63 research outputs found
Modeling the Formation of Clouds in Brown Dwarf Atmospheres
Because the opacity of clouds in substellar mass object (SMO) atmospheres
depends on the composition and distribution of particle sizes within the cloud,
a credible cloud model is essential for accurately modeling SMO spectra and
colors. We present a one--dimensional model of cloud particle formation and
subsequent growth based on a consideration of basic cloud microphysics. We
apply this microphysical cloud model to a set of synthetic brown dwarf
atmospheres spanning a broad range of surface gravities and effective
temperatures (g_surf = 1.78 * 10^3 -- 3 * 10^5 cm/s^2 and T_eff = 600 -- 1600
K) to obtain plausible particle sizes for several abundant species (Fe,
Mg2SiO4, and Ca2Al2SiO7). At the base of the clouds, where the particles are
largest, the particle sizes thus computed range from ~5 microns to over 300
microns in radius over the full range of atmospheric conditions considered. We
show that average particle sizes decrease significantly with increasing brown
dwarf surface gravity. We also find that brown dwarfs with higher effective
temperatures have characteristically larger cloud particles than those with
lower effective temperatures. We therefore conclude that it is unrealistic when
modeling SMO spectra to apply a single particle size distribution to the entire
class of objects.Comment: 25 pages; 8 figures. We have added considerable detail describing the
physics of the cloud model. We have also added discussions of the issues of
rainout and the self-consistent coupling of clouds with brown dwarf
atmospheric models. We have updated figures 1, 3, and 4 with new vertical
axis labels and new particle sizes for forsterite and gehlenite. Accepted to
the Astrophysical Journal, Dec. 2, 200
L and T Dwarf Models and the L to T Transition
Using a model for refractory clouds, a novel algorithm for handling them, and
the latest gas-phase molecular opacities, we have produced a new series of L
and T dwarf spectral and atmosphere models as a function of gravity and
metallicity, spanning the \teff range from 2200 K to 700 K. The correspondence
with observed spectra and infrared colors for early- and mid-L dwarfs and for
mid- to late-T dwarfs is good. We find that the width in infrared
color-magnitude diagrams of both the T and L dwarf branches is naturally
explained by reasonable variations in gravity and, therefore, that gravity is
the "second parameter" of the L/T dwarf sequence. We investigate the dependence
of theoretical dwarf spectra and color-magnitude diagrams upon various cloud
properties, such as particle size and cloud spatial distribution. In the region
of the LT transition, we find that no one cloud-particle-size and gravity
combination can be made to fit all the observed data. Furthermore, we note that
the new, lower solar oxygen abundances of Allende-Prieto, Lambert, & Asplund
(2002) produce better fits to brown dwarf data than do the older values.
Finally, we discuss various issues in cloud physics and modeling and speculate
on how a better correspondence between theory and observation in the
problematic LT transition region might be achieved.Comment: accepted to the Astrophysical Journal, 21 figures (20 in color);
spectral models in electronic form available at
http://zenith.as.arizona.edu/~burrow
A Possible Bifurcation in Atmospheres of Strongly Irradiated Stars and Planets
We show that under certain circumstances the differences between the
absorption mean and Planck mean opacities can lead to multiple solutions for an
LTE atmospheric structure. Since the absorption and Planck mean opacities are
not expected to differ significantly in the usual case of radiative
equilibrium, non-irradiated atmospheres, the most interesting situations where
the effect may play a role are strongly irradiated stars and planets, and also
possibly structures where there is a significant deposition of mechanical
energy, such as stellar chromospheres and accretion disks. We have presented an
illustrative example of a strongly irradiated giant planet where the
bifurcation effect is predicted to occur for a certain range of distances from
the star.Comment: 22 pages, 6 figures, submitted to Ap
Spectra and Diagnostics for the Direct Detection of Wide-Separation Extrasolar Giant Planets
We calculate as a function of orbital distance, mass, and age the theoretical
spectra and orbit-averaged planet/star flux ratios for representative
wide-separation extrasolar giant planets (EGPs) in the optical, near-infrared,
and mid-infrared. Stellar irradiation of the planet's atmosphere and the
effects of water and ammonia clouds are incorporated and handled in a
consistent fashion. We include predictions for 12 specific known EGPs. In the
process, we derive physical diagnostics that can inform the direct EGP
detection and remote sensing programs now being planned or proposed.
Furthermore, we calculate the effects of irradiation on the spectra of a
representative companion brown dwarf as a function of orbital distance.Comment: submitted to the Astrophysical Journal, 19 pages, 11 color figure
Beyond the T Dwarfs: Theoretical Spectra, Colors, and Detectability of the Coolest Brown Dwarfs
We explore the spectral and atmospheric properties of brown dwarfs cooler
than the latest known T dwarfs. Our focus is on the yet-to-be-discovered
free-floating brown dwarfs in the \teff range from 800 K to 130 K
and with masses from 25 to 1 \mj. This study is in anticipation of the new
characterization capabilities enabled by the launch of SIRTF and the eventual
launch of JWST. We provide spectra from 0.4 \mic to 30 \mic, highlight
the evolution and mass dependence of the dominant HO, CH, and NH
molecular bands, consider the formation and effects of water-ice clouds, and
compare our theoretical flux densities with the sensitivities of the
instruments on board SIRTF and JWST. The latter can be used to determine the
detection ranges from space of cool brown dwarfs. In the process, we determine
the reversal point of the blueward trend in the near-infrared colors with
decreasing \teff, the \teffs at which water and ammonia clouds appear, the
strengths of gas-phase ammonia and methane bands, the masses and ages of the
objects for which the neutral alkali metal lines are muted, and the increasing
role as \teff decreases of the mid-infrared fluxes longward of 4 \mic. These
changes suggest physical reasons to expect the emergence of at least one new
stellar class beyond the T dwarfs. Our spectral models populate, with cooler
brown dwarfs having progressively more planet-like features, the theoretical
gap between the known T dwarfs and the known giant planets. Such objects likely
inhabit the galaxy, but their numbers are as yet unknown.Comment: Includes 14 figures, most in color; accepted to the Astrophysical
Journa
BIGRE: a low cross-talk integral field unit tailored for extrasolar planets imaging spectroscopy
Integral field spectroscopy (IFS) represents a powerful technique for the
detection and characterization of extrasolar planets through high contrast
imaging, since it allows to obtain simultaneously a large number of
monochromatic images. These can be used to calibrate and then to reduce the
impact of speckles, once their chromatic dependence is taken into account. The
main concern in designing integral field spectrographs for high contrast
imaging is the impact of the diffraction effects and the non-common path
aberrations together with an efficient use of the detector pixels. We focus our
attention on integral field spectrographs based on lenslet-arrays, discussing
the main features of these designs: the conditions of appropriate spatial and
spectral sampling of the resulting spectrograph's slit functions and their
related cross-talk terms when the system works at the diffraction limit. We
present a new scheme for the integral field unit (IFU) based on a dual-lenslet
device (BIGRE), that solves some of the problems related to the classical TIGER
design when used for such applications. We show that BIGRE provides much lower
cross-talk signals than TIGER, allowing a more efficient use of the detector
pixels and a considerable saving of the overall cost of a lenslet-based
integral field spectrograph.Comment: 17 pages, 18 figures, accepted for publication in Ap
Ks-band detection of thermal emission and color constraints to CoRoT-1b: A low-albedo planet with inefficient atmospheric energy redistribution and a temperature inversion
We report the detection in Ks-band of the secondary eclipse of the hot
Jupiter CoRoT-1b, from time series photometry with the ARC 3.5-m telescope at
Apache Point Observatory. The eclipse shows a depth of 0.336+/-0.042 percent
and is centered at phase 0.5022 (+0.0023,-0.0027), consistent with a zero
eccentricity orbit ecos{\omega} = 0.0035 (+0.0036,-0.0042). We perform the
first optical to near-infrared multi-band photometric analysis of an
exoplanet's atmosphere and constrain the reflected and thermal emissions by
combining our result with the recent 0.6, 0.71, and 2.09 micron secondary
eclipse detections by Snellen et al. (2009), Gillon et al. (2009), and Alonso
et al. (2009a). Comparing the multi-wavelength detections to state-of-the-art
radiative-convective chemical-equilibrium atmosphere models, we find the
near-infrared fluxes difficult to reproduce. The closest blackbody-based and
physical models provide the following atmosphere parameters: a temperature T =
2454 (+84,-170) K, a very low Bond albedo A_B = 0.000 (+0.087,-0.000), and an
energy redistribution parameter P_n = 0.1, indicating a small but nonzero
amount of heat transfer from the day- to night-side. The best physical model
suggests a thermal inversion layer with an extra optical absorber of opacity
kappa_e =0.05cm^2g^-1, placed near the 0.1-bar atmospheric pressure level. This
inversion layer is located ten times deeper in the atmosphere than the
absorbers used in models to fit mid-infrared Spitzer detections of other
irradiated hot Jupiters.Comment: accepted for publication on Ap
Association of Cumulative Lead Exposure with Parkinson's Disease
BACKGROUND. Research using reconstructed exposure histories has suggested an association between heavy metal exposures, including lead, and Parkinson's disease (PD), but the only study that used bone lead, a biomarker of cumulative lead exposure, found a nonsignificant increase in risk of PD with increasing bone lead. OBJECTIVES. We sought to assess the association between bone lead and PD. METHODS. Bone lead concentrations were measured using 109Cd excited K-shell X-ray fluorescence from 330 PD patients (216 men, 114 women) and 308 controls (172 men, 136 women) recruited from four clinics for movement disorders and general-community cohorts. Adjusted odds ratios (ORs) for PD were calculated using logistic regression. RESULTS. The average age of cases and controls at bone lead measurement was 67 (SD = 10) and 69 (SD = 9) years of age, respectively. In primary analyses of cases and controls recruited from the same groups, compared with the lowest quartile of tibia lead, the OR for PD in the highest quartile was 3.21 [95% confidence interval (CI), 1.17-8.83]. Results were similar but slightly weaker in analyses restricted to cases and controls recruited from the movement disorders clinics only (fourth-quartile OR = 2.57; 95% CI, 1.11-5.93) or when we included controls recruited from sites that did not also contribute cases (fourth-quartile OR = 1.91; 95% CI, 1.01-3.60). We found no association with patella bone lead. CONCLUSIONS. These findings, using an objective biological marker of cumulative lead exposure among typical PD patients seen in our movement disorders clinics, strengthen the evidence that cumulative exposure to lead increases the risk of PD.National Institutes of Health (R01-ES010798, K01-ES01265
Phase Functions and Light Curves of Wide Separation Extrasolar Giant Planets
We calculate self-consistent extrasolar giant planet (EGP) phase functions
and light curves for orbital distances ranging from 0.2 AU to 15 AU. We explore
the dependence on wavelength, cloud condensation, and Keplerian orbital
elements. We find that the light curves of EGPs depend strongly on wavelength,
the presence of clouds, and cloud particle sizes. Furthermore, the optical and
infrared colors of most EGPs are phase-dependent, tending to be reddest at
crescent phases in and . Assuming circular orbits, we find that at
optical wavelengths most EGPs are 3 to 4 times brighter near full phase than
near greatest elongation for highly-inclined (i.e., close to edge-on) orbits.
Furthermore, we show that the planet/star flux ratios depend strongly on the
Keplerian elements of the orbit, particularly inclination and eccentricity.
Given a sufficiently eccentric orbit, an EGP's atmosphere may make periodic
transitions from cloudy to cloud-free, an effect that may be reflected in the
shape and magnitude of the planet's light curve. Such elliptical orbits also
introduce an offset between the time of the planet's light curve maximum and
the time of full planetary phase, and for some sets of orbital parameters, this
light curve maximum can be a steeply increasing function of eccentricity. We
investigate the detectability of EGPs by proposed space-based direct-imaging
instruments.Comment: submitted to Astrophysical Journa
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