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

ciated with star-grazing planetesimals which produce, at comparable distance to thestar, gas comae similar to those routinely observed in the 0 Pic system. This is consistentwith the inferred clumpiness of the near-stellar circumstellar gas in other HAeBestars (Graham 1992, 1994), and may explain why boundary layer emission, such aswould be expected from accretion of predominantly gaseous material, is not typicallyobserved in association with the less-embedded HAeBe stars. The HD 100546 planetesimaldiffers from those seen toward /3 Pic by retaining mildly refractory specieswhich sublimate at temperatures above 700 K (Jenkins 1987), and by having largeamounts of carbon, nitrogen, and oxygen. The refractory species seen in the UVspectra are consistent with the silicates in this system being dominated by Mg-richolivenes and show a deficiency of siderophile elements.If the planetesimal seen toward this object is representative of 2 Myr-old bodiesassociated with other intermediate-mass PMS stars, comparison of the HD 100546data with 0 Pic suggests that substantial modification of planetesimals which becomestar-grazers occurs between 2-3 Myr and 12 Myr, the inferred age of the ,8 Pic system(Lanz et al. 1995).Acknowledgements. Support for this study was provided by the NASA Long-Term Space Astrophysics Program to Eureka Scientific under NASA Contract NASW4756. Data analysis facilities were provided by the IUE Data Analysis Center locatedin the Laboratory for Astronomy & Solar Physics, NASA/GSFC.ReferencesBeust, H. et al. 1989, Astr. Ap-223, 304.Beust, H. 1995 in Circumstellar Dust Disks and Planet Formation, ed. R. Ferlet,and A. Vidal-Madjar (Paris: Editions Frontieres), p. 35.Bohm, T. and Catala, C. 1995 1 Astr. Ap.301, 155.Grady, C.A. et al. 1996, Astr. Ap. Suppl. Ser.(in press).Graham, J.A. 1992, Pub. A. S. P., 104, 479.Graham, J.A. 1994, ASP Conf. Ser. 62, 363.Grinin, V.P. et al. 1994, Astr. Ap.292, 165.Grinin, V.P., and Rostopchina, A.N. 1996, Astron. Rep. 40, 171.Henning, T. et al. 1994 Astr. Ap.291, 546.Hu, J.Y., The, P.-S., and de Winter, D. 1989, Astr. Ap.208, 213.Jenkins, E.B. 1987 in Interstellar Processes, eds. D.J. Hollenbach and H.A. Thronson(Dordrecht: Reidel), p. 533.Lanz, T. et al. 1995, Ap. d.447,L41.Sitko, M.L. et al. 1995 in Circumstellar Dust Disks and Planet Formation, ed. R.Ferlet, and A. Vidal-Madjar (Paris: Editions Frontieres), p, 389.Sitko, M.L. et al. 1996 (this conference).Vidal-Madjar, A. et al. 1994 Astr. Ap.290,245.Waelkens, C. 1996 (this conference).30