CONVECTION, CHROMOSPHERIC HEATING AND MIXING OF MATERIAL IN MAIN-SEQUENCE F-TYPE STARS

Abstract

ABSTRACT This thesis falls into the general category of studies of the outer layers of stars on the mid to lower main sequence. It deals specifically with the properties of the convective envelopes of F-type stars, and how these relate to mixing mechanisms in their interiors and energy deposition in their atmospheres. The observational context of the study includes a spectroscopic derivation, using NLTE analysis, of the oxygen abundances in 50 stars belonging to the Hyades cluster and the Ursa Major group, and some additional field stars. The aim was to set restrictions on the mechanism of microscopic diffusion which has been proposed to explain the presence of the "lithium gap" in F stars. The results show a notable uniformity in the abundances derived, across the relevant temperature range: from 5800 K to 7400 K. The data could just be indicating a small dip, of at most 0.1 dex, in the region of the lithium gap. The equivalent widths of the He I D3 line were also measured, in absorption, in 36 stars from the same open clusters and the general field. A data base is presented for 145 stars, of spectral types between late A and K, comprising measurements of several indices of chromospheric and coronal activity, taken from the literature and from our own observations. The investigation of the dependence of activity on spectral type, the correlations between the different indices, and their variations with Rossby number confirm previous suggestions that the dissipation of accoustic waves generated at the top of the convective envelope is the mechanism for heating the chromospheres of early F-type stars. In the theoretical section of the thesis, a mechanism is proposed and developed for mixing material in a stellar interior in order to explain the lithium gap. Mixing is induced by the presence and propagation of internal gravity waves. These waves are generated by the fluctuating pressure applied by the cells at the base of the convection zone to the radiative interior of the star. Our mechanism is capable of accounting for the observations of lithium in F-type stars of the Pleiades and the Hyades, as well as of beryllium in the latter cluster.