We present a series of systematic abundance measurements for 89 hydrogen atmosphere (DA-type) white dwarfs with temperatures spanning 16 000-77 000 K drawn from the FUSE spectral archive. This is the ...largest study to date of white dwarfs where radiative forces are significant, exceeding our earlier work, based mainly on International Ultraviolet Explorer and Hubble Space Telescope data, by a factor 3. Using heavy element blanketed non-local thermodynamic equilibrium stellar atmosphere calculations, we have addressed the heavy element abundance patterns making completely objective measurements of abundance values and their error ranges using a χ2 fitting technique. We are able to establish the broad range of abundances seen in a given temperature range and establish the incidence of stars which appear, in the optical, to be atmospherically devoid of any material other than H. We compare the observed abundances to predictions of radiative levitation calculations, revealing little agreement. We propose that the supply of heavy elements is accreted from external sources rather than being intrinsic to the star. These elements are then retained in the white dwarf atmospheres by radiative levitation, a model that can explain both the diversity of measured abundances for stars of similar temperature and gravity, including cases with apparently pure H envelopes, and the presence of photospheric metals at temperatures where radiative levitation is no longer effective.
The calcium and hydrogen abundances, Galactic positions and kinematics of 146 DZ white dwarfs from the Sloan Digital Sky Survey are analysed to constrain the possible origin of their externally ...polluted atmospheres. There are no correlations found between their accreted calcium abundances and spatial–kinematical distributions relative to interstellar material. Furthermore, two thirds of the stars are currently located above the Galactic gas and dust layer, and their kinematics indicate multi-Myr residences in this region where interstellar material is virtually absent. Where detected, the hydrogen abundances for 37 DZA stars show little or no correlation with accreted calcium or spatial–kinematical distributions, though there is a general trend with cooling age. It is found that Eddington-type accretion of interstellar hydrogen can reproduce the observed hydrogen abundances, yet simultaneously fails to account for calcium. The calcium-to-hydrogen ratios for the DZA stars are dominated by supersolar values, as are the lower limits for the remaining 109 DZ stars. All together, these polluted white dwarfs currently contain 1020±2 g of calcium in their convective envelopes, commensurate with the masses of calcium inferred for large asteroids. A census of current Teff≲ 12 000 K, helium-rich stars from the Sloan Digital Sky Survey suggests the DZ and DC white dwarfs belong to the same stellar population, with similar basic atmospheric compositions, effective temperatures, spatial distributions and Galactic space velocities. Based on this result, pollution by the interstellar medium cannot simultaneously account for both the polluted and non-polluted subpopulations. Rather, it is probable that these white dwarfs are contaminated by circumstellar matter; the rocky remains of terrestrial planetary systems. In this picture, two predictions emerge. First, at least 3.5 per cent of all white dwarfs harbour the remnants of terrestrial planetary systems; this is a concrete lower limit and the true fraction is almost certainly, and perhaps significantly, higher. Therefore, one can infer that at least 3.5 per cent of main-sequence A- and F-type stars build terrestrial planets. Secondly, the DZA stars are externally polluted by both metals and hydrogen, and hence constrain the frequency and mass of water rich, extrasolar planetesimals.
Abstract
The gravitational potential φ = GM/Rc2 at the surface of the white dwarf G191-B2B is 10,000 times stronger than that at the Earth’s surface. Numerous photospheric absorption features are ...detected, making this a suitable environment to test theories in which the fundamental constants depend on gravity. We have measured the fine structure constant, α, at the white dwarf surface, used a newly calibrated Hubble Space Telescope STIS spectrum of G191-B2B, two new independent sets of laboratory Fe V wavelengths, and new atomic calculations of the sensitivity parameters that quantify Fe V wavelength dependency on α. The two results obtained are: Δα/α0 = (6.36 ± 0.35stat ± 1.84sys) × 10−5 and Δα/α0 = (4.21 ± 0.48stat ± 2.25sys) × 10−5. The measurements hint that the fine structure constant increases slightly in the presence of strong gravitational fields. A comprehensive search for systematic errors is summarised, including possible effects from line misidentifications, line blending, stratification of the white dwarf atmosphere, the quadratic Zeeman effect and electric field effects, photospheric velocity flows, long-range wavelength distortions in the HST spectrum, and variations in the relative Fe isotopic abundances. None fully account for the observed deviation but the systematic uncertainties are heavily dominated by laboratory wavelength measurement precision.
We present a near-infrared photometric search for unresolved substellar companions and debris discs around white dwarfs in the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey. We ...cross-correlate the Sloan Digital Sky Survey (SDSS) DR4 and McCook & Sion catalogues of white dwarfs with the UKIDSS DR8 producing 3109 and 163 unique matches, respectively. Cooling models are fitted to the optical photometry of a subsample of DA white dwarfs and extended to the near-infrared. A comparison is then made with the observed photometry to identify those stars with a near-infrared excess consistent with the presence of a cool companion or debris disc. Where present, we have estimated the approximate spectral type of any putative companion, or an upper limit on the temperature of a debris disc. In total we identify 14-16 new candidate white dwarf + very low mass stellar systems, 9-11 candidate white dwarf + brown dwarf systems, and three candidate white dwarf + debris discs. We place lower limits on the unresolved (<2 arcsec) companions to all DA white dwarfs and thus assess the sensitivity of UKIDSS to such objects. We use this result to estimate unresolved binary fractions of f
WD + dL≥ 0.4 ± 0.3 per cent, f
WD + dT≥ 0.2 per cent and f
WD + BD≥ 0.5 ± 0.3 per cent.
WASP-39b is a hot Saturn-mass exoplanet with a predicted clear atmosphere based on observations in the optical and infrared. Here, we complete the transmission spectrum of the atmosphere with ...observations in the near-infrared (NIR) over three water absorption features with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) G102 (0.8-1.1 m) and G141 (1.1-1.7 m) spectroscopic grisms. We measure the predicted high-amplitude H2O feature centered at 1.4 m and the smaller amplitude features at 0.95 and 1.2 m, with a maximum water absorption amplitude of 2.4 planetary scale heights. We incorporate these new NIR measurements into previously published observational measurements to complete the transmission spectrum from 0.3 to 5 m. From these observed water features, combined with features in the optical and IR, we retrieve a well constrained temperature Teq = 1030 K, and atmospheric metallicity solar, which is relatively high with respect to the currently established mass-metallicity trends. This new measurement in the Saturn-mass range hints at further diversity in the planet formation process relative to our solar system giants.
Abstract
We present a thermal emission spectrum of the bloated hot Jupiter HAT-P-32Ab from a single eclipse observation made in spatial scan mode with the Wide Field Camera 3 (WFC3) aboard the Hubble ...Space Telescope (HST). The spectrum covers the wavelength regime from 1.123 to 1.644 μm which is binned into 14 eclipse depths measured to an averaged precision of 104 parts-per million. The spectrum is unaffected by a dilution from the close M-dwarf companion HAT-P-32B, which was fully resolved. We complemented our spectrum with literature results and performed a comparative forward and retrieval analysis with the 1D radiative-convective ATMO model. Assuming solar abundance of the planet atmosphere, we find that the measured spectrum can best be explained by the spectrum of a blackbody isothermal atmosphere with Tp = 1995 ± 17 K, but can equally well be described by a spectrum with modest thermal inversion. The retrieved spectrum suggests emission from VO at the WFC3 wavelengths and no evidence of the 1.4 μm water feature. The emission models with temperature profiles decreasing with height are rejected at a high confidence. An isothermal or inverted spectrum can imply a clear atmosphere with an absorber, a dusty cloud deck or a combination of both. We find that the planet can have continuum of values for the albedo and recirculation, ranging from high albedo and poor recirculation to low albedo and efficient recirculation. Optical spectroscopy of the planet's day-side or thermal emission phase curves can potentially resolve the current albedo with recirculation degeneracy.
Where are all the Sirius-like binary systems? Holberg, J. B; Oswalt, T. D; Sion, E. M ...
Monthly notices of the Royal Astronomical Society,
11/2013, Letnik:
435, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Approximately 70 per cent of the nearby white dwarfs appear to be single stars, with the remainder being members of binary or multiple star systems. The most numerous and most easily identifiable ...systems are those in which the main-sequence companion is an M star, since even if the systems are unresolved the white dwarf either dominates or is at least competitive with the luminosity of the companion at optical wavelengths. Harder to identify are systems where the non-degenerate component has a spectral type earlier than M0 and the white dwarf becomes the less luminous component. Taking Sirius as the prototype, these latter systems are referred to here as 'Sirius like'. There are currently 98 known Sirius-like systems. Studies of the local white dwarf population within 20 pc indicate that approximately 8 per cent of all white dwarfs are members of Sirius-like systems, yet beyond 20 pc the frequency of known Sirius-like systems declines to between 1 and 2 per cent, indicating that many more of these systems remain to be found. Estimates are provided for the local space density of Sirius-like systems and their relative frequency among both the local white dwarf population and the local population of A to K main-sequence stars. The great majority of currently unidentified Sirius-like systems will likely turn out to be closely separated and unresolved binaries. Ways to observationally detect and study these systems are discussed.
Gaia Early Data Release 3 Riello, M.; De Angeli, F.; Evans, D. W. ...
Astronomy and astrophysics (Berlin),
05/2021, Letnik:
649
Journal Article
Recenzirano
Odprti dostop
Context. Gaia
Early Data Release 3 (
Gaia
EDR3) contains astrometry and photometry results for about 1.8 billion sources based on observations collected by the European Space Agency
Gaia
satellite ...during the first 34 months of its operational phase.
Aims.
In this paper, we focus on the photometric content, describing the input data, the algorithms, the processing, and the validation of the results. Particular attention is given to the quality of the data and to a number of features that users may need to take into account to make the best use of the
Gaia
EDR3 catalogue.
Methods.
The processing broadly followed the same procedure as for
Gaia
DR2, but with significant improvements in several aspects of the blue and red photometer (BP and RP) preprocessing and in the photometric calibration process. In particular, the treatment of the BP and RP background has been updated to include a better estimation of the local background, and the detection of crowding effects has been used to exclude affected data from the calibrations. The photometric calibration models have also been updated to account for flux loss over the whole magnitude range. Significant improvements in the modelling and calibration of the
Gaia
point and line spread functions have also helped to reduce a number of instrumental effects that were still present in DR2.
Results. Gaia
EDR3 contains 1.806 billion sources with
G
-band photometry and 1.540 billion sources with
G
BP
and
G
RP
photometry. The median uncertainty in the
G
-band photometry, as measured from the standard deviation of the internally calibrated mean photometry for a given source, is 0.2 mmag at magnitude
G
= 10–14, 0.8 mmag at
G
≈ 17, and 2.6 mmag at
G
≈ 19. The significant magnitude term found in the
Gaia
DR2 photometry is no longer visible, and overall there are no trends larger than 1 mmag mag
−1
. Using one passband over the whole colour and magnitude range leaves no systematics above the 1% level in magnitude in any of the bands, and a larger systematic is present for a very small sample of bright and blue sources. A detailed description of the residual systematic effects is provided. Overall the quality of the calibrated mean photometry in
Gaia
EDR3 is superior with respect to DR2 for all bands.
Context.
The latest
Gaia
data release in July 2022, DR3, in addition to the refinement of the astrometric and photometric parameters from DR2, added a number of important data products to those ...available in earlier releases, including radial velocity data, information on stellar multiplicity, and XP spectra of a selected sample of stars.
Gaia
has proved to be an important search tool for white dwarf stars, which are readily identifiable from their absolute
G
magnitudes as low luminosity objects in the Hertzsprung–Russell (H–R) diagram. Each data release has yielded large catalogs of white dwarfs, containing several hundred thousand objects, far in excess of the numbers known from all previous surveys (∼40 000). While the normal
Gaia
photometry (
G
,
G
BP
, and
G
RP
bands) and astrometry can be used to identify white dwarfs with high confidence, it is much more difficult to parameterize the stars and determine the white dwarf spectral type from this information alone. Observing all stars in these catalogs with follow-up spectroscopy and photometry is also a huge logistical challenge with current facilities.
Aims.
The availability of the XP spectra and synthetic photometry presents an opportunity for a more detailed spectral classification and measurement of the effective temperature and surface gravity of
Gaia
white dwarfs.
Methods.
A magnitude limit of
G
< 17.6 was applied to the routine production of XP spectra for
Gaia
sources, which would have excluded most white dwarfs. Therefore, we created a catalog of 100 000 high-quality white dwarf identifications for which XP spectra were processed, with a magnitude limit of
G
< 20.5. Synthetic photometry was computed for all these stars, from the XP spectra, in Johnson, SDSS, and J-PAS, published as the
Gaia
Synthetic Photometry Catalog – White Dwarfs (GSPC-WD). We took this catalog and applied machine learning techniques to provide a classification of all the stars from the XP spectra. We have then applied an automated spectral fitting program, with
χ
-squared minimization, to measure their physical parameters (effective temperature and log
g
) from which we could estimate the white dwarf masses and radii.
Results.
We present the results of this work, demonstrating the power of being able to classify and parameterize such a large sample of ≈100 000 stars. We describe what we can learn about the white dwarf population from this dataset. We also explored the uncertainties in the process and the limitations of the dataset.
The gravitational redshift of Sirius B Joyce, S R G; Barstow, M A; Holberg, J B ...
Monthly notices of the Royal Astronomical Society,
12/2018, Letnik:
481, Številka:
2
Journal Article
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