The carbon-to-oxygen ratio in stars with planets

In some recent works, the C/O abundance ratio in high-metallicity stars with planets is found to vary from 0.4 to about 1.0. This has led to discussions about the existence of terrestrial planets with a carbon-dominated composition that is very different from the composition of the Earth. The C/O values were obtained by determining carbon abundances from high-excitation CI lines and oxygen abundances from the forbidden [OI] line at 6300 A. This weak line is, however, strongly affected by a nickel blend at high metallicities. Aiming for more precise C/O ratios, oxygen abundances in this paper are derived from the high-excitation OI triplet at 7774 A and carbon abundances from the CI lines at 5052 and 5380 A using MARCS model atmospheres and including non-LTE corrections. The results do not confirm the high C/O ratios previously found. C/O shows a tight, slightly increasing dependence on metallicity from C/O=0.58 at [Fe/H]=0.0 to C/O=0.70 at [Fe/H] =0.4 with an rms scatter of only 0.06. Assuming that the composition of a proto-planetary disk is the same as that of the host star, the C/O values found in this paper lend no support to the existence of carbon-rich planets. The small scatter of C/O among thin-disk stars suggests that the nucleosynthesis products of Type II supernovae and low- to intermediate-mass stars are well mixed in the interstellar medium.Comment: 10 pages, 11 figures, accepted for publication in A&

High-precision abundances of elements in solar twin stars: Trends with stellar age and elemental condensation temperature

HARPS spectra with S/N > 600 for 21 solar twin stars are used to determine very precise (sigma ~ 0.01 dex) differential abundances of C, O, Na, Mg, Al, Si, S, Ca, Ti, Cr, Fe, Ni, Zn, and Y in order to see how well [X/Fe] is correlated with elemental condensation temperature, Tc. In addition, precise (sigma < 0.8 Gyr) stellar ages are obtained by interpolating between Yonsei-Yale isochrones in the logg - Teff diagram. It is confirmed that the ratio between refractory and volatile elements is lower in the Sun than in most of the solar twins, but for many stars, the relation between [X/Fe] and Tc is not well defined. For several elements there is, instead, an astonishingly tight correlation between [X/Fe] and stellar age with amplitudes up to 0.2 dex over an age interval of 8 Gyr in contrast to the lack of correlation between [Fe/H] and age. While [Mg/Fe] increases with age, the s-process element yttrium shows the opposite behavior so that [Y/Mg] can be used as a sensitive chronometer for Galactic evolution. [Na/Fe] and [Ni/Fe] are not well correlated with stellar age, but define a tight Ni-Na relation similar to that previously found for more metal-poor stars. These results provide new constraints on supernovae yields and Galactic evolution. Furthermore, it is found that the C/O ratio evolves very little with time, which is of interest for discussions of the composition of exoplanets.Comment: 13 pages, 14 figures, and 2 on-line tables. To appear in A&

For the Public Good

[Excerpt] This early warning meeting began the labor and community struggle in Hammond, Indiana to save LaSalle Steel, a 350-worker specialty products steel mill owned by Texas-based Quanex Corporation. A year later, the plant would expand rather than shut down, the union at LaSalle would be stronger than ever, and the labor-community coalition led by the Calumet Project for Industrial Jobs would celebrate its biggest anti-shutdown victory yet