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Newly-computed collisional rate coefficients for the excitation of C$_2$ in collisions with H$_2$, presented recently by Najar and Kalugina (2020), are significantly larger than the values adopted previously in models for the excitation of the C$_2$ molecule, a widely used probe of the interstellar gas density. With these new rate coefficients, we have modeled the C$_2$ rotational distributions inferred from visible and ultraviolet absorption observations of electronic transitions of C$_2$ towards a collection of 46 nearby background sources. The inferred gas densities in the foreground interstellar clouds responsible for the observed C$_2$ absorption are a factor 4 to 7 smaller than those inferred previously, a direct reflection of the larger collisional rate coefficients computed by Najar and Kalugina (2020). These lower density estimates are generally in good agreement with the peak densities inferred from 3D extinction maps for the relevant sightlines. In cases where H$_3^+$ absorption has also been observed and used to estimate the cosmic-ray ionization rate (CRIR), our estimates of the latter will also decrease accordingly because the H$_3^+$ abundance is a function of the ratio of the CRIR to the gas density., Comment: 46 pages, 11 figures, 1 figure set with 23 figures, accepted for publication in the Astrophysical Journal. Version 2: An error is corrected in Table 2, in which values listed in two of the columns were jumbled in Version 1. There are no other changes

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 ApJ

We discuss the interstellar absorption from many atomic and molecular species seen in high-resolution $HST$/STIS UV and high-S/N optical spectra of the moderately reddened B3-5~V star HD~62542. This remarkable sight line exhibits both very steep far-UV extinction and a high fraction of hydrogen in molecular form -- with strong absorption from CH, C$_2$, CN, and CO, but weak absorption from CH$^+$ and most of the commonly observed diffuse interstellar bands. Most of the material resides in a single narrow velocity component -- offering a rare opportunity to probe the primarily molecular core of a single interstellar cloud with little associated diffuse atomic gas. Detailed analyses of the spectra indicate that: (1) the molecular fraction in the main cloud is high [$f$(H$_2$) $>$ 0.8]; (2) the gas is fairly cold ($T_{\rm k}$ = 40--43 K, from the rotational excitation of H$_2$ and C$_2$); (3) the local hydrogen density $n_{\rm H}$ $\sim$ 1500 cm$^{-3}$ (from the C$_2$ excitation, the fine-structure excitation of C$^0$, and simple chemical models); (4) the unusually high excitation temperatures for $^{12}$CO and $^{13}$CO may be largely due to radiative excitation; (5) $N$(C$^+$):$N$(CO):$N$(C) $\sim$ 100:10:1; (6) the depletions of many elements are more severe than those seen in any other sight line, and the detailed pattern of depletions differs from those derived from larger samples of Galactic sight lines; and (7) the various neutral/first ion ratios do not yield consistent estimates for the electron density, even when the effects of grain-assisted recombination and low-temperaure dielectronic recombination are considered., Comment: accepted to ApJ, 89 pages, 13 tables, 26 figures

Aiming for a new and more comprehensive DIB catalog between 4,000 and 9,000{\AA}, we revisited the Atlas Catalog based on the observations of HD 183143 and HD 204827 (Hobbs et al. 2008 and 2009). Twenty-five medium-to-highly reddened sight lines were selected, sampling a variety of spectral types of the background star and the interstellar environments. The median SNR of these spectra is ~ 1,300 around 6,400{\AA}. Compared to the Atlas Catalog, 22 new DIBs were found, and the boundaries of 27 (sets) of DIBs were adjusted, resulting in an updated catalog containing 559 DIBs that we refer to as the Apache Point Observatory Catalog of Optical Diffuse Interstellar Bands. Measurements were then made based on this catalog. We found our survey most sensitive between 5,500 and 7,000{\AA} due largely to the local SNR of the spectra, the relative absence of interfering stellar lines, and the weakness of telluric residuals. For our data sample, the number of DIBs detected in a given sight line is mostly dependent on EB-V and less on the spectral type of the background star. Some dependence on fH2 is observed, but less well-determined due to the limited size of the data sample. The variations of the wavelengths of each DIB in different sight lines are generally larger than those of the known interstellar lines CH+, CH, and K I. Those variations could be due to the inherent error in the measurement, or to differences in the velocity components among sight lines.

We study the behavior of eight diffuse interstellar bands (DIBs) in different interstellar environments, as characterized by the fraction of hydrogen in molecular form [$f$(H$_2$)], with comparisons to the corresponding behavior of various known atomic and molecular species. The equivalent widths of the five "normal" DIBs ($\lambda\lambda$5780.5, 5797.1, 6196.0, 6283.8, and 6613.6), normalized to $E(B-V)$, show a "Lambda-shaped" behavior: they increase at low $f$(H$_2$), peak at $f$(H$_2$) ~ 0.3, and then decrease. The similarly normalized column densities of Ca, Ca$^+$, Ti$^+$, and CH$^+$ also decline for $f$(H$_2$) > 0.3. In contrast, the normalized column densities of Na, K, CH, CN, and CO increase monotonically with $f$(H$_2$), and the trends exhibited by the three C$_2$ DIBs ($\lambda\lambda$4726.8, 4963.9, and 4984.8) lie between those two general behaviors. These trends with $f$(H$_2$) are accompanied by cosmic scatter, the dispersion at any given $f$(H$_2$) being significantly larger than the individual errors of measurement. The Lambda-shaped trends suggest the balance between creation and destruction of the DIB carriers differs dramatically between diffuse atomic and diffuse molecular clouds; additional processes besides ionization and shielding are needed to explain those observed trends. Except for several special cases, the highest $W$(5780)/$W$(5797) ratios, characterizing the so-called "sigma-zeta effect", occur only at $f$(H$_2$) < 0.2. We propose a sequence of DIBs based on trends in their pair-wise strength ratios with increasing $f$(H$_2$). In order of increasing environmental density, we find the $\lambda$6283.8 and $\lambda$5780.5 DIBs, the $\lambda$6196.0 DIB, the $\lambda$6613.6 DIB, the $\lambda$5797.1 DIB, and the C$_2$ DIBs., Comment: accepted to ApJ; 53 pages; 12 figures; 3 tables

We report additional detections of the chloronium molecular ion, H$_2$Cl$^+$, toward four bright submillimeter continuum sources: G29.96, W49N, W51, and W3(OH). With the use of the HIFI instrument on the Herschel Space Observatory, we observed the $2_{12}-1_{01}$ transition of ortho-H$_2^{35}$Cl$^+$ at 781.627 GHz in absorption toward all four sources. Much of the detected absorption arises in diffuse foreground clouds that are unassociated with the background continuum sources and in which our best estimates of the $N({\rm H_2Cl^+})/N({\rm H})$ ratio lie in the range $(0.9 - 4.8) \times 10^{-9}$. These chloronium abundances relative to atomic hydrogen can exceed the predictions of current astrochemical models by up to a factor of 5. Toward W49N, we have also detected the $2_{12}-1_{01}$ transition of ortho-H$_2^{37}$Cl$^+$ at 780.053 GHz and the $1_{11}-0_{00}$ transition of para-H$_2^{35}$Cl$^+$ at 485.418 GHz. These observations imply $\rm H_2^{35}Cl^+/H_2^{37}Cl^+$ column density ratios that are consistent with the solar system $^{35}$Cl/$^{37}$Cl isotopic ratio of 3.1, and chloronium ortho-to-para ratios consistent with 3, the ratio of spin statistical weights., Comment: 31 pages, including 7 figures. Accepted for publication in the ApJ

Anomalously broad diffuse interstellar bands (DIBs) at 5780.5, 5797.1, 6196.0, and 6613.6 A are found in absorption along the line of sight to Herschel 36, the star illuminating the bright Hourglass region of the H II region Messier 8. Interstellar absorption from excited CH+ in the J=1 level and from excited CH in the J=3/2 level is also seen. To our knowledge, neither those excited molecular lines nor such strongly extended DIBs have previously been seen in absorption from interstellar gas. These unusual features appear to arise in a small region near Herschel 36 which contains most of the neutral interstellar material in the sight line. The CH+ and CH in that region are radiatively excited by strong far-IR radiation from the adjacent infrared source Her 36 SE. Similarly, the broadening of the DIBs toward Herschel 36 may be due to radiative pumping of closely spaced high-J rotational levels of relatively small, polar carrier molecules. If this picture of excited rotational states for the DIB carriers is correct and applicable to most DIBs, the 2.7 degree cosmic microwave background may set the minimum widths (about 0.35 A) of known DIBs, with molecular processes and/or local radiation fields producing the larger widths found for the broader DIBs. Despite the intense local UV radiation field within the cluster NGC 6530, no previously undetected DIBs stronger than 10 mA in equivalent width are found in the optical spectrum of Herschel 36, suggesting that neither dissociation nor ionization of the carriers of the known DIBs by this intense field creates new carriers with easily detectable DIB-like features. Possibly related profile anomalies for several other DIBs are noted., Comment: 36 pages; 8 figures; accepted to ApJ; higher quality versions of figures 1 and 2 may be found at http://astro.uchicago.edu/~dwelty/Her36/

Hydrocarbons are ubiquitous in the interstellar medium, observed in diverse environments ranging from diffuse to molecular dark clouds and strong photon-dominated regions near HII regions. Recently, two broad diffuse interstellar bands (DIBs) at 4881{\AA} and 5450{\AA} were attributed to the linear version of propynylidene l-C3H2, a species whose more stable cyclic conformer c-C3H2 has been widely observed in the diffuse interstellar medium at radio wavelengths. This attribution has already been criticized on the basis of indirect plausibility arguments because the required column densities are quite large, N(l-C3H2)/EB-V = 4 \times 1014 cm-2 mag-1. Here we present new measurements of N(l-C3H2) based on simultaneous 18-21 GHz VLA absorption profiles of cyclic and linear C3H2 taken along sightlines toward extragalactic radiocontinuum background sources with foreground Galactic reddening EB-V = 0.1 - 1.6 mag. We find that N(l-C3H2)/N(c-C3H2) ? 1/15 - 1/40 and N(l-C3H2)/EB-V ? 2 \pm 1 \times 1011 cm-2 mag-1, so that the column densities of l-C3H2 needed to explain the diffuse interstellar bands are some three orders of magnitude higher than what is observed. We also find N(C4H)/EB-V < 1.3 \times 1013 cm-2 mag-1 and N(C4H-)/EB-V < 1 \times 1011 cm-2 mag-1 (3?). Using available data for CH and C2H we compare the abundances of small hydrocarbons in diffuse and dark clouds as a guide to their ability to contribute as DIB carriers over a wide range of conditions in the interstellar medium., Comment: Accepted for ApJ Letters

Using the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared (HIFI), we have observed para-chloronium (H2Cl+) toward six sources in the Galaxy. We detected interstellar chloronium absorption in foreground molecular clouds along the sight-lines to the bright submillimeter continuum sources Sgr A (+50 km/s cloud) and W31C. Both the para-H2-35Cl+ and para-H2-37Cl+ isotopologues were detected, through observations of their 1(11)-0(00) transitions at rest frequencies of 485.42 and 484.23 GHz, respectively. For an assumed ortho-to-para ratio of 3, the observed optical depths imply that chloronium accounts for ~ 4 - 12% of chlorine nuclei in the gas phase. We detected interstellar chloronium emission from two sources in the Orion Molecular Cloud 1: the Orion Bar photodissociation region and the Orion South condensation. For an assumed ortho-to-para ratio of 3 for chloronium, the observed emission line fluxes imply total beam-averaged column densities of ~ 2.0E+13 cm-2 and ~ 1.2E+13 cm-2, respectively, for chloronium in these two sources. We obtained upper limits on the para-H2-35Cl+ line strengths toward H2 Peak 1 in the Orion Molecular cloud and toward the massive young star AFGL 2591. The chloronium abundances inferred in this study are typically at least a factor ~10 larger than the predictions of steady-state theoretical models for the chemistry of interstellar molecules containing chlorine. Several explanations for this discrepancy were investigated, but none has proven satisfactory, and thus the large observed abundances of chloronium remain puzzling., Comment: Accepted for publication in the Astrophysical Journal

We establish correlations between equivalent widths of eight diffuse interstellar bands (DIBs), and examine their correlations with atomic hydrogen, molecular hydrogen, and EB-V . The DIBs are centered at \lambda\lambda 5780.5, 6204.5, 6283.8, 6196.0, 6613.6, 5705.1, 5797.1, and 5487.7, in decreasing order of Pearson\^as correlation coefficient with N(H) (here defined as the column density of neutral hydrogen), ranging from 0.96 to 0.82. We find the equivalent width of \lambda 5780.5 is better correlated with column densities of H than with E(B-V) or H2, confirming earlier results based on smaller datasets. We show the same is true for six of the seven other DIBs presented here. Despite this similarity, the eight strong DIBs chosen are not well enough correlated with each other to suggest they come from the same carrier. We further conclude that these eight DIBs are more likely to be associated with H than with H2, and hence are not preferentially located in the densest, most UV shielded parts of interstellar clouds. We suggest they arise from different molecules found in diffuse H regions with very little H (molecular fraction f<0.01). Of the 133 stars with available data in our study, there are three with significantly weaker \lambda 5780.5 than our mean H-5780.5 relationship, all of which are in regions of high radiation fields, as previously noted by Herbig. The correlations will be useful in deriving interstellar parameters when direct methods are not available. For instance, with care, the value of N(H) can be derived from W{\lambda}(5780.5)., Comment: Accepted for publication in The Astrophysical Journal; 37 pages, 11 figures, 6 tables

Optical nebular emission lines are commonly used to estimate the star formation rate of galaxies and the black hole accretion rate of their central active nucleus. The accuracy of the conversion from line strengths to physical properties depends upon the accuracy to which the lines can be corrected for dust attenuation. For studies of single galaxies with normal amounts of dust, most dust corrections result in the same derived properties within the errors. However, for statistical studies of populations of galaxies, or for studies of galaxies with higher dust contents such as might be found in some classes of "transition" galaxies, significant uncertainty arises from the dust attenuation correction. We compare the strength of the predominantly unobscured mid-IR [NeII]15.5um + [NeIII]12.8um emission lines to the optical H alpha emission lines in four samples of galaxies: (i) ordinary star forming galaxies, (ii) optically selected dusty galaxies, (iii) ULIRGs, (iv) Seyfert 2 galaxies. We show that a single dust attenuation curve applied to all samples can correct H alpha emission for dust attenuation to a factor better than 2. Similarly, we compare mid-IR [OIV] and optical [OIII] luminosities to find that [OIII] can be corrected to a factor better than 3. This shows that the total dust attenuation suffered by the AGN narrow line region is not significantly different to that suffered by the starforming HII regions in the galaxy. We provide explicit dust attenuation corrections, together with errors, for [OII], [OIII] and H alpha. The best-fit average attenuation curve is slightly greyer than the Milky-Way extinction law, indicating either that external galaxies have slightly different typical dust properties to the Milky Way, or that there is a significant contribution from scattering. Finally, we uncover an intriguing correlation between Silicate absorption and Balmer decrement., Comment: 19 pages, 9 figures, accepted for publication in MNRAS

CONTEXT: Water vapour maser emission from evolved oxygen-rich stars remains poorly understood. Additional observations, including polarisation studies and simultaneous observation of different maser transitions may ultimately lead to greater insight. AIMS: We have aimed to elucidate the nature and structure of the VY CMa water vapour masers in part by observationally testing a theoretical prediction of the relative strengths of the 620.701 GHz and the 22.235 GHz maser components of ortho water vapour. METHODS: In its high-resolution mode (HRS) the Herschel Heterodyne Instrument for the Infrared (HIFI) offers a frequency resolution of 0.125 MHz, corresponding to a line-of-sight velocity of 0.06 km/s, which we employed to obtain the strength and linear polarisation of maser spikes in the spectrum of VY CMa at 620.701 GHz. Simultaneous ground based observations of the 22.235 GHz maser with the Max-Planck-Institut f\"ur Radioastronomie 100-meter telescope at Effelsberg, provided a ratio of 620.701 GHz to 22.235 GHz emission. RESULTS:We report the first astronomical detection to date of water vapour maser emission at 620.701 GHz. In VY CMa both the 620.701 and the 22.235 GHz polarisation are weak. At 620.701 GHz the maser peaks are superposed on what appears to be a broad emission component, jointly ejected asymmetrically from the star. We observed the 620.701 GHz emission at two epochs 21 days apart, both to measure the potential direction of linearly polarised maser components and to obtain a measure of the longevity of these components. Although we do not detect significant polarisation levels in the core of the line, they rise up to approximately 6% in its wings.

We consider five indicators for intrinsic AGN luminosity: the luminosities of the [OIII]$\lambda$5007 line, the [OIV]25.89$\mu$m line, the mid-infrared (MIR) continuum emission by the torus, and the radio and hard X-ray (E $>$ 10keV) continuum emission. We compare these different proxies using two complete samples of low-redshift type 2 AGN selected in a homogeneous way based on different indicators: an optically selected [OIII] sample and a mid-infrared selected 12$\mu$m sample. We examine the correlations between all five different proxies, and find better agreement for the [OIV], MIR, and [OIII] luminosities than for the hard X-ray and radio luminosities. Next, we compare the ratios of the fluxes of the different proxies to their values in unobscured Type 1 AGN. The agreement is best for the ratio of the [OIV] and MIR fluxes, while the ratios of the hard X-ray to [OIII], [OIV], and MIR fluxes are systematically low by about an order-of-magnitude in the Type 2 AGN, indicating that hard X-ray selected samples do not represent the full Type 2 AGN population. In a similar spirit, we compare different optical and MIR diagnostics of the relative energetic contributions of AGN and star formation processes in our samples of Type 2 AGN. We find good agreement between the various diagnostic parameters, such as the equivalent width of the MIR polycyclic aromatic hydrocarbon features, the ratio of the MIR [OIV]/[NeII] emission-lines, the spectral index of the MIR continuum, and the commonly used optical emission-line ratios. Finally, we test whether the presence of cold gas associated with star-formation leads to an enhanced conversion efficiency of AGN ionizing radiation into [OIII] or [OIV] emission. We find that no compelling evidence exists for this scenario for the luminosities represented in this sample (L$_{bol}$ $\approx$ 10$^{9}$ - 8 $\times$ 10$^{11}$ L$_{\sun}$). (abridged), Comment: accepted for publication in ApJ; 58 pages

We report the detection of absorption lines by the reactive ions OH+, H2O+ and H3O+ along the line of sight to the submillimeter continuum source G10.6$-$0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH+ at 971 GHz, H2O+ at 1115 and 607 GHz, and H3O+ at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6$-$0.4 as well as in the molecular gas directly associated with that source. The OH+ spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H2O+ lines remains lower than 1 in the same velocity range, and the H3O+ line shows only weak absorption. For LSR velocities between 7 and 50 kms$^{-1}$ we estimate total column densities of $N$(OH+) $> 2.5 \times 10^{14}$ cm$^{-2}$, $N$(H2O+) $\sim 6 \times 10^{13}$ cm$^{-2}$ and $N$(H3O+) $\sim 4.0 \times 10^{13}$ cm$^{-2}$. These detections confirm the role of O$^+$ and OH$^+$ in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH+ by the H2O+ column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.

Large optical surveys provide an unprecedented census of galaxies in the local Universe, forming an invaluable framework into which more detailed studies of objects can be placed. But how useful are optical surveys for understanding the co-evolution of black holes and galaxies, given their limited wavelength coverage, selection criteria, and depth? In this conference paper I present work-in-progress comparing optical and mid-IR diagnostics of three "unusual" low redshift populations (luminous Seyferts, dusty Balmer-strong AGN, ULIRGs) with a set of ordinary star-forming galaxies from the SDSS. I address the questions: How well do the mid-infrared and optical diagnostics of star formation and AGN strength agree? To what extent do optical surveys allow us to include extreme, dusty, morphologically disturbed galaxies in our "complete" census of black hole-galaxy co-evolution?, Comment: Proceedings of contributed talk at "Co-Evolution of Central Black Holes and Galaxies" (eds. B.M. Peterson, R.S. Somerville, and T. Storchi-Bergmann), IAU symposium 267, August 2009. 6 pages, 5 figures

(abridged)The majority of Active Galactic Nuclei (AGN) suffer from significant obscuration by surrounding dust and gas. X-ray surveys in the 2-10 keV band will miss the most heavily-obscured AGN in which the absorbing column density exceeds $\sim10^{24}$cm$^{-2}$ (the Compton-thick AGN). It is therefore vital to know the fraction of AGN that are missed in such X-rays surveys and to determine if these AGN represent some distinct population in terms of the fundamental properties of AGN and/or their host galaxies. In this paper we present the analysis of \textit{XMM-Newton} X-ray data for a complete sample of 17 low-redshift Type 2 Seyfert galaxies chosen from the Sloan Digital Sky Survey based solely on the high observed flux of the [OIII]$\lambda$5007 emission-line. This line is formed in the Narrow Line Region hundreds of parsecs away from the central engine. Thus, unlike the X-ray emission, it is not affected by obscuration due to the torus surrounding the black hole. It therefore provides a useful isotropic indicator of the AGN luminosity. As additional indicators of the intrinsic AGN luminosity, we use the Spitzer Space Telescope to measure the luminosities of the mid-infrared continuum and the [OIV]25.89$\mu$m narrow emission-line. We then use the ratio of the 2-10 keV X-ray luminosity to the [OIII], [OIV], and mid-infrared luminosities to assess the amount of X-ray obscuration and to distinguish between Compton-thick and Compton-thin objects. We find that the majority of the sources suffer significant amounts of obscuration: the observed 2-10 keV emission is depressed by more than an order-of-magnitude in 11 of the 17 cases (as expected for Compton-thick sources)., Comment: accepted for publication to ApJ; 48 pages, 15 figures

We present mid-infrared spectral maps of the NGC 1333 star forming region, obtained with the the Infrared Spectrometer on board the Spitzer Space Telescope. Eight pure H2 rotational lines, from S (0) to S (7), are detected and mapped. The H2 emission appears to be associated with the warm gas shocked by the multiple outflows present in the region. A comparison between the observed intensities and the predictions of detailed shock models indicates that the emission arises in both slow (12 - 24 km/s) and fast (36 - 53 km/s) C-type shocks with an initial ortho-to-para ratio of ~ 1. The present H2 ortho-to-para ratio exhibits a large degree of spatial variations. In the post-shocked gas, it is usually about 2, i.e. close to the equilibrium value (~ 3). However, around at least two outflows, we observe a region with a much lower (~ 0.5) ortho-to-para ratio. This region probably corresponds to gas which has been heated-up recently by the passage of a shock front, but whose ortho-to-para has not reached equilibrium yet. This, together with the low initial ortho-to-para ratio needed to reproduce the observed emission, provide strong evidence that H2 is mostly in para form in cold molecular clouds. The H2 lines are found to contribute to 25 - 50% of the total outflow luminosity, and thus can be used to ascertain the importance of star formation feedback on the natal cloud. From these lines, we determine the outflow mass loss rate and, indirectly, the stellar infall rate, the outflow momentum and the kinetic energy injected into the cloud over the embedded phase. The latter is found to exceed the binding energy of individual cores, suggesting that outflows could be the main mechanism for core disruption., Comment: Accepted for publication in the Astrophysical Journal

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 Series

We have used archival FUSE data to complete a survey of interstellar HD in 41 lines of sight with a wide range of extinctions. This follow up to an earlier survey was made to further assess the utility of HD as a cosmological probe; to analyze the HD formation process; and to see what trends with other interstellar properties were present in the data. We employed the curve-of-growth method, supported by line profile fitting, to derive accurate column densities of HD. We find that the N(HD)/2N(H2) ratio is substantially lower than the atomic D/H ratio and conclude that the molecular ratio has no bearing on cosmology, because local processes are responsible for the formation of HD. Based on correlations with E(B-V), H2, CO, and iron depletion, we find that HD is formed in the densest portion of the clouds; the slope of the logN(HD)/log(H2) correlation is greater than 1.0, caused by the destruction rate of HD declining more slowly than that of H2; and, as a sidelight, that the depletions are density dependent., Comment: 30 pages, 13 figures; Accepted for publication in ApJ

We report the results of spectroscopic mapping observations carried out toward a ~1 min x 1 min region within the northern lobe of the outflow from NGC 2071 using the Infrared Spectrograph (IRS) of the Spitzer Space Telescope. These observations covered the 5.2-37 um spectral region and have led to the detection of a number of ionic, atomic, and molecular lines, including fine-structure emission of Si+, Fe+, S++, S, the S(0)-S(7) pure rotational lines of H2, the R(3) and R(4) transitions of HD, and at least 11 transitions of H2O. In addition, the 6.2, 7.4, 7.6, 7.9, 8.6 and 11.3 um PAH emission bands were also observed and several transitions of OH were tentatively detected. Most of the detected line transitions were strong enough to map including, for the first time, three transitions of hot H2O. We find that: (1) the water emission is extended; (2) the extended emission is aligned with the outflow; and, (3) the spatial distribution of the water emission generally follows that observed for H2. Based on the measured line intensities, we derive an HD abundance relative to H2 of 1.1-1.8 10^-5 and an H2O number density of 12-2 cm^3. The H2 density in the water-emitting region is not well constrained by our observations, but is likely between 3 10^4 and 10^6 cm^3, yielding an H2O abundance relative to H2 of between 2 10^-5 and 6 10^-4. Future observations planned for the Herschel Space Observatory should greatly improve the density estimate, and thus our knowledge of the H2O abundance, for the water-emitting regions reported here. Finally, we note a possible departure from the H2O ortho-to-para ratio of 3:1 expected for water formed in hot post-shocked gas, suggesting that a significant fraction of the water vapor we detect may arise from H2O sputtered from cold dust grains., Comment: 35 pages, 15 figures, 4 tables, accepted for publication in ApJ

We report the results of spectroscopic mapping observations carried out toward small (1 x 1 arcmin) regions within the supernova remnants W44, W28, IC443, and 3C391 using the Infrared Spectrograph of the Spitzer Space Telescope. These observations, covering the 5.2 - 37 micron spectral region, have led to the detection of a total of 15 fine structure transitions of Ne+, Ne++, Si+, P+, S, S++, Cl+, Fe+, and Fe++; the S(0) - S(7) pure rotational lines of molecular hydrogen; and the R(3) and R(4) transitions of hydrogen deuteride. In addition to these 25 spectral lines, the 6.2, 7.7, 8.6, 11.3 and 12.6 micron PAH emission bands were also observed. Most of the detected line transitions have proven strong enough to map in several sources, providing a comprehensive picture of the relative distribution of the various line emissions observable in the Spitzer/IRS bandpass. A principal component analysis of the spectral line maps reveals that the observed emission lines fall into five distinct groups, each of which may exhibit a distinct spatial distribution: (1) lines of S and H2 (J > 2); (2) the H2 S(0) line; (3) lines of ions with appearance potentials less than 13.6 eV; (4) lines of ions with appearance potentials greater than 13.6 eV, not including S++; (5) lines of S++. Lines of group (1) likely originate in molecular material subject to a slow, nondissociative shock that is driven by the overpressure within the supernova remnant, and lines in groups (3) - (5) are associated primarily with dissociative shock fronts with a range of (larger) shock velocities. The H2 S(0) line shows a low-density diffuse emission component, and - in some sources - a shock-excited component., Comment: 43 pages, including 21 figures. Accepted for publication in ApJ

We report the detection of interstellar hydrogen deuteride (HD) toward the supernova remnant IC443, and the tentative detection of HD toward the Herbig Haro objects HH54 and HH7 and the star forming region GGD37 (Cepheus A West). Our detections are based upon spectral line mapping observations of the R(3) and R(4) rotational lines of HD, at rest wavelengths of 28.502 and 23.034 micron respectively, obtained using the Infrared Spectrograph onboard the Spitzer Space Telescope. The HD R(4)/R(3) line intensity ratio promises to be a valuable probe of the gas pressure in regions where it can be observed. The derived HD/H2 abundance ratios are 1.19(+0.35/-0.24)E-5, 1.80(+0.54/-0.32)E-5, and 1.41(+0.46/-0.33)E-5 respectively (68.3% confidence limits, based upon statistical errors alone) for IC443 (clump C), HH54, and HH7. If HD is the only significant reservoir of gas-phase deuterium in these sources, the inferred HD/H2 ratios are all consistent with a gas-phase elemental abundance [n(D)/n(H)](gas) ~ 7.5E-6, a factor 2 - 3 below the values obtained previously from observations of atomic deuterium in the local bubble and the Galactic halo. However, similarly low gas-phase deuterium abundances have been inferred previously for molecular gas clouds in the Orion region, and in atomic clouds along sight-lines within the Galactic disk to stars more distant than 500 pc from the Sun., Comment: 12 pages, including 5 figures. Accepted for publication in ApJ Letters

We report the results of spectroscopic mapping observations carried out toward the Herbig-Haro objects HH7-11 and HH54 over the 5.2 - 37 micron region using the Infrared Spectrograph of the Spitzer Space Telescope. These observations have led to the detection and mapping of the S(0) - S(7) pure rotational lines of molecular hydrogen, together with emissions in fine structure transitions of Ne+, Si+, S, and Fe+. The H2 rotational emissions indicate the presence of warm gas with a mixture of temperatures in the range 400 - 1200 K, consistent with the expected temperature behind nondissociative shocks of velocity ~ 10 - 20 km/s, while the fine structure emissions originate in faster shocks of velocity 35 - 90 km/s that are dissociative and ionizing. Maps of the H2 line ratios reveal little spatial variation in the typical admixture of gas temperatures in the mapped regions, but show that the H2 ortho-to-para ratio is quite variable, typically falling substantially below the equilibrium value of 3 attained at the measured gas temperatures. The non-equilibrium ortho-to-para ratios are characteristic of temperatures as low as ~ 50 K, and are a remnant of an earlier epoch, before the gas temperature was elevated by the passage of a shock. Correlations between the gas temperature and H2 ortho-to-para ratio show that ortho-to-para ratios < 0.8 are attained only at gas temperatures below ~ 900 K; this behavior is consistent with theoretical models in which the conversion of para- to ortho-H2 behind the shock is driven by reactive collisions with atomic hydrogen, a process which possesses a substantial activation energy barrier (E_A/k ~ 4000 K) and is therefore very inefficient at low temperature., Comment: 45 pages, including 16 figures. Accepted for publication in ApJ

We report the first ensemble results from the FUSE survey of molecular hydrogen in lines of sight with A_V $\gtrsim$ 1 mag. We have developed techniques for fitting computed profiles to the low-J lines of H2, and thus determining column densities for J = 0 and J = 1, which contain $\gtrsim$99% of the total H2. From these column densities and ancillary data we have derived the total H2 column densities, hydrogen molecular fractions, and kinetic temperatures for 23 lines of sight. This is the first significant sample of molecular hydrogen column densities of 10^21 cm^-2, measured through UV absorption bands. We have also compiled a set of extinction data for these lines of sight, which sample a wide range of environments. We have searched for correlations of our H2-related quantities with previously published column densities of other molecules and extinction parameters. We find strong correlations between H2 and molecules such as CH, CN, and CO, in general agreement with predictions of chemical models. We also find the expected correlations between hydrogen molecular fraction and various density indicators such as kinetic temperature, CN abundance, the steepness of the far-UV extinction rise, and the width of the 2175A bump. Despite the relatively large molecular fractions, we do not see the values greater than 0.8 expected in translucent clouds. With the exception of a few lines of sight, we see little evidence for the presence of individual translucent clouds in our sample. We conclude that most of the lines of sight are actually composed of two or more diffuse clouds similar to those found toward targets like zeta Oph. We suggest a modification in terminology to distinguish between a "translucent line of sight" and a "translucent cloud.", Comment: 64 pages in AASTeX preprint format, includes 12 figures and 11 tables; accepted for publication in ApJ

We explore here the question of how cloud physics can be more directly probed when one observes the majority of cooling emissions from molecular gas. For this purpose we use results from a recent Spitzer Space Telescope study of the young cluster of embedded objects in NGC1333. For this study we mapped the emission from eight pure H2 rotational lines, from S(0) to S(7). The H2 emission appears to be associated with the warm gas shocked by the multiple outflows present in the region. The H2 lines are found to contribute to 25 - 50% of the total outflow luminosity, and can be used to more directly ascertain the importance of star formation feedback on the natal cloud. From these lines, we determine the outflow mass loss rate and, indirectly, the stellar infall rate, the outflow momentum and the kinetic energy injected into the cloud over the embedded phase. The latter is found to exceed the binding energy of individual cores, suggesting that outflows could be the main mechanism for cores disruption. Given the recent launch of Herschel and the upcoming operational lifetime of SOFIA we discuss how studies of molecular cooling can take a step beyond understanding thermal balance to exploring the origin, receipt, and transfer of energy in atomic and molecular gas in a wide range of physical situations.