700 research outputs found
Spectral Variability from the Patchy Atmospheres of T and Y Dwarfs
Brown dwarfs of a variety of spectral types have been observed to be
photometrically variable. Previous studies have focused on objects at the L/T
transition, where the iron and silicate clouds in L dwarfs break up or
dissipate. However, objects outside of this transitional effective temperature
regime also exhibit variability. Here, we present models for mid-late T dwarfs
and Y dwarfs. We present models that include patchy salt and sulfide clouds as
well as water clouds for the Y dwarfs. We find that for objects over 375 K,
patchy cloud opacity would generate the largest amplitude variability within
near-infrared spectral windows. For objects under 375 K, water clouds also
become important and generate larger amplitude variability in the mid-infrared.
We also present models in which we perturb the temperature structure at
different pressure levels of the atmosphere to simulate hot spots. These models
show the most variability in the absorption features between spectral windows.
The variability is strongest at wavelengths that probe pressure levels at which
the heating is the strongest. The most illustrative types of observations for
understanding the physical processes underlying brown dwarf variability are
simultaneous, multi-wavelength observations that probe both inside and outside
of molecular absorption features.Comment: 6 pages, 5 figures, Accepted for publication in ApJ Letter
Detecting Water In the atmosphere of HR 8799 c with L-band High Dispersion Spectroscopy Aided By Adaptive Optics
High dispersion spectroscopy of brown dwarfs and exoplanets enables exciting
science cases, e.g., mapping surface inhomogeneity and measuring spin rate.
Here, we present band observations of HR 8799 c using Keck NIRSPEC
(R=15,000) in adaptive optics (AO) mode (NIRSPAO). We search for molecular
species (HO and CH) in the atmosphere of HR 8799 c with a template
matching method, which involves cross correlation between reduced spectrum and
a template spectrum. We detect HO but not CH, which suggests
disequilibrium chemistry in the atmosphere of HR 8799 c, and this is consistent
with previous findings. We conduct planet signal injection simulations to
estimate the sensitivity of our AO-aided high dispersion spectroscopy
observations. We conclude that contrast can be reached in band.
The sensitivity is mainly limited by the accuracy of line list used in modeling
spectra and detector noise. The latter will be alleviated by the NIRSPEC
upgrade.Comment: 14 pages, 5 figures, 5 tables, accepted for publication on AJ,
references update
A Comparison of Near-Infrared Photometry and Spectra for Y Dwarfs with a New Generation of Cool Cloudy Models
We present YJHK photometry, or a subset, for the six Y dwarfs discovered in
WISE data by Cushing et al.. The data were obtained using NIRI on the Gemini
North telescope. We also present a far-red spectrum obtained using GMOS-North
for WISEPC J205628.90+145953.3. We compare the data to Morley et al. (2012)
models, which include cloud decks of sulfide and chloride condensates. We find
that the models with these previously neglected clouds can reproduce the energy
distributions of T9 to Y0 dwarfs quite well, other than near 5um where the
models are too bright. This is thought to be because the models do not include
departures from chemical equilibrium caused by vertical mixing, which would
enhance the abundance of CO, decreasing the flux at 5um. Vertical mixing also
decreases the abundance of NH_3, which would otherwise have strong absorption
features at 1.03um and 1.52um that are not seen in the Y0 WISEPC
J205628.90+145953.3. We find that the five Y0 to Y0.5 dwarfs have 300 < T_eff K
< 450, 4.0 < log g < 4.5 and f_sed ~ 3. These temperatures and gravities imply
a mass range of 5 - 15 M_Jupiter and ages around 5 Gyr. We suggest that WISEP
J182831.08+265037.8 is a binary system, as this better explains its luminosity
and color. We find that the data can be made consistent with observed trends,
and generally consistent with the models, if the system is composed of a T_eff
= 325 K and log g ~ 4.0
secondary, corresponding to masses of 10 and 7 M_Jupiter and an age around 2
Gyr. If our deconvolution is correct, then the T_eff = 300 K cloud-free model
fluxes at K and W2 are too faint by 0.5 - 1.0 magnitudes. We will address this
discrepancy in our next generation of models, which will incorporate water
clouds and mixing.Comment: 39 pages, 10 Figures, 8 Tables. Accepted by ApJ. This revision
replaces Figures 9 and 10 with B & W versions, corrects figure captions for
color online only, corrects references. Text is unchanged. Tables 3, 4 and 8
are available at http://www.gemini.edu/staff/sleggett, other model data are
available at http://www.ucolick.org/~cmorley/cmorley/Data.htm
Water Clouds in Y Dwarfs and Exoplanets
The formation of clouds affects brown dwarf and planetary atmospheres of
nearly all effective temperatures. Iron and silicate condense in L dwarf
atmospheres and dissipate at the L/T transition. Minor species such as sulfides
and salts condense in mid-late T dwarfs. For brown dwarfs below Teff=450 K,
water condenses in the upper atmosphere to form ice clouds. Currently over a
dozen objects in this temperature range have been discovered, and few previous
theoretical studies have addressed the effect of water clouds on brown dwarf or
exoplanetary spectra. Here we present a new grid of models that include the
effect of water cloud opacity. We find that they become optically thick in
objects below Teff=350-375 K. Unlike refractory cloud materials, water ice
particles are significantly non-gray absorbers; they predominantly scatter at
optical wavelengths through J band and absorb in the infrared with prominent
features, the strongest of which is at 2.8 microns. H2O, NH3, CH4, and H2 CIA
are dominant opacity sources; less abundant species such as may also be
detectable, including the alkalis, H2S, and PH3. PH3, which has been detected
in Jupiter, is expected to have a strong signature in the mid-infrared at 4.3
microns in Y dwarfs around Teff=450 K; if disequilibrium chemistry increases
the abundance of PH3, it may be detectable over a wider effective temperature
range than models predict. We show results incorporating disequilibrium
nitrogen and carbon chemistry and predict signatures of low gravity in
planetary- mass objects. Lastly, we make predictions for the observability of Y
dwarfs and planets with existing and future instruments including the James
Webb Space Telescope and Gemini Planet Imager.Comment: 23 pages, 20 figures, Revised for Ap
Thermal Emission and Albedo Spectra of Super Earths with Flat Transmission Spectra
Planets larger than Earth and smaller than Neptune are some of the most
numerous in the galaxy, but observational efforts to understand this population
have proved challenging because optically thick clouds or hazes at high
altitudes obscure molecular features (Kreidberg et al. 2014b). We present
models of super Earths that include thick clouds and hazes and predict their
transmission, thermal emission, and reflected light spectra. Very thick, lofted
clouds of salts or sulfides in high metallicity (1000x solar) atmospheres
create featureless transmission spectra in the near-infrared. Photochemical
hazes with a range of particle sizes also create featureless transmission
spectra at lower metallicities. Cloudy thermal emission spectra have muted
features more like blackbodies, and hazy thermal emission spectra have emission
features caused by an inversion layer at altitudes where the haze forms. Close
analysis of reflected light from warm (~400-800 K) planets can distinguish
cloudy spectra, which have moderate albedos (0.05-0.20), from hazy models,
which are very dark (0.0-0.03). Reflected light spectra of cold planets (~200
K) accessible to a space-based visible light coronagraph will have high albedos
and large molecular features that will allow them to be more easily
characterized than the warmer transiting planets. We suggest a number of
complementary observations to characterize this population of planets,
including transmission spectra of hot (>1000 K) targets, thermal emission
spectra of warm targets using the James Webb Space Telescope (JWST), high
spectral resolution (R~10^5) observations of cloudy targets, and reflected
light spectral observations of directly-imaged cold targets. Despite the dearth
of features observed in super Earth transmission spectra to date, different
observations will provide rich diagnostics of their atmospheres.Comment: 23 pages, 23 figures. Revised for publication in The Astrophysical
Journa
Forward and Inverse Modeling of the Emission and Transmission Spectrum of GJ 436b: Investigating Metal Enrichment, Tidal Heating, and Clouds
The Neptune-mass GJ 436b is one of the most-studied transiting exoplanets
with repeated measurements of both its thermal emission and transmission
spectra. We build on previous studies to answer outstanding questions about
this planet, including its potentially high metallicity and tidal heating of
its interior. We present new observations of GJ 436b's thermal emission at 3.6
and 4.5 micron, which reduce uncertainties in estimates of GJ 436b's flux at
those wavelengths and demonstrate consistency between Spitzer observations
spanning more than 7 years. We analyze the Spitzer thermal emission photometry
and Hubble WFC3 transmission spectrum in tandem. We use a powerful dual-pronged
modeling approach, comparing these data to both self-consistent and retrieval
models. We vary the metallicity, intrinsic luminosity from tidal heating,
disequilibrium chemistry, and heat redistribution. We also study the effect of
clouds and photochemical hazes on the spectra, but do not find strong evidence
for either. The self-consistent and retrieval modeling combine to suggest that
GJ 436b has a high atmospheric metallicity, with best fits at or above several
hundred times solar metallicity, tidal heating warming its interior with
best-fit intrinsic effective effective temperatures around 300--350 K, and
disequilibrium chemistry. High metal-enrichments (>600x solar) can only occur
from the accretion of rocky, rather than icy, material. Assuming Tint~300--350
K, we find that Q'~2x10^5--10^6, larger than Neptune's Q', and implying a long
tidal circularization timescale for the planet's orbit. We suggest that
Neptune-mass planets may be a more diverse class than previously imagined, with
metal-enhancements potentially spanning several orders of magnitude, to perhaps
over 1000x solar metallicity. High fidelity observations with instruments like
JWST will be critical for characterizing this diversity.Comment: 15 pages, 18 figures. Revised for publication in Ap
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