MILLIMETER PHOTOMETRY AND INFRARED-SPECTROSCOPY OF VEGA-EXCESS STARS

JCMT millimetre-wave detections have been obtained for 11 Vega-excess stars having spectral types ranging from B9 to K5. UKIRT 10-mum spectr a have been obtained for two of the sources, SAO 179815 and SAO 186777 . The spectrum of SAO 179815 shows an unusually broad and diffuse sili cate emission feature, whilst SAO 186777 shows the unidentified infrar ed (UIR) features, which are usually attributed to hydrocarbon vibrati onal modes. The mm photometry, along with optical, IRAS and near-IR ph otometry (much of the latter recently obtained by the authors), have b een used to define the spectral energy distributions of the objects. A number of them show a 1-5 mum excess in addition to the longer wavele ngth excess. Values of the fractional excess luminosity, L(dust)/L, h ave been derived from the spectral energy distributions; they exhibit a substantial range, from approximately 10(-5) up to almost 0.5, the t heoretical maximum for a passive optically thick flared disc. Radiativ e transfer models have been constructed for several sources. One needs a well defined overall energy distribution, 10- and/or 20-mum spectra , and sub-mm and mm photometry in order to significantly constrain the model free parameters (disc density distribution, grain size power-la w index, minimum and maximum grain radii).