From Stardust to Planetesimals: Contributed Papers - NASA Lunar ...

MODELLING THE DUST AROUND VEGA-LIKE STARSROGER J. SYLVESTER Department of Physics and Astronomy, UniversityCollege London, Gower Street, London WCIE 6BT, UKC.J. SKINNER Space Telescope Science Institute, 3700 San Martin Drive,Baltimore, MD21218, USAM.J. BARLOW Department of Physics and Astronomy, University CollegeLondon, Gower Street, London WCI E 6BT, UKABSTRACT. Models are presented of four Vega-like stars: main-sequence starswith infrared emission from circumstellar dust. The dusty environments of the fourstars are rather diverse, as shown by their spectral energy distributions. Good fits tothe observations were obtained for all four stars.1. INTRODUCTIONVega-like stars are main-sequence stars with excess mid- and far-infrared emissiondue to circumstellar dust, which is thought to be distributed in discs. We have recentlyundertaken an observational survey of Vega-like stars, taken mainly from thelist of Walker & Wolstencroft (1988). Our observations include optical, near-IR andmillimetre-wave photometry, and mid-IR spectroscopy (Sylvester et al 1996). Similarobservations of a number of these sources have also been published by Walker & Butner(1995) and Butner et al (1996). In parallel with our observing programme, wehave modelled several of our targets using a radiative transfer code based on that developedby Skinner, Barlow & Justtanont (1992) to model SAO 179815 (=HD 98800).2. TECHNIQUE AND RESULTSThe model uses multiple grain sizes (typically 15 sizes are used) and dust materials(usually silicate and/or amorphous carbon). Power laws are used to describe thegrain-size distribution and the variation of dust density in the disc with distancefrom the star. The power-law indices are treated as free parameters. The grainoptical properties, such as absorption efficiency, are calculated from the bulk opticalconstants using Mie theory, and the discs are treated as optically thin. Some resultsof the modelling are presented in Table 1 and Figures 1-4. A fuller account of themodel technique and results may be found in Sylvester & Skinner (1996).The four stars presented here have rather different spectral energy distributions(SEDs), and give some idea of the variety of properties displayed by Vega-like stars.SAO 158350 has the smallest fractional excess luminosity (L,.,1L * ), and the smallestderived dust mass (see Table 1). Its mid-IR spectrum is consistent with there being