155 research outputs found
Model atmospheres of sub-stellar mass objects
We present an outline of basic assumptions and governing structural equations
describing atmospheres of substellar mass objects, in particular the extrasolar
giant planets and brown dwarfs. Although most of the presentation of the
physical and numerical background is generic, details of the implementation
pertain mostly to the code CoolTlusty. We also present a review of numerical
approaches and computer codes devised to solve the structural equations, and
make a critical evaluation of their efficiency and accuracy.Comment: 31 pages, 10 figure
A Systematic Study of Departures from Chemical Equilibrium in the Atmospheres of Substellar Mass Objects
We present a systematic study of the spectral consequences of departures from
chemical equilibrium in the atmospheres of L and T dwarfs, and for even cooler
dwarfs. The temperature/pressure profiles of the non-equilibrium models are
fully consistent with the non-equilibrium chemistry. Our grid of
non-equilibrium models includes spectra for effective temperatures from 200 K
to 1800 K, three surface gravities, four possible values of the coefficient of
eddy diffusion in the radiative zone, and three different CO/CH chemical
reaction prescriptions. We find that the non-equilibrium overabundance of CO
translates into flux suppressions in the M (5 m) band of at most
40% between effective temperatures of 600 and 1800 K. The effect is
largest around K. The underabundance of ammonia due
to non-equilibrium chemistry translates into flux enhancements of no more than
20% for the range from 300 to 1800 K, with the largest
effects at the lowest values of . The magnitude of the departure
from chemical equilibrium increases with decreasing gravity, with increasing
eddy diffusion coefficient, and with decreasing speed of the CO/CH
reaction. Though these effects are modest, they lead to better fits with the
measured T dwarf spectra. Furthermore, the suppression in the M band due to
non-equilibrium enhancements in the CO abundance disappears below 500 K,
and is only partial above 500 K, preserving the M band flux as a useful
diagnostic of cool atmospheres and maintaining its importance for searches for
brown dwarfs cooler than T dwarfs.Comment: Accepted to the Astrophysical Journal; 21 figures in a total of 26
emulateapj page
Continuum Spectra of Quasar Accretion Disk Models
We have calculated the spectrum and polarization of a standard thin accretion
disk with parameters appropriate for a bright quasar. This model improves upon
previous work by including ultraviolet metal line opacities, assumed for now to
be in LTE. Though not yet fully self-consistent, our calculations demonstrate
that metal lines can change the spectral slope, reduce the polarization, and
reduce the Lyman edge feature in accretion disk spectra. Some observational
differences between quasar spectra and accretion disk models might be
reconciled with the inclusion of metal lines.Comment: 4 pages, 3 figures, to appear in "Accretion Processes in
Astrophysical Systems: Some Like it Hot," proceedings of the 8th Annual
October Astrophysics Conference in Marylan
Chemical enrichment and physical conditions in I Zw 18
Abridged.
Low-metallicity star-forming dwarf galaxies are prime targets to understand
the chemical enrichment of the interstellar medium. The HI region provides
important constraints on the dispersal and mixing of heavy elements released by
successive star-formation episodes. Our primary objective is to study the
enrichment of the HI region and the interplay between star-formation history
and metallicity evolution.
We observed the most metal-poor star-forming galaxy in the Local Universe, I
Zw 18, with Hubble/COS. The abundances in the neutral gas are derived from
far-UV absorption-lines (HI, CII, CII*, NI, OI, ...) and are compared to the
abundances in the HII region. Models are constructed to calculate the
ionization structure and the thermal processes. We investigate the gas cooling
in the HI region through physical diagnostics drawn from the fine-structure
level of C+.
We find that HI region abundances are lower by a factor of ~2 as compared to
the HII region. There is no differential depletion on dust between the HI and
HII region. Using sulfur as a metallicity tracer, we calculate a metallicity of
1/46 solar (vs. 1/31 in the HII region). From the study of abundance ratios, we
propose that C, N, O, and Fe are mainly produced in massive stars. We argue
that the HI envelope may contain pockets of pristine gas with a metallicity
essentially null. Finally, we derive the physical conditions in the HI region
by investigating the CII* absorption line. The cooling rate derived from CII*
is consistent with collisions with H atoms in the diffuse neutral gas. We
calculate the star-formation rate from the CII* cooling rate assuming that
photoelectric effect on dust is the dominant gas heating mechanism. Our
determination is in good agreement with the values in the literature if we
assume a low dust-to-gas ratio (~2000 times lower than the Milky Way value).Comment: Accepted for publication in A&A. Fixed typos and reference
Spectrum and atmosphere models of irradiated transiting extrasolar giant planets
We show that a consistent fit to observed secondary eclipse data for several
strongly irradiated transiting planets demands a temperature inversion
(stratosphere) at altitude. Such a thermal inversion significantly influences
the planet/star contrast ratios at the secondary eclipse,their wavelength
dependences, and, importantly, the day-night flux contrast during a planetary
orbit. The presence of the thermal inversion/stratosphere seems to roughly
correlate with the stellar flux at the planet. Such temperature inversions
might caused by an upper-atmosphere absorber whose exact nature is still
uncertainComment: 7 pages, 3 figures. To appear in the Proceedings of the 253rd IAU
Symposium: "Transiting Planets", May 2008, Cambridge, M
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