163 research outputs found
Proper motion of very low mass stars and brown dwarfs in the Pleiades cluster
We report proper motion measurements for 25 very-low mass (VLM) star and
brown dwarf (BD) candidates of the Pleiades cluster previously identified by
Bouvier et al. (1998). Proper motions are measured with an accuracy of 9
mas/yr, compared to an expected tangential motion of about 50 mas/yr for
Pleiades members. Of the 25 candidates, 15 have a membership probability of 95%
or more and 7 are rejected as being field dwarfs. The 3 remaining candidates
exhibit independent evidence for membership (lithium absorption or long-term
proper motion). From the firm identification of Pleiades VLM and BD members,
the cluster's substellar mass function is revised to dN/dM \propto M^{-0.5} in
the mass range from 0.04 to 0.3 M_solar.Comment: 7 pages, 2 tables, 2 figures, accepted by A&
Modeling the Formation of Giant Planet Cores I: Evaluating Key Processes
One of the most challenging problems we face in our understanding of planet
formation is how Jupiter and Saturn could have formed before the the solar
nebula dispersed. The most popular model of giant planet formation is the
so-called 'core accretion' model. In this model a large planetary embryo formed
first, mainly by two-body accretion. This is then followed by a period of
inflow of nebular gas directly onto the growing planet. The core accretion
model has an Achilles heel, namely the very first step. We have undertaken the
most comprehensive study of this process to date. In this study we numerically
integrate the orbits of a number of planetary embryos embedded in a swarm of
planetesimals. In these experiments we have included: 1) aerodynamic gas drag,
2) collisional damping between planetesimals, 3) enhanced embryo cross-sections
due to their atmospheres, 4) planetesimal fragmentation, and 5) planetesimal
driven migration. We find that the gravitational interaction between the
embryos and the planetesimals lead to the wholesale redistribution of material
- regions are cleared of material and gaps open near the embryos. Indeed, in
90% of our simulations without fragmentation, the region near that embryos is
cleared of planetesimals before much growth can occur. The remaining 10%,
however, the embryos undergo a burst of outward migration that significantly
increases growth. On timescales of ~100,000 years, the outer embryo can migrate
~6 AU and grow to roughly 30 Earth-masses. We also find that the inclusion of
planetesimal fragmentation tends to inhibit growth.Comment: Accepted to AJ, 62 pages 11 figure
A Survey for Massive Giant Planets in Debris Disks with Evacuated Inner Cavities
The commonality of collisionally replenished debris around main sequence
stars suggests that minor bodies are frequent around Sun-like stars. Whether or
not debris disks in general are accompanied by planets is yet unknown, but
debris disks with large inner cavities - perhaps dynamically cleared - are
considered to be prime candidates for hosting large-separation massive giant
planets. We present here a high-contrast VLT/NACO angular differential imaging
survey for eight such cold debris disks. We investigated the presence of
massive giant planets in the range of orbital radii where the inner edge of the
dust debris is expected. Our observations are sensitive to planets and brown
dwarfs with masses >3 to 7 Jupiter mass, depending on the age and distance of
the target star. Our observations did not identify any planet candidates. We
compare the derived planet mass upper limits to the minimum planet mass
required to dynamically clear the inner disks. While we cannot exclude that
single giant planets are responsible for clearing out the inner debris disks,
our observations constrain the parameter space available for such planets. The
non-detection of massive planets in these evacuated debris disks further
reinforces the notion that the giant planet population is confined to the inner
disk (<15 AU).Comment: Accepted for publication in Ap
ASTRA: ASTrometry and phase-Referencing Astronomy on the Keck interferometer
ASTRA (ASTrometric and phase-Referencing Astronomy) is an upgrade to the
existing Keck Interferometer which aims at providing new self-phase referencing
(high spectral resolution observation of YSOs), dual-field phase referencing
(sensitive AGN observations), and astrometric (known exoplanetary systems
characterization and galactic center general relativity in strong field regime)
capabilities. With the first high spectral resolution mode now offered to the
community, this contribution focuses on the progress of the dual field and
astrometric modes.Comment: 10 pages, 6 figures, 2 tables, SPIE 201
Employing a New BVIc Photometric Survey of IC 4665 to Investigate the Age of this Young Open Cluster
We present a new, BVIc photometric survey of the young open cluster IC4665,
which improves on previous studies of this young cluster by incorporating a
rigorous standardization procedure, thus providing high-fidelity colors and
magnitudes for cluster members. We use this new photometric dataset to
reevaluate the properties (age and distance) of IC4665. Namely, using a
statistical approach incorporating Tau^2 CMD modeling, we measure a
pre-main-sequence isochrone age and distance of 36+-9 Myr and 360+-12 pc, as
well as a upper-main-sequence turn-off age and distance of 42+-12 Myr and
357+-12 pc. These ages and distances are highly dependent on the isochrone
model and color used for the fitting procedure, with a possible range of ~10-20
Myr in age and ~20 pc in distance. This spread in calculated ages and distances
seen between colors and models is likely due to limitations in the individual
membership catalogs and/or systematic differences in the predicted stellar
parameters from the different sets of models. Interestingly, when we compare
the isochrone ages for IC4665 to the published lithium depletion boundary age,
28+-5 Myr, we observe that this cluster does not appear to follow the trend of
isochrone ages being 1.5 times smaller than lithium depletion boundary ages. In
addition, comparing the overall magnetic activity (X-ray and H-alpha emission)
in IC4665 with other well studied open clusters, we find the observed activity
distributions for this young cluster are best characterized by assuming an age
of 30-40 Myr, thus in agreement with our pre-main-sequence and turn-off
isochrone ages for IC4665. Overall, although some age discrepancies do exist,
particularly in the ages measured from pre-main-sequence isochrones, the range
of possible IC4665 ages derived from the various dating techniques employed
here is relatively small compared to that found for other well studied open
clusters.Comment: 17 pages, 13 figures, 6 tables; Accepted for publication in the
Astronomical Journa
Are Debris Disks and Massive Planets Correlated?
Using data from the Spitzer Space Telescope Legacy Science Program
``Formation and Evolution of Planetary Systems'' (FEPS), we have searched for
debris disks around 9 FGK stars (2-10 Gyr), known from radial velocity (RV)
studies to have one or more massive planets. Only one of the sources, HD 38529,
has excess emission above the stellar photosphere; at 70 micron the
signal-to-noise ratio in the excess is 4.7 while at wavelengths < 30 micron
there is no evidence of excess. The remaining sources show no excesses at any
Spitzer wavelengths. Applying survival tests to the FEPS sample and the results
for the FGK survey published in Bryden et al. (2006), we do not find a
significant correlation between the frequency and properties of debris disks
and the presence of close-in planets. We discuss possible reasons for the lack
of a correlation.Comment: 24 pages, 3 figures. Accepted to Astrophysical Journa
Clinical efficacy of omalizumab in chronic spontaneous urticaria is associated with a reduction of FcεRI-positive cells in the skin
Background. Treatment with omalizumab, a humanized recombinant monoclonal
anti-IgE antibody, results in clinical efficacy in patients with Chronic
Spontaneous Urticaria (CSU). The mechanism of action of omalizumab in CSU has
not been elucidated in detail. Objectives. To determine the effects of
omalizumab on levels of high affinity IgE receptor-positive (FcεRI+) and IgE-
positive (IgE+) dermal cells and blood basophils. Treatment efficacy and
safety were also assessed. Study design. In a double-blind study, CSU patients
aged 18‑75 years were randomized to receive 300 mg omalizumab (n=20) or
placebo (n=10) subcutaneously every 4 weeks for 12 weeks. Changes in disease
activity were assessed by use of the weekly Urticaria Activity Score (UAS7).
Circulating IgE levels, basophil numbers and levels of expression of FcεRI+
and IgE+ cells in the skin and in blood basophils were determined. Results.
Patients receiving omalizumab showed a significantly greater decrease in UAS7
compared with patients receiving placebo. At Week 12 the mean difference in
UAS7 between treatment groups was -14.82 (p=0.0027), consistent with previous
studies. Total IgE levels in serum were increased after omalizumab treatment
and remained elevated up to Week 12. Free IgE levels decreased after
omalizumab treatment. Mean levels of FcεRI+ skin cells in patients treated
with omalizumab 300 mg were decreased at Week 12 compared with baseline in the
dermis of both non-lesional and lesional skin, reaching levels comparable with
those seen in healthy volunteers (HVs). There were no statistically
significant changes in mean FcɛRI+ cell levels in the placebo group. Similar
results were seen for changes in IgE+ cells, although the changes were not
statistically significant. The level of peripheral blood basophils increased
immediately after treatment start and returned to Baseline values after the
follow-up period. The levels of FcεRI and IgE expression on peripheral blood
basophils were rapidly reduced by omalizumab treatment up to Week 12.
Conclusions. Treatment with omalizumab resulted in rapid clinical benefits in
patients with CSU. Treatment with omalizumab was associated with reduction in
FcɛRI+ and IgE+ basophils and intradermal cells
Evolution of Mid-IR Excess Around Sun-like Stars: Constraints on Models of Terrestrial Planet Formation
We report observations from the Spitzer Space Telescope (SST) regarding the
frequency of 24 micron excess emission toward sun-like stars. Our unbiased
sample is comprised of 309 stars with masses 0.7-2.2 Msun and ages from <3 Myr
to >3 Gyr that lack excess emission at wavelengths <=8 microns. We identify 30
stars that exhibit clear evidence of excess emission from the observed 24/8
micron flux ratio. The implied 24 micron excesses of these candidate debris
disk systems range from 13 % (the minimum detectable) to more than 100 %
compared to the expected photospheric emission. The frequency of systems with
evidence for dust debris emitting at 24 micron ranges from 8.5-19 % at ages
<300 Myr to < 4 % for older stars. The results suggest that many, perhaps most,
sun-like stars might form terrestrial planets.Comment: Accepted for publication in the Astrophysical Journal Letter
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