173 research outputs found
Neutral atomic absorption lines and far-UV extinction: Possible implications for depletions and grain parameters
Researchers examine nine lines of sight within the Galaxy and one in the Large Magellanic Cloud (LMC) for which data on both neutral atomic absorption lines (Snow 1984; White 1986; Welty, Hobbs, and York 1989) and far UV extinction (Bless and Savage 1972; Jenkins, Savage, and Spitzer 1986) are available, in order to test the assumption that variations in gamma/alpha will cancel in taking ratios of the ionization balance equation, and to try to determine to what extent that assumption has affected the aforementioned studies of depletions and grain properties
Interstellar Ti II in the Milky Way and Magellanic Clouds
We discuss several sets of Ti II absorption-line data, which probe a variety
of interstellar environments in our Galaxy and in the Magellanic Clouds.
Comparisons of high-resolution (FWHM ~ 1.3-1.5 km/s) Ti II spectra of Galactic
targets with corresponding high-resolution spectra of Na I, K I, and Ca II
reveal both similarities and differences in the detailed structure of the
absorption-line profiles -- reflecting component-to-component differences in
the ionization and depletion behaviour of those species. Moderate-resolution
(FWHM ~ 3.4-4.5 km/s) spectra of more heavily reddened Galactic stars provide
more extensive information on the titanium depletion in colder, denser clouds
-- where more than 99.9 per cent of the Ti may be in the dust phase.
Moderate-resolution (FWHM ~ 4.5-8.7 km/s) spectra of stars in the Magellanic
Clouds suggest that the titanium depletion is generally much less severe in the
LMC and SMC than in our Galaxy [for a given N(H_tot), E(B-V), or molecular
fraction f(H_2)] -- providing additional evidence for differences in depletion
patterns in those two lower-metallicity galaxies. We briefly discuss possible
implications of these results for the interpretation of gas-phase abundances in
QSO absorption-line systems and of variations in the D/H ratio in the local
Galactic ISM.Comment: 56 pages, 26 figures, accepted to MNRA
Detection of Interstellar C_2 and C_3 in the Small Magellanic Cloud
We report the detection of absorption from interstellar C_2 and C_3 toward
the moderately reddened star Sk 143, located in the near 'wing' region of the
SMC, in optical spectra obtained with the ESO VLT/UVES. These detections of C_2
(rotational levels J=0-8) and C_3 (J=0-12) absorption in the SMC are the first
beyond our Galaxy. The total abundances of C_2 and C_3 (relative to H_2) are
similar to those found in diffuse Galactic molecular clouds -- as previously
found for CH and CN -- despite the significantly lower average metallicity of
the SMC. Analysis of the rotational excitation of C_2 yields an estimated
kinetic temperature T_k ~ 25 K and a moderately high total hydrogen density n_H
~ 870 cm^-3 -- compared to the T_01 ~ 45 K and n_H ~ 85-300 cm^-3 obtained from
H_2. The populations of the lower rotational levels of C_3 are consistent with
an excitation temperature of about 34 K.Comment: accepted to MNRAS; 10 pages, 6 figure
Spatial and Temporal Variations in Interstellar Absorption toward HD 72127AB
New optical spectra of Ca II and Na I toward HD 72127AB provide additional
evidence for both spatial and temporal variations in the complex interstellar
absorption along the two sight lines; archival UV spectra yield information on
the abundances, depletions, and physical conditions in the gas toward HD
72127A. Similarities in the strengths of various tracers of interstellar
material in the two lines of sight suggest that the total hydrogen column
densities [N(H) ~ 2.5 x 10^{20} cm^{-2}] and the depletions and ionization in
the main components at low LSR velocities also are similar. Toward HD 72127A,
the main components are relatively cool (T < 900 K), but with depletions
resembling those found in warm, diffuse disc clouds; the generally weaker
components at higher velocities have much milder depletions, more like those
found in halo clouds. Several trace neutral species -- Ca I, Cr I, and Fe I --
are much stronger toward HD 72127B, however. The column density of Cr I, for
example, is about 30 times the value determined toward zeta Oph (the only
previous detection of that species in the ISM). Dielectronic recombination in
warmer gas (T > 5000 K) may be largely responsible for the enhanced abundances
of those trace neutral species toward HD 72127B. If the main components toward
HD 72127AB are associated with material in the Vela SNR, the differences in
abundances and physical conditions occur on scales of about 1100 AU.Comment: 13 pages, 4 figures, accepted to MNRA
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