78 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
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
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
New constraints on carbonate diagenesis from integrated Sr and S isotopic and rare earth element data, Jurassic Smackover Formation, U.S. Gulf Coast: a reply
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30291/1/0000693.pd
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
Short-lived Nuclei in the Early Solar System: Possible AGB Sources
(Abridged) We review abundances of short-lived nuclides in the early solar
system (ESS) and the methods used to determine them. We compare them to the
inventory for a uniform galactic production model. Within a factor of two,
observed abundances of several isotopes are compatible with this model. I-129
is an exception, with an ESS inventory much lower than expected. The isotopes
Pd-107, Fe-60, Ca-41, Cl-36, Al-26, and Be-10 require late addition to the
solar nebula. Be-10 is the product of particle irradiation of the solar system
as probably is Cl-36. Late injection by a supernova (SN) cannot be responsible
for most short-lived nuclei without excessively producing Mn-53; it can be the
source of Mn-53 and maybe Fe-60. If a late SN is responsible for these two
nuclei, it still cannot make Pd-107 and other isotopes. We emphasize an AGB
star as a source of nuclei, including Fe-60 and explore this possibility with
new stellar models. A dilution factor of about 4e-3 gives reasonable amounts of
many nuclei. We discuss the role of irradiation for Al-26, Cl-36 and Ca-41.
Conflict between scenarios is emphasized as well as the absence of a global
interpretation for the existing data. Abundances of actinides indicate a
quiescent interval of about 1e8 years for actinide group production in order to
explain the data on Pu-244 and new bounds on Cm-247. This interval is not
compatible with Hf-182 data, so a separate type of r-process is needed for at
least the actinides, distinct from the two types previously identified. The
apparent coincidence of the I-129 and trans-actinide time scales suggests that
the last actinide contribution was from an r-process that produced actinides
without fission recycling so that the yields at Ba and below were governed by
fission.Comment: 92 pages, 14 figure files, in press at Nuclear Physics
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