Can Composite Fluffy Dust Particles Solve the Interstellar Carbon Crisis?

Recent observations suggest that the abundance of carbon in the interstellar medium is only ∼ 60% of its solar value, and that other heavy elements may be depleted by a similar amount. Furthermore, more than half of the interstellar carbon is observed to be in the gas in the form of C, leaving less than ∼ 40% of the solar carbon abundance available for the dust phase. These observations have created the so-caled interstellar ”carbon crisis”, since traditional interstellar dust models require about twice that value to be tied up in graphite grains in order to explain the interstellar extinction curve. Recently, Mathis (1996) suggested a possible solution to this crisis. In his newly-proposed dust model the majority of the interstellar carbon is contained in composite and fluffy grains that are made up of silicates and amorphous carbon grains, with 45% of their volume consisting of vacuum. Per unit mass, these grains produce more UV extinction, and can therefore account for the interstellar extinction curve with about half the carbon required in traditional dust models. This paper presents an detailed assessment of the newly-proposed dust model, and concludes that it falls short in solving the carbon crisis, in providing a fit to the UV-optical interstellar extinction curve. It also predicts a far-infrared emissivity in excess of that observed with the COBE/DIRBE and FIRAS instruments from the diffuse interstellar medium. This excess infrared emission is a direct consequence of the lower albedo of the composite fluffy dust particles, compared to that of the traditional MRN mixture of bare silicate and graphite grains. The failure of the new model highlights the interrelationships between the various dust properties and their observational consequences, and the need to satisfy them all simultaneously in any comprehensive interstellar dust model. In light of these problems, the paper examines other possible solutions to the carbon crisis. Subject headings: interstellar: grains, abundances infrared: radiation spectrum, ISM: dust, extinction Laboratory for Astronomy and Solar Physics, Code 685, NASA/Goddard Space Flight Center, Greenbelt, MD 20771. e-mail address: [email protected]