348 research outputs found
NLTE effects on oxygen lines
The NLTE effects affecting oxygen-abundance determinations of solar-type
stars are discussed. LTE is perfectly safe for the forbidden lines. The
permitted triplet at 777 nm is expected to show NLTE effects on the order of a
few tenths of a dex (always in the sense that LTE overestimates the abundance),
but the magnitude of the effects is dependent on the still very uncertain cross
sections of collisional excitation by collisions with neutral hydrogen atoms.
Little is known about the NLTE effects on molecular line formation.Comment: 8 pages, 2 figures, to be published in New Astronomy Reviews as
proceedings of JD8 of IAU GA 2000, eds. B. Barbuy, P.E. Nissen, R. Peterson,
F. Spit
3D non-LTE line formation in the solar photosphere and the solar oxygen abundance
We study the formation of O I and OH spectral lines in three-dimensional
hydrodynamic models of the solar photosphere. The line source function of the O
I 777 nm triplet is allowed to depart from local thermodynamic equilibrium
(LTE), within the two-level-atom approximation. Comparison with results from 1D
models show that the 3D models alleviate, but do not remove, the discrepancy
between the oxygen abundances reported from non-LTE work on the 777 nm triplet
and from the [O I] 630 nm and OH lines. Results for the latter two could imply
that the solar oxygen abundance is below 8.8 (lg(H) = 12). If this is
confirmed, the discrepancy between theory and observation for the 777 nm
triplet lines might fall within the range of errors in equivalent width
measurements and f-values. The line source function of the 777 nm triplet in
the 1.5D approximation is shown to differ insignificantly from the full 3D
non-LTE result.Comment: 10 pages, uuencoded compressed PostScript file including figures, to
appear in Astronomy & Astrophysics. Also available at
ftp://www.astro.su.se/pub/da
Boron Abundances in the Galactic Disk
When compared to lithium and beryllium, the absence of boron lines in the
optical results in a relatively small data set of boron abundances measured in
Galactic stars to date. In this paper we discuss boron abundances published in
the literature and focus on the evolution of boron in the Galaxy as measured
from pristine boron abundances in cool stars as well as early-type stars in the
Galactic disk. The trend of B with Fe obtained from cool F-G dwarfs in the disk
is found to have a slope of 0.87 +/- 0.08 (in a log-log plot). This slope is
similar to the slope of B with Fe found for the metal poor halo stars and there
seems to be a smooth connection between the halo and disk in the chemical
evolution of boron. The disk trend of boron with oxygen has a steeper slope of
~1.5. This slope suggests an intermediate behavior between primary and
secondary production of boron with respect to oxygen. The slope derived for
oxygen is consistent with the slope obtained for Fe provided that [O/Fe]
increases as [Fe/H] decreases, as observed in the disk.Comment: 6 pages, 3 figures, IAUS268 Proceeding
Solar off-limb emission of the OI 7772 \AA\ line
The aim of this paper is to understand the formation of the OI line at 7772
\AA\ in the solar chromosphere. We used SST/CRISP observations to observe OI
7772 \AA\ in several places around the solar limb. We compared the observations
with synthetic spectra calculated with the RH code in the one-dimension
spherical geometry mode. New accurate hydrogen collisional rates were included
for the RH calculations. The observations reveal a dark gap in the lower
chromosphere, which is caused by variations in the line opacity as shown by our
models. The lower level of the 7772 \AA\ transition is populated by a downward
cascade from the continuum. We study the effect of Lyman- pumping and
hydrogen collisions between the triplet and quintet system in OI. Both have a
small but non-negligible influence on the line intensity.Comment: 9 pages, 12 figures, Accepted for publication in A&
The Observed Trend of Boron and Oxygen in Field Stars of the Disk
Oxygen abundances are derived in a sample of 13 field F and G dwarfs and
subgiants with metallicities in the range of -0.75 < [Fe/H] < +0.15. This is
the same sample of stars for which boron abundances have been derived earlier
from archived spectra obtained with the Hubble Space Telescope. In a log-log
comparison of the B versus the O abundances, a slope of m(BO)=1.39 is found,
indicating that in the disk, the abundance of B relative to O is intermediate
between primary and secondary production (hybrid behavior). This relation of B
versus O for disk stars is compared to the same relation for halo stars.Comment: 14 pages, 3 figures, 1 table. In press to The Astronomical Journal
(July 2001
Non-LTE neutral carbon spectral line formation in late-type stars
We present non-Local Thermodynamic Equilibrium (non-LTE) calculations for
neutral carbon spectral line formation, carried out for a grid of model
atmospheres covering the range of late-type stars. The results of our detailed
calculations suggest that the carbon non-LTE corrections in these stars are
higher than usually adopted, remaining substantial even at low metallicity. For
the most metal-poor stars in the sample of Akerman et al. (2004), the non-LTE
abundance corrections are of the order of -0.35...-0.45 dex (when neglecting H
collisions). Applying our results to those observations, the apparent [C/O]
upturn seen in their LTE analysis is no longer present, thus revealing no need
to invoke contributions from Pop. III stars to the carbon nucleosynthesis.Comment: 2 pages, 1 figure. To appear in the Proceedings of IAU Symposium 228
"From Li to U: Elemental Tracers of Early Cosmic Evolution", eds. V. Hill, P.
Francois and F. Primas, Cambridge University Press. Replacement with minor
textual correction
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