292 research outputs found
A Physical Model-based Correction for Charge Traps in the Hubble Space Telescope's Wide Field Camera 3 Near-IR Detector and Applications to Transiting Exoplanets and Brown Dwarfs
The Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) near-IR channel
is extensively used in time-resolved observations, especially for transiting
exoplanet spectroscopy and brown dwarf and directly imaged exoplanet rotational
phase mapping. The ramp effect is the dominant source of systematics in the
WFC3 for time-resolved observations, which limits its photometric precision.
Current mitigation strategies are based on empirical fits and require
additional orbits "to help the telescope reach a thermal equilibrium". We show
that the ramp effect profiles can be explained and corrected with high fidelity
using charge trapping theories. We also present a model for this process that
can be used to predict and to correct charge trap systematics. Our model is
based on a very small number of parameters that are intrinsic to the detector.
We find that these parameters are very stable between the different datasets,
and we provide best-fit values. Our model is tested with more than 120 orbits
( visits) of WFC3 observations and is proved to be able to provide near
photon noise limited corrections for observations made with both staring and
scanning modes of transiting exoplanets as well as for starting-mode
observations of brown dwarfs. After our model correction, the light curve of
the first orbit in each visit has the same photometric precision as subsequent
orbits, so data from the first orbit need no longer be discarded. Near IR
arrays with the same physical characteristics (e.g., JWST/NIRCam) may also
benefit from the extension of this model, if similar systematic profiles are
observed.Comment: 16 pages, 13 figures, accepted to Astronomical Journa
Weather on Other Worlds. IV. H emission and photometric variability are not correlated in L0T8 dwarfs
Recent photometric studies have revealed that surface spots that produce flux
variations are present on virtually all L and T dwarfs. Their likely magnetic
or dusty nature has been a much-debated problem, the resolution to which has
been hindered by paucity of diagnostic multi-wavelength observations. To test
for a correlation between magnetic activity and photometric variability, we
searched for H emission among eight L3T2 ultra-cool dwarfs with
extensive previous photometric monitoring, some of which are known to be
variable at 3.6 m or 4.5 m. We detected H only in the
non-variable T2 dwarf 2MASS J125453930122474. The remaining seven objects do
not show H emission, even though six of them are known to vary
photometrically. Combining our results with those for 86 other L and T dwarfs
from the literature show that the detection rate of H emission is very
high (94) for spectral types between L0 and L3.5 and much smaller (20)
for spectral types L4, while the detection rate of photometric variability
is approximately constant (3055) from L0 to T8 dwarfs. We conclude
that chromospheric activity, as evidenced by H emission, and
large-amplitude photometric variability are not correlated. Consequently, dust
clouds are the dominant driver of the observed variability of ultra-cool dwarfs
at spectral types at least as early as L0.Comment: 12 pages, 4 figures, accepted for publication in Ap
The Onset of Planet Formation in Brown Dwarf Disks
The onset of planet formation in protoplanetary disks is marked by the growth
and crystallization of sub-micron-sized dust grains accompanied by dust
settling toward the disk mid-plane. Here we present infrared spectra of disks
around brown dwarfs and brown dwarf candidates. We show that all three
processes occur in such cool disks in a way similar or identical to that in
disks around low- and intermediate-mass stars. These results indicate that the
onset of planet formation extends to disks around brown dwarfs, suggesting that
planet formation is a robust process occurring in most young circumstellar
disks.Comment: Published in Science 2005, vol 310, 834; 3 pages in final format, 4
figures + 8 pages Supporting Online Material. For final typeset, see
http://www.sciencemag.org/cgi/content/abstract/310/5749/834?eto
Earths in Other Solar Systems N-body simulations: the Role of Orbital Damping in Reproducing the Kepler Planetary Systems
The population of exoplanetary systems detected by Kepler provides
opportunities to refine our understanding of planet formation. Unraveling the
conditions needed to produce the observed exoplanets will sallow us to make
informed predictions as to where habitable worlds exist within the galaxy. In
this paper, we examine using N-body simulations how the properties of planetary
systems are determined during the final stages of assembly. While accretion is
a chaotic process, trends in the ensemble properties of planetary systems
provide a memory of the initial distribution of solid mass around a star prior
to accretion. We also use EPOS, the Exoplanet Population Observation Simulator,
to account for detection biases and show that different accretion scenarios can
be distinguished from observations of the Kepler systems. We show that the
period of the innermost planet, the ratio of orbital periods of adjacent
planets, and masses of the planets are determined by the total mass and radial
distribution of embryos and planetesimals at the beginning of accretion. In
general, some amount of orbital damping, either via planetesimals or gas,
during accretion is needed to match the whole population of exoplanets.
Surprisingly, all simulated planetary systems have planets that are similar in
size, showing that the "peas in a pod" pattern can be consistent with both a
giant impact scenario and a planet migration scenario. The inclusion of
material at distances larger than what Kepler observes has a profound impact on
the observed planetary architectures, and thus on the formation and delivery of
volatiles to possible habitable worlds.Comment: Resubmitted to ApJ. Planet formation models available online at
http://eos-nexus.org/genesis-database
The First Detailed Look at a Brown Dwarf Disk
The combination of mid-infrared and recent submm/mm measurements allows us to
set up the first comprehensive spectral energy distribution (SED) of the
circumstellar material around a young Brown Dwarf. Simple arguments suggest
that the dust is distributed in the form of a disk. We compare basic models to
explore the disk parameters. The modeling shows that a flat disk geometry fits
well the observations. A flared disk explains the SED only if it has a
puffed-up inner rim and an inner gap much larger than the dust sublimation
radius. Similarities and differences with disks around T Tauri stars are
discussed.Comment: 11 pages, 1 figur
Crystalline silicates as a probe of disk formation history
We present a new perspective on the crystallinity of dust in protoplanetary
disks. The dominant crystallization by thermal annealing happens in the very
early phases of disk formation and evolution. Both the disk properties and the
level of crystallinity are thereby directly linked to the properties of the
molecular cloud core from which the star+disk system was formed. We show that,
under the assumption of single star formation, rapidly rotating clouds produce
disks which, after the main infall phase (i.e. in the optically revealed class
II phase), are rather massive and have a high accretion rate but low
crystallinity. Slowly rotating clouds, on the other hand, produce less massive
disks with lower accretion rate, but high levels of crystallinity. Cloud
fragmentation and the formation of multiple stars complicates the problem and
necessitates further study. The underlying physics of the model is
insufficiently understood to provide the precise relationship between
crystallinity, disk mass and accretion rate. But the fact that with `standard'
input physics the model produces disks which, in comparison to observations,
appear to have either too high levels of crystallinity or too high disk masses,
demonstrates that the comparison of these models to observations can place
strong contraints on the disk physics. The question to ask is not why some
sources are so crystalline, but why some other sources have such a low level of
crystallinity.Comment: Accepted for publication in ApJ
Evolution of Young Brown Dwarf Disks in the Mid-Infrared
We have imaged two bona-fide brown dwarfs with TReCS/GEMINI-S and find
mid-infrared excess emission that can be explained by optically thick dust disk
models. In the case of the young (2Myr) Cha H1 we measure
fluxes at 10.4m and 12.3m that are fully consistent with a standard
flared disk model and prominent silicate emission. For the 10Myr old
brown dwarf 2MASS1207-3932 located in the TW Hydrae association we find excess
emission at 8.7m and 10.4m with respect to its photosphere, and
confirm disk accretion as likely cause of its strong activity. Disks around
brown dwarfs likely last at least as long as their low-mass stellar
counterparts in the T-Tauri phase. Grain growth, dust settling, and evolution
of the geometry of brown dwarfs disks may appear on a timescale of 10Myr and
can be witnessed by observations in the mid-infrared.Comment: 6 pages, 4 figure
Stellar-Mass-Dependent Disk Structure in Coeval Planet-Forming Disks
Previous studies suggest that the planet-forming disks around very-low-mass
stars/brown dwarfs may be flatter than those around more massive stars, in
contrast to model predictions of larger scale heights for gas-disks around
lower-mass stars. We conducted a statistically robust study to determine
whether there is evidence for stellar-mass-dependent disk structure in
planet-forming disks. We find a statistically significant difference in the
Spitzer/IRAC color distributions of disks around very-low-mass and low-mass
stars all belonging to the same star-forming region, the Chamaeleon I
star-forming region. We show that self consistently calculated disk models
cannot fit the median spectral energy distributions (SEDs) of the two groups.
These SEDs can be only explained by flatter disk models, consistent with the
effect of dust settling in disks. We find that relative to the disk structure
predicted for flared disks the required reduction in disk scale height is
anti-correlated with the stellar mass, i.e. disks around lower-mass stars are
flatter. Our results show that the initial and boundary conditions of planet
formation are stellar-mass-dependent, an important finding that must be
considered in planet formation models.Comment: Astrophysical Journal, in pres
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