Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38

We used the ARO 12m antenna to observe emission from the J=1-0 lines of carbon monoxide, \hcop\ and HNC and the J=2-1 line of CS toward and around the continuum peak used for absorption studies and we compare them with CH, HNC, C\p\ and other absorption spectra from PRISMAS. We develop a kinematic analysis that allows a continuous description of the spectral properties and relates them to viewing geometry in the Galaxy. As for CH, HF, C\p, \hcop\ and other species observed in absorption, mm-wave emission in CO, \hcop, HNC and CS is continuous over the full velocity range expected for material between the Sun and W31 4.95 kpc away. CO emission is much stronger than average in the Galactic molecular ring and the mean \HH\ density derived from CH, 4 \pccc \la 2$$ \la 10 \pccc at 4 \la R \la 6.4 kpc, is similarly elevated. The CO-\HH\ conversion factor falls in a narrow range \XCO\ = 1-2\times10^{20}~\HH\ \pcc~({\rm K}-\kms)^{-1} if the emitting gas is mostly on the near side of the sub-central point, as we suggest. The brightnesses of \hcop, HNC, and CS are comparable (0.83\%, 0.51\% and 1.1\% respectively relative to CO) and have no variation in galactocentric radius with respect to CO. Comparison of the profile-averaged \hcop\ emission brightness and optical depth implies local densities n(H) \approx 135\pm25\pccc with most of excitation of \hcop\ from electrons. At such density, a consistent picture of the \HH-bearing gas, accounting also for the CO emission, has a volume filling factor 3\% and a 5 pc clump or cloud size.Comment: Accepted to A&

Stellar Feedback in the ISM Revealed by Wide-Field Far-Infrared Spectral-Imaging

The radiative and mechanical interaction of stars with their environment drives the evolution of the ISM and of galaxies as a whole. The far-IR emission (lambda ~30 to 350 microns) from atoms and molecules dominates the cooling of the warm gas in the neutral ISM, the material that ultimately forms stars. Far-IR lines are thus the most sensitive probes of stellar feedback processes, and allow us to quantify the deposition and cycling of energy in the ISM. While ALMA (in the (sub)mm) and JWST (in the IR) provide astonishing sub-arcsecond resolution images of point sources and their immediate environment, they cannot access the main interstellar gas coolants, nor are they designed to image entire star-forming regions (SFRs). Herschel far-IR photometric images of the interstellar dust thermal emission revealed the ubiquitous large-scale filamentary structure of SFRs, their mass content, and the location of thousands of prestellar cores and protostars. These images, however, provide a static view of the ISM: not only they dont constrain the cloud dynamics, moreover they cannot reveal the chemical composition and energy transfer within the cloud, thus giving little insight into the regulation process of star formation by stellar feedback. In this white paper we emphasize the need of a space telescope with wide-field spectral-imaging capabilities in the critical far-IR domain.Comment: White Paper submitted to the Astro 2020 Decadal Survey on Astronomy and Astrophysics (National Academies of Science, Engineering, and Medicine

The Dark Neutral Medium is (Mostly) Molecular Hydrogen

We acquired ALMA ground state absorption profiles of HCO+ and other molecules toward 33 extragalactic continuum sources seen toward the Galactic anticenter, deriving N(H2) = N(HCO+)/3x10^{-9}. We observed J=1-0 CO emission with the IRAM 30m in directions where HCO+ was newly detected. HCO+ absorption was detected in 28 of 33 new directions and CO emission along 19 of those 28. The 5 sightlines lacking detectable HCO+ have 3 times lower mean EBV and N(DNM). Binned in EBV, N(H2) and N(DNM) are strongly correlated and vary by factors of 50-100 over the observed range EBV~0.05-1 mag, while N(HI) varies by factors of only 2-3. On average N(DNM) and N(H2) are well matched, and detecting HCO+ absorption adds little/no H2 in excess of the previously inferred DNM. There are 5 cases where 2N(H2) < N(DNM)/2 indicates saturation of the HI emission. For sightlines with \WCO > 1 K-\kms the CO-H2 conversion factor N(H2)/\WCO\ = 2-3x10^{20}\pcc/K-\kms is higher than derived from studies of resolved clouds in gamma-rays. Our work sampled primarily atomic gas with a mean H2 fraction ~1/3, but the DNM is almost entirely molecular. CO fulfills its role as an H2 tracer when its emission is strong, but large-scale CO surveys are not sensitive to H2 columns associated with typical values N(DNM) = 2-6x10^{20}\pcc. Lower \XCO\ values from $\gamma$-ray studies arise in part from different definitions and usage. Sightlines with \WCO\ \ge 1 K-\kms\ represent 2/3 of the H2 detected in HCO+ and detecting 90% of the H2 would require detecting CO at levels \WCO\~0.2-0.3 K-\kms For full abstract see the paperComment: Accepted to Astronomy and Astrophysics (Main Journal

Molecular hydrogen and its proxies HCO+^+ and CO in the diffuse interstellar medium

There is a robust polyatomic chemistry in diffuse, partially-molecular interstellar gas that is readily accessible in absorption at radio/mm/sub-mm wavelengths. Accurate column densities are derived owing to the weak internal excitation, so relative molecular abundances are well known with respect to each other but not with respect to H2. Here we consider the use of proxies for hydrogen column densities N(H2) and N(H) = N(HI)+2N(H2) based on measurements of HCO+ absorption and CO emission and absorption, and we compare these with results obtained by others when observing HI, H2 and CO toward stars and AGN. We consider the use of HCO+ as a proxy for H2 and show that the assumption of a relative abundance N(H2) = N(HCO+)/3x10^{-9} gives the same view of the atomic-molecular hydrogen transition that is seen in UV absorption toward stars. CO on the other hand shows differences between the radio and optical regimes because emission is always detected when N(\hcop) > 6x10^{11}\pcc or N(H2) > 2x10^20\pcc. Wide variations in the integrated CO {J=1-0} brightness W_CO and N(CO)/N(H2) imply equivalent variations in the CO-H2 conversion factor even while the ensemble mean is near the usual Galactic values. Gas/reddening ratios found in absorption toward stars, N(H)/E(B-V) = 6.2x10^21 H \pcc/mag overall or 6.8x10^21 H \pcc/mag for sightlines at E(B-V) <= 0.08 mag lacking H2 are well below the Galactic mean measured at low reddening and high Galactic latitude, 8.3x10^21 H \pcc/mag.Comment: Accepted for The Astrophysical Journa

Interstellar Hydrides

Interstellar hydrides -- that is, molecules containing a single heavy element atom with one or more hydrogen atoms -- were among the first molecules detected outside the solar system. They lie at the root of interstellar chemistry, being among the first species to form in initially-atomic gas, along with molecular hydrogen and its associated ions. Because the chemical pathways leading to the formation of interstellar hydrides are relatively simple, the analysis of the observed abundances is relatively straightforward and provides key information about the environments where hydrides are found. Recent years have seen rapid progress in our understanding of interstellar hydrides, thanks largely to far-IR and submillimeter observations performed with the Herschel Space Observatory. In this review, we will discuss observations of interstellar hydrides, along with the advanced modeling approaches that have been used to interpret them, and the unique information that has thereby been obtained.Comment: Accepted for publication in Annual Review of Astronomy and Astrophysics 2016, Vol. 5

The Horsehead nebula, a template source for interstellar physics and chemistry

We present a summary of our previous investigations of the physical and chemical structure of the Horsehead nebula, and discuss how these studies led to advances on the understanding of the impact of FUV radiation on the structure of dense interstellar clouds. Specific molecular tracers can be used to isolate different environments, that are more sensitive to changes in the FUV radiation or density than the classical tracers of molecular gas : the CO isotopologues or the dust (sub)millimeter continuum emission. They include the HCO or CCH radicals for the FUV illuminated interfaces, or the molecular ions H$^{13}$CO$^+$, DCO$^+$ and other deuterated species (DNC, DCN) for the cold dense core. We discuss future prospects in the context of Herschel and ALMA

Blind decomposition of Herschel-HIFI spectral maps of the NGC 7023 nebula

Large spatial-spectral surveys are more and more common in astronomy. This calls for the need of new methods to analyze such mega- to giga-pixel data-cubes. In this paper we present a method to decompose such observations into a limited and comprehensive set of components. The original data can then be interpreted in terms of linear combinations of these components. The method uses non-negative matrix factorization (NMF) to extract latent spectral end-members in the data. The number of needed end-members is estimated based on the level of noise in the data. A Monte-Carlo scheme is adopted to estimate the optimal end-members, and their standard deviations. Finally, the maps of linear coefficients are reconstructed using non-negative least squares. We apply this method to a set of hyperspectral data of the NGC 7023 nebula, obtained recently with the HIFI instrument onboard the Herschel space observatory, and provide a first interpretation of the results in terms of 3-dimensional dynamical structure of the region.Comment: Proceedings of the 2012 meeting of the french astronomical society (SF2A) in Nic

Spatially extended OH+ emission from the Orion Bar and Ridge

We report the first detection of a Galactic source of OH+ line emission: the Orion Bar, a bright nearby photon-dominated region. Line emission is detected over ~1' (0.12 pc), tracing the Bar itself as well as the Southern tip of the Orion Ridge. The line width of ~4 km/s suggests an origin of the OH+ emission close to the PDR surface, at a depth of A_V ~0.3-0.5 into the cloud where most hydrogen is in atomic form. Steady-state collisional and radiative excitation models require unrealistically high OH+ column densities to match the observed line intensity, indicating that the formation of OH+ in the Bar is rapid enough to influence its excitation. Our best-fit OH+ column density of ~1x10^14 cm^-2 is similar to that in previous absorption line studies, while our limits on the ratios of OH+/H2O+ (>~40) and OH+/H3O+ (>~15) are higher than seen before. The column density of OH+ is consistent with estimates from a thermo-chemical model for parameters applicable to the Orion Bar, given the current uncertainties in the local gas pressure and the spectral shape of the ionizing radiation field. The unusually high OH+/H2O+ and OH+/H3O+ ratios are probably due to the high UV radiation field and electron density in this object. In the Bar, photodissociation and electron recombination are more effective destroyers of OH+ than the reaction with H2, which limits the production of H2O+. The appearance of the OH+ lines in emission is the result of the high density of electrons and H atoms in the Orion Bar, since for these species, inelastic collisions with OH+ are faster than reactive ones. In addition, chemical pumping, far-infrared pumping by local dust, and near-UV pumping by Trapezium starlight contribute to the OH+ excitation. Similar conditions may apply to extragalactic nuclei where OH+ lines are seen in emission.Comment: Accepted by A&A; 10 pages, 5 figure