Annu. Rev. Astron. Astrophys. 1990. 28: 37-70
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6. OTHER DIAGNOSTICS OF INTERSTELLAR DUST

6.1 Depletions in Interstellar Gas

The strengths of the interstellar absorption lines of various ions in the spectra of background stars indicate the ionic column densities in the gas phase, and by inference the amounts of the elements in grains. Jenkins (78) reviews the method, results, and several pitfalls. The principal results of depletions as regards grains are:

1. The elements O, N, and Zn are slightly depleted with respect to solar abundances, and their depletion does not vary measurably between dense and diffuse gas. The errors are such that they could be undepleted, or depleted by a factor of two.

2. Depletions of other elements increase significantly with average gas density along the line of sight. The elements P, Mg, and Cl are almost undepleted in diffuse gas (<n(H)> approx 0.1 cm-3), and are depleted by about an order of magnitude when <n(H)> approx 10 cm-3.

3. The elements Fe, Cr, and Si are depleted about one order of magnitude in the diffuse ISM, and the depletions go roughly as the square root of the mean gas density, so that their depletion is about a factor of 10 more than P, Mg, and Cl, over a wide range in mean gas density (80). Thus, these depletions are two orders of magnitude along lines of sight through dense gas. This difference in depletion between outer-cloud dust and diffuse dust implies that grains evolve as they go from one environment to another. Ca, Ti, and Al have similar depletions in low-density gas but have a steeper dependence of depletion on mean gas density.

4. The depletion of C is, unfortunately, not reliably determined except for one line of sight: towards delta Sco with the Copernicus satellite (73). For observations with IUE all lines of C+, the dominant ionization stage in H I regions, lie on the ``flat'' (insensitive) portion of the curve of growth. For delta Sco, about 25% of the C is in the gas phase, which is about the amount found in CO in molecular clouds. The gas-phase carbon in the diffuse ISM is apparently simply converted to CO in molecular clouds, while the solid fraction remains approximately fixed.

There is some confusion in the literature because the C depletion can be crudely estimated from the well-determined gas-phase abundance of neutral C by estimating C0 / C+. Unfortunately, the ionization corrections are factors of hundreds to thousands!

5. At a given mean gas density, there is a surprisingly small dispersion of the depletions (about ± 0.3 dex), whereas some depletions vary by one or two orders of magnitude. Much of this dispersion must arise from averaging various local conditions along the line of sight. Again we see that the state of grains must be quite well described by only one parameter (perhaps local gas density).

6. Depletions are a function of z, the height above the plane of the Galaxy (50). Probably the extinction law also depends on z (83).

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