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

OH+ in Diffuse Molecular Clouds

Near ultraviolet observations of OH+ and OH in diffuse molecular clouds reveal a preference for different environments. The dominant absorption feature in OH+ arises from a main component seen in CH+ (that with the highest CH+/CH column density ratio), while OH follows CN absorption. This distinction provides new constraints on OH chemistry in these clouds. Since CH+ detections favor low-density gas with small fractions of molecular hydrogen, this must be true for OH+ as well, confirming OH+ and H2O+ observations with the Herschel Space Telescope. Our observed correspondence indicates that the cosmic ray ionization rate derived from these measurements pertains to mainly atomic gas. The association of OH absorption with gas rich in CN is attributed to the need for high enough density and molecular fraction before detectable amounts are seen. Thus, while OH+ leads to OH production, chemical arguments suggest that their abundances are controlled by different sets of conditions and that they coexist with different sets of observed species. Of particular note is that non-thermal chemistry appears to play a limited role in the synthesis of OH in diffuse molecular clouds.Comment: 15 pages, 4 figures, to appear in 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 $\lambda$ Ori, 1 Sco, and $\delta$ 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 NpNnN_pN_n Scheme and the Structure of the Valence Space

The $N_pN_n$ 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

Oscillator strengths for transitions to Rydberg levels in 12C16O^{12}C^{16}O, 13C16O^{13}C^{16}O and 13C18O^{13}C^{18}O between 967 and 972 A

Absorption oscillator strengths have been determined from high-resolution spectra in the 967-972 \AA region of three CO isotopomers for transitions to the Rydberg levels 4{\it p$\pi$}(0), 3{\it d$\pi$}(1) and 4{\it p$\sigma$}(0), as well as to the mixed {\it E(6)} level recently characterized by Eidelsberg et al. (2004). Synchrotron radiation from the Super-ACO electron storage ring at Orsay (LURE) was used as a light source. Oscillator strengths were extracted from the recorded spectra by least-squares fitting of the experimental profiles with synthetic spectra taking into account the homogeneous and heterogeneous interactions of the four levels. Column densities were derived from fits to the 3{\it p$\pi$}(0) absorption band whose oscillator strength is well established. These are the first reported measurements for $^{13}$C$^{18}$O. For $^{12}$C$^{16}$O, our results are consistent with the larger values obtained in the most recent laboratory and astronomical studies.Comment: 9 pages 7 figures 3 tables. Accepted in A&A, date of acceptance 11/05/200

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$_2$, which arises from the rapid conversion of Cl$^+$ to Cl$^0$ in regions where H$_2$ 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$_2$-bearing gas in each direction. These results indicate that the molecular fraction in the H$_2$-bearing gas is at least 10% for all sight lines with $\log N({\rm H}_2)\gtrsim18$ and that the gas is essentially fully molecular at $\log N({\rm H}_2)\approx21$.Comment: 44 pages, 21 figures, accepted for publication in Ap