554 research outputs found
A comparison of UV surface brightness and HI surface densities for spiral galaxies
Shaya and Federman (1987) suggested that the ambient ultraviolet flux at 1000 A permeating a spiral galaxy controls the neutral hydrogen (HI) surface density in the galaxy. They found that the atomic envelopes surrounding small molecular clouds, because of their great number, provide the major contribution to the HI surface density over the stellar disk. The increase in HI surface density with later Hubble types was ascribed to the stronger UV fields from more high-mass stars in later Hubble types. These hypotheses are based on the observations of nearby diffuse interstellar clouds, which show a sharp atomic-to-molecular transition (Savage et al. 1977), and on the theoretical framework introduced by Federman, Glassgold, and Kwan (1979). Atomic envelopes around interstellar clouds in the solar neighborhood arise when a steady state is reached between photodissociation of H2 and the formation of H2 on grains. The photodissociation process involves photons with wavelengths between 912 A and 1108 A. Shaya and Federman used H-alpha flux as an approximate measure for the far UV flux and made their comparisons based on averages over Hubble type. Here, researchers compare, on an individual basis, UV data obtained with space-borne and balloon-borne instruments for galaxies with measurements of HI surface density (Warmels 1988a, b). The comparisons substantiate the conclusion of Shaya and Federman that the far UV field controls the HI content of spiral galaxies
Density Variations over Subparsec Scales in Diffuse Molecular Gas
We present high-resolution observations of interstellar CN, CH, CH^{+},
\ion{Ca}{1}, and \ion{Ca}{2} absorption lines toward the multiple star systems
HD206267 and HD217035. Substantial variations in CN absorption are observed
among three sight lines of HD206267, which are separated by distances of order
10,000 AU; smaller differences are seen for CH, CH^{+}, and \ion{Ca}{1}. Gas
densities for individual velocity components are inferred from a chemical
model, independent of assumptions about cloud shape. While the component
densities can differ by factors of 5.0 between adjacent sightlines, the
densities are always less than 5000 cm^{-3}. Calculations show that the derived
density contrasts are not sensitive to the temperature or reaction rates used
in the chemical model. A large difference in the CH^{+} profiles (a factor of 2
in column density) is seen in the lower density gas toward HD217035.Comment: 9 pages, 2 figures. Accepted for publication in ApJ
Revisiting the Chlorine Abundance in Diffuse Interstellar Clouds from Measurements with the Copernicus Satellite
We reanalyzed interstellar Cl I and Cl II spectra acquired with the
Copernicus satellite. The directions for this study come from those of Crenny &
Federman and sample the transition from atomic to molecular rich clouds where
the unique chemistry leading to molecules containing chlorine is initiated. Our
profile syntheses relied on up-to-date laboratory oscillator strengths and
component structures derived from published high-resolution measurements of K I
absorption that were supplemented with Ca II and Na I D results. We obtain
self-consistent results for the Cl I lines at 1088, 1097, and 1347 A from which
precise column densities are derived. The improved set of results reveals
clearer correspondences with H2 and total hydrogen column densities. These
linear relationships arise from rapid conversion of Cl^+ to Cl^0 in regions
where H2 is present.Comment: 17 pp, 2 tables, and 3 figures, to appear in The Astrophysical
Journa
High-Resolution Measurements of Intersystem Bands of Carbon Monoxide toward X Persei
In an echelle spectrum of X Per acquired with the Space Telescope Imaging
Spectrograph we have identified individual rotational lines of 11
triplet-singlet (intersystem) absorption bands of ^12CO. Four bands provide
first detections for interstellar clouds. From a comparison with the zeta Oph
sight line we find that X Per is obscured by a higher 12CO column density of
1.4 x 10^16 cm-2. Together with the high spectral resolution of 1.3 km s-1,
this allows (i) an improved measurement of previously published f-values for
seven bands, and (ii) an extraction of the first astrophysical oscillator
strengths for d-X (8-0), (9-0), and (10-0), as well as for e-X (12-0). The
^13CO d-X (12-0) band, previously suspected to exist toward zeta Oph, is now
readily resolved and modeled. Our derived intersystem f-values for ^12CO
include a few mild (leq 34%) disagreements with recent predictions from a
perturbation analysis calculated for the interstellar excitation temperature.
Overall, the comparison confirms the superiority of employing multiple singlet
levels in the calculations of mixing coefficients over previous single-level
predictions.Comment: 11 pages (incl. 1 figure). Accepted by ApJ Letter
Atomic Physics with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. V. Oscillator Strengths for Neutral Carbon Lines below 1200 Angstrom
We analyzed high resolution spectra of interstellar neutral carbon absorption
toward Ori, 1 Sco, and Sco that were obtained with the
Goddard High Resolution Spectrograph on the Hubble Space Telescope. Several
multiplets were detected within the wavelength interval 1150 to 1200 A, where
most neutral carbon lines have ill-defined oscillator strength; multiplets at
longer wavelengths with well-defined atomic parameters were also seen. We
extracted accurate column densities and Doppler parameters from lines with
precise laboratory-based f-values. These column densities and b-values were
used to obtain a self-consistent set of f-values for all the observed neutral
carbon lines. For many of the lines with wavelength below 1200 A, the derived
f-values differ appreciably from the values quoted in the compilation by Morton
(1991). The present set of f-values extends and in some cases supersedes those
given in Zsargo et al. (1997), which were based on lower resolution data.Comment: 8 pages, 2 figures, 6 tables, accepted for publication in Ap
Generalization of the Scheme and the Structure of the Valence Space
The scheme, which has been extensively applied to even-even nuclei,
is found to be a very good benchmark for odd-even, even-odd, and doubly-odd
nuclei as well. There are no apparent shifts in the correlations for these four
classes of nuclei. The compact correlations highlight the deviant behavior of
the Z=78 nuclei, are used to deduce effective valence proton numbers near Z=64,
and to study the evolution of the Z=64 subshell gap.Comment: 10 pages, 4 figure
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