High-resolution spectroscopy of Vega-like stars - I. Effective temperatures, gravities and photospheric abundances
Vega-like stars are young main-sequence stars exhibiting an excess emission of infrared radiation. Modelling this excess depends not only on the parameters assigned to the grains, but on those assigned to the stars themselves. In an effort to update and improve the information available on this class of star, we have analysed 13 stars classed as Vega-like, having an infrared excess attributable to dust emission, along with two spectral standards which have also been found to show excess emission from dust. In this, the first of two papers, we derive stellar properties (spectral type, effective temperature and log g) and photospheric abundances. The spectral types derived revealed that one of the sample was a luminosity class III giant, ruling it out of the Vega-like class, and two others underwent a significant reclassification. The remainder had their type confirmed. All but two programme stars have been found to be emission-line stars - their emission-line properties are discussed in Paper II. Attention has recently been drawn to the possible link between Vega-like stars and the photospheric metal-depleted class of A-type stars, the lambda Bootis stars. These latter stars are hypothesized to have obtained their underabundances by the accretion of depleted circumstellar gas on to the photosphere of the star. Since Vega-like stars are expected to have discs of dust, it might be expected that accretion may cause this same phenomenon. We have analysed four A-type stars in our sample and two A-type standards, deriving photospheric abundances for up to 10 elements. No pattern of underabundance similar to lambda Bootis stars was found, although a depletion of silicon was found in two stars (up to 0.86 dex below solar) and of magnesium in one star (0.56 dex lower). The depletion could be attributable to the accretion of those elements on to grains in the circumstellar environment of these stars.