Millimetre photometry and infrared spectroscopy of Vega-excess stars

Abstract

JCMT millimetre-wave detections have been obtained for 11 Vega-excess stars having spectral types ranging from B9 to K5. UKIRT 10-µm spectra have been obtained for two of the sources, SAO 179815 and SAO 186777. The spectrum of SAO 179815 shows an unusually broad and diffuse silicate emission feature, whilst SAO 186777 shows the unidentified infrared (UIR) features, which are usually attributed to hydrocarbon vibrational modes. The mm photometry, along with optical, IRAS and near-IR photometry (much of the latter recently obtained by the authors), have been used to define the spectral energy distributions of the objects. A number of them show a 1–5µm excess in addition to the longer wavelength excess. Values of the fractional excess luminosity,L _dust /L _⋆, have been derived from the spectral energy distributions; they exhibit a substantial range, from ∼ 10⁻⁵ up to almost 0.5, the theoretical maximum for a passive optically thick flared disc. Radiative transfer models have been constructed for several sources. One needs a well defined overall energy distribution, 10- and/or 20-µm spectra, and sub-mm and mm photometry in order to significantly constrain the model free parameters (disc density distribution, grain size power-law index, minimum and maximum grain radii).