Stellar atmosphere

For the spectroscopic analyses, I identify three kinds of environments stellar atmospheres, stellar ejecta and interstellar gas, and stellar systems. 

Stellar atmospheres. Stellar spectroscopy, most particularly stellar photospheric spectroscopy, is a major source of the abundance data used to address questions of nucleosynthesis. 

Maser emission has been discovered in different objects such as in molecular clouds, comets, planetary atmospheres, stellar atmospheres (as an example see Fig. 7.4). 

The astrochemistty of ions may be divided into topics of interstellar clouds, stellar atmospheres, planetary atmospheres and comets. There are many areas of astrophysics (stars, planetary nebulae, novae, supemovae) where highly ionized species are important, but beyond the scope of ion chemistry . (Still, molecules, including H2O, are observed in solar spectra [155] and a surprise in the study of Supernova 1987A was the identification of molecular species, CO, SiO and possibly ITf[156. 157]. ) In the early universe, after expansion had cooled matter to the point that molecules could fonn, the small fraction of positive and negative ions that remained was crucial to the fomiation of molecules, for example [156]... 

In addition to these laboratory-based experiments it is interesting to note that the Swan bands of C2 are important in astrophysics. They have been observed in the emission spectra of comets and also in the absorption spectra of stellar atmospheres, including that of the sun, in which the interior of the star acts as the continuum source. 

Chandrasekhar, S., Revs. Modern Phys. 16, 301, "The negative ions of hydrogen and oxygen in stellar atmospheres."... 

Abstract. In this contribution we present the results based on high-resolution spectra of 45 clump stars of the Galactic field. The main atmospheric parameters and abundances of 12C, 13C, N, O and other mixing sensitive chemical elements were investigated. Elemental ratios in the sample of field stars are compared to the results available for evolved stars in open clusters and to the theoretical prediction of extra mixing in stellar interiors. 

Abstract. AGB stars, in particular those of carbon types, are excellent laboratories to constraint the theory of stellar structure, evolution and nucleosynthesis. Despite the uncertainties still existing in the chemical analysis of these stars, the determination of the abundances of several key species in their atmospheres (lithium, s-elements, carbon and magnesium isotopic ratios etc.) is an useful tool to test these theories and the mixing processes during the AGB phase. This contribution briefly review some recent advances on this subject. 

Kurucz, R. L., 1993, CD-ROMs, ATLAS9 Stellar Atmospheres Programs... 

Our multi-level carbon model atom is adapted from D. Kiselman (private communication), with improved atomic data and better sampling of some absorption lines. The statistical equilibrium code MULTI (Carlsson 1986), together with ID MARCS stellar model atmospheres for a grid of 168 late-type stars with varying Tefj, log g, [Fe/H] and [C/Fe], were used in all Cl non-LTE spectral line formation calculations, to solve radiative-transfer and rate equations and to find the non-LTE solution for the multi-level atom. We put particular attention in the study of the permitted Cl lines around 9100 A, used by Akerman et al. (2004). 

The aim of this paper is to re-analyze the stars studied by AP04 with a technique which discriminates a-enhanced (Non Solar Scaled Abundance, NSSA) stars from those with Solar Scaled Abundances (SSA) without requiring any assumption about the stellar atmospheric parameter values. 

R.L. Kurucz CD-ROM 13, ATLAS9 Stellar Atmospheres Program and 2 km/s Grid (Smithsonian Astrop. Obs., Cambridge 1993)... 

The spectra have been reduced with the GIRAFFE BLDRS pipeline developed at the Geneva Observatory. EW have been measured with DAOSPEC [2], based on a linelist produced with the Vienna Atomic Line Database (VALD) [3]. Preliminary estimates of the stellar parameters Te//, log g, vt and [M/H] have been obtained from the WFI photometry published by [4] and the color-temperature calibration by [5]. MARCS model stellar atmospheres [6] have been... 

Be is a very interesting element, produced by spallation of galactic cosmic rays. The only two usable lines are in the extreme UV (313 nm), in a crowded spectral region, and the stellar radiation is heavily absorbed by the earth s atmosphere, so that their observations are challenging in faint stars. Only very recently (Pasquini et al. 2004) the first Be observations became available, in 2 TO stars of the nearby NGC6397. 

The abundance patterns of individual stars of different ages and environments enable us to unlock the evolutionary history of galaxies. Many physical characteristics of a galaxy may change over time, such as shape and colour, however the metal content and abundance ratios of stellar atmospheres are not so easy to tamper with. Stars retain the chemical imprint of the interstellar gas out of which they formed, and metals can only increase with time. This method to study galaxy evolution has been elegantly named Chemical Tagging [2],... 

AGB stars constitute excellent laboratories to test the theory of stellar evolution and nucleosynthesis. Their particular internal structure allows two important processes to occur in them. First is the so-called 3(,ldredge-up (3DUP), a mixing mechanism in which the convective envelope penetrates the interior of the star after each thermal instability in the He-shell (thermal pulse, TP). The other is the activation of the s-process synthesis from alpha captures on 13C or/and 22Ne nuclei that generate the necessary neutrons which are subsequently captured by iron-peak nuclei. The repeated operation of TPs and the 3DUP episodes enriches the stellar envelope in newly synthesized elements and transforms the star into a carbon star, if the quantity of carbon added into the envelope is sufficient to increase the C/O ratio above unity. In that way, the atmosphere becomes enriched with the ashes of the above nucleosynthesis processes which can then be detected spectroscopically. 

We see that the models which best reproduce the location of all the six data points are the tracks which do not fit the solar location. The models whose convection is calibrated on the 2D simulation make a poor job, as the FST models and other models with efficient convection do therefore this result can not be inputed to the fact that we employ local convection models. A possibility is that we are in front of an opacity problem, more that in front of a convection problem. Actually we would be inclined to say that opacities are not a problem (we have shown this in Montalban et al. (2004), by comparing models computed with Heiter et al (2002) or with AH97 model atmospheres), but something can still be badly wrong, as implied by the recent redetermination of solar metallicity (Asplund et al., 2004). A further possibility is that the inefficient convection in PMS requires the introduction of a second parameter -linked to the stellar rotation and magnetic field, as we have suggested in the past (Ventura et al., 1998 D Antona et al., 2000), but this remains to be worked out. 

D Hydrodynamical Simulations of Convection in Red-Giants Stellar Atmospheres... 

Abstract. We present preliminary results of 3D hydrodynamical simulations of surface convection in red giants stars. We investigate the main differences between static ID and 3D time-dependent model stellar atmospheres of red giants for a range of metallicities between solar and [Fe/H] = —3 focusing in particular on the impact of 3D spectral line formation on the derivation of stellar abundances. 

In this contribution we present preliminary estimates of the effect of convection in 3D model stellar atmospheres of red giants on the formation of spectral lines and on the derivation of chemical abundances in stars. 

We then use a Feautrier scheme [4] to perform spectral line formation calculations in local thermodynamic equilibrium approximation (LTE) for the species indicated in table 1. At this stage we consider only rays in the vertical direction and a single snapshot per 3D simulation. Abundance corrections are computed differentially by comparing the predictions from 3D models with the ones from ID MARCS model stellar atmospheres ([2]) generated for the same stellar parameters (a microturbulence = 2.0 km s-1 is applied to calculations with ID models). 

Preliminary simulations of convection in red giants stellar atmospheres indicate that cooler surface layers are expected in 3D models than in ID. Therefore, in the LTE case, for lines forming in those layers corrections to abundances derived with ID analysis have to be applied in the same direction as for dwarfs and subgiants. The magnitude of the corrections though appears to be lower for red giants. 

It is just a half-century ago that the concept of real abundance variations became well-established. It is less that 50 years since the famous B2FH paper was published. Quantitative CCD-based spectroscopy is only some 20 years old, while quantitative multi-object spectroscopy has really begun only in the last decade. These rapid observational advances were enabled by impressive advances in instrumentation, combined with increasing software power and complexity. In parallel, significant advances in stellar atmospheric modelling, and the requisite atomic and molecular data, have allowed analyses of superb precision for large numbers of stars. 

The black body radiation profiles for a planet and the Sun have significantly different maximum temperatures with different spectral characteristics planetary emission is principally in the infrared whereas stellar emission is dominated by the visible. Molecules present in the atmosphere may absorb the infrared radiation and re-radiate the radiation back to Earth. This was thought to be the role of glass... 

The bulk of stellar radiation comes from the surface layers or atmosphere of a star, more particularly the photosphere , which is defined as the region having optical depths for continuum radiation between about 0.01 and a few. The optical depth ti is measured inwards from the surface and represents the number of mean free paths of radiation travelling vertically outwards before it escapes from the star. It is related to the geometrical height z above some arbitrary layer by... 

---