Composition of interstellar clouds in the disk and halo. 2: $\gamma^2$ Velorum
E. L. Fitzpatrick, L. Spitzer;
ApJ, 1994, 427, 232
ABSTRACT:High-resolution observations of gamma(sup 2) Vel with the Goddard High-Resolution Spectrograph (GHRS) echelle on the
Hubble Space Telescope reveal the presence of seven narrow absorption
components, with LSR velocities between -23 and +9 km s(exp -1).
Three of these show
column density ratios N(S(++))/N(S(+)) and N(P(++))/N(P+)) of about 1 or
more, and can be identified as H II regions, while the other four are H I
regions, consistent with the O I profile and with the overall H(sup 0) column
density of 5.9 x 10(exp 19) cm(exp -2), given the usual assumptions that S is
undepleted while O has a depletion D(O) = -0.3 dex.
The depletions of Fe, Si, and Mn,
which could be measure accurately for two of the four H I regions (components 6
and 7), differ somewhat from the values of D(sub ws) found for slowly moving
warm clouds in HD 93521; in particular, for the component at 4.0 km s(exp -1)
(No.
6), absolute of D exceeds absolute of D(sub ws) by 0.1-0.4 dex, while for
that at 9.3 km s(exp -1) (No.
7), absolute of D equals absolute of D(sub ws) on
the average.
The observed ratio of Fe + Mg atoms to Si atoms in the grains of
component 6 is 2.04 +/-0.10, consistent with an olivine grain composition; the
Fe/Mg ratio is 1.5 +/- 0.2.
The electron density in component 6, determined
from the C II(sup *) feature, is 0.075 +/- 0.013 cm (exp -3), about two-thirds
of that found for clouds of this velocity in HD 93521.
In the two
conspicuous H II regions, components 3 and 4, n(sub e), determined from the Si II(sup
*) feature, is about 1 cm(exp -3).
From the column density of S(+) + S(++)
in these two components, the total H II path length is about 40 pc.
With the
radius of a wind-blown bubble around gamma(sup 2) Vel set equal to 60 pc, the
effective Stromgren radius is about 100 pc, requiring that T approx.
equal to
50,000 K for the Wolf-Rayet component of the gamma(sup 2) Vel binary.
Since
zeta Pup is a comparable source of ionizing radiation, this temperature is
an upper limit.
The profiles of the strongest H2 absorption features,
from Copernicus archives, indicate that the absorbing molecules have a
mean velocity identical with that of the strongest H II component
(No.
4).
We have no explanation for the possible presence of these H2 molecules in a
region of ionized H.
Alternatively, the H2 profiles can be explained by
molecules in the two adjacent (in velocity) H I regions, components 2 and 5,
provided their H I gas has densities and temperatures typical of normal cold
clouds.
The GHRS data show absorption by highly ionized atoms Si(3+) and C(3+),
N(4+) in broad features, in addition to the narrow-line absorption by Si(3+)
and C(3+) observed in the dominant H II components, Nos.
3 and 4.
The broad
C(3+) and N(4+) features have widths corresponding to T in the range (4-8) x
10(exp 5) K, consistent with the broad O(5+) line shown in Copernicus
data.
Despite some observational uncertainties, the ratios of column densities in
the broad C(3+), N(4+), and O(5+) features agree to +/- 0.1 dex with
theoretical values for warm gas, heating and evaporating by thermal conduction
from an adjacent hot region.
Outward evaporation from an isolated cloud in
a hot ambient gas cannot be distinguished, on the basis of these data,
from inward evaporation of a warm shell, compressed by an expanding, hot
stellar-wind bubble.
For several halo stars, the C IV/O VI ratio has a quite
different average value, perhaps consistent with cooling of infalling hot gas
instead of conductive heating and evaporation.
KEYWORDS: atmospheric density, data reduction, galaxies, halos, interstellar gas, stellar spectra, stellar winds, ultraviolet spectra, absorption spectra, atmospheric temperature, echelle gratings, emission spectra, hubble space telescope, infrared astronomy satellite, iue, line of sight, optical thickness, radial velocity, spectrographs, thermal conductivity
PERSOKEY:absorption, uv, HST, h_i, H+, ,
CODE: fitzpatrick94