389 research outputs found
The Free-Free Opacity in Warm, Dense, and Weakly Ionized Helium
We investigate the ionization and the opacity of warm, dense helium under
conditions found in the atmospheres of cool white dwarf stars. Our particular
interest is in densities up to and temperatures from 1000K to
10000K. For these physical conditions various approaches for modeling the
ionization equilibrium predict ionization fractions that differ by orders of
magnitudes. Furthermore, estimates of the density at which helium
pressure-ionizes vary from to . In this context, the
value of the electron-atom inverse bremsstrahlung absorption is highly
uncertain. We present new results obtained from a non-ideal chemical model for
the ionization equilibrium, from Quantum Molecular Dynamics (QMD) simulations,
and from the analysis of experimental data to better understand the ionization
fraction in fluid helium in the weak ionization limit.Comment: 4 pages, 3 figures, 1 table. Accepted for publication in the
Proceedings of the 14th APS Topical Conference on Shock Compression of
Condensed Matter, Baltimore, M
Spectroscopic Detection of Carbon Monoxide in Two Late-type T Dwarfs
M band spectra of two late-type T dwarfs, 2MASS J09373487+2931409, and Gliese
570D, confirm evidence from photometry that photospheric CO is present at
abundance levels far in excess of those predicted from chemical equilibrium.
These new and unambiguous detections of CO, together with an earlier
spectroscopic detection of CO in Gliese 229B and existing M band photometry of
a large selection of T dwarfs, suggest that vertical mixing in the photosphere
drives the CO abundance out of chemical equilibrium and is a common, and likely
universal feature of mid-to-late type T dwarfs. The M band spectra allow
determinations of the time scale of vertical mixing in the atmosphere of each
object, the first such measurements of this important parameter in late T
dwarfs. A detailed analysis of the spectral energy distribution of 2MASS
J09373487+2931409 results in the following values for metallicity, temperature,
surface gravity, and luminosity: [M/H]~-0.3, T_eff=925-975K, log g=5.20-5.47,
log L/L_sun=-5.308 +/- 0.027. The age is 3-10 Gyr and the mass is in the range
45-69 M_Jup.Comment: 36 pages incl. 12 figures and 3 tables, accepted by Ap
Near-UV absorption in very cool DA white dwarfs
The atmospheres of very cool, hydrogen-rich white dwarfs (Teff <6000 K) are
challenging to models because of the increased complexity of the equation of
state, chemical equilibrium, and opacity sources in a low-temperature, weakly
ionized dense gas. In particular, many models that assume relatively simple
models for the broadening of atomic levels and mostly ideal gas physics
overestimate the flux in the blue part of their spectra. A solution to this
problem that has met with some success is that additional opacity at short
wavelengths comes for the extreme broadening of the Lyman alpha line of atomic
H by collisions primarily with H2. For the purpose of validating this model
more rigorously, we acquired Hubble Space Telescope STIS spectra of 8 very cool
white dwarfs (5 DA and 3 DC stars). Combined with their known parallaxes,
BVRIJHK and Spitzer IRAC photometry, we analyze their entire spectral energy
distribution (from 0.24 to 9.3 micron) with a large grid of model atmospheres
and synthetic spectra. We find that the red wing of the Lyman alpha line
reproduces the rapidly decreasing near-UV flux of these very cool stars very
well. We determine better constrained values of Teff and gravity as well as
upper limits to the helium abundance in their atmospheres.Comment: 41 pages, 9 figures. Accepted for publication in the Ap
SDSS J141624.08+134826.7: Blue L Dwarfs and Non-Equilibrium Chemistry
We present an analysis of the recently discovered blue L dwarf SDSS
J141624.08+134826.7. We extend the spectral coverage of its published spectrum
to ~4 microns by obtaining a low-resolution L band spectrum with SpeX on the
NASA IRTF. The spectrum exhibits a tentative weak CH4 absorption feature at 3.3
microns but is otherwise featureless. We derive the atmospheric parameters of
SDSS J141624.08+134826.7 by comparing its 0.7-4.0 micron spectrum to the
atmospheric models of Marley and Saumon which include the effects of both
condensate cloud formation and non-equilibrium chemistry due to vertical mixing
and find the best fitting model has Teff=1700 K, log g=5.5 [cm s-2], fsed=4,
and Kzz=10^4 cm2 s-1. The derived effective temperature is significantly cooler
than previously estimated but we confirm the suggestion by Bowler et al. that
the peculiar spectrum of SDSS J141624.08+134826.7 is primarily a result of thin
condensate clouds. In addition, we find strong evidence of vertical mixing in
the atmosphere of SDSS J141624.08+134826.7 based on the absence of the deep 3.3
micron CH4 absorption band predicted by models computed in chemical
equilibrium. This result suggests that observations of blue L dwarfs are an
appealing way to quantitatively estimate the vigor of mixing in the atmospheres
of L dwarfs because of the dramatic impact such mixing has on the strength of
the 3.3 micron CH4 band in the emergent spectra of L dwarfs with thin
condensate clouds.Comment: Accepted for publication in the Astronomical Journa
Properties of the T8.5 Dwarf Wolf 940 B
We present 7.5-14.2um low-resolution spectroscopy, obtained with the Spitzer
Infrared Spectrograph, of the T8.5 dwarf Wolf 940 B, which is a companion to an
M4 dwarf with a projected separation of 400 AU. We combine these data with
previously published near-infrared spectroscopy and mid-infrared photometry, to
produce the spectral energy distribution for the very low-temperature T dwarf.
We use atmospheric models to derive the bolometric correction and obtain a
luminosity of log L/Lsun = -6.01 +/- 0.05. Evolutionary models are used with
the luminosity to constrain the values of effective temperature (T_eff) and
surface gravity, and hence mass and age for the T dwarf. We further restrict
the allowed range of T_eff and gravity using age constraints implied by the M
dwarf primary, and refine the physical properties of the T dwarf by comparison
of the observed and modelled spectroscopy and photometry. This comparison
indicates that Wolf 940 B has a metallicity within 0.2 dex of solar, as more
extreme values give poor fits to the data - lower metallicity produces a poor
fit at lambda > 2um while higher metallicity produces a poor fit at lambda <
2um. This is consistent with the independently derived value of [m/H] = +0.24
+/- 0.09 for the primary star, using the Johnson & Apps (2008) M_K:V-K
relationship. We find that the T dwarf atmosphere is undergoing vigorous
mixing, with an eddy diffusion coefficient K_zz of 10^4 to 10^6 cm^2 s^-1. We
derive an effective temperature of 585 K to 625 K, and surface gravity log g =
4.83 to 5.22 (cm s^-2), for an age range of 3 Gyr to 10 Gyr, as implied by the
kinematic and H alpha properties of the M dwarf primary. The lower gravity
corresponds to the lower temperature and younger age for the system, and the
higher value to the higher temperature and older age. The mass of the T dwarf
is 24 M_Jupiter to 45 M_Jupiter for the younger to older age limit.Comment: 24 pages which include 5 Figures and 3 Tables. Accepted for
publication in the Astrophysical Journal July 2 201
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