916 research outputs found
Evidence for an external origin of heavy elements in hot DA white dwarfs
We present a series of systematic abundance measurements for 89 hydrogen
atmosphere (DA-type) white dwarfs with temperatures spanning 16000-77000K 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 IUE and HST data, by a factor three. Using heavy element
blanketed non-LTE stellar atmosphere calculations, we have addressed the heavy
element abundance patterns making completely objective measurements of
abundance values and their error ranges using a \c{hi}2 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.Comment: 23 pages. 13 Figures, 4 Tables. Accepted for publication in the
Monthly Notices of the Royal Astronomical Societ
Effective area calibration of the reflection grating spectrometers of XMM-Newton. II. X-ray spectroscopy of DA white dwarfs
White dwarf spectra have been widely used as a calibration source for X-ray
and EUV instruments. The in-flight effective area calibration of the reflection
grating spectrometers (RGS) of XMM-Newton depend upon the availability of
reliable calibration sources. We investigate how well these white dwarf spectra
can be used as standard candles at the lowest X-ray energies in order to gauge
the absolute effective area scale of X-ray instruments. We calculate a grid of
model atmospheres for Sirius B and HZ 43A, and adjust the parameters using
several constraints until the ratio of the spectra of both stars agrees with
the ratio as observed by the low energy transmission grating spectrometer
(LETGS) of Chandra. This ratio is independent of any errors in the effective
area of the LETGS. We find that we can constrain the absolute X-ray spectrum of
both stars with better than 5 % accuracy. The best-fit model for both stars is
close to a pure hydrogen atmosphere, and we put tight limits to the amount of
helium or the thickness of a hydrogen layer in both stars. Our upper limit to
the helium abundance in Sirius B is 4 times below the previous detection based
on EUVE data. We also find that our results are sensitive to the adopted
cut-off in the Lyman pseudo-continuum opacity in Sirius B. We get best
agreement with a long wavelength cut-off. White dwarf model atmospheres can be
used to derive the effective area of X-ray spectrometers in the lowest energy
band. An accuracy of 3-4 % in the absolute effective area can be achieved.Comment: 15 pages, 7 figures, accepted for publication in Astronomy &
Astrophysics, main journa
High ions towards white dwarfs: circumstellar line shifts and stellar temperature
Based on a compilation of OVI, CIV, SiIV and NV data from IUE, FUSE, GHRS,
STIS, and COS, we derive an anti- correlation between the stellar temperature
and the high ion velocity shift w.r.t. to the photosphere, with positive (resp.
negative) velocity shifts for the cooler (resp. hotter) white dwarfs. This
trend probably reflects more than a single process, however such a dependence
on the WD's temperature again favors a CS origin for a very large fraction of
those ion absorptions, previously observed with IUE, HST-STIS, HST-GHRS, FUSE,
and now COS, selecting objects for which absorption line radial velocities,
stellar effective temperature and photospheric velocity can be found in the
literature. Interestingly, and gas in near-equilibrium in the star vicinity. It
is also probably significant that the temperature that corresponds to a null
radial velocity, i.e. \simeq 50,000K, also corresponds to the threshold below
which there is a dichotomy between pure or heavy elements atmospheres as well
as some temperature estimates for and a form of balance between radiation
pressure and gravitation. This is consistent with ubiquitous evaporation of
orbiting dusty material. Together with the fact that the fraction of stars with
(red-or blue-) shifted lines and the fraction of stars known to possess heavy
species in their atmosphere are of the same order, such a velocity-temperature
relationship is consistent with quasi-continuous evaporation of orbiting CS
dusty material, followed by accretion and settling down in the photosphere. In
view of these results, ion measurements close to the photospheric or the IS
velocity should be interpreted with caution, especially for stars at
intermediate temperatures. While tracing CS gas, they may be erroneously
attributed to photospheric material or to the ISM, explaining the difficulty of
finding a coherent pattern of the high ions in the local IS 3D distribution.Comment: Accepted by A&A. Body of paper identical to v1. This submission has a
more appropriate truncation of the original abstrac
The status and future of EUV astronomy
The Extreme Ultraviolet wavelength range was one of the final windows to be
opened up to astronomy. Nevertheless, it provides very important diagnostic
tools for a range of astronomical objects, although the opacity of the
interstellar medium restricts the majority of observations to sources in our
own galaxy. This review gives a historical overview of EUV astronomy, describes
current instrumental capabilities and examines the prospects for future
facilities on small and medium-class satellite platforms.Comment: Published in Advances in Space Researc
Exoplanets with JWST: degeneracy, systematics and how to avoid them
The high sensitivity and broad wavelength coverage of the James Webb Space Telescope will transform the field of exoplanet transit spectroscopy. Transit spectra are inferred from minute, wavelength-dependent variations in the depth of a transit or eclipse as the planet passes in front of or is obscured by its star, and the spectra contain information about the composition, structure and cloudiness of exoplanet atmospheres. Atmospheric retrieval is the preferred technique for extracting information from these spectra, but the process can be confused by astrophysical and instrumental systematic noise. We present results of retrieval tests based on synthetic, noisy JWST spectra, for clear and cloudy planets and active and inactive stars. We find that the ability to correct for stellar activity is likely to be a limiting factor for cloudy planets, as the effects of unocculted star spots may mimic the presence of a scattering slope due to clouds. We discuss the pros and cons of the available JWST instrument combinations for transit spectroscopy, and consider the effect of clouds and aerosols on the spectra. Aerosol high in a planet’s atmosphere obscures molecular absorption features in transmission, reducing the information content of spectra in wavelength regions where the cloud is optically thick. We discuss the usefulness of particular wavelength regions for identifying the presence of cloud, and suggest strategies for solving the highly-degenerate retrieval problem for these objects
Beyond the iron group: heavy metals in hot subdwarfs
We report the discovery of strong photospheric resonance lines of Ga III, Ge
IV, Sn IV and Pb IV in the UV spectra of more than two dozen sdB and sdOB stars
at temperatures ranging from 22000 K to 40000 K. Lines of other heavy elements
are also detected, however in these cases more atomic data are needed. Based on
these discoveries, we present a hypothesis to explain the apparent lack of
silicon in sdB stars hotter than ~32000 K. The existence of triply ionised Ge,
Sn, and Pb suggests that rather than silicon sinking deep into the photosphere,
it is removed from the star in a fractionated stellar wind. This hypothesis
provides a challenge to diffusion models of sdB stars.Comment: 5 pages, 3 figures, to appear in A&A Letter
Metal abundances in PG1159 stars from Chandra and FUSE spectroscopy
We investigate FUSE spectra of three PG1159 stars and do not find any
evidence for iron lines. From a comparison with NLTE models we conclude a
deficiency of 1-1.5 dex. We speculate that iron was transformed into heavier
elements. A soft X-ray Chandra spectrum of the unique H- and He-deficient star
H1504+65 is analyzed. We find high neon and magnesium abundances and confirm
that H1504+65 is the bare core of either a C-O or a O-Ne-Mg white dwarf.Comment: To be published in: Proceedings 13th European Workshop on White
Dwarfs, NATO Science Series, 4 pages, 1 figur
Chandra grating spectroscopy of three hot white dwarfs
High-resolution soft X-ray spectroscopic observations of single hot white
dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have
observed two white dwarfs, one is of spectral type DA (LB 1919) and the other
is a non-DA of spectral type PG1159 (PG 1520+525). The spectra of both stars
are analyzed, together with an archival Chandra spectrum of another DA white
dwarf (GD 246). The soft X-ray spectra of the two DA white dwarfs are
investigated in order to study the effect of gravitational settling and
radiative levitation of metals in their photospheres. LB 1919 is of interest
because it has a significantly lower metallicity than DAs with otherwise
similar atmospheric parameters. GD 246 is the only white dwarf known that shows
identifiable individual iron lines in the soft X-ray range. For the PG1159
star, a precise effective temperature determination is performed in order to
confine the position of the blue edge of the GW Vir instability region in the
HRD. (abridged)Comment: A&A, in pres
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