1,306 research outputs found
Sub-millimeter images of a dusty Kuiper belt around eta Corvi
We present sub-millimeter and mid-infrared images of the circumstellar disk
around the nearby F2V star eta Corvi. The disk is resolved at 850um with a size
of ~100AU. At 450um the emission is found to be extended at all position
angles, with significant elongation along a position angle of 130+-10deg; at
the highest resolution (9.3") this emission is resolved into two peaks which
are to within the uncertainties offset symmetrically from the star at 100AU
projected separation. Modeling the appearance of emission from a narrow ring in
the sub-mm images shows the observed structure cannot be caused by an edge-on
or face-on axisymmetric ring; the observations are consistent with a ring of
radius 150+-20AU seen at 45+-25deg inclination. More face-on orientations are
possible if the dust distribution includes two clumps similar to Vega; we show
how such a clumpy structure could arise from the migration over 25Myr of a
Neptune mass planet from 80-105AU. The inner 100AU of the system appears
relatively empty of sub-mm emitting dust, indicating that this region may have
been cleared by the formation of planets, but the disk emission spectrum shows
that IRAS detected an additional hot component with a characteristic
temperature of 370+-60K (implying a distance of 1-2AU). At 11.9um we found the
emission to be unresolved with no background sources which could be
contaminating the fluxes measured by IRAS. The age of this star is estimated to
be ~1Gyr. It is very unusual for such an old main sequence star to exhibit
significant mid-IR emission. The proximity of this source makes it a perfect
candidate for further study from optical to mm wavelengths to determine the
distribution of its dust.Comment: 22 pages, 4 figures. Scheduled for publication in ApJ 10 February
2005 issu
Highest weight Macdonald and Jack Polynomials
Fractional quantum Hall states of particles in the lowest Landau levels are
described by multivariate polynomials. The incompressible liquid states when
described on a sphere are fully invariant under the rotation group. Excited
quasiparticle/quasihole states are member of multiplets under the rotation
group and generically there is a nontrivial highest weight member of the
multiplet from which all states can be constructed. Some of the trial states
proposed in the literature belong to classical families of symmetric
polynomials. In this paper we study Macdonald and Jack polynomials that are
highest weight states. For Macdonald polynomials it is a (q,t)-deformation of
the raising angular momentum operator that defines the highest weight
condition. By specialization of the parameters we obtain a classification of
the highest weight Jack polynomials. Our results are valid in the case of
staircase and rectangular partition indexing the polynomials.Comment: 17 pages, published versio
The Detection of Crystalline Silicates in Ultra-Luminous Infrared Galaxies
Silicates are an important component of interstellar dust and the structure
of these grains -- amorphous versus crystalline -- is sensitive to the local
physical conditions. We have studied the infrared spectra of a sample of
ultra-luminous infrared galaxies. Here, we report the discovery of weak, narrow
absorption features at 11, 16, 19, 23, and 28 microns, characteristic of
crystalline silicates, superimposed on the broad absorption bands at 10 and 18
microns due to amorphous silicates in a subset of this sample. These features
betray the presence of forsterite (Mg_2SiO_4), the magnesium-rich end member of
the olivines. Previously, crystalline silicates have only been observed in
circumstellar environments. The derived fraction of forsterite to amorphous
silicates is typically 0.1 in these ULIRGs. This is much larger than the upper
limit for this ratio in the interstellar medium of the Milky Way, 0.01. These
results suggest that the timescale for injection of crystalline silicates into
the ISM is short in a merger-driven starburst environment (e.g., as compared to
the total time to dissipate the gas), pointing towards massive stars as a
prominent source of crystalline silicates. Furthermore, amorphization due to
cosmic rays, which is thought to be of prime importance for the local ISM, lags
in vigorous starburst environments.Comment: 7 pages, 5 figures, accepted for publication in Ap
2.4 - 197 micron spectroscopy of OH/IR stars: The IR characteristics of circumstellar dust in O-rich environments
Infrared spectra of a number of evolved O-rich stars have been obtained with
the Short- and Long- Wavelength spectrometers on board the Infrared Space
Observatory. The very broad wavelength coverage (2.4-197 micron) obtained by
combining observations made with the two spectrometers includes practically all
of the flux emitted by the sources, and allows us to determine the emission and
absorption features of the dense circumstellar dust shells. Agreement between
the fluxes obtained by the two instruments is generally very good; the largest
discrepancies are probably due to source variability. Our sample of oxygen-rich
AGB stars exhibits a wealth of spectral features due to crystalline silicates
and crystalline water ice in emission and absorption. In this study a
qualitative overview of all features due to crystalline silicates and water ice
in these high mass loss rate objects is presented. It seems that there is a
certain onset value for the mass loss rate above which these features appear in
the spectrum. Moreover, crystalline silicate emission features have been
detected for the first time at wavelengths where the amorphous silicates are
still in absorption, implying different spatial distributions for the two
materials. A spherically symmetric and an axi-symmetric geometry are proposed.Comment: 7 figures, 15 pages. Accepted by A&A. A version with high resolution
figures can be obtained from ftp://ceres.astro.uva.nl in directory /pub/ciska
and is named H1687.p
The composition of circumstellar gas and dust in 51 Oph
We analyze ISO archive data of the nearby bright emission-line star 51 Oph,
previously classified as a proto-planetary system similar to beta Pic. The
infrared spectrum reveals the presence of gas-phase emission bands of hot
(approx 850 K) CO, CO_2, H_2O and NO. In addition to this, partially
crystalline silicate dust is present. The solid-state bands and the energy
distribution are indicative of dust that has formed recently, rather than of
debris dust. The presence of hot molecular gas and the composition of the
circumstellar dust are highly unusual for a young star and are reminiscent of
what is found around evolved (AGB) stars, although we exclude the possibility
of 51 Oph belonging to this group. We suggest several explanations for the
nature of 51 Oph, including a recent episode of mass loss from a Be star, and
the recent destruction of a planet-sized body around a young star.Comment: 4 pages, 3 figures, accepted by A&A (letters
Spitzer/MIPS Observations of Stars in the Beta Pictoris Moving Group
We present Multiband Imaging Photometer for Spitzer (MIPS) observations at 24
and 70 microns for 30 stars, and at 160 microns for a subset of 12 stars, in
the nearby (~30 pc), young (~12 Myr) Beta Pictoris Moving Group (BPMG). In
several cases, the new MIPS measurements resolve source confusion and
background contamination issues in the IRAS data for this sample. We find that
7 members have 24 micron excesses, implying a debris disk fraction of 23%, and
that at least 11 have 70 micron excesses (disk fraction of >=37%). Five disks
are detected at 160 microns (out of a biased sample of 12 stars observed), with
a range of 160/70 flux ratios. The disk fraction at 24 and 70 microns, and the
size of the excesses measured at each wavelength, are both consistent with an
"inside-out" infrared excess decrease with time, wherein the shorter-wavelength
excesses disappear before longer-wavelength excesses, and consistent with the
overall decrease of infrared excess frequency with stellar age, as seen in
Spitzer studies of other young stellar groups. Assuming that the infrared
excesses are entirely due to circumstellar disks, we characterize the disk
properties using simple models and fractional infrared luminosities. Optically
thick disks, seen in the younger TW Hya and eta Cha associations, are entirely
absent in the BPMG.
Additional flux density measurements at 24 and 70 microns are reported for
nine Tucanae-Horologium Association member stars. Since this is <20% of the
association membership, limited analysis on the complete disk fraction of this
association is possible.Comment: Accepted for Ap
The Maximal Denumerant of a Numerical Semigroup
Given a numerical semigroup S = and n in S, we
consider the factorization n = c_0 a_0 + c_1 a_1 + ... + c_t a_t where c_i >=
0. Such a factorization is maximal if c_0 + c_1 + ... + c_t is a maximum over
all such factorizations of n. We provide an algorithm for computing the maximum
number of maximal factorizations possible for an element in S, which is called
the maximal denumerant of S. We also consider various cases that have
connections to the Cohen-Macualay and Gorenstein properties of associated
graded rings for which this algorithm simplifies.Comment: 13 Page
A Sub-Millimeter Search of Nearby Young Stars for Cold Dust: Discovery of Debris Disks around Two Low-Mass Stars
(Abridged) We present results from a JCMT/SCUBA 850 um search for cold dust
around nearby young stars belonging to the beta Pic (t~12 Myr) and the Local
Association (t~50 Myr) moving groups. Unlike most past sub-mm studies, our
sample was chosen on the basis of stellar age. Our observations achieve about
an order of magnitude greater sensitivity in dust mass compared to previous
work in this age range. We detected two of the three M dwarfs in our sample at
850 um, GJ 182 and GJ 803. GJ 182 may also possess a 25 um excess, indicative
of warm dust in the inner few AU of its disk. For GJ 803 (AU Mic), sub-mm
mapping finds that the 850 um emission is unresolved. A non-detection of the CO
3-2 line indicates the system is gas-poor, and the SED suggests the presence of
a large inner disk hole (~17 AU = 1.7 arcsec in radius). These are possible
indications that planets at large separations can form around M dwarfs within
\~10 Myr. In a companion paper (Kalas, Liu & Matthews 2004), we confirm the
existence of a dust disk around GJ 803 using optical coronagraphic imaging.
Given its youthfulness, proximity, and detectability, the GJ 803 disk will be a
valuable system for studying disk, and perhaps planet, formation in great
detail. Overall, sub-mm measurements of debris disks point to a drop in dust
mass by a factor of about 10^3 within the first ~10 Myr, with the subsequent
decline in the masses of sub-mm detected disks consistent with t^{-0.5} to
t^{-1}.Comment: 9 pages, ApJ, in press. Minor changes made to reflect final published
manuscrip
Atomic and molecular interstellar absorption lines toward the high galactic latitude stars HD~141569 and HD~157841 at ultra-high resolution
We present ultra-high resolution (0.32 km/s) spectra obtained with the 3.9m
Anglo-Australian Telescope (AAT) and Ultra-High-Resolution Facility (UHRF), of
interstellar NaI D1, D2, Ca II K, K I and CH absorption toward two high
galactic latitude stars HD141569 and HD157841. We have compared our data with
21-cm observations obtained from the Leiden/Dwingeloo HI survey. We derive the
velocity structure, column densities of the clouds represented by the various
components and identify the clouds with ISM structures seen in the region at
other wavelengths. We further derive abundances, linear depletions and H2
fractional abundances for these clouds, wherever possible. Toward HD141569, we
detect two components in our UHRF spectra : a weak, broad component at - 15
km/s, seen only in CaII K absorption and another component at 0 km/s, seen in
NaI D1, D2, Ca II K, KI and CH absorption. In the case of the HD157841
sightline, a total of 6 components are seen on our UHRF spectra in NaI D1, D2
Ca II K, K I and CH absorption. 2 of these 6 components are seen only in a
single species.Comment: 16 pages, Latex, 4 figures, ps files Astrophysical Journal (in press
The circumstellar disc around the Herbig AeBe star HD169142
We present 7 mm and 3.5 cm wavelength continuum observations toward the
Herbig AeBe star HD169142 performed with the Very Large Array (VLA) with an
angular resolution of ~1". We find that this object exhibits strong (~4.4 mJy),
unresolved (~1") 7 mm continuum emission, being one of the brightest isolated
Herbig AeBe stars ever detected with the VLA at this wavelength. No emission is
detected at 3.5 cm continuum, with a 3 sigma upper limit of ~0.08 mJy. From
these values, we obtain a spectral index of ~2.5 in the 3.5 cm to 7 mm
wavelength range, indicating that the observed flux density at 7mm is most
likely dominated by thermal dust emission coming from a circumstellar disc. We
use available photometric data from the literature to model the spectral energy
distribution (SED) of this object from radio to near-ultraviolet frequencies.
The observed SED can be understood in terms of an irradiated accretion disc
with low mass accretion rate, 10^{-8} solar masses per year, surrounding a star
with an age of ~10 Myr. We infer that the mass of the disc is ~0.04 solar
masses, and is populated by dust grains that have grown to a maximum size of 1
mm everywhere, consistent with the lack of silicate emission at 10 microns.
These features, as well as indications of settling in the wall at the dust
destruction radius, led us to speculate the disc of HD169142 is in an advanced
stage of dust evolution, particularly in its inner regions.Comment: 13 pages, 2 figures. Accepted by MNRA
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