Hydride spectroscopy of the diffuse interstellar medium: new clues on the gas fraction in molecular form and cosmic ray ionization rate in relation to H
            <sub>3</sub>
            <sup>+</sup>

Annu. Rev. Astron. Astrophys.

Neufeld

Goicoechea

2016

129

171

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.

Section: Ism: Interstellar Mediummentioning

confidence: 99%

Interstellar Hydrides

Annu. Rev. Astron. Astrophys.

Neufeld

Goicoechea

2016

129

171

“…Godard et al 2010;Gerin et al 2011;Flagey et al 2013). The first results of the ionised and neutral carbon observations have recently been presented in Gerin et al (2012Gerin et al ( , 2014.…”

Section: Introductionmentioning

confidence: 99%

First detection of [N II] 205μm absorption in interstellar gas

Persson

Mookerjea

et al. 2014

A&A

Self Cite

We present high resolution [N ii] 205 μm ( 3 P 1 − 3 P 0 ) spectra obtained with Herschel-HIFI towards a small sample of far-infrared bright star forming regions in the Galactic plane: W 31C (G10.6−0.4), W 49N (G43.2−0.1), W 51 (G49.5−0.4), and G34.3+0.1. The detection of the 205 μm line in absorption emphasises the importance of a high spectral resolution, and also offers a new tool for investigation of the WIM.

“…Observations of hydride progressions (Gerin et al 2012) like OH + , OH 2 + , and OH 3 + (Gerin et al 2010a) or NH, NH 2 , and NH 3 (Persson et al 2012) are sensitive to the presence of gas over a wide range of H 2 fractions.…”

Section: Introductionmentioning

confidence: 99%

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

Liszt

2015

A&A

Self Cite

Context. The sightline to W31C G10.62-0.38 was extensively observed in absorption under the PRISMAS program on Herschel. Aims. We relate absorbing material to the older view of Galactic molecules gained from CO emission. Methods. We used the Arizona Radio Observatory Kitt Peak 12m antenna to observe emission from the J = 1−0 lines of carbon monoxide, HCO + , 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 + , 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. Results. As it is for CH, HF, C + , HCO + , and other species observed in absorption, mm-wave emission in CO, HCO + , 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 H 2 density derived from CH, 4 cm −3 < ∼ 2 n(H 2 ) < ∼ 10 cm −3 at 4 < ∼ R < ∼ 6.4 kpc, is similarly elevated. The CO-H 2 conversion factor falls in a narrow range X CO = 1−2 × 10 20 H 2 cm −2 (K − km s −1 ) −1 if the emitting gas is mostly on the near side of the subcentral point, as we suggest. The brightnesses of HCO + , 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 HCO + emission brightness and optical depth implies local densities n(H) ≈ 135 ± 25 cm −3 with most of the excitation of HCO + from electrons. At this level of density, a consistent picture of the H 2 -bearing gas, which also accounts for the CO emission, has a volume filling factor of 3% and a 5 pc clump or cloud size.