THE GAS AND DUST ABUNDANCES OF DIFFUSE HALO CLOUDS IN THE MILKY WAY

摘要

We present Goddard High-Resolution Spectrograph observations of the interstellar Mg II, Si II, P II, S II, Mn II, Cr II, Fe II, Ni II, Zn II, and Ge II lines toward HD 116852, a star at-1.3 kpc from the Galactic plane and a distance of 4.8 kpc in the direction l = 304.9°, b = -16.1°. The intermediate-resolution (FWHM ≈ 11-18 km s~(-1)) observations of these species have S/N ≈ 30-90. The high-resolution (FWHM = 3.5 km s~(-1)) observations of Mn II and Fe II reveal a rich interstellar velocity structure extending from ≈ -70 to +35 km s~(-1). The velocity separation of the absorption components caused by differential Galactic rotation allows us to study the abundances of halo clouds below the Sagittarius and Norma spiral arms. We find that the gas-phase abundances of Mg, Si; S, Mn, Cr, Fe, and Ni relative to Zn in the HD 116852 halo clouds are very similar to those of clouds in the low halo (0.3 ≤ ∣z∣ ≤ 1.5 kpc) toward high-latitude stars in the solar neighborhood. There is a progression toward increasing gas-phase abundance from the Galactic disk to halo. For the entire cloud sample, the average logarithmic gas-phase abundance ratios relative to solar abundance ratios are [Mg/ Zn] = -0.52, [Si/Zn] = -0.26, [S/Zn] = -0.05, [Mn/Zn] = -0.61, [Cr/Zn] = -0.51, [Fe/Zn] = -0.64, and [Ni/Zn] = -0.84. Since Zn is not depleted in the warm neutral gas, these abundance values should be the same as [X/H], There are no systematic differences in these abundances for halo clouds with galactocentric distances ≈7-10 kpc. We find that dust grains contain approximately 70% of the Mg, 45% of the Si, and 77% of the Fe in the halo clouds. The small variation in the halo cloud abundances strongly supports the idea that the cores of the dust grains are resilient and difficult to destroy completely by the processes that circulate gas from the disk to the halo. The gas- and dust-phase abundance patterns in the halo clouds are consistent with more severe grain destruction in the clouds at greater distances from the plane. We calculate (Mg + Fe)/ Si = 3.26 ± 0.64 in the dust grains if solar abundances are used as references and (Mg ± Fe)/Si = 3.91 ± 0.64 if B-star reference abundances are used. These ratios imply that there must be grains composed of Fe-oxides, or perhaps pure Fe, in the halo clouds. Since pure Fe grains are destroyed much more rapidly than silicates behind fast shocks, we favor silicates and oxides as the likely constituents of grain material in the halo clouds.