4,193 research outputs found
Spitzer IRAC Observations of White Dwarfs. I. Warm Dust at Metal-Rich Degenerates
This paper presents the results of a Spitzer IRAC 3-8 micron photometric
search for warm dust orbiting 17 nearby, metal-rich white dwarfs, 15 of which
apparently have hydrogen dominated atmospheres (type DAZ). G166-58, G29-38, and
GD 362 manifest excess emission in their IRAC fluxes and the latter two are
known to harbor dust grains warm enough to radiate detectable emission at
near-infrared wavelengths as short as 2 micron. Their IRAC fluxes display
differences compatible with a relatively larger amount of cooler dust at GD
362. G166-58 is presently unique in that it appears to exhibit excess flux only
at wavelengths longer than about 5 micron. Evidence is presented that this
mid-infrared emission is most likely associated with the white dwarf,
indicating that G166-58 bears circumstellar dust no warmer than T~400 K. The
remaining 14 targets reveal no reliable mid-infrared excess, indicating the
majority of DAZ stars do not have warm debris disks sufficiently opaque to be
detected by IRAC.Comment: Accepted to ApJ, 10 figures, 6 table
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
Spitzer IRAC Observations of White Dwarfs. II. Massive Planetary and Cold Brown Dwarf Companions to Young and Old Degenerates
This paper presents a sensitive and comprehensive IRAC 3-8 m photometric
survey of white dwarfs for companions in the planetary mass regime with
temperatures cooler than the known T dwarfs. The search focuses on descendents
of intermediate mass stars with M\ga3 whose inner, few hundred AU
regions cannot be probed effectively for massive planets and brown dwarfs by
any alternative existing method. Furthermore, examination for mid-infrared
excess explores an extensive range of orbital semimajor axes, including the
intermediate 5-50 AU range poorly covered and incompletely accessible by other
techniques at main sequence or evolved stars. Three samples of white dwarfs are
chosen which together represent relatively young as well as older populations
of stars: 9 open cluster white dwarfs, 22 high mass field white dwarfs, and 17
metal-rich field white dwarfs. In particular, these targets include: 7 Hyads
and 4 field white dwarfs of similar age; 1 Pleiad and 19 field white dwarfs of
similar age; van Maanen 2 and 16 similarly metal-rich white dwarfs with ages
between 1 and 7 Gyr. No substellar companion candidates were identified at any
star. By demanding a 15% minimum photometric excess at 4.5 m to indicate a
companion detection, upper limits in the planetary mass regime are established
at 34 of the sample white dwarfs, 20 of which have limits below 10
according to substellar cooling models. Specifically, limits below the minimum
mass for deuterium burning are established at all Pleiades and Hyadeswhite
dwarfs, as well as similarly young field white dwarfs, half a dozen of which
receive limits at or below 5 . Two IRAC epochs at vMa 2 rule out
T\ga200 K proper motion companions within 1200 AU.Comment: 41 pages, accepted to Ap
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
SACY - a Search for Associations Containing Young stars
The scientific goal of the SACY (Search for Associations Containing
Young-stars) was to identify possible associations of stars younger than the
Pleiades Association among optical counterparts of the ROSAT X-ray bright
sources. High-resolution spectra for possible optical counterparts later than
G0 belonging to HIPPARCOS and/or TYCHO-2 catalogs were obtained in order to
assess both the youth and the spatial motion of each target. More than 1000
ROSAT sources were observed, covering a large area in the Southern Hemisphere.
The newly identified young stars present a patchy distribution in UVW and XYZ,
revealing the existence of huge nearby young associations. Here we present the
associations identified in this survey.Comment: 8 pages, 2 figures, to appear in the Proceedings of Open Issues in
Local Formation and Early Stellar Evolution, Ouro Preto, Brazi
The Circumstellar Disk of HD 141569 Imaged with NICMOS
Coronagraphic imaging with the Near Infrared Camera and Multi Object
Spectrometer on the Hubble Space Telescope reveals a large, ~400 AU (4'')
radius, circumstellar disk around the Herbig Ae/Be star HD 141569. A reflected
light image at 1.1 micron shows the disk oriented at a position angle of 356
+/- 5 deg and inclined to our line of sight by 51 +/- 3 deg; the intrinsic
scattering function of the dust in the disk makes the side inclined toward us,
the eastern side, brighter. The disk flux density peaks 185 AU (1.''85) from
the star and falls off to both larger and smaller radii. A region of depleted
material, or a gap, in the disk is centered 250 AU from the star. The dynamical
effect of one or more planets may be necessary to explain this morphology.Comment: 4 pages, LaTeX with emulateapj.sty and epsfig.sty, 4 postscript
figures, Accepted to ApJ Letter
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