Tracers of the ionization fraction in dense and translucent gas

Bron, Emeric; Roueff, E.; Gérin, M.; Pety, J.; Gratier, P.; Petit, Franck; Guzmán, Viviana V.; Orkisz, Jan H.; Magalhaes, Victor de Souza; Gaudel, Mathilde; Vono, Maxime; Bardeau, S.; Chainais, Pierre; Goicoechea, J. R.; Hughes, Annie; Kainulainen, J.; Languignon, David; Bourlot, J. Le; Levrier, F.; Liszt, H. S.; Öberg, Karin I.; Peretto, N.; Roueff, Antoine; Sievers, A.

doi:10.1051/0004-6361/202038040

Cited by 19 publications

(20 citation statements)

References 63 publications

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“…This means that detecting extended HCN J = 2-1 emission may not always imply the presence of very dense gas, n cr (H 2 ; J = 2-1) a few 10 6 cm −3 , but lower density gas with high ionization fractions. Hence, a precise measurement of H 2 densities and χ(HCN) abundances would greatly benefit from an estimation of the electron abundance of the gas where HCN emits (e.g., Bron et al 2021).…”

Section: Role Of Electron Excitationmentioning

confidence: 99%

“…Observations of sulfur radio recombination lines imply χ(S + ) 1.4 × 10 −5 in the Orion Bar PDR . Deeper inside the cloud, in the cold cores where star-formation actually takes place, the ionization fraction decreases to χ e 10 −8 , and n e is negligibly low (e.g., Guelin et al 1982;Caselli et al 1998;Maret & Bergin 2007;Goicoechea et al 2009;Bron et al 2021).…”

Section: Introductionmentioning

confidence: 99%

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Anomalous HCN emission from warm giant molecular clouds

Goicoechea

Lique

Santa-Maria

2022

A&A

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Hydrogen cyanide (HCN) is considered a good tracer of the dense molecular gas that serves as fuel for star formation. However, recent large-scale surveys of giant molecular clouds (GMCs) have detected extended HCN rotational line emission far from star-forming cores. Such observations often spectroscopically resolve the HCN J = 1–0 (partially also the J = 2–1 and 3–2) hyperfine structure (HFS). A precise determination of the physical conditions of the gas requires treating the HFS line overlap effects. Here, we study the HCN HFS excitation and line emission using nonlocal radiative transfer models that include line overlaps and new HFS-resolved collisional rate coefficients for inelastic collisions of HCN with both para-H2 and ortho-H2 (computed via the scaled-infinite order sudden approximation up to Tk = 500 K). In addition, we account for the role of electron collisions in the HFS level excitation. We find that line overlap and opacity effects frequently produce anomalous HCN J = 1–0 HFS line intensity ratios (i.e., inconsistent with the common assumption of the same Tex for all HFS lines) as well as anomalous HFS line width ratios. Line overlap and electron collisions also enhance the excitation of the higher J rotational lines. Our models explain the anomalous HCN J = 1–0 HFS spectra observed in the Orion Bar and Horsehead photodissociation regions. As shown in previous studies, electron excitation becomes important for molecular gas with H2 densities below a few 105 cm−3 and electron abundances above ~10−5. We find that when electron collisions are dominant, the relative intensities of the HCN J = 1–0 HFS lines can be anomalous too. In particular, electron excitation can produce low-surface-brightness HCN emission from very extended but low-density gas in GMCs. The existence of such a widespread HCN emission component may affect the interpretation of the extragalactic relationship HCN luminosity versus star-formation rate. Alternatively, extended HCN emission may arise from dense star-forming cores and become resonantly scattered by large envelopes of lower density gas. There are two scenarios – namely, electron-assisted (weakly) collisionally excited versus scattering – that lead to different HCN J = 1–0 HFS intensity ratios, which can be tested on the basis of observations.

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Section: Role Of Electron Excitationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Anomalous HCN emission from warm giant molecular clouds

Goicoechea

Lique

Santa-Maria

2022

A&A

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“…We rederive the line properties and gas temperature at the spatial resolution of the CRRL observations using pySpecKit (Ginsburg & Mirocha 2011). We also use the HCN(1-0) and C 2 H(1-0) (J = 1/2-1/2) data cubes of Melnick et al (2011), as their ratio can be used as a probe of x(e − ) (Bron et al 2021). The C 2 H(1-0) data cube includes the fine structure components F = 0-1 and F = 1-1 within the observed range of frequencies.…”

Section: Molecular Gasmentioning

confidence: 99%

The ionization fraction in OMC-2 and OMC-3

Salas¹,

Rugel²,

Emig³

et al. 2021

A&A

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Context. The electron density (ne−) plays an important role in setting the chemistry and physics of the interstellar medium. However, measurements of ne− in neutral clouds have been directly obtained only toward a few lines of sight or they rely on indirect determinations. Aims. We use carbon radio recombination lines and the far-infrared lines of C+ to directly measure ne− and the gas temperature in the envelope of the integral shaped filament (ISF) in the Orion A molecular cloud. Methods. We observed the C102α (6109.901 MHz) and C109α (5011.420 MHz) carbon radio recombination lines (CRRLs) using the Effelsberg 100 m telescope at ≈2′ resolution toward five positions in OMC-2 and OMC-3. Since the CRRLs have similar line properties, we averaged them to increase the signal-to-noise ratio of the spectra. We compared the intensities of the averaged CRRLs, and the 158 μm-[CII] and [13CII] lines to the predictions of a homogeneous model for the C+/C interface in the envelope of a molecular cloud and from this comparison we determined the electron density, temperature and C+ column density of the gas. Results. We detect the CRRLs toward four positions, where their velocity (vLSR ≈ 11 km s−1) and widths (σv ≈ 1 km s−1) confirms that they trace the envelope of the ISF. Toward two positions we detect the CRRLs, and the 158 μm-[CII] and [13CII] lines with a signal-to-noise ratio ≥5, and we find ne− = 0.65 ± 0.12 cm−3 and 0.95 ± 0.02 cm−3, which corresponds to a gas density nH ≈ 5 × 103 cm−3 and a thermal pressure of pth ≈ 4 × 105 K cm−3. We also constrained the ionization fraction in the denser portions of the molecular cloud using the HCN(1–0) and C2H(1–0) lines to x(e−) ≤ 3 × 10−6. Conclusions. The derived electron densities and ionization fraction imply that x(e−) drops by a factor ≥100 between the C+ layer and the regions probed by HCN(1–0). This suggests that electron collisional excitation does not play a significant role in setting the excitation of HCN(1–0) toward the region studied, as it is responsible for only ≈10% of the observed emission.

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“…the same volumes of gas, with C 2 H tracing gas closer to the source of radiation (the Trapezium stars). Since the analytical expressions derived by Bron et al (2021) are based on a one zone model (i.e., there is no spatial information), this implies that the ionization fractions derived from this ratio will be an average of the ionization fractions in the volumes probed by C 2 H and HCN, and thus can only be used as upper limits to x(e − ) in the gas probed by the HCN(1-0) line.…”

Section: Ionization Fraction Of the Molecular Gasmentioning

confidence: 99%

The ionization fraction in OMC-2 and OMC-3

Salas,

Rugel,

Emig

et al. 2021

Preprint

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Context. The electron density (n e − ) plays an important role in setting the chemistry and physics of the interstellar medium. However, measurements of n e − in neutral clouds have been directly obtained only toward a few lines of sight or they rely on indirect determinations. Aims. We use carbon radio recombination lines and the far-infrared lines of C + to directly measure n e − and the gas temperature in the envelope of the integral shaped filament (ISF) in the Orion A molecular cloud. Methods. We observed the C102α (6109.901 MHz) and C109α (5011.420 MHz) carbon radio recombination lines (CRRLs) using the Effelsberg 100m telescope at ≈2 resolution toward five positions in OMC-2 and OMC-3. Since the CRRLs have similar line properties, we averaged them to increase the signal-to-noise ratio of the spectra. We compared the intensities of the averaged CRRLs, and the 158 µm-[CII] and [ 13 CII] lines to the predictions of a homogeneous model for the C + /C interface in the envelope of a molecular cloud and from this comparison we determined the electron density, temperature and C + column density of the gas. Results. We detect the CRRLs toward four positions, where their velocity ( LSR ≈11 km s −1 ) and widths (σ ≈1 km s −1 ) confirms that they trace the envelope of the ISF. Toward two positions we detect the CRRLs, and the 158 µm-[CII] and [ 13 CII] lines with a signal-to-noise ratio ≥ 5, and we find n e − =0.65 ± 0.12 cm −3 and 0.95 ± 0.02 cm −3 , which corresponds to a gas density n H ≈5 × 10 3 cm −3 and a thermal pressure of p th ≈4 × 10 5 K cm −3 . We also constrained the ionization fraction in the denser portions of the molecular cloud using the HCN(1-0) and C 2 H(1-0) lines to x(e − )≤3 × 10 −6 . Conclusions. The derived electron densities and ionization fraction imply that x(e − ) drops by a factor ≥100 between the C + layer and the regions probed by HCN(1-0). This suggests that electron collisional excitation does not play a significant role in setting the excitation of HCN(1-0) toward the region studied, as it is responsible for only ≈10% of the observed emission.

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Tracers of the ionization fraction in dense and translucent gas

Cited by 19 publications

References 63 publications

Anomalous HCN emission from warm giant molecular clouds

Anomalous HCN emission from warm giant molecular clouds

The ionization fraction in OMC-2 and OMC-3

The ionization fraction in OMC-2 and OMC-3

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