Composition of Interstellar Clouds in the Disk and Halo. IV. HD 215733

In this paper we continue our investigation of diffuse clouds in the interstellar medium with an analysis of the line of sight toward the star HD 215733, located in the Galactic halo some 1700 pc below the plane. As in our previous papers, we utilize the component-fitting technique to determine velocities, velocity widths, and column densities for a variety of ions in each of the absorbing regions detected. Our data include a large number of ultraviolet absorption lines observed with the Goddard High Resolution Spectrograph and ground-based observations of Ca II K absorption and H I 21 cm emission. We detect 23 components (absorbing regions) in the low-ionization species toward HD 215733 and seven components in the highly ionized species (i.e., Si³⁺, C³⁺, and N⁴⁺). The low-ion components arise in H I gas. Gas-phase abundances measured for these components follow the pattern seen for halo stars in our previous studies. These have been interpreted as indicating either that a nearly indestructible population of interstellar grains is present or that the intrinsic abundances of a number of elements in the ISM are significantly subsolar.

Kinetic temperatures are estimated for 16 low-ion components. Of these, four (with the highest values of |v_LSR|) are warm, with T > 1000 K; six are cold, with T < 300 K. Extensive diagnostic information includes data on the excited atoms C⁺ and C⁰, showing that in the cold clouds n(H⁰) ~= 25 cm^-3, with n(H⁰)T ~= 2500 K cm^-3. The ionization equilibrium of C⁰, Mg⁰, S⁰, and Ca⁺ gives log n_e values differing systematically by up to 1 dex between these different species. Correction for these differences, together with a somewhat uncertain overall calibration with excited C⁺, gives values of log n_e in the range from -2.1 to -2.7 for the cold clouds in the four best determined cases, and from -1.2 to -1.8 for the 3 warm clouds with measured n_e. The cold cloud values yield n_e/n(H⁰) ~= 2 × 10^-4, which suggests ionization of the heavier elements only (with all H neutral), but values higher by half a dex would also be consistent with the data. Strong absorption features of Si³⁺ and C³⁺ both appear in three components. For two of these, the ratio of b values between these two species equals the square root of the mass ratio, suggesting thermal broadening at temperatures of 6 × 10⁵ K and 5 × 10⁴ K. The column density ratios would require a temperature of ~8 × 10⁴ K in collisional equilibrium.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, under NASA contract NAS 5-2655.