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Register a new userDisentangling Composite Spectrum Hot Subdwarfs
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We give preliminary results from a spectroscopic study of composite spectrum hot subdwarfs (sd+late-type). We obtained spectra of a sample of hot subdwarfs selected from the Catalogue of Spectroscopically Identified Hot Subdwarfs on the basis of near-infrared photometry from the Two Micron All Sky Survey (2MASS). The sample consists of 20 photometric and spectroscopic single and 54 composite hot subdwarfs, 6 resolved (or barely resolved) visual doubles, and 5 objects with emission lines or broad absorption lines with emission cores. Spectra of 84 standard (single late-type) stars with Hipparcos parallaxes were also obtained for calibration. These observations cover 4600-8900 Ang with 3 Ang resolution. We measured equivalent width-like indices around Mg I b, Na I D, the Ca II infrared triplet, H-alpha, and H-beta. Using the single late-type star observations combined with model energy distributions, we explore how the measured indices of a composite spectrum vary as the temperature and luminosity of the late-type companion are varied and as the temperature and radius of the hot subdwarf are varied. We use the measured indices of the composite systems to estimate the temperature and gravity of the late-type star, taking into account the dilution of its spectral features by light from the hot subdwarf.
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In the last decade or so, there have been numerous searches for hot subdwarfs in close binaries. There has been little to no attention paid to wide binaries however. The advantages of understanding these systems can be many. The stars can be assumed to be coeval, which means they have common properties. The distance and metallicity, for example, are both unknown for the subdwarf component, but may be determinable for the secondary, allowing other properties of the subdwarf to be estimated. With this in mind, we have started a search for common proper motion pairs containing a hot subdwarf component. We have uncovered several promising candidate systems, which are presented here.
Thanks to the high sensitivity of the instruments on board the XMM-Newton and Chandra satellites, it has become possible to explore the properties of the X-ray emission from hot subdwarfs. The small but growing sample of hot subdwarfs detected in X-rays includes binary systems, in which the X-rays result from wind accretion onto a compact companion (white dwarf or neutron star), as well as isolated sdO stars in which X-rays are probably due to shock instabilities in the wind. X-ray observations of these low mass stars provide information which can be useful also for our understanding of the winds of more luminous and massive early-type stars and can lead to the discovery of particularly interesting binary systems.
Hot subdwarfs are evolved low--mass stars that have survived core helium ignition and are now in (or recently finished with) the core helium burning stage. At the hot end of the Horizontal Branch (HB), many of these stars are multiperiodic pulsators. These pulsations have revealed details of their global and internal structure, and provide important constraints on the origin of hot HB stars. While many features of their structure deduced from seismic fits have confirmed what we expected from evolutionary considerations, there have been some surprises as well.
Using high-cadence observations from the Zwicky Transient Facility at low Galactic latitudes, we have discovered a new class of pulsating, hot, compact stars. We have found four candidates, exhibiting blue colors ($g-rleq-0.1$ mag), pulsation amplitudes of $>5%$, and pulsation periods of $200 - 475$ sec. Fourier transforms of the lightcurves show only one dominant frequency. Phase-resolved spectroscopy for three objects reveals significant radial velocity, T$_{rm eff}$ and log(g) variations over the pulsation cycle, consistent with large amplitude radial oscillations. The mean T$_{rm eff}$ and log(g) for these stars are consistent with hot subdwarf B (sdB) effective temperatures and surface gravities. We calculate evolutionary tracks using MESA and adiabatic pulsations using GYRE for low-mass helium-core pre-white dwarfs and low mass helium-burning stars. Comparison of low-order radial oscillation mode periods with the observed pulsation periods show better agreement with the pre-white dwarf models. Therefore, we suggest that these new pulsators and Blue Large-Amplitude Pulsators (BLAPs) could be members of the same class of pulsators, composed of young $approx0.25-0.35$ M$_odot$ helium-core pre-white dwarfs.
We present the results of a study of the late-type companions in hot subdwarf composite spectrum binaries. The exact nature of these late-type companions has been disputed in the literature -- some argue that they are main sequence stars, and others have claimed they are subgiants. To determine the properties of the late-type companions, we first conducted a survey utilizing the Two Micron All Sky Survey (2MASS) All-Sky Data Release Catalog to identify composite-colored binaries in the Catalogue of Spectroscopically Identified Hot Subdwarfs (Kilkenny, Heber, & Drilling 1988, 1992). We then conducted a spectroscopic study of a sub-sample of the 2MASS composite-colored hot subdwarfs. The sample consists of photometrically and spectroscopically single and composite hot subdwarfs (14 single and 51 composite). We also obtained spectra of 59 single late-type stars with Hipparcos parallaxes for calibration. We used measured equivalent width (EW) indices from the composite systems to estimate the temperature and gravity of the late-type star, taking into account the dilution of its spectral features by light from the hot subdwarf. Results from combining the spectroscopic data with model energy distributions indicate that the late-type companions in composite-spectrum systems are best described by main sequence companions overall.
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