473 research outputs found
The White Dwarfs within 20 Parsecs of the Sun: Kinematics and Statistics
We present the kinematical properties, distribution of spectroscopic
subtypes, stellar population subcomponents of the white dwarfs within 20 pc of
the sun. We find no convincing evidence of halo white dwarfs in the total 20 pc
sample of 129 white dwarfs nor is there convincing evidence of genuine thick
disk subcomponent members within 20 parsecs. Virtually the entire 20 pc sample
likely belongs to the thin disk. The total DA to non-DA ratio of the 20 pc
sample is 1.6, a manifestation of deepening envelope convection which
transforms DA stars with sufficiently thin H surface layers into non-DAs. The
addition of 5 new stars to the 20 pc sample yields a revised local space
density of white dwarfs of M_{\sun}/yr and a
corresponding mass density of M_{\sun}/pc.
We find that at least 15% of the white dwarfs within 20 parsecs of the sun (the
DAZ and DZ stars) have photospheric metals that possibly originate from
accretion of circumstellar material (debris disks) around them. If this
interpretation is correct, this suggests the possibility that the same
percentage have planets or asteroid-like bodies orbiting them.Comment: Accepted for publication in The Astronomical Journa
Hydrogen emission from Jupiter: Hydrogen emission from sunlit atmosphere of Saturn
Successful IUE observations of the equatorial sunlit atmosphere of Jupiter and Saturn have been obtained. Spectra containing atomic and molecular hydrogen and solar reflection continuum emissions have been analyzed, with the purpose of determining the long term temporal behavior of the electroglow process. Quantitative estimates have been established for the first time using a model analysis of the short wavelength region of the spectrum. Both systems show varying degrees of long term variability in hydrogen emission rate, but the time scale is too short to determine whether there is a dependence on solar cycle activity. As part of the emission modeling program, a preliminary point source spreading function for the IUE SWP instrument has been established, suggesting a wavelength dependence in spectral line width different from previous analyses. Further IUE observations are planned for both Jupiter and Saturn
Near-UV absorption in very cool DA white dwarfs
The atmospheres of very cool, hydrogen-rich white dwarfs (Teff <6000 K) are
challenging to models because of the increased complexity of the equation of
state, chemical equilibrium, and opacity sources in a low-temperature, weakly
ionized dense gas. In particular, many models that assume relatively simple
models for the broadening of atomic levels and mostly ideal gas physics
overestimate the flux in the blue part of their spectra. A solution to this
problem that has met with some success is that additional opacity at short
wavelengths comes for the extreme broadening of the Lyman alpha line of atomic
H by collisions primarily with H2. For the purpose of validating this model
more rigorously, we acquired Hubble Space Telescope STIS spectra of 8 very cool
white dwarfs (5 DA and 3 DC stars). Combined with their known parallaxes,
BVRIJHK and Spitzer IRAC photometry, we analyze their entire spectral energy
distribution (from 0.24 to 9.3 micron) with a large grid of model atmospheres
and synthetic spectra. We find that the red wing of the Lyman alpha line
reproduces the rapidly decreasing near-UV flux of these very cool stars very
well. We determine better constrained values of Teff and gravity as well as
upper limits to the helium abundance in their atmospheres.Comment: 41 pages, 9 figures. Accepted for publication in the Ap
A search for hidden white dwarfs in the ROSAT EUV survey II: Discovery of a distant DA+F6/7V binary system in a direction of low density neutral hydrogen
We report the results of our final search for hot white dwarfs in unresolved,
Sirius-type, binary systems with IUE. One new system, RE J0500-364 (DA+F6/7V),
has been identified. This star appears to lie at a distance of between
500-1000pc, making it one of the most distant white dwarfs, if not the most
distant, to be detected in the EUV surveys. The very low line-of-sight neutral
hydrogen volume density to this object could place a lower limit on the length
of the Beta CMa interstellar tunnel of diffuse gas, which stretches away from
the Local Bubble in a similar direction to RE J0500-364.Comment: 1 LaTex file plus 15 figures; accepted for publication in Monthly
Notices of the Royal Astronomical Societ
IUE observations of a hot DAO white dwarf: Implications for diffusion theory and photospheric stratification
Observations of the DAO white dwarf PG1210+533, including the first high dispersion spectrum of a hybrid H-He object of this nature were obtained by IUE. In contrast with hot DAs in the 50,000 K temperature range, PG1210+533 shows no narrow interstellar-like metal lines, in spite of an optically observed He/H abundance of 0.1. This lack of metal makes accretion from the ISM an unlikely source for the He in the PG1210+533 photosphere. A significant discovery in the high dispersion spectrum is the existence of a sharp, non-LTE like, core seen in the He II 1640 line. Such features are detected in DO white dwarfs. A small aperture SWP low dispersion observation reveals the Lyman alpha profile of PG1210+533 to be surprisingly weak and narrow. Fits of this profile using pure H models yielded a T(eff) = 56,000 K. Fits of the Balmer H gamma profile however, yield T(eff) = 42,300 K and log g = 8.5 + or - 0.5 for the same models. It is unlikely that homogeneously mixed H-He atmospheres can resolve the inconsistency between the Lyman alpha and H gamma features in this star. Stratified models involving thin H photospheres may be necessary to explain these results
Large time scale variation in hydrogen emission from Jupiter and Saturn
The IUE and Voyager spacecraft observations of Jupiter and Saturn were combined to obtain a consistent measurement of temporal variation of the equatorial subsolar hydrogen emission. The outer planets appear to have rather independent behavior over time scales of the order of 10 yr, particularly in emission from the H Ly alpha line. The time interval from 1978 to the present shows variation of mean equatorial H Ly alpha brightness of 2 at Jupiter and 5 at Saturn. The relative magnitudes of the variations is sufficiently different to suggest that response to input from the Sun is at least nonlinear. The brightness of H2 band emission appears to be relatively more stable than H Ly alpha. There is evidence in IUE observations of a moderate increase in H2 band brightness with increasing time at Jupiter, in opposition to the variation in H Ly alpha
Towards a standardised line list for G191-B2B, and other DA type objects
We present a comprehensive analysis of the far UV spectrum of G191-B2B over
the range of 900-1700{\AA} using co-added data from the FUSE and STIS archives.
While previous identifications made by Holberg et al. (2003) are reaffirmed in
this work, it is found that many previously unidentified lines can now be
attributed to Fe, Ni, and a few lighter metals. Future work includes extending
this detailed analysis to a wider range of DA objects, in the expectation that
a more complete analysis of their atmospheres can be realised.Comment: 4 pages, 2 figures, 1 table: To appear in the proceedings of the
"18th European White Dwarf Workshop" in Krakow, Poland, 201
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
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