189 research outputs found
Primordial helium recombination II: two-photon processes
Interpretation of precision measurements of the cosmic microwave background
(CMB) will require a detailed understanding of the recombination era, which
determines such quantities as the acoustic oscillation scale and the Silk
damping scale. This paper is the second in a series devoted to the subject of
helium recombination, with a focus on two-photon processes in He I. The
standard treatment of these processes includes only the spontaneous two-photon
decay from the 2^1S level. We extend this treatment by including five
additional effects, some of which have been suggested in recent papers but
whose impact on He I recombination has not been fully quantified. These are:
(i) stimulated two-photon decays; (ii) two-photon absorption of redshifted HeI
line radiation; (iii) two-photon decays from highly excited levels in HeI (n^1S
and n^1D, with n>=3); (iv) Raman scattering; and (v) the finite width of the
2^1P^o resonance. We find that effect (iii) is highly suppressed when one takes
into account destructive interference between different intermediate states
contributing to the two-photon decay amplitude. Overall, these effects are
found to be insignificant: they modify the recombination history at the level
of several parts in 10^4.Comment: 19 pages, 11 figures, to be submitted to PR
Radiative recombination data for modeling dynamic finite-density plasmas
We have calculated partial final-state resolved radiative recombination (RR) rate coefficients from the initial ground and metastable levels of all elements up to and including Zn, plus Kr, Mo, and Xe, for all isoelectronic sequences up to Na-like forming Mg-like. The data are archived according to the Atomic Data and Analysis Structure (ADAS) data class adf48, which spans a temperature range of z2(101-107) K, where z is the initial ion charge. Fits to total rate coefficients have been determined, for both the ground and metastable levels, and those for the ground are presented here. Comparison is made both with previous RR rate coefficients and with (background) R-matrix photoionization cross sections. This RR database complements a dielectronic recombination (DR) database already produced, and both are being used to produce updated ionization balances for both (electron) collisionally ionized and photoionized plasmas
Atomic and Molecular Opacities for Brown Dwarf and Giant Planet Atmospheres
We present a comprehensive description of the theory and practice of opacity
calculations from the infrared to the ultraviolet needed to generate models of
the atmospheres of brown dwarfs and extrasolar giant planets. Methods for using
existing line lists and spectroscopic databases in disparate formats are
presented and plots of the resulting absorptive opacities versus wavelength for
the most important molecules and atoms at representative temperature/pressure
points are provided. Electronic, ro-vibrational, bound-free, bound-bound,
free-free, and collision-induced transitions and monochromatic opacities are
derived, discussed, and analyzed. The species addressed include the alkali
metals, iron, heavy metal oxides, metal hydrides, , , , ,
, , , and representative grains. [Abridged]Comment: 28 pages of text, plus 22 figures, accepted to the Astrophysical
Journal Supplement Series, replaced with more compact emulateapj versio
Argon Abundances in the Solar Neighborhood: Non-LTE Analysis of Orion Association B-type Stars
Argon abundances have been derived for a sample of B main-sequence stars in
the Orion association. The abundance calculations are based on NLTE metal
line-blanketed model atmospheres calculated with the NLTE code TLUSTY and an
updated and complete argon model atom. We derive an average argon abundance for
this young population of A(Ar) = 6.66 +- 0.06. While our result is in excellent
agreement with a recent analysis of the Orion nebula, it is significantly
higher than the currently recommended solar value which is based on abundance
measurements in the solar corona. Moreover, the derived argon abundances in the
Orion B stars agree very well with a measurement from a solar impulsive flare
during which unmodified solar photospheric material was brought to flare
conditions. We therefore argue that the argon abundances obtained independently
for both the Orion B stars and the Orion nebula are representative of the disk
abundance value in the solar neighborhood. The lower coronal abundance may
reflect a depletion related to the FIP effect. We propose a new reference value
for the abundance of argon in the solar neighborhood, A(Ar) = 6.63 +- 0.10,
corresponding to Ar/O = 0.009.Comment: to appear in Astrophysical Journal, 24 pages, 3 figures; minor
corrections after referee's repor
Theoretical He I Emissivities in the Case B Approximation
We calculate the He I case B recombination cascade spectrum using improved
radiative and collisional data. We present new emissivities over a range of
electron temperatures and densities. The differences between our results and
the current standard are large enough to have a significant effect not only on
the interpretation of observed spectra of a wide variety of objects but also on
determinations of the primordial helium abundance.Comment: Accepted to ApJ
Primordial helium recombination III: Thomson scattering, isotope shifts, and cumulative results
Upcoming precision measurements of the temperature anisotropy of the cosmic
microwave background (CMB) at high multipoles will need to be complemented by a
more complete understanding of recombination, which determines the damping of
anisotropies on these scales. This is the third in a series of papers
describing an accurate theory of HeI and HeII recombination. Here we describe
the effect of Thomson scattering, the He isotope shift, the contribution of
rare decays, collisional processes, and peculiar motion. These effects are
found to be negligible: Thomson and He scattering modify the free electron
fraction at the level of several . The uncertainty in the
rate is significant, and for conservative estimates gives
uncertainties in of order . We describe several convergence
tests for the atomic level code and its inputs, derive an overall
error budget, and relate shifts in to the changes in , which
are at the level of 0.5% at . Finally, we summarize the main
corrections developed thus far. The remaining uncertainty from known effects is
in .Comment: 19 pages, 15 figures, to be submitted to PR
Chemical Abundances from the Continuum
The calculation of solar absolute fluxes in the near-UV is revisited,
discussing in some detail recent updates in theoretical calculations of
bound-free opacity from metals. Modest changes in the abundances of elements
such as Mg and the iron-peak elements have a significant impact on the
atmospheric structure, and therefore self-consistent calculations are
necessary. With small adjustments to the solar photospheric composition, we are
able to reproduce fairly well the observed solar fluxes between 200 and 270 nm,
and between 300 and 420 nm, but find too much absorption in the 270-290 nm
window. A comparison between our reference 1D model and a 3D time-dependent
hydrodynamical simulation indicates that the continuum flux is only weakly
sensitive to 3D effects, with corrections reaching <10% in the near-UV, and <2%
in the optical.Comment: 10 pages, 5 figures, to appear in the proceedings of the conference A
Stellar Journey, a symposium in celebration of Bengt Gustafsson's 65th
birthday, June 23-27, 2008, Uppsal
On the Indirect Detection of Sodium in the Atmosphere of the Planetary Companion to HD 209458
Using a self-consistent atmosphere code, we construct a new model of the
atmosphere of the transiting extrasolar giant planet HD 209458b to investigate
the disparity between the observed strength of the sodium absorption feature at
589 nm and the predictions of previous models. For the atmospheric
temperature-pressure profile we derive, silicate and iron clouds reside at a
pressure of several mbar in the planet's atmosphere. These clouds have
significant vertical extent and optical depth due to our slant viewing geometry
and lead to increased absorption in bands directly adjacent to the sodium line
core. Using a non-LTE sodium ionization model that includes photoionization by
stellar UV flux, collisional processes with H_2, and radiative recombination,
we show that the ionization depth in the planet's atmosphere reaches ~1/2 mbar
at the day/night terminator. Ionization leads to a slight weakening of the
sodium feature. We present our baseline model, including ionization and clouds,
which falls near the observational error bars. The sensitivity of our
conclusions to the derived atmospheric temperature-pressure profile is
discussed.Comment: 15 pages, 9 figures -- Accepted to The Astrophysical Journal -- The
paper is significantly revised and expande
Time Dependent Monte Carlo Radiative Transfer Calculations For 3-Dimensional Supernova Spectra, Lightcurves, and Polarization
We discuss Monte-Carlo techniques for addressing the 3-dimensional
time-dependent radiative transfer problem in rapidly expanding supernova
atmospheres. The transfer code SEDONA has been developed to calculate the
lightcurves, spectra, and polarization of aspherical supernova models. From the
onset of free-expansion in the supernova ejecta, SEDONA solves the radiative
transfer problem self-consistently, including a detailed treatment of gamma-ray
transfer from radioactive decay and with a radiative equilibrium solution of
the temperature structure. Line fluorescence processes can also be treated
directly. No free parameters need be adjusted in the radiative transfer
calculation, providing a direct link between multi-dimensional hydrodynamical
explosion models and observations. We describe the computational techniques
applied in SEDONA, and verify the code by comparison to existing calculations.
We find that convergence of the Monte Carlo method is rapid and stable even for
complicated multi-dimensional configurations. We also investigate the accuracy
of a few commonly applied approximations in supernova transfer, namely the
stationarity approximation and the two-level atom expansion opacity formalism.Comment: 16 pages, ApJ accepte
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