P. de Laverny — Page 8

arXiv2001

VLT Observations of Turnoff stars in the Globular Cluster NGC 6397

VLT-UVES high resolution spectra of seven turnoff stars in the metal-poor globular cluster NGC 6397 have been obtained. Atmospheric parameters and abundances of several elements (Li, Na, Mg, Ca, Sc, Ti, Cr, Fe, Ni, Zn and Ba) were derived for program stars. The mean iron abundance is [Fe/H] = -2.02, with no star-to-star variation. The mean abundances of the alpha-elements (Ca, Ti) and of the iron-peak elements (Sc, Cr, Ni) are consistent with abundances derived for field stars of similar metallicity. Magnesium is also almost solar, consistent with the values found by Idiart & Thévenin (2000) when non-LTE effects (NLTE hereafter) are taken into account. The sodium abundance derived for five stars is essentially solar, but one object (A447) is clearly Na deficient. These results are compatible with the expected abundance range estimated from the stochastic evolutionary halo model by Argast et al. (2000) when at the epoch of [Fe/H] $\sim$ -2 the interstellar medium is supposed to become well-mixed.

astro-ph↗

arXiv2000

Lithium and rotation on the subgiant branch II. Theoretical analysis of observations

Lithium abundances and rotation, determined for 120 subgiant stars in Lebre et al. (1999) are analyzed. To this purpose, the evolutionary status of the sample as well as the individual masses have been determined using the HIPPARCOS trigonometric parallax measurements to locate very precisely our sample stars in the HR diagram. We look at the distributions of A(Li) and Vsini with mass when stars evolve from the main sequence to the subgiant branch. For most of the stars in our sample we find good agreement with the dilution predictions. However, the more massive cool stars with upper limits of Li abundances show a significant discrepancy with the theoretical predictions, even if the Non-LTE effects are taken into account. For the rotation behaviour, our analysis confirms that low mass stars leave the main sequence with a low rotational rate, while more massive stars are slowed down only when reaching the subgiant branch. We also checked the connection between the observed rotation behaviour and the magnetic braking due to the deepening of the convective envelope. Our results shed new light on the lithium and rotation discontinuities in the evolved phase.

astro-ph↗