710 research outputs found
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
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
Atomic Physics with the Goddard High Resolution Spectrograph on the Hubble Space Telescope
High quality spectra of interstellar absorption from C I toward beta(sup 1) S(sub co), rho O(sub ph) A, and chi O(sub ph) were obtained with the Goddard High Resolution Spectrograph on HST. Many weak lines were detected within the observed wavelength intervals: 1150-1200 A for beta(sup 1) S(sub co) and 1250-1290 A for rho O(sub ph) A and chi O(sub ph). Curve-of-growth analyses were performed in order to extract 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 C I lines. A particularly important constraint was the need to reproduce data for more than one line of sight. For about 50% of the lines, the derived f-values differ appreciably from the values quoted by Morton
A Reexamination of Phosphorus and Chlorine Depletions in the Diffuse Interstellar Medium
We present a comprehensive examination of interstellar P and Cl abundances
based on an analysis of archival spectra acquired with the Space Telescope
Imaging Spectrograph of the Hubble Space Telescope and the Far Ultraviolet
Spectroscopic Explorer. Column densities of P II, Cl I, and Cl II are
determined for a combined sample of 107 sight lines probing diffuse atomic and
molecular gas in the local Galactic interstellar medium (ISM). We reevaluate
the nearly linear relationship between the column densities of Cl I and H,
which arises from the rapid conversion of Cl to Cl in regions where
H is abundant. Using the observed total gas-phase P and Cl abundances, we
derive depletion parameters for these elements, adopting the methodology of
Jenkins. We find that both P and Cl are essentially undepleted along sight
lines showing the lowest overall depletions. Increasingly severe depletions of
P are seen along molecule-rich sight lines. In contrast, gas-phase Cl
abundances show no systematic variation with molecular hydrogen fraction.
However, enhanced Cl (and P) depletion rates are found for a subset of sight
lines showing elevated levels of Cl ionization. An analysis of neutral chlorine
fractions yields estimates for the amount of atomic hydrogen associated with
the H-bearing gas in each direction. These results indicate that the
molecular fraction in the H-bearing gas is at least 10% for all sight lines
with and that the gas is essentially fully
molecular at .Comment: 44 pages, 21 figures, accepted for publication in Ap
Constraining the Environment of CH+ Formation with CH3+ Observations
The formation of CH+ in the interstellar medium has long been an outstanding
problem in chemical models. In order to probe the physical conditions of the
ISM in which CH+ forms, we propose the use of CH3+ observations. The pathway to
forming CH3+ begins with CH+, and a steady state analysis of CH3+ and the
reaction intermediary CH2+ results in a relationship between the CH+ and CH3+
abundances. This relationship depends on the molecular hydrogen fraction, f_H2,
and gas temperature, T, so observations of CH+ and CH3+ can be used to infer
the properties of the gas in which both species reside. We present observations
of both molecules along the diffuse cloud sight line toward Cyg OB2 No. 12.
Using our computed column densities and upper limits, we put constraints on the
f_H2 vs. T parameter space in which CH+ and CH3+ form. We find that average,
static, diffuse molecular cloud conditions (i.e. f_H2>0.2, T~60 K) are excluded
by our analysis. However, current theory suggests that non-equilibrium effects
drive the reaction C+ + H_2 --> CH+ + H, endothermic by 4640 K. If we consider
a higher effective temperature due to collisions between neutrals and
accelerated ions, the CH3+ partition function predicts that the overall
population will be spread out into several excited rotational levels. As a
result, observations of more CH3+ transitions with higher signal-to-noise
ratios are necessary to place any constraints on models where magnetic
acceleration of ions drives the formation of CH+.Comment: 7 pages, 3 figures, 2 tables, accepted for publication in Ap
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