Physical conditions in the ISM towards HD185418

We have developed a complete model of the hydrogen molecule as part of the spectral simulation code Cloudy. Our goal is to apply this to spectra of high-redshift star-forming regions where H2 absorption is seen, but where few other details are known, to understand its implication for star formation. The microphysics of H2 is intricate, and it is important to validate these numerical simulations in better-understood environments. This paper studies a well-defined line-of-sight through the Galactic interstellar medium (ISM) as a test of the microphysics and methods we use. We present a self-consistent calculation of the observed absorption-line spectrum to derive the physical conditions in the ISM towards HD185418, a line-of-sight with many observables. We deduce density, temperature, local radiation field, cosmic ray ionization rate, chemical composition and compare these conclusions with conditions deduced from analytical calculations. We find a higher density, similar abundances, and require a cosmic ray flux enhanced over the Galactic background value, consistent with enhancements predicted by MHD simulations.Comment: 31 pages, accepted for publication in Ap

Molecular Hydrogen in the FUSE Translucent Lines of Sight: The Full Sample

We report total abundances and related parameters for the full sample of the FUSE survey of molecular hydrogen in 38 translucent lines of sight. New results are presented for the "second half" of the survey involving 15 lines of sight to supplement data for the first 23 lines of sight already published. We assess the correlations between molecular hydrogen and various extinction parameters in the full sample, which covers a broader range of conditions than the initial sample. In particular, we are now able to confirm that many, but not all, lines of sight with shallow far-UV extinction curves and large values of the total-to-selective extinction ratio, $R_V$ = $A_V$ / $E(B-V)$ -- characteristic of larger than average dust grains -- are associated with particularly low hydrogen molecular fractions ($f_{\rm H2}$). In the lines of sight with large $R_V$, there is in fact a wide range in molecular fractions, despite the expectation that the larger grains should lead to less H$_2$ formation. However, we see specific evidence that the molecular fractions in this sub-sample are inversely related to the estimated strength of the UV radiation field and thus the latter factor is more important in this regime. We have provided an update to previous values of the gas-to-dust ratio, $N$(H$_{\rm tot}$)/$E(B-V)$, based on direct measurements of $N$(H$_2$) and $N$(H I). Although our value is nearly identical to that found with Copernicus data, it extends the relationship by a factor of 2 in reddening. Finally, as the new lines of sight generally show low to moderate molecular fractions, we still find little evidence for single monolithic "translucent clouds" with $f_{\rm H2}$ $\sim$ 1.Comment: 35 pages, 5 tables, 7 figures, accepted for publication in The Astrophysical Journal Supplements Serie

FUSE Observations of Molecular Hydrogen in Translucent Interstellar Clouds: The Line of Sight Toward HD 73882

We report the results of initial FUSE observations of molecular hydrogen (H2) in translucent clouds. These clouds have greater optical depth than any of the diffuse clouds previously observed for far-UV H2 absorption, and provide new insights into the physics and chemistry of such regions. Our initial results involve observations of HD 73882, a well-studied southern hemisphere star lying behind substantial interstellar material (E(B-V) = 0.72; A_V = 2.44). We find a total H2 column density, N(H2) = 1.2 x 10^{21} cm^{-2}, about three times larger than the values for diffuse clouds previously measured in the far-UV. The gas kinetic temperature indicated by the ratio N(J=1)/N(J=0) is 58 +/- 10 K. With the aid of ground-based data to calculate an appropriate multi-component curve of growth, we have determined column densities for all rotational levels up to J = 7. The J >= 2 states can be reasonably fitted with a rotational excitation temperature of 307 +/- 23 K. Both the kinetic and rotational temperatures are similar to those found in previous investigations of diffuse clouds. The ratios of carbonaceous molecules to hydrogen molecules are also similar to ratios in diffuse clouds, suggesting a similar chemistry for this line of sight.Comment: 7 pages, 3 figures, to appear in ApJ Letters (FUSE first-results issue

FUSE Observations of the HD Molecule toward HD 73882

The Lyman and Werner band systems of deuterated molecular hydrogen (HD) occur in the far UV range below 1200 A. The high sensitivity of the FUSE mission can give access, at moderate resolution, to hot stars shining through translucent clouds, in the hope of observing molecular cores in which deuterium is essentially in the form of HD. Thus, the measurement of the HD/H2 ratio may become a new powerful tool to evaluate the deuterium abundance, D/H, in the interstellar medium. We report here on the detection of HD toward the high extinction star HD 73882 [E(B-V)=0.72]. A preliminary analysis is presented.Comment: 4 pages + 4 .ps figures. This paper will appear in a special issue of Astrophysical Journal Letters devoted to the first scientific results from the FUSE missio

Directional Radiation and Photodissociation Regions in Molecular Hydrogen Clouds

Some astrophysical observations of molecular hydrogen point to a broadening of the velocity distribution for molecules at excited rotational levels. This effect is observed in both Galactic and high redshift clouds. Analysis of H_2, HD, and CI absorption lines has revealed the broadening effect in the absorption system of QSO 1232+082 (z_{abs}=2.33771). We analyze line broadening mechanisms by considering in detail the transfer of ultraviolet radiation (in the resonance lines of the Lyman and Werner H_2 molecular bands) for various velocity distributions at excited rotational levels. The mechanism we suggest includes the saturation of the lines that populate excited rotational levels (radiative pumping) and manifests itself most clearly in the case of directional radiation in the medium. Based on the calculated structure of a molecular hydrogen cloud in rotational level populations, we have considered an additional mechanism that takes into account the presence of a photodissociation region. Note that disregarding the broadening effects we investigated can lead to a significant systematic error when the data are processed.Comment: 14 pages, 10 figure

A Catalog of Diffuse Interstellar Bands in the Spectrum of HD 204827

Echelle spectra of the double-lined spectroscopic binary HD 204827 were obtained on five nights, at a resolving power R = 38,000 and with a S/N ratio = 750 near 6000 A in the final, combined spectrum. The stars show E(B-V) = 1.11 and spectral types near O9.5V and B0.5III. A catalog is presented of 380 diffuse interstellar bands (DIBs) measured between 3900 and 8100 A in the stars' spectrum. The central wavelengths, the widths (FWHM), and the equivalent widths of nearly all of the bands are tabulated, along with the minimum uncertainties in the latter. The reliable removal of very weak stellar lines from the catalog, and of some stellar lines from the less severe blends with DIBs, is made generally easy by the highly variable radial velocities of both stars. The principal result of this investigation is that the great majority of the bands in the catalog are very weak and relatively narrow. Typical equivalent widths amount to a few mA, and the band widths (FWHM) are most often near 0.55 A. Therefore, most of these DIBs can be detected only in spectra obtained at a resolving power and a S/N ratio at least comparable to those used here. In addition, the anomalous interstellar reddening and the very high value of the ratio N(C2)/E(B-V) seen toward HD 204827 indicate that the physical conditions in one or more of the several interstellar clouds seen in this direction differ significantly from those found toward the prototypical DIB target HD 183143, for example. Probably primarily for these reasons, 113 of the 380 bands, or 30%, were not detected in four previous, modern surveys of the DIBs seen in the spectra of stars other than HD 204827. Both the tabulated data for the DIBs and a plot of our spectrum of HD 204827 are available online at http://dibdata.orgComment: 85 pages (17 of text, 14 of tables, 54 of figures). To appear in the Astrophysical Journa

HD/H2 Molecular Clouds in the Early Universe: The Problem of Primordial Deuterium

We have detected new HD absorption systems at high redshifts, z_abs=2.626 and z_abs=1.777, identified in the spectra of the quasars J0812+3208 and Q1331+170, respectively. Each of these systems consists of two subsystems. The HD column densities have been determined: log(N(HD),A)=15.70+/-0.07 for z_A=2.626443(2) and log(N(HD),B)=12.98+/-0.22 for z_B=2.626276(2) in the spectrum of J0812+3208 and log(N(HD),C)=14.83+/-0.15 for z_C=1.77637(2) and log(N(HD),D)=14.61+/-0.20 for z_D=1.77670(3) in the spectrum of Q1331+170. The measured HD/H2 ratio for three of these subsystems has been found to be considerably higher than its values typical of clouds in our Galaxy. We discuss the problem of determining the primordial deuterium abundance, which is most sensitive to the baryon density of the Universe \Omega_{b}. Using a well-known model for the chemistry of a molecular cloud, we have estimated the isotopic ratio D/H=HD/2H_2=(2.97+/-0.55)x10^{-5} and the corresponding baryon density \Omega_{b}h^2=0.0205^{+0.0025}_{-0.0020}. This value is in good agreement with \Omega_{b}h^2=0.0226^{+0.0006}_{-0.0006} obtained by analyzing the cosmic microwave background radiation anisotropy. However, in high-redshift clouds, under conditions of low metallicity and low dust content, hydrogen may be incompletely molecularized even in the case of self-shielding. In this situation, the HD/2H_2 ratio may not correspond to the actual D/H isotopic ratio. We have estimated the cloud molecularization dynamics and the influence of cosmological evolutionary effects on it