14 research outputs found
On the Radii of Brown Dwarfs Measured with AKARI Near-Infrared Spectroscopy
We derive the radii of 16 brown dwarfs observed by AKARI using their
parallaxes and the ratios of observed to model fluxes. We find that the brown
dwarf radius ranges between 0.64-1.13 RJ with an average radius of 0.83 RJ. We
find a trend in the relation between radii and Teff; the radius is at a minimum
at Teff~1600 K, which corresponds to the spectral types of mid-to late-L. The
result is interpreted by a combination of radius-mass and radius-age relations
that are theoretically expected for brown dwarfs older than 10^8 yr.Comment: 22 pages, 3 figures; submitted 2012 October 14 and accepted 2013
February 12 for publication to The Astrophysical Journa
Akari Observations of Brown Dwarfs. II CO2 as Probe of Carbon and Oxygen Abundances in Brown Dwarfs
Recent observations with the infrared astronomical satellite AKARI have shown
that the CO2 bands at 4.2 micron in three brown dwarfs are much stronger than
expected from the unified cloudy model (UCM) based on recent solar C & O
abundances. This result has been a puzzle, but we now find that this is simply
an abundance effect: We show that these strong CO2 bands can be explained with
the UCMs based on the classical C & O abundances (log Ac and log Ao), which are
about 0.2 dex larger compared to the recent values. Since three other brown
dwarfs could be well interpreted with the recent solar C & O abundances, we
require at least two model sequences based on the different chemical
compositions to interpret all the AKARI spectra. The reason for this is that
the CO2 band is especially sensitive to C & O abundances, since the CO2
abundance depends approximately on AcAo^2 --- the cube of C & O abundances. For
this reason, even low resolution spectra of very cool dwarfs, especially of CO2
cannot be understood unless a model with proper abundances is applied. For the
same reason, CO2 is an excellent indicator of C & O abundances, and we can now
estimate C & O abundances of brown dwarfs: Three out of six brown dwarfs
observed with AKARI should have high C & O abundances similar to the classical
solar values (e.g. logAc = 8.60 and logAo = 8.92), but the other three may have
low C & O abundances similar to the recent solar values (e.g. logAc = 8.39 and
logAo = 8.69). This result implies that three out of six brown dwarfs are
highly metal rich relative to the Sun if the recent solar C & O abundances are
correct.Comment: 12 pages, 6 figures, To appear in ApJ June 20 issu
Deep Thermal Infrared Imaging of HR 8799 bcde: New Atmospheric Constraints and Limits on a Fifth Planet
We present new (3.8 ) and Br- (4.05 ) data
and reprocessed archival data for the young, planet-hosting star HR
8799 obtained with Keck/NIRC2, VLT/NaCo and Subaru/IRCS. We detect all four HR
8799 planets in each dataset at a moderate to high signal-to-noise (SNR
6-15). We fail to identify a fifth planet, "HR 8799 f", at 15
at a 5- confidence level: one suggestive, marginally significant
residual at 0.2" is most likely a PSF artifact. Assuming companion ages of 30
and the Baraffe (Spiegel \& Burrows) planet cooling models, we rule out
an HR 8799 f with mass of 5 (7 ), 7 (10 ), and 12
(13 ) at 12 , 9 , and 5 ,
respectively. All four HR 8799 planets have red early T dwarf-like -
[4.05] colors, suggesting that their SEDs peak in between the and
broadband filters. We find no statistically significant difference
in HR 8799 cde's colors. Atmosphere models assuming thick, patchy clouds appear
to better match HR 8799 bcde's photometry than models assuming a uniform cloud
layer. While non-equilibrium carbon chemistry is required to explain HR 8799
bc's photometry/spectra, evidence for it from HR 8799 de's photometry is
weaker. Future, deep IR spectroscopy/spectrophotometry with the Gemini Planet
Imager, SCExAO/CHARIS, and other facilities may clarify whether the planets are
chemically similar or heterogeneous.Comment: 18 pages, 6 Tables, and 9 Figures. Fig. 1a is the key figure.
Accepted for publication in Ap
CHARIS Science: Performance Simulations for the Subaru Telescope's Third-Generation of Exoplanet Imaging Instrumentation
We describe the expected scientific capabilities of CHARIS, a high-contrast
integral-field spectrograph (IFS) currently under construction for the Subaru
telescope. CHARIS is part of a new generation of instruments, enabled by
extreme adaptive optics (AO) systems (including SCExAO at Subaru), that promise
greatly improved contrasts at small angular separation thanks to their ability
to use spectral information to distinguish planets from quasistatic speckles in
the stellar point-spread function (PSF). CHARIS is similar in concept to GPI
and SPHERE, on Gemini South and the Very Large Telescope, respectively, but
will be unique in its ability to simultaneously cover the entire near-infrared
, , and bands with a low-resolution mode. This extraordinarily broad
wavelength coverage will enable spectral differential imaging down to angular
separations of a few , corresponding to 0.\!\!''1. SCExAO
will also offer contrast approaching at similar separations,
0.\!\!''1--0.\!\!''2. The discovery yield of a CHARIS survey will
depend on the exoplanet distribution function at around 10 AU. If the
distribution of planets discovered by radial velocity surveys extends unchanged
to 20 AU, observations of 200 mostly young, nearby stars targeted
by existing high-contrast instruments might find 1--3 planets. Carefully
optimizing the target sample could improve this yield by a factor of a few,
while an upturn in frequency at a few AU could also increase the number of
detections. CHARIS, with a higher spectral resolution mode of , will
also be among the best instruments to characterize planets and brown dwarfs
like HR 8799 cde and And b.Comment: 13 pages, 7 figures, proceedings from SPIE Montrea
A DATA-DRIVEN APPROACH FOR RETRIEVING TEMPERATURES AND ABUNDANCES IN BROWN DWARF ATMOSPHERES
Brown dwarf spectra contain a wealth of information about their molecular
abundances, temperature structure, and gravity. We present a new data driven
retrieval approach, previously used in planetary atmosphere studies, to extract
the molecular abundances and temperature structure from brown dwarf spectra.
The feasibility of the approach is first demonstrated on a synthetic brown
dwarf spectrum. Given typical spectral resolutions, wavelength coverage, and
noise properties precisions of tens of percent can be obtained for the
molecular abundances and 10s-100s K on the temperature profile. The technique
is then applied to the well studied brown dwarf, Gl 570D. From this spectral
retrieval the spectroscopic radius is constrained to be 0.75 - 0.83
, to be 5.13 - 5.46 and to be
between 804 and 849 K. Estimates for the range of abundances and allowed
temperature profiles are also derived. The results from our retrieval approach
are in agreement with the self-consistent grid modeling results of Saumon et al
(2006). This new approach will allow us to address issues of compositional
differences between brown dwarfs and possibly their formation environments,
disequilibrium chemistry, missing physics in current grid modeling approaches
as well as a many other issues.Comment: Accepted to ApJ Methods discussion expanded, conclusions unchange