by means of a statistical resampling analysis. For all stars, atmospheric parameters were derived by fitting synthetic spectra to the newly obtained low-resolution spectra. We have computed stellar evolution models using {tt LPCODE} stellar evolution code, and computed $l = 1$ g-mode frequencies with the adiabatic non-radial pulsation code {tt LP-PUL}. Derived observational mean period spacings are then compared to the mean period spacings from detailed stellar evolution computations coupled with the adiabatic pulsation computations of g-modes. The atmospheric parameters derived from spectroscopic data are typical of long-period pulsating sdB stars with the effective temperature ranging from 23,700,K to 27,600,K and surface gravity spanning from 5.3,dex to 5.5,dex. In agreement with the expectations from theoretical arguments and previous asteroseismological works, we find that the mean period spacings obtained for models with small convective cores, as predicted by a pure Schwarzschild criterion, are incompatible with the observations. We find that models with a standard/modest convective boundary mixing at the boundary of the convective core are in better agreement with the observed mean period spacings and are therefore more realistic.
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