60 research outputs found
Deep 10 and 18 micron Imaging of the HR 4796A Circumstellar Disk: Transient Dust Particles & Tentative Evidence for a Brightness Asymmetry
We present new 10.8 and 18.2 micron images of HR 4796A, a young A0V star that
was recently discovered to have a spectacular, nearly edge-on, circumstellar
disk prominent at ~20 microns (Jayawardhana et al. 1998; Koerner et al. 1998).
These new images, obtained with OSCIR at Keck II, show that the disk's size at
10 microns is comparable to its size at 18 microns. Therefore, the 18
micron-emitting dust may also emit some, or all, of the 10 micron radiation.
Using these multi-wavelength images, we determine a "characteristic" diameter
of 2-3 microns for the mid-infrared-emitting dust particles if they are
spherical and composed of astronomical silicates. Particles this small are
expected to be blown out of the system by radiation pressure in a few hundred
years, and therefore these particles are unlikely to be primordial. Dynamical
modeling of the disk (Wyatt et al. 2000) indicates that the disk surface
density is relatively sharply peaked near 70 AU, which agrees with the mean
annular radius deduced by Schneider et al. (1999) from their NICMOS images. We
present evidence (~1.8 sigma significance) for a brightness asymmetry that may
result from the presence of the hole and the gravitational perturbation of the
disk particle orbits by the low-mass stellar companion or a planet. This
"pericenter glow," which must still be confirmed, results from a very small (a
few AU) shift of the disk's center of symmetry relative to the central star HR
4796A; one side of the inner boundary of the annulus is shifted towards HR
4796A, thereby becoming warmer and more infrared-emitting. The possible
detection of pericenter glow implies that the detection of even complex
dynamical effects of planets on disks is within reach.Comment: 18 pages. 9 GIF images. Total size ~800 kB. High resolution images
available upon request. Accepted for publication in the Astrophysical Journal
(scheduled for January 10, 2000
A dust disk surrounding the young A star HR4796A
We report the codiscovery of the spatially-resolved dust disk of the
Vega-like star HR 4796A. Images of the thermal dust emission at m show an elongated structure approximately 200 AU in diameter surrounding
the central A0V star. The position angle of the disk, , is consistent to the position angle of the M companion star,
, suggesting that the disk-binary system is being seen nearly
along its orbital plane. The surface brightness distribution of the disk is
consistent with the presence of an inner disk hole of approximately 50 AU
radius, as was originally suggested by Jura et al. on the basis of the infrared
spectrum. HR 4796 is a unique system among the Vega-like or Pictoris
stars in that the M star companion (a weak-emission T Tauri star) shows that
the system is relatively young, Myr. The inner disk hole may
provide evidence for coagulation of dust into larger bodies on a timescale
similar to that suggested for planet formation in the solar system.Comment: 12 pages, 3 PostScript figures, accepted for publication in
Astrophysical Journal Letter
A Large-Area Search for Low Mass Objects in Upper Scorpius I: The Photometric Campaign and New Brown Dwarfs
We present a wide-field photometric survey covering ~200 deg^2 toward the
Upper Scorpius OB association. Data taken in the R and I bands with the Quest-2
camera on the Palomar 48-inch telescope were combined with the 2MASS JHK survey
and used to select candidate pre-main sequence stars. Follow-up spectroscopy
with the Palomar 200-inch telescope of 62 candidate late-type members
identified 43 stars that have surface gravity signatures consistent with
association membership. From the optical/near-infrared photometry and derived
spectral types we construct an HR diagram for the new members and find 30
likely new brown dwarfs, nearly doubling the known substellar population of the
Upper Scorpius OB association. Continuation of our spectroscopic campaign
should reveal hundreds on new stellar and substellar members.Comment: 36 pages including 14 figures and 2 tables. Accepted for publication
in A
Mid-infrared imaging of the young binary star Hen 3-600: Evidence for a dust disk around the primary
We present high-resolution mid-infrared observations of the nearby late-type
young binary system Hen 3-600. The binary, at a distance of 50 pc, could
be a member of the TW Hydrae Association, the nearest known group of young
stars, with an age of a few million years. Our images make it possible for the
first time to determine which star in the pair, separated by 1.4'', harbors the
mid-infrared excess detected by IRAS. In the near-infrared, where the radiation
is primarily photospheric, Hen 3-600A (M3) and Hen 3-600B (M3.5) have a flux
ratio of 1.6. At 4.8m, 10.8m, and 18.2m, the primary becomes
increasingly dominant over the secondary, suggesting that most of the
circumstellar dust in the system resides around Hen 3-600A. Comparison of the
spectral energy distribution (SED) of Hen 3-600A to the median SED of classical
T Tauri stars suggests that its disk may be truncated by the secondary and
provides tentative evidence for a central disk hole. The distribution of dust
in the Hen 3-600 system may provide important clues to the formation and
evolution of protoplanetary disks in close binaries.Comment: 9 pages, 2 PostScript figures, accepted for publication in The
Astrophysical Journal Letter
Injection of Radioactivities into the Forming Solar System
Meteorite studies have revealed the presence of short-lived radioactivities
in the early solar system. The current data suggests that the origin of at
least some of the radioactivities requires contribution from recent
nucleosynthesis at a stellar site. This sets a strict time limit on the time
available for the formation of the solar system and argues for the theory of
the triggered origin of the solar system. According to this scenario, the
formation of our planetary system was initiated by the impact of an
interstellar shock wave on a molecular cloud core. The shock wave originated
from a nearby explosive stellar event and carried with it radioactivities
produced in the stellar source. In addition to triggering the collapse of the
molecular cloud core, the shock wave also deposited some of the freshly
synthesized radioactivities into the collapsing system. The radioactivities
were then incorporated into the first solar system solids, in this manner
leaving a record of the event in the meteoritic material. The viability of the
scenario can be investigated through numerical simulations studying the
processes involved in mixing shock wave material into the collapsing system.
The high-resolution calculations presented here show that injection occurs
through Rayleigh-Taylor instabilities, the injection efficiency is
approximately 10%, and temporal and spatial heterogeneities in the abundances
of the radioactivities existed at the time of their arrival in the forming
solar system.Comment: 13 pages, including 3 figures. Better-quality figures available at
http://www.public.asu.edu/~hvanhal/pubs
Planetary Torques as the Viscosity of Protoplanetary Disks
We revisit the idea that density-wave wakes of planets drive accretion in
protostellar disks. The effects of many small planets can be represented as a
viscosity if the wakes damp locally, but the viscosity is proportional to the
damping length. Damping occurs mainly by shocks even for earth-mass planets.
The excitation of the wake follows from standard linear theory including the
torque cutoff. We use this as input to an approximate but quantitative
nonlinear theory based on Burger's equation for the subsequent propagation and
shock. Shock damping is indeed local but weakly so. If all metals in a
minimum-mass solar nebula are invested in planets of a few earth masses each,
dimensionless viscosities [alpha] of order dex(-4) to dex(-3) result. We
compare this with observational constraints. Such small planets would have
escaped detection in radial-velocity surveys and could be ubiquitous. If so,
then the similarity of the observed lifetime of T Tauri disks to the
theoretical timescale for assembling a rocky planet may be fate rather than
coincidence.Comment: 23 pages, 3 figures. Uses aastex50
Molybdenum Evidence for Inherited Planetary Scale Isotope Heterogeneity of the Protosolar Nebula
Isotope anomalies provide important information about early solar system
evolution. Here we report molybdenum isotope abundances determined in samples
of various meteorite classes. There is no fractionation of molybdenum isotopes
in our sample set within 0.1 permil and no contribution from the extinct
radionuclide 97Tc at mass 97 (97Tc/92Mo<3E-6). Instead, we observe clear
anomalies in bulk iron meteorites, mesosiderites, pallasites, and chondrites
characterized by a coupled excess in p- and r- or a mirror deficit in s-process
nuclides (Mo-HL). This large scale isotope heterogeneity of the solar system
observed for molybdenum must have been inherited from the interstellar
environment where the sun was born, illustrating the concept of ``cosmic
chemical memory''. The presence of molybdenum anomalies is used to discuss the
filiation between planetesimals.Comment: 7 pages, 2 figures, 1 table, accepted in Ap
Mid-Infrared Imaging of Candidate Vega-Like Systems
We have conducted deep mid-infrared imaging of a relatively nearby sample of
candidate Vega-like stars using the OSCIR instrument on the CTIO 4-meter and
Keck II 10-meter telescopes. Our discovery of a spatially-resolved disk around
HR 4796A has already been reported (Jayawardhana et al. 1998). Here we present
imaging observations of the other members of the sample, including the
discovery that only the primary in the HD 35187 binary system appears to harbor
a substantial circumstellar disk and the possible detection of extended disk
emission around 49 Ceti. We derive global properties of the dust disks, place
constraints on their sizes, and discuss several interesting cases in detail.
Although our targets are believed to be main sequence stars, we note that
several have large infrared excesses compared to prototype Vega-like systems,
and may therefore be somewhat younger. The disk size constraints we derive, in
many cases, imply emission from relatively large ( 10m) particles
at mid-infrared wavelengths.Comment: 15 pages and 2 PostScript figures, accepted for publication in The
Astronomical Journa
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