- ID:
- ivo://CDS.VizieR/J/AJ/157/63
- Title:
- Radius relations for low-metallicity M-dwarf stars
- Short Name:
- J/AJ/157/63
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- M subdwarfs are low-metallicity M dwarfs that typically inhabit the halo population of the Galaxy. Metallicity controls the opacity of stellar atmospheres; in metal-poor stars, hydrostatic equilibrium is reached at a smaller radius, leading to smaller radii for a given effective temperature. We compile a sample of 88 stars that span spectral classes K7 to M6 and include stars with metallicity classes from solar-metallicity dwarf stars to the lowest metallicity ultra subdwarfs to test how metallicity changes the stellar radius. We fit models to Palomar Double Spectrograph (DBSP) optical spectra to derive effective temperatures (T_eff_) and we measure bolometric luminosities (L_bol_) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the T_eff_ and L_bol_ using the Stefan-Boltzman law. We find that for a given temperature, ultra subdwarfs can be as much as five times smaller than their solar-metallicity counterparts. We present color-radius and color-surface brightness relations that extend down to [Fe/H] of -2.0 dex, in order to aid the radius determination of M subdwarfs, which will be especially important for the WFIRST exoplanetary microlensing survey.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/845/171
- Title:
- Rapidly pulsating sdB stars search with GALEX
- Short Name:
- J/ApJ/845/171
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- NASA's Galaxy Evolution Explorer (GALEX) provided near- and far-UV observations for approximately 77% of the sky over a 10-year period; however, the data reduction pipeline initially only released single NUV and FUV images to the community. The recently released Python module gPhoton changes this, allowing calibrated time-series aperture photometry to be extracted easily from the raw GALEX data set. Here we use gPhoton to generate light curves for all hot subdwarf B (sdB) stars that were observed by GALEX, with the intention of identifying short-period, p-mode pulsations. We find that the spacecraft's short visit durations, uneven gaps between visits, and dither pattern make the detection of hot subdwarf pulsations difficult. Nonetheless, we detect UV variations in four previously known pulsating targets and report their UV pulsation amplitudes and frequencies. Additionally, we find that several other sdB targets not previously known to vary show promising signals in their periodograms. Using optical follow-up photometry with the Skynet Robotic Telescope Network, we confirm p-mode pulsations in one of these targets, LAMOST J082517.99+113106.3, and report it as the most recent addition to the sdBVr class of variable stars.
- ID:
- ivo://CDS.VizieR/J/ApJ/804/30
- Title:
- Robo-AO observed cool subdwarf companions
- Short Name:
- J/ApJ/804/30
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Cool subdwarfs are the oldest members of the low-mass stellar population. Mostly present in the galactic halo, subdwarfs are characterized by their low-metallicity. Measuring their binary fraction and comparing it to solar-metallicity stars could give key insights into the star formation process early in the Milky Way's history. However, because of their low luminosity and relative rarity in the solar neighborhood, binarity surveys of cool subdwarfs have suffered from small sample sizes and incompleteness. Previous surveys have suggested that the binary fraction of red subdwarfs is much lower than for their main-sequence cousins. Using the highly efficient Robo-AO system, we present the largest high-resolution survey of subdwarfs, sensitive to angular separations ({rho}>=0.15") and contrast ratios ({Delta}_mi_<=6) invisible in past surveys. Of 344 target cool subdwarfs, 43 are in multiple systems, 19 of which are newly discovered, for a binary fraction of 12.5+/-1.9%. We also discovered seven triple star systems for a triplet fraction of 2.0+/-0.8%. Comparisons to similar surveys of solar-metallicity dwarf stars gives a ~3{sigma} disparity in luminosity between companion stars, with subdwarfs displaying a shortage of low-contrast companions. We also observe a lack of close subdwarf companions in comparison to similar-mass dwarf multiple systems.
- ID:
- ivo://CDS.VizieR/J/A+A/623/L12
- Title:
- sdOB star Feige 46 spectrum
- Short Name:
- J/A+A/623/L12
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the discovery of long-period, low-amplitude, g-mode pulsations in the intermediate He-rich hot subdwarf (sdOB) star Feige 46. So far, only one other He-enriched sdOB star (LS IV -14116) was known to exhibit such pulsations. From our ground-based light curves of Feige 46, we extracted five independent periodicities ranging from 2294s to 3400s. We fit our optical spectrum of the star with our grid of non-local thermodynamic equilibrium (NLTE) model atmospheres and derived the following atmospheric parameters: Teff=36120+/-230K, logg=5.93+/-0.04, and log N(He)/N(H)=-0.32+/-0.03 (formal fitting errors only). These parameters are very similar to those of LS IV -14116 and place Feige 46 well outside of the instability strip where the hydrogen-rich g-mode sdB pulsators are found. We used the Gaia parallax and proper motion of Feige 46 to perform a kinematic analysis of this star and found that it likely belongs to the Galactic halo population. This is most certainly an intriguing and interesting result given that LS IV -14116 is also a halo object.The mechanism responsible for the pulsations in these two peculiar objects remains unclear, but a possible scenario involves the epsilon-mechanism. Although they are the only two members in their class of variable stars, these pulsators appear to have more in common than just their pulsation properties.
- ID:
- ivo://CDS.VizieR/J/ApJS/167/40
- Title:
- SDSS4 confirmed white dwarfs catalog
- Short Name:
- J/ApJS/167/40
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a catalog of 9316 spectroscopically confirmed white dwarfs from the Sloan Digital Sky Survey Data Release 4. We have selected the stars through photometric cuts and spectroscopic modeling, backed up by a set of visual inspections. About 6000 of the stars are new discoveries, roughly doubling the number of spectroscopically confirmed white dwarfs. We analyze the stars by performing temperature and surface gravity fits to grids of pure hydrogen and helium atmospheres. Among the rare outliers are a set of presumed helium-core DA white dwarfs with estimated masses below 0.3M_{sun}_, including two candidates that may be the lowest-mass yet found. We also present a list of 928 hot subdwarfs.
- ID:
- ivo://CDS.VizieR/J/A+A/650/A205
- Title:
- Search for transiting planets around sdBs
- Short Name:
- J/A+A/650/A205
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Hot subdwarfs experienced strong mass loss on the red giant branch (RGB) and are now hot and small He-burning objects. These stars constitute excellent opportunities for addressing the question of the evolution of exoplanetary systems directly after the RGB phase of evolution. In this project we aim to perform a transit survey in all available light curves of hot subdwarfs from space-based telescopes (Kepler, K2, TESS, and CHEOPS) with our custom-made pipeline SHERLOCK in order to determine the occurrence rate of planets around these stars as a function of orbital period and planetary radius. We also aim to determine whether planets that were previously engulfed in the envelope of their red giant host star can survive, even partially, as a planetary remnant. For this first paper, we performed injection-and-recovery tests of synthetic transits for a selection of representative Kepler, K2, and TESS light curves to determine which transiting bodies in terms of object radius and orbital period we will be able to detect with our tools. We also provide estimates for CHEOPS data, which we analyzed with the pycheops package. Transiting objects with a radius <~1.0R_{Earth}_ can be detected in most of the Kepler, K2, and CHEOPS targets for the shortest orbital periods (1d and shorter), reaching values as low as ~0.3R_{Earth}_ in the best cases. Sub-Earth-sized bodies are only reached for the brightest TESS targets and for those that were observed in a significant number of sectors. We also give a series of representative results for larger planets at greater distances, which strongly depend on the target magnitude and on the length and quality of the data. The TESS sample will provide the most important statistics for the global aim of measuring the planet occurrence rate around hot subdwarfs. The Kepler, K2, and CHEOPS data will allow us to search for planetary remnants, that is, very close and small (possibly disintegrating) objects.
- ID:
- ivo://CDS.VizieR/J/A+A/617/A6
- Title:
- Seven new pulsating sdA stars
- Short Name:
- J/A+A/617/A6
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The nature of the recently identified "sdA" spectroscopic class of star is not well understood. The thousands of known sdAs have H-dominated spectra, spectroscopic surface gravities intermediate to main sequence stars and isolated white dwarfs, and effective temperatures below the lower limit for He-burning subdwarfs. Most are likely products of binary stellar evolution, whether extremely low-mass white dwarfs and their precursors, or blue stragglers in the halo. Stellar eigenfrequencies revealed through time series photometry of pulsating stars sensitively probe stellar structural properties. The properties of pulsations exhibited by any sdA stars would contribute importantly to our developing understanding of this class. We extend our photometric campaign to discover pulsating extremely low-mass white dwarfs from McDonald Observatory to target sdA stars classified from SDSS spectra. We also obtain follow-up time series spectroscopy to search for binary signatures from four new pulsators. Out of 23 sdA stars observed, we clearly detect stellar pulsations in seven. Dominant pulsation periods range from 4.6 minutes to 12.3 hours, with most on hour timescales. We argue specific classifications for some of the new variables, identifying both compact and likely main sequence dwarf pulsators, along with a candidate low-mass RR Lyrae star. With dominant pulsation periods spanning orders of magnitude, the pulsational evidence supports the emerging narrative that the sdA class consists of multiple stellar populations. Since multiple types of sdA exhibit stellar pulsations, follow-up asteroseismic analysis can be used to probe the precise evolutionary natures and stellar structures of these individual subpopulations.
- ID:
- ivo://CDS.VizieR/J/A+A/615/A78
- Title:
- Spectral models for binary products
- Short Name:
- J/A+A/615/A78
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Stars stripped of their hydrogen-rich envelope through interaction with a binary companion are generally not considered when accounting for ionizing radiation from stellar populations, despite the expectation that stripped stars emit hard ionizing radiation, form frequently, and live 10-100 times longer than single massive stars. We compute the first grid of evolutionary and spectral models specially made for stars stripped in binaries for a range of progenitor masses (2-20M_{sun}_) and metallicities ranging from solar to values representative for pop II stars. For stripped stars with masses in the range 0.3-7M_{sun}_, we find consistently high effective temperatures (20000-100000K, increasing with mass), small radii (0.2-1R_{sun}_), and high bolometric luminosities, comparable to that of their progenitor before stripping. The spectra show a continuous sequence that naturally bridges subdwarf-type stars at the low-mass end and Wolf-Rayet-like spectra at the high-mass end. For intermediate masses we find hybrid spectral classes showing a mixture of absorption and emission lines. These appear for stars with mass-loss rates of 10^-8^-10^-6^M_{sun}_/yr, which have semi-transparent atmospheres. At low metallicity, substantial hydrogen-rich layers are left at the surface and we predict spectra that resemble O-type stars instead. We obtain spectra undistinguishable from subdwarfs for stripped stars with masses up to 1.7M_{sun}_, which questions whether the widely adopted canonical value of 0.47M_{sun}_ is uniformly valid. Only a handful of stripped stars of intermediate mass have currently been identified observationally. Increasing this sample will provide necessary tests for the physics of interaction, internal mixing, and stellar winds. We use our model spectra to investigate the feasibility to detect stripped stars next to an optically bright companion and recommend systematic searches for their UV excess and possible emission lines, most notably HeII {lambda}4686 in the optical and HeII {lambda}1640 in the UV. Our models are publicly available for further investigations or inclusion in spectral synthesis simulations.
- ID:
- ivo://CDS.VizieR/J/A+A/579/A58
- Title:
- Spectra of two mid-L subdwarfs
- Short Name:
- J/A+A/579/A58
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We obtained low- to intermediate-resolution (R~2500-9000) optical (~560-770nm) spectra of two mid-L subdwarfs, SDSS J125637.13-022452.4 (SDSS1256; sdL3.5) and 2MASS J162620.14+392519.5 (2MASS1626; sdL4). We report the presence of a feature at the nominal position of the lithium absorption doublet at 6707.8{AA} in the spectrum of SDSS1256, with an equivalent width of 66+/-27{AA} at 2.4{sigma}, which we identify as arising from a CaH molecular transition based on atmosphere models. We do not see any feature at the position of the lithium feature in the spectrum of 2MASS1626. The existence of overlapping molecular absorption sets a confusion detection limit of [Li/H]=-3 for equivalently-typed L subdwarfs. We provided improved radial velocity measurements of -126+/-10km/s and -239+/-12km/s for SDSS1256 and 2MASS1626, respectively, as well as revised Galactic orbits. We implemented adjusting factors for the CaH molecule in combination with the NextGen atmosphere models to fit the optical spectrum of SDSS1256 in the 6200-7300{AA} range. We also estimate the expected Li abundance from interstellar accretion ([Li/H]=-5), place limits on circumstellar accretion (10^9^g/yr), and discuss the prospects of Li searches in cooler L and T subdwarfs.
- ID:
- ivo://CDS.VizieR/III/137
- Title:
- Spectroscopically Identified Hot Subdwarf Stars
- Short Name:
- III/137
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The catalog contains data for 1225 spectroscopically classified hot subdwarf stars. It excludes central stars of planetary nebulae but includes stars in binary systems. The data assembled include, where available, alternative designations, accurate coordinates, UBV and uvby photometry, spectral types and determinations of effective temperature and surface gravity.
