THE INTERSTELLAR CARBON BUDGET AND THE ROLE OF CARBON IN DUST AND LARGE MOLECULES

Published data on stellar composition show that carbon in the sun is s ubstantially more abundant than in other stars. A carbon abundance of 225 carbon atoms per 10(6) hydrogen atoms is representative of galacti c stars, whereas Published values for the sun range from 350 to 470 ca rbon atoms per 10(6) hydrogen atoms. Other elements are also present i n enhanced quantities in the solar system, consistent with suggestions that a supernova event was closely associated with the formation of t he solar system. The overabundance of carbon in the solar system has m any important implications, including new constraints on nucleosynthes is models for supernovae and substantial modification of the so-called ''cosmic'' composition normally adopted in discussions of galactic an d interstellar abundances. A reduction in the galactic carbon budget, as suggested by the stellar composition data, strongly constrains the quantity of carbon that is available for the formation of interstellar dust, and some dust models now appear implausible because they requir e more carbon than is available.