From interstellar abundances to grain composition: the major dust constituents Mg, Si, and Fe

We analyse observational correlations for three elements entering into the composition of interstellar silicate and oxide grains. Using current solar abundances, we converted the gas phase abundances into dust phase abundances for 196 sightlines. We deduce a sharp difference in abundances for sightlines located at low (|b| < 30 degrees) and high (|b| > 30 degrees) galactic latitudes. For high-latitude stars, the ratios Mg/Si and Fe/Si in dust are close to 1.5. For disk stars they are reduced to Mg/Si similar to 1.2 and Fe/Si similar to 1.05. The derived numbers indicate that 1) the dust grains cannot be the mixture of silicates with olivine and pyroxene composition only, and some amount of magnesium or iron (or both) should be in another population and that 2) the destruction of Mg-rich grains in the warm medium is more effective than for Fe-rich grains. We reveal a decrease in dust phase abundances and correspondingly an increase in gas phase abundances with distance D for stars with D greater than or similar to 400 pc. We attribute this to an observational selection effect: a systematic trend toward lower observed hydrogen column density for distant stars. We find differences in abundances for disk stars with low (E(B - V) less than or similar to 0.2) and high (E(B-V) greater than or similar to 0.2) reddenings that reflect the distinction between the sightlines passing through diffuse and translucent interstellar clouds. For Scorpius-Ophiuchus, we detect a uniform increase in dust phase abundances of Mg and Si with an increase in the ratio of total to selective extinction R(V) and a decrease in the strength of the far-UV extinction. This is the first evidence of growth of Mg-Si grains due to accretion in the interstellar medium.