Composition of Interstellar Clouds in the Disk and Halo. IV. HD 215733
E. L. Fitzpatrick, L. Spitzer;
ApJ, 1997, 475, 623
ABSTRACT: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 |vLSR|) 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 ne 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 ne 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 ne.
The cold cloud values yield ne/n(H0) ~= 2 x 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 x 105 K
and 5 x 104 K.
The column density ratios would require a temperature of ~8 x
104 K in collisional equilibrium.
KEYWORDS: ism: abundances, ism: clouds, ism: dust, extinction, galaxy: halo, stars: individual henry draper number: hd 215733, ultraviolet: ism
PERSOKEY:absorption, hst, h_i, 21 cm, uv, h+, ,
CODE: fitzpatrick97