Session 16 - Interstellar Grains, PAH's and the Interstellar Bands.
Display session, Monday, January 13
Metropolitan Ballroom,

## [16.05] The Abundance of Interstellar Carbon

U. J. Sofia, K. Guerin (Villanova Univ), D. M. Meyer (Northwestern Univ)

The abundance of carbon in the interstellar medium is important for determining the composition of interstellar dust. Most dust grain models require that a substantial fraction of the C abundace resides in interstellar grains and/or polycyclic aromatic hydrocarbons (PAHs). Cardelli et al. (1996, ApJ, 467, 334) showed that for moderately dense lines of sight the measured interstellar gas-phase C/H is quite constant. We use high signal-to-noise (> 1000) data from the Hubble Space Telescope's Goddard High Resolution Spectrograph to extend these measurements to the diffuse line of sight toward \tau CMa. Since many elements show a measureable return from dust to the gas-phase in more diffuse sight lines, the abundance measurement toward \tau CMa will place strong limits on the variability of gas-phase C/H in the interstellar medium. We interpret a varying gas-phase C/H ratio as varying dust-phase abundances of carbon. Therefore, these observations will show the resiliency of carbon-based grains or PAHs.

This work is supported by the HST grant GO-06542.01-95A and the NASA LTSARP grant NAGW-4607.