Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userStatistical equilibrium of silicon in the atmospheres of metal-poor stars
179
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Aims. The statistical equilibrium of neutral and ionized silicon in the atmospheres of metal-poor stars is discussed. Non-local thermodynamic equilibrium effects are investigated and the silicon abundances in metal-poor stars determined. Methods. We have used high resolution, high signal to noise ratio spectra from the UVES spectragraph at the ESO VLT telescope. Line formation calculations of Si i and Si ii in the atmospheres of metal-poor stars are presented for atomic models of silicon including 174 terms and 1132 line transitions. Recent improved calculations of Si i and Si ii photoionization cross-sections are taken into account, and the influence of the free-free quasi-molecular absorption in the Ly alpha wing is investigated by comparing theoretical and observed fluxes of metal-poor stars. All abundance results are derived from LTE and NLTE statistical equilibrium calculations and spectrum synthesis methods. Results. It is found that the extreme ultraviolet radiation is very important for metal-poor stars, especially for the high temperature, very metal-poor stars. The radiative bound-free cross-sections also play a very important role for these stars. Conclusions. NLTE effects for Si are found to be important for metal-poor stars, in particular for warm metal-poor stars. It is found that these effects depend on the temperature. For warm metal-poor stars, the NLTE abundance correction reaches ~ 0.2 dex relative to standard LTE calculations. Our results indicate that Si is overabundant for metal-poor stars.
rate research
Read More
We present our latest 3D model atmospheres for carbon-enhanced metal-poor (CEMP) stars computed with the CO5BOLD code. The stellar parameters are representative of hot turn-off objects (Teff ~ 6250 K, log g=4.0, [Fe/H]=-3.0). The main purpose of these models is to investigate the role of 3D effects on synthetic spectra of the CH G-band (4140-4400 A), the CN BX-band (3870-3890 A), and several UV OH transitions (3122-3128 A). By comparison with the synthetic spectra from standard 1D model atmospheres (assuming local thermodynamic equilibrium, LTE), we derive 3D abundance corrections for carbon and oxygen of up to -0.5 and -0.7 dex, respectively.
The presence of 6Li in the atmospheres of metal-poor halo stars is usually inferred from the detection of a subtle extra depression in the red wing of the 7Li doublet line at 670.8 nm. However, the intrinsic line asymmetry caused by convective flows in the photospheres of cool stars is almost indistinguishable from the asymmetry produced by a weak 6Li blend on a (presumed) symmetric 7Li profile. Previous determinations of the 6Li/ 7Li isotopic ratio based on 1D model atmospheres, ignoring the convection-induced line asymmetry, must therefore be considered as upper limits. By comparing synthetic 1D LTE and 3D non-LTE line profiles of the Li 670.8 nm feature, we quantify the differential effect of the convective line asymmetry on the derived 6Li abundance as a function of effective temperature, gravity, and metallicity. As expected, we find that the asymmetry effect systematically reduces the resulting 6Li/7Li ratios. Depending on the stellar parameters, the 3D-1D offset in 6Li/7Li ranges between -0.005 and -0.020. When this purely theoretical correction is taken into account for the Asplund 2006 sample of stars, the number of significant 6Li detections decreases from 9 to 5 (2 sigma criterion), or from 5 to 2 (3 sigma criterion). We also present preliminary results of a re-analysis of high-resolution, high S/N spectra of individual metal-poor turn-off stars, to see whether the second Lithium problem actually disappears when accounting properly for convection and non-LTE line formation in 3D stellar atmospheres. Out of 8 stars, HD84937 seems to be the only significant (2 sigma) detection of 6Li. In view of our results, the existence of a 6Li plateau appears questionable.
Convective line asymmetries in the optical spectrum of two metal-poor stars, Gmb1830 and HD140283, are compared to those observed for solar metallicity stars. The line bisectors of the most metal-poor star, the subgiant HD140283, show a significantly larger velocity span that the expectations for a solar-metallicity star of the same spectral type and luminosity class. The enhanced line asymmetries are interpreted as the signature of the lower metal content, and therefore opacity, in the convective photospheric patterns. These findings point out the importance of three-dimensional convective velocity fields in the interpretation of the observed line asymmetries in metal-poor stars, and in particular, urge for caution when deriving isotopic ratios from observed line shapes and shifts using one-dimensional model atmospheres. The mean line bisector of the photospheric atomic lines is compared with those measured for the strong Mg I b1 and b2 features. The upper part of the bisectors are similar, and assuming they overlap, the bottom end of the stronger lines, which are formed higher in the atmosphere, goes much further to the red. This is in agreement with the expected decreasing of the convective blue-shifts in upper atmospheric layers, and compatible with the high velocity redshifts observed in the chromosphere, transition region, and corona of late-type stars.
The origin of carbon-enhanced metal-poor (CEMP) stars plays a key role in characterising the formation and evolution of the first stars and the Galaxy since the extremely-poor (EMP) stars with [Fe/H] leq -2.5 share the common features of carbon enhancement in their surface chemical compositions. The origin of these stars is not yet established due to the controversy of the origin of CEMP stars without the enhancement of s-process element abundances, i.e., so called CEMP-no stars. In this paper, we elaborate the s-process nucleosynthesis in the EMP AGB stars and explore the origin of CEMP stars. We find that the efficiency of the s-process is controlled by O rather than Fe at [Fe/H] lesssim -2. We demonstrate that the relative abundances of Sr, Ba, Pb to C are explained in terms of the wind accretion from AGB stars in binary systems.
We report the discovery of one extremely metal-poor (EMP; [Fe/H]<-3) and one ultra metal-poor (UMP; [Fe/H]<-4) star selected from the SDSS/SEGUE survey. These stars were identified as EMP candidates based on their medium-resolution (R~2,000) spectra, and were followed-up with high-resolution (R~35,000) spectroscopy with the Magellan-Clay Telescope. Their derived chemical abundances exhibit good agreement with those of stars with similar metallicities. We also provide new insights on the formation of the UMP stars, based on comparison with a new set of theoretical models of supernovae nucleosynthesis. The models were matched with 20 UMP stars found in the literature, together with one of the program stars (SDSS J1204+1201), with [Fe/H]=-4.34. From fitting their abundances, we find that the supernovae progenitors, for stars where carbon and nitrogen are measured, had masses ranging from 20.5 M_sun to 28 M_sun and explosion energies from 0.3 to 0.9x10^51 erg. These results are highly sensitive to the carbon and nitrogen abundance determinations, which is one of the main drivers for future high-resolution follow-up of UMP candidates. In addition, we are able to reproduce the different CNO abundance patterns found in UMP stars with a single progenitor type, by varying its mass and explosion energy.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
