Browsing by Subject "hydrodynamical model atmospheres"

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Granulation Signatures in the Spectrum of the Very Metal-Poor Red Giant HD 122563
(2010-12) Ramirez, I.; Collet, R.; Lambert, David L.; Prieto, Carlos Allende; Asplund, M.; Lambert, D. L.
A very high resolution (R = lambda/Delta lambda = 200,000), high signal-to-noise ratio (S/N similar or equal to 340) blue-green spectrum of the very metal-poor ([Fe/H] similar or equal to -2.6) red giant star HD 122563 has been obtained by us at McDonald Observatory. We measure the asymmetries and core wavelengths of a set of unblended Fe I lines covering a wide range of line strength. Line bisectors exhibit the characteristic C-shape signature of surface convection (granulation) and they span from about 100 m s(-1) in the strongest Fe I features to 800 m s(-1) in the weakest ones. Core wavelength shifts range from about -100 to -900 m s(-1), depending on line strength. In general, larger blueshifts are observed in weaker lines, but there is increasing scatter with increasing residual flux. Assuming local thermodynamic equilibrium (LTE), we synthesize the same set of spectral lines using a state-of-the-art three-dimensional (3D) hydrodynamic simulation for a stellar atmosphere of fundamental parameters similar to those of HD 122563. We find good agreement between model predictions and observations. This allows us to infer an absolute zero point for the line shifts and radial velocity. Moreover, it indicates that the structure and dynamics of the simulation are realistic, thus providing support to previous claims of large 3D-LTE corrections to elemental abundances and fundamental parameters of very metal-poor red giant stars obtained with standard 1D-LTE spectroscopic analyses, as suggested by the hydrodynamic model used here.
An Upper Limit On The Sulphur Abundance In HE 1327-2326
(2012-08) Bonifacio, P.; Caffau, E.; Venn, K. A.; Lambert, D. L.; Lambert, D. L.
Context. Star HE 1327-2326 is a unique object, with the lowest measured iron abundance ([Fe/H] similar to -6) and a peculiar chemical composition that includes large overabundances of C, N, and O with respect to iron. One important question is whether the chemical abundances in this star reflect the chemical composition of the gas cloud from which it was formed or if they have been severely affected by other processes, such as dust-gas winnowing. Aims. We measure or provide an upper limit to the abundance of the volatile element sulphur, which can help to discriminate between the two scenarios. Methods. We observed HE 1327-2326 with the high resolution infra-red spectrograph CRIRES at the VLT to observe the S i lines of Multiplet 3 at 1045 nm. Results. We do not detect the S i line. A 3 sigma upper limit on the equivalent width (EW) of any line in our spectrum is EW < 0.66 pm. Using either one-dimensional static or three-dimensional hydrodynamical model-atmospheres, this translates into a robust upper limit of [S/H] < -2.6. Conclusions. This upper limit does not provide conclusive evidence for or against dust-gas winnowing, and the evidence coming from other elements (e. g., Na and Ti) is also inconclusive or contradictory. The formation of dust in the atmosphere versus an origin of the metals in a metal-poor supernova with extensive >fall-back> are not mutually exclusive. It is possible that dust formation distorts the peculiar abundance pattern created by a supernova with fall-back, thus the abundance ratios in HE 1327-2326 may be used to constrain the properties of the supernova(e) that produced its metals, but with some caution.