- ID:
- ivo://CDS.VizieR/J/AJ/138/606
- Title:
- Hot subdwarf stars in rejected PG catalog
- Short Name:
- J/AJ/138/606
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- 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 CaII 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.
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Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/569/A15
- Title:
- KIC 10553698A asteroseismology
- Short Name:
- J/A+A/569/A15
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The subdwarf-B pulsator, KIC10553698A, is one of 16 such objects observed with one-minute sampling for most of the duration of the Kepler Mission. Like most of these stars, it displays a rich g-mode pulsation spectrum with several clear multiplets that maintain regular frequency splitting. We identify these pulsation modes as components of rotationally split multiplets in a star rotating with a period of ~41d. From 162 clearly significant periodicities, we are able to identify 156 as likely components of l=1 or l=2 multiplets. For the first time we are able to detect l=1 modes that interpose in the asymptotic period sequences and that provide a clear indication of mode trapping in a stratified envelope, as predicted by theoretical models. A clear signal is also present in the Kepler photometry at 3.387d. Spectroscopic observations reveal a radial-velocity amplitude of 64.8km/s. We find that the radial-velocity variations and the photometric signal have phase and amplitude that are perfectly consistent with a Doppler-beaming effect and conclude that the unseen companion, KIC10553698B, must be a white dwarf most likely with a mass close to 0.6M_{sun}_.
- ID:
- ivo://CDS.VizieR/J/ApJS/61/569
- Title:
- KPD UV-excess objects
- Short Name:
- J/ApJS/61/569
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A survey for ultraviolet-excess objects has been performed for over about 1000sq.deg of the Galactic plane. Photographic blue and ultraviolet plates were obtained to select ultraviolet-excess candidates, and spectroscopic observations were used to determine the nature of those objects. Over 70 white dwarfs and subdwarfs were identified from the survey plates, and these objects were analyzed statistically to determine scale heights and space densities. The sample was complete to B=15.3mag, with the space density of white dwarfs for Mv<12.25 equal to 0.72+/-0.25 per 1000pc^3^, slightly higher than that from the Palomar-Green survey. The average absolute magnitude determined for both the subdwarf O and the subdwarf B stars was considerably fainter than previous determinations, while the scale height for the subdwarfs was lower than earlier suggested values. The space density for the subdwarf O stars is about 7x10^-7^pc^-3^, while the value for the subdwarf B stars is about 2x10^-6^pc^-3, both values considerably higher than previous estimates. The space densities can be used to place evolutionary constraints on these objects.
- ID:
- ivo://CDS.VizieR/J/MNRAS/506/2122
- Title:
- Light curves and eclipse times of HW Vir
- Short Name:
- J/MNRAS/506/2122
- Date:
- 17 Jan 2022 00:24:15
- Publisher:
- CDS
- Description:
- A growing number of eclipsing binary systems of the HW Virginis (HW Vir) kind (i.e. composed by a subdwarf-B/O primary star and an M dwarf secondary) show variations in their orbital period, also called eclipse time variations (ETVs). Their physical origin is not yet known with certainty: While some ETVs have been claimed to arise from dynamical perturbations due to the presence of circumbinary planetary companions, other authors suggest that the Applegate effect or other unknown stellar mechanisms could be responsible for them. In this work, we present 28 unpublished high-precision light curves of one of the most controversial of these systems, the prototype HW Vir. We homogeneously analysed the new eclipse timings together with historical data obtained between 1983 and 2012, demonstrating that the planetary models previously claimed do not fit the new photometric data, besides being dynamically unstable. In an effort to find a new model able to fit all the available data, we developed a new approach based on a global-search genetic algorithm and eventually found two new distinct families of solutions that fit the observed timings very well, yet dynamically unstable at the 10^5^-yr time-scale. This serves as a cautionary tale on the existence of formal solutions that apparently explain ETVs but are not physically meaningful, and on the need of carefully testing their stability. On the other hand, our data confirm the presence of an ETV on HW Vir that known stellar mechanisms are unable to explain, pushing towards further observing and modeling efforts.
- ID:
- ivo://CDS.VizieR/J/AJ/126/1455
- Title:
- 2MASS photometry of hot subdwarf stars
- Short Name:
- J/AJ/126/1455
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- 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 by Kilkenny et al. (1988SAAOC..12....1K). 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 Stroemgren uvby) available in the literature, we show that it is possible to identify hot subdwarf stars that exhibit atypically red IR colors, which 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. (1988SAAOC..12....1K), about 40% in a magnitude-limited sample have colors that are consistent with the presence of an unresolved late-type companion. Binary stars are overrepresented in a magnitude-limited sample. In an approximately volume-limited sample the fraction of composite-color binaries is about 30%.
- ID:
- ivo://CDS.VizieR/J/A+A/549/A110
- Title:
- Metal abundances of sdB stars
- Short Name:
- J/A+A/549/A110
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Hot subdwarf B stars (sdBs) are considered to be core helium-burning stars with very thin hydrogen envelopes situated on or near the extreme horizontal branch (EHB). The formation of sdBs is still unclear as well as the chemical composition of their atmospheres. The observed helium depletion is attributed to atmospheric diffusion. Metal abundances have been determined for about a dozen sdBs only resulting in puzzling patterns with enrichment of heavy metals and depletion of lighter ones. In this paper we present a detailed metal abundance analysis of 106 sdBs. From high resolution spectra we measured elemental abundances of up to 24 different ions per star. A semi-automatic analysis pipeline was developed to calculate and fit LTE models to a standard set of spectral lines. A general trend of enrichment was found with increasing temperature for most of the heavier elements. The lighter elements like carbon, oxygen and nitrogen are depleted and less affected by temperature. Although there is considerable scatter from star to star, the general abundance patterns in most sdBs are similar. State-of-the-art diffusion models predict such patterns and are in qualitative agreement with our results. However, the highest enrichments measured cannot not be explained with these models. Peculiar line shapes of the strongest metal lines in some stars indicate vertical stratification to be present in the atmospheres. Such effects are not accounted for in current diffusion models and may be responsible for some of the yet unexplained abundance anomalies.
37. M-Subdwarfs
- ID:
- ivo://CDS.VizieR/J/AJ/113/806
- Title:
- M-Subdwarfs
- Short Name:
- J/AJ/113/806
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a spectroscopic classification system for M-dwarfs and M-subdwarfs based on quantitative measures of TiO and CaH features in the region 6200 - 7400 Angstroms. Our sample of cool stars covers the range from solar metallicity stars to the most extreme subdwarfs known. Using synthetic spectra computed by Allard and Hauschildt (1995), we derive metallicities for the stars. Stars are classified as dwarfs (M V), subdwarfs (sdM), or extreme subdwarfs (esdM). These classifications correspond to [m/H] = 0.0, -1.2, and -2.0 respectively. Our metallicity scale agrees with theoretical HR diagrams and HST globular cluster measurements. We discuss some nearby subdwarfs of particular interest in light of our metallicity scale. We include photometry compiled from the literature.
- ID:
- ivo://CDS.VizieR/J/A+A/564/A90
- Title:
- M subdwarfs VLT/UVES high resolution spectra
- Short Name:
- J/A+A/564/A90
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- M subdwarfs are metal poor and cool stars. They are important probes of the old galactic populations. However, they remain elusive due to their low luminosity. Observational and modeling efforts are required to fully understand their physics and to investigate the effects of metallicity in their cool atmospheres. We perform a detailed study of a sample of subdwarfs to determine their stellar parameters and constrain the stat-of-the art atmospheric models. We present UVES/VLT high resolution spectra of three late-K subdwarfs and 18 M subdwarfs. Our atlas covers the optical region from 6400{AA} up to the near infrared at 8900{AA}. We show spectral details of cool atmospheres at very high resolution (R~40000) and compare with synthetic spectra computed from the recent BT-Settl atmosphere models. Our comparison shows that molecular features (TiO, VO, CaH), and atomic features (FeI, TiI, NaI, KI) are well fitted by current models. We produce an effective temperature versus spectral type relation all over the subdwarf spectral sequence. Thanks to the high resolution of our spectra, we perform a detailed comparison of line profiles of individual elements such as FeI, CaII, TiI, and are able to determine accurate metallicities of these stars. These determinations contribute to calibrate the relation between metallicity and molecular band strength indices from low-resolution spectra. This work shows that the new generation of models are able to reproduce various spectral features of M subdwarfs. Working with these high resolution spectra allowed us to disentangle the atmospheric parameters (effective temperature, gravity, metallicity), which is not possible when using low resolution spectroscopy or photometry.
- ID:
- ivo://CDS.VizieR/J/other/RAA/16.G7
- Title:
- Nearby M subdwarfs from LAMOST DR2
- Short Name:
- J/other/RAA/16.G
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We identify 108 M subdwarfs (sdMs) out of more than two hundred thousand M type spectra from the second data release (DR2) of the LAMOST regular survey. This sample, among which 58 members are identified for the first time, includes 33 extreme subdwarfs (esdMs) and 11 ultra subdwarfs (usdMs). The selection is based on the usual ratio of absorption depth of CaH2, CaH3 and TiO5 band systems. We also emphasize the use of the CaH1 band. We provide estimates of spectral subtype (SPT), Lepine metallicity index zeta, effective temperature and [Fe/H]. Both zeta-[Fe/H] and SPT-Teff figures show reasonable consistency; compared to PHOENIX model spectra, average rounded values of [Fe/H] for sdMs, esdMs and usdMs are respectively -0.5, -1 and -1.5. The photometric distances are estimated, indicating that most sources are located within 500pc of the Sun and 350pc of the Galactic disk. Velocities and 3D Galactic motions are also briefly discussed. Among the 108 subdwarfs, seven stars appear to be active with a significant H{alpha} emission line. The source LAMOST J104521.52+482823.3 is a white dwarf - M subdwarf binary, while LAMOST J123045.52+410943.8, also active, exhibits carbon features in red.
- ID:
- ivo://CDS.VizieR/J/MNRAS/457/3396
- Title:
- New hot subdwarf stars
- Short Name:
- J/MNRAS/457/3396
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Recent massive sky surveys in different bandwidths are providing new opportunities to modern astronomy. The Virtual Observatory (VO) represents the adequate framework to handle the huge amount of information available and filter out data according to specific requirements. In this work, we applied a selection strategy to find new, uncatalogued hot subdwarfs making use of VO tools. We used large area catalogues like GALEX, Sloan Digital Sky Survey (SDSS), SuperCosmos and Two Micron All Sky Survey (2MASS) to retrieve photometric and astrometric information of stellar objects. To these objects, we applied colour and proper motion filters, together with an effective temperature cutoff, aimed at separating hot subdwarfs from other blue objects such as white dwarfs, cataclysmic variables or main-sequence OB stars. As a result, we obtained 437 new, uncatalogued hot subdwarf candidates. Based on previous results, we expect our procedure to have an overall efficiency of at least 80 per cent. Visual inspection of the 68 candidates with SDSS spectrum showed that 65 can be classified as hot subdwarfs: 5 sdOs, 25 sdOBs and 35 sdBs. This success rate above 95 per cent proves the robustness and efficiency of our methodology. The spectral energy distribution of 45 per cent of the subdwarf candidates showed infrared excesses, a signature of their probable binary nature. The stellar companions of the binary systems so detected are expected to be late-type main-sequence stars. A detailed determination of temperatures and spectral classification of the cool companions will be presented in a forthcoming work.
