Spectroscopic Observations of Convective Patterns in the Atmospheres of Metal‐poor Stars

Convective line asymmetries in the optical spectrum of two metal-poor stars,Gmb1830 and HD140283, are compared to those observed for solar metallicitystars. The line bisectors of the most metal-poor star, the subgiant HD140283,show a significantly larger velocity span that the expectations for asolar-metallicity star of the same spectral type and luminosity class. Theenhanced line asymmetries are interpreted as the signature of the lower metalcontent, and therefore opacity, in the convective photospheric patterns. Thesefindings point out the importance of three-dimensional convective velocityfields in the interpretation of the observed line asymmetries in metal-poorstars, and in particular, urge for caution when deriving isotopic ratios fromobserved line shapes and shifts using one-dimensional model atmospheres. The mean line bisector of the photospheric atomic lines is compared withthose measured for the strong Mg I b1 and b2 features. The upper part of thebisectors are similar, and assuming they overlap, the bottom end of thestronger lines, which are formed higher in the atmosphere, goes much further tothe red. This is in agreement with the expected decreasing of the convectiveblue-shifts in upper atmospheric layers, and compatible with the high velocityredshifts observed in the chromosphere, transition region, and corona oflate-type stars.Comment: 27 pages, LaTeX; 10 Figures (14 PostScript files); to be published in The Astrophysical Journa