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.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1996