23 research outputs found
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
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
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
Line-blanketed model atmospheres for R Coronae Borealis stars and hydrogen-deficient carbon stars
We have constructed line-blanketed model atmospheres for the hydrogen-deficient and carbon-rich R Coronae Borealis (RCrB) stars, as well as for the similar hydrogen-deficient carbon (HdC) stars and the cool extreme helium (EHe) stars. Improved continuum opacities have been used together with realistic line absorption data for atomic and molecular transitions. The observed dereddened fluxes of R CrB are compared with the calculated model fluxes and found to agree best with a model effective temperature of 6900K, while the infrared flux method gives between 6600 and 6900K, depending on the nature of the flux excess in the J and H bands compared to the model fluxes. The excess may correspond to a recently formed dust cloud close to the star, with a typical temperature around 2000K and a dust mass of ~10^-11^M_{sun}_. The agreement for the ultraviolet flux distribution is also very satisfactory as seen from IUE spectra of RCrB. Theoretical broad band photometry is presented and effective temperatures of RCrB and HdC stars estimated. The constructed models show a significantly steeper temperature gradient compared to previously existing models as a result of the line opacity. Due to the cool surface and high abundance of carbon, molecular bands of e.g. C_2_ and CO are visible in the spectra even at as high effective temperatures as 7000K. Furthermore, the high temperatures encountered at depth explain the observed Hei and CII lines for T_eff_ down to ~7000K. In the inner layers ({tau}_Ross_ > 3) the models show density inversions related to the ionization zone of helium. For certain low gravity models the luminosity exceeds the local Eddington limit and hence gas pressure inversions occur as well, which could be related to the decline events of RCrB stars
Oxygen lines in solar granulation. I. Testing 3D models against new observations with high spatial and spectral resolution
Aims: we seek to provide additional tests of the line formation of
theoretical 3D solar photosphere models. In particular, we set out to test the
spatially-resolved line formation at several viewing angles, from the solar
disk-centre to the limb and focusing on atomic oxygen lines. The purpose of
these tests is to provide additional information on whether the 3D model is
suitable to derive the solar oxygen abundance. We also aim to empirically
constrain the NLTE recipes for neutral hydrogen collisions, using the
spatially-resolved observations of the OI 777 nm lines. Methods: using the
Swedish 1-m Solar Telescope we obtained high-spatial-resolution observations of
five atomic oxygen lines (along with lines for other species) for five
positions on the solar disk. These observations have a high spatial and
spectral resolution, and a continuum intensity contrast up to 9% at 615 nm. The
theoretical line profiles were computed using the 3D model, with a full 3D NLTE
treatment for oxygen and LTE for the other lines. Results: at disk-centre we
find an excellent agreement between predicted and observed line shifts,
strengths, FWHM and asymmetries. At other viewing angles the agreement is also
good, but the smaller continuum intensity contrast makes a quantitative
comparison harder. We use the disk-centre observations we constrain S_H, the
scaling factor for the efficiency of collisions with neutral hydrogen. We find
that S_H=1 provides the best match to the observations. Conclusions: overall
there is a very good agreement between predicted and observed line properties
over the solar granulation. This further reinforces the view that the 3D model
is realistic and a reliable tool to derive the solar oxygen abundance.Comment: 16 pages, 16 figures, accepted for publication in A&
Formation of Li I lines in photospheric granulation
The possibility of significant systematic errors due to the use of 1D
homogeneous atmospheres in lithium-abundance determinations of cool stars
motivates a study of non-local-thermodynamic-equilibrium (NLTE) effects on Li I
line formation in a 3D solar-granulation simulation snapshot. The NLTE effect
on the equivalent width of the 671 nm resonance line is small in 1D models or
in integrated light from the granulation model. The line-strength variations
over the granulation pattern are however markedly different in NLTE compared to
LTE -- observations of this may provide diagnostics to NLTE effects. The
effects of horizontal photon exchange found in the granulation model are
moderate and due entirely to bound-bound processes, ultraviolet overionization
is unimportant.Comment: 9 pages Latex (AASTeX using aaspp4.sty) with 3 figures (PS). The
former EPS figures have been replaced with safer PS due to technical problems
encountered by some users. No change in content. Accepted for publication in
Astrophysical Journal Letter