A study of Vega-like stars and their circumstellar dust.

摘要

Fifteen Vega-like stars have been studied with the aid of optical, infrared and millimetre wave data. In many cases, the optical characteristics of these Vega-like stars have been studied for the very first time. Attempts have been made to determine their effective temperatures, surface gravities, photospheric abundances and the ages of the later type stars through a lithium abundance analysis. Four of the stars have had their spectral types reclassified. Mg and Si underabundances have been found in the photosphere of one star, and a Si depletion in one other. Two of the later type stars in the sample have been found to be young enough to be classed as T Tauri stars. The sample's emission line characteristics have been analysed and evidence for circumstellar gas sought through analysis of narrow absorption lines superimposed on the photospheric Ca II K, Na I D and Fe II fines. The emission-fine activity of several of the stars are seen to exhibit similar characteristics to their respective pre-main sequence counterparts (the Herbig Ae/Be and T Tauri stars). Circumstellar gas has been detected towards two stars, HD 144432 and 51 Oph. The two primary components of the HD 35187 system have been spectrally resolved for the first time. Analysis of the two show that HD 35187B is surrounded by a disk of circumstellar dust, whilst the other composnent is not. Both stars are still pre-main sequence objects, although very close to the zero-age main sequence, aged ~10 million years. Optically thick radiative transfer modelling has been undertaken, and spherical vs. non-spherical geometries have been explored. It is found that the majority of the Vega-like stars in this sample cannot be associated with spherical envelopes, but instead are likely to be surrounded by disks or toroids of dust. The spectral energy distribution of HD 144432 has been modelled in its entirety for the first time using a single density distribution. It has been found that a population of mm-sized grains is required to account for the sub-mm and mm fluxes observed in this system, and in the HD 142666 system. The near-IR excesses observed in some Vega-like stars are compared to those of the pre-main sequence HAeBe stars, and their evolutionary status discussed. It is shown that there is a definite trend in the near-IR excesses found for the Herbig Ae/Be stars to the Vega-likes, strongly suggesting that the two groups are related. The ages and spectral characteristics of some of the Vega-like stars would lead to them being classed as Herbig Ae stars were it not for the fact that they are field stars, generally quite far from known star-forming regions.