Composition of interstellar clouds in the disk and halo. 3: HD 149881
L. Spitzer, E. L. Fitzpatrick;
ApJ, 1995, 445, 196
ABSTRACT:The spectrum of HD 149881, a halo star 1300 pc above the Galactic plane, has been analyzed, based on
high-resolution GHRS data from Hubble Space Telescope (HST) together with 21 cm
emission observations and profiles of Na I and Ca II features.
The results
reveal 11 absorption components, of which nine are produced in H I regions,
with log N(H(sup 0) in each region mostly approximately 19.5; one component
clearly originates in an H II region.
Detailed column densities show that
(D(Zn)), the absolute value of the depletion D, for Zn, like that for S, does not
much exceed 0.1 dex.
With Zn used as a standard of reference, D(Fe) varies
from -1.1 to -0.6 dex; (D(Cr)) and (D(Mn)) average less than (D(Fe)) by about
0.1 and 0.2 dex, respectively, with dispersions of about 0.1 dex in this
difference.
As in Papers I and II, the measured D(Si) is consistent with a 2 to 1 ratio of
Fe to Si atoms in grains.
The kinetic temperature is these H I regions
varies widely among components.
Maximum temperatures, found from the
widths of 21 cm emission components, range from about 100 to 10,000 K, while
actual temperatures, found by comparing the H I widths with the b-values for
the ultraviolet features, are about equal or less than 1000 K for six of the H
I components.
In contrast, toward the halo star HD 93521, temperatures
of about 6000 K were found for most of the components.
With use of these HD
149881 temperatures, values of n(sub e) were obtained from the ionization
equilibrium of Ca(+) and also of Na(0).
These values range over an order of
magnitude, from about 4.9 x 10(exp -2) to 1.3 x 10(exp -3)/cu cm, systematically
smaller (but also less certain) than in HD 93521.
For the low values of n(sub e)
found in the coldest components, the electrons can be accounted for by
ionization of C atoms, if n(sub H) approximately equals 10/cu cm.
For the warmest
component, an ionization probability of beta(H(sup 0)) approximately equals 3 x
10(exp -15)/s is required, with lower values for other H I components.
KEYWORDS: abundance, b stars, binary stars, galactic halos, interstellar extinction, interstellar gas, milky way galaxy, ultraviolet spectra, absorption spectra, electron density (concentration), hubble space telescope, ionization, spectral line width, spectrum analysis, temperature distribution
PERSOKEY:absorption, hst, h_i, 21 cm, uv, h+, ,
CODE: spitzer95