Interstellar Dust - B.T. Draine

7. DUST ABUNDANCES VS. DEPLETION PATTERNS

Jenkins (2003) finds that observed depletions on many sightlines suggest two types of grain material: a "core" material which is returned to the gas phase only in very high velocity shock waves, and a "mantle" material which is more easily stripped. For low velocity gas, variations in depletion from one sightline to another are interpreted as due to invariant grain cores plus varying amounts of mantle material.

What do Jenkins' abundances of elements in the cores and mantles suggest as regards the volumes of cores and mantles? Table 1 shows one possible mix of minerals which could reproduce the overall elemental compositions deduced by Jenkins for sightlines with "depletion multiplier" F_* = 1, representative of "cool disk" material. The aims of this exercise are to estimate (1) how much oxygen is likely to be in the grain material, and (2) the volumes of core and mantle material which would be consistent with the depletion patterns found by Jenkins, for comparison with the grain volume required by physical dust models. The compositions in Table 1 are purely illustrative, and should not be taken to be realistic.

**Table 1.** Jenkins (2003) Grain Composition: One Illustrative Possibility

Material	C^a	O^a	Mg^a	Si^a	Al^a	Ca^a	Fe^a	Ni^a	_b	V^c

Grain Cores
C,PAH,HAC,...	71	-	-	-	-	-	-	-	2.2	6.5
MgFeSiO₄ olivine	-	52	13	13	-	-	13	-	3.8	9.8
CaMgSiO₄ monticellite	-	8	2	2	-	2	-	-	3.2	1.6
Fe₂O₃ hematite	-	18	-	-	-	-	12	-	5.3	3.0
Al₂O₃ corundum	-	4.5	-	-	3	-	-	-	4.02	0.6
Ni₂O₃ dinickel trioxide	-	2.4	-	-	-	-	-	1.6	4.84	0.5

Illustrative Core Total	71	85	15	15	3	2	25	1.6	3.5	22.1
Observed Core Total^d	71_-71⁺⁶¹	53_-53⁺⁴⁹	15	14	3.0	2.2	25	1.6

Grain Mantles
C,PAH,HAC,...	35	-	-	-	-	-	-	-	2.2	3.2
Mg_0.9Fe_0.1SiO₃ pyroxene	-	57	17	19	-	-	2	-	3.3	9.9

Illustrative Mantle Total	35	57	17	19	-	-	2	-	3.5	13.1

Cores + Mantles
C,PAH,HAC,...	106	-	-	-	-	-	-	-	2.2	9.7^e
silicates	-	117	32	34	-	2	15	-	3.5	21.4^e
other	-	24	-	-	3	-	12	1.6	5.2	4.0

Illustrative Core + Mantle Total	106	142	32	34	3	2	27	1.6	3.5	35.2^e
Observed Core + Mantle Total^d	106_-20⁺¹⁶	134_-23⁺²²	32	33	3.0	2.2	28	1.8

^a Atomic abundance (ppm) per total H.
^b Solid density (g cm^-3).
^c Grain volume per total H (10^-28 cm³).
^d From Jenkins (2003). Quoted uncertainties do not include uncertainties in assumed total abundances.
^e Models that reproduce the observed interstellar extinction per H require a greater volume of grain material than provided by the depletions found by Jenkins (2003). A model based on carbonaceous grains plus silicate grains (see Draine 2003a) has V = 37 × 10^-28 cm³/H for silicate grains and V = 21 × 10^-28 cm³ / H for carbonaceous grains (see text).

In Table 1 the overall composition of the core material is consistent with Jenkins' results. The amounts of Mg, Fe, and Si in the mantle require ~ 57 ppm O if combined into pyroxene - consistent with the O/H = 81_-58⁺⁵⁴ ppm which Jenkins assigns to the mantle.

Studies of total depletions infer the amount of grain material based on an assumed value for the total abundance of each element; Jenkins adopted current estimates for solar abundances. It is important to remember that estimates of solar abundances have varied considerably over time; the review by Anders & Grevesse (1989) had C/H = 363 ± 35 ppm and O/H = 851 ± 72 ppm, whereas the most recent redeterminations find C/H = 246 ± 23 ppm (Allende Prieto et al. 2001) and O/H = 490 ± 47 ppm (Allende Prieto et al. 2002) - C/H has gone down by a factor 1.5, and O/H by a factor 1.7! Tomorrow's "solar abundance" values may differ from today's. Furthermore, interstellar abundances may not be equal to solar, or even to the abundances in young stars, as discussed in Section 10 below.

The total grain volume indicated by Jenkins' abundance estimates is 35 × 10^-28 cm³ / H, only ~ 60 % of the total grain volume for the physical dust model of Weingartner & Draine (2001a). The dust modeling has approximated the dust as solid spheres, and it is expected that nonspherical porous grains would allow the extinction to be reproduced with a slightly reduced total solid volume, but it isn't clear that this would reduce the required solid volume by 40%.

One is compelled to consider seriously the possibility that interstellar abundances of the depleted elements - especially C - may exceed solar abundances. This will be discussed further in Section 10.