Interstellar Depletion onto Very Small Dust Grains

Abstract

We consider the depletion of elements from the interstellar gas onto a population of very small dust grains. Adopting a grain model in which D4% of the cosmic C abundance is in grains with radii ¹10we Ðnd that the rate of accretion onto these grains is fast enough to account for the observed large A , depletions of elements like Ti, without invoking unreasonably high rates of mass transfer between interstellar phases or low grain destruction rates. If these grains are composed of arene rings, then only a limited number of metal atoms can be locked up in them. The depletion would be quenched when this limit is reached, unless there is a mechanism for transferring the metals to larger grains and refreshing the very small grain populationÈfor example, by grain coagulation and shattering in the di †use interstellar medium (ISM). If Fe depletes onto the very small grains, then for reasonable coagulation rates there is at least one metal atom per Ðve C atoms in the very small grain population. Furthermore, B60% of the cosmic Fe is associated with the carbonaceous grain population. It is unclear whether this scenario is compatible with observations. However, if there is another population of very small grains, with a large capacity for holding Fe atoms, it might be the sink for the most heavily depleted elements.

The turbulent life of dust grains in the supernova-driven, multiphase interstellar medium

“…Their scheme of mass exchange is also not supported by our results as we observe intense mass transfer from the warm to the molecular phase in the simulations with mixed driving and from the molecular to the cold phase in the case of random driving, which are absent in Weingartner & Draine (1999). The transition rates included in their scheme of the order of a few 10 −8 yr −1 are roughly similar to the values predicted by our simulations, except the last 20 Myr for the mixed and peak driving simulations, when the dominant mass exchange rates exceed 10 −7 yr −1 .…”

“…It controls the circulation of gas from the WNM, where dust abundances are reduced by destruction in interstellar shocks, and matter enriched with dust due to accretion on grain surfaces at higher gas densities. Several models of dust evolution in the idealised two-and three-phase ISM have been proposed to explain the observed differences between element depletion in the warm medium and cold clouds (Draine 1990;O'Donnell & Mathis 1997;Tielens 1998;Weingartner & Draine 1999). Models with a two-phase ISM neglect the difference between the molecular and diffuse H i clouds and consider mass circulation only between the ambient warm medium and clouds (O'Donnell & Mathis 1997;Tielens 1998).…”

“…O'Donnell & Mathis (1997) showed that a three-phase ISM model with an additional mass exchange with the molecular clouds reproduces the observations better. Assuming a steady state for the interchange between phases and timescales of dust destruction and accretion, one can estimate the rates of mass exchange required to reproduce the observed element depletion (Draine 1990;Weingartner & Draine 1999). The resulting rates, however, depend on the adopted scheme of mass transfer between phases that in the case of the three-phase ISM can occur via different routes.…”

“…For random and mixed driving, we study the implications of our measured transition rates for predictions of element depletion on dust grains with the simple model of Weingartner & Draine (1999). They used the observed depletion to infer the transition rates between the phases in their mass transfer scheme.…”

“…We compute the Ti depletion for Model A and B (Table 3 in Weingartner & Draine 1999) that differ by the size distribution of small grains and the destruction timescale τ d . Both models assume extensions of the MRN power law (Mathis et al 1977) dngr(a) da ∼ a −K (6) with K = 3.5.…”