632 research outputs found
Strengthening the Case for Asteroidal Accrection: Evidence for Subtle and Diverse Disks at White Dwarfs
Spitzer Space Telescope IRAC 3-8 micron and AKARI IRC 2-4 micron photometry
are reported for ten white dwarfs with photospheric heavy elements; nine
relatively cool stars with photospheric calcium, and one hotter star with a
peculiar high carbon abundance. A substantial infrared excess is detected at HE
2221-1630, while modest excess emissions are identified at HE 0106-3253 and HE
0307+0746, implying these latter two stars have relatively narrow (Delta r <
0.1 Rsol) rings of circumstellar dust. A likely 7.9 micron excess is found at
PG 1225-079 and may represent, together with G166-58, a sub-class of dust ring
with a large inner hole. The existence of attenuated disks at white dwarfs
substantiates the connection between their photospheric heavy elements and the
accretion of disrupted minor planets, indicating many polluted white dwarfs may
harbor orbiting dust, even those lacking an obvious infrared excess.Comment: 13 pages, emulateapj, accepted to Ap
Copernicus observations of C I and CO in diffuse interstellar clouds
Copernicus was used to observe absorption lines of C I in its ground state and excited fine structure levels and CO toward 29 stars. We use the C I data to infer densities and pressures within the observed clouds, and because our results are of higher precision than previous work, much more precise estimates of the physical conditions in clouds are obtained. In agreement with previous work, the interstellar thermal pressure appears to be variable, with most clouds having values of p/k between 1000/cu cm K and 10,000/cu cm K, but there are some clouds with p/k as high as 100,000/cu cm K. Our results are consistent with the view that the interstellar thermal pressure is so variable that the gas undergoes continuous dynamic evolution. Our observations provide useful constraints on the physical processes on the surfaces of grains. In particular, we find that grains are efficient catalysts of interstellar H2 in the sense that at least half of the hydrogen atoms that strike grains come off as part of H2. Results place strong constraints on models for the formation and destruction of interstellar CO. In many clouds, an order of magnitude less CO than predicted in some models was found
Spectral synthesis of circumstellar disks - application to white dwarf debris disks
Gas and dust disks are common objects in the universe and can be found around
various objects, e.g. young stars, cataclysmic variables, active galactic
nuclei, or white dwarfs. The light that we receive from disks provides us with
clues about their composition, temperature, and density. In order to better
understand the physical and chemical dynamics of these disks, self-consistent
radiative transfer simulations are inevitable. Therefore, we have developed a
1+1D radiative transfer code as an extension to the well-established model
atmosphere code \verb!PHOENIX!. We will show the potential of the application
of our code to model the spectra of white dwarf debris disks.Comment: 4 pages, 4 figures, to appear in: Proceedings of the 16th European
Workshop on White Dwarf
Externally-polluted white dwarfs with dust disks
We report Spitzer Space Telescope photometry of eleven externally-polluted
white dwarfs. Of the nine stars for which we have IRAC photometry, we find that
GD 40, GD 133 and PG 1015+161 each has an infrared excess that can be
understood as arising from a flat, opaque, dusty disk. GD 56 also has an
infrared excess characteristic of circumstellar dust, but a flat-disk model
cannot reproduce the data unless there are grains as warm as 1700 K and perhaps
not even then. Our data support the previous suggestion that the metals in the
atmosphere of GD 40 are the result of accretion of a tidally-disrupted asteroid
with a chondritic composition.Comment: ApJ, in pres
A Dusty Disk Around GD 362, a White Dwarf With a Uniquely High Photospheric Metal Abundance
Eighteen years after an infrared excess was discovered associated with the
white dwarf G29-38, we report ground-based measurements (JHKL'N') with
mJy-level sensitivity of GD 362 that show it to be a second single white dwarf
with an infrared excess. As a first approximation, the excess around GD 362,
which amounts to about 3% of the total stellar luminosity, can be explained by
emission from a passive, flat, opaque dust disk that lies within the Roche
radius of the white dwarf. The dust may have been produced by the tidal
disruption of a large parent body such as an asteroid. Accretion from this
circumstellar disk could account for the remarkably high abundance of metals in
the star's photosphere.Comment: 12 pages, 2 figures. ApJ Letters, in pres
Spectral modeling of gaseous metal disks around DAZ white dwarfs
We report on our attempt for the first non-LTE modeling of gaseous metal
disks around single DAZ white dwarfs recently discovered by Gaensicke et al.
and thought to originate from a disrupted asteroid. We assume a Keplerian
rotating viscous disk ring composed of calcium and hydrogen and compute the
detailed vertical structure and emergent spectrum. We find that the observed
infrared CaII emission triplet can be modeled with a hydrogen-deficient gas
ring located at R=1.2 R_sun, inside of the tidal disruption radius, with Teff
about 6000 K and a low surface mass density of about 0.3 g/cm**2. A disk having
this density and reaching from the central white dwarf out to R=1.2 R_sun would
have a total mass of 7 10**21 g, corresponding to an asteroid with about 160 km
diameter.Comment: Proceedings, 16th European White Dwarf Workshop, Barcelona, 200
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