Excitation of Molecular Hydrogen in the Orion Bar PhotodissociationRegion from a Deep Near-infrared IGRINS Spectrum

We report high angular resolution (4.9 × 3.0 ) images of reactive ions SH + , HOC + , and SO + toward the Orion Bar photodissociation region (PDR). We used ALMA-ACA to map several rotational lines at 0.8 mm, complemented with multi-line observations obtained with the IRAM 30 m telescope. The SH + and HOC + emission is restricted to a narrow layer of 2 -to 10 -width (≈800 to 4000 AU depending on the assumed PDR geometry) that follows the vibrationally excited H * 2 emission. Both ions efficiently form very close to the H/H 2 transition zone, at a depth of A V 1 mag into the neutral cloud, where abundant C + , S + , and H * 2 coexist. SO + peaks slightly deeper into the cloud. The observed ions have low rotational temperatures (T rot ≈ 10−30 K T k ) and narrow line-widths (∼2−3 km s −1 ), a factor of 2 narrower that those of the lighter reactive ion CH + . This is consistent with the higher reactivity and faster radiative pumping rates of CH + compared to the heavier ions, which are driven relatively more quickly toward smaller velocity dispersion by elastic collisions and toward lower T rot by inelastic collisions. We estimate column densities and average physical conditions from an excitation model (n(H 2 ) ≈ 10 5 −10 6 cm −3 , n(e − ) ≈ 10 cm −3 , and T k ≈ 200 K). Regardless of the excitation details, SH + and HOC + clearly trace the most exposed layers of the UV-irradiated molecular cloud surface, whereas SO + arises from slightly more shielded layers.

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“…In fact, H * 2 emission from very high vibrational levels (up to v = 10 or E/k ≈ 50 000 K) has recently been reported (Kaplan et al 2017).…”

Section: Results: Morphology and Emission Propertiesmentioning

confidence: 99%

Spatially resolved images of reactive ions in the Orion Bar

Goicoechea

Cuadrado

Pety

et al. 2017

A&A

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“…These FUV photons radiatively pump H 2 molecules to vibrationally excited states (e.g., Hollenbach & Tielens 1997, and references therein) at the edge of the irradiated cloud, leading to bright near-IR (NIR) H 2 ( v ≥ 1) emission that is detected at large-spatial scales (Luhman et al 1994). In particular, H 2 lines from vibrational levels up to v =10 (or E u / k ≈50,000 K) have been detected toward the Orion Bar (e.g., Kaplan et al 2017).…”

Section: Discussionmentioning

confidence: 99%

Molecular tracers of radiative feedback in Orion (OMC-1)

Goicoechea

Santa-Maria

Bron

et al. 2019

A&A

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Young massive stars regulate the physical conditions, ionization, and fate of their natal molecular cloud and surroundings. It is important to find tracers that help quantifying the stellar feedback processes that take place at different spatial scales. We present ~85 arcmin2 (~1.3 pc2) velocity-resolved maps of several submillimeter molecular lines, taken with Herschel/HIFI, toward the closest high-mass star-forming region, the Orion molecular cloud 1 core (OMC-1). The observed rotational lines include probes of warm and dense molecular gas that are difficult, if not impossible, to detect from ground-based telescopes: CH+ (J = 1–0), CO (J = 10–9), HCO+ (J = 6–5) and HCN (J = 6–5), and CH (N, J =1, 3/2–1, 1/2). These lines trace an extended but thin layer (AV ≃3–6 mag or ~1016 cm) of molecular gas at high thermal pressure, Pth = nH · Tk ≈ 107 – 109 cm−3 K, associated with the far ultraviolet (FUV) irradiated surface of OMC-1. The intense FUV radiation field, emerging from massive stars in the Trapezium cluster, heats, compresses and photoevaporates the cloud edge. It also triggers the formation of specific reactive molecules such as CH+. We find that the CH+ (J = 1–0) emission spatially correlates with the flux of FUV photons impinging the cloud: G0 from ~103 to ~105. This correlation is supported by constant-pressure photodissociation region (PDR) models in the parameter space Pth/G0 ≈ [5 · 103 – 8 · 104] cm−3 K where many observed PDRs seem to lie. The CH+ (J = 1–0) emission spatially correlates with the extended infrared emission from vibrationally excited H2 (v ≥ 1), and with that of [C ii] 158 μm and CO J = 10–9, all emerging from FUV-irradiated gas. These correlations link the presence of CH+ to the availability of C+ ions and of FUV-pumped H2 (v ≥ 1) molecules. We conclude that the parsec-scale CH+ emission and narrow-line (Δv ≃ 3 km s−1) mid-J CO emission arises from extended PDR gas and not from fast shocks. PDR line tracers are the smoking gun of the stellar feedback from young massive stars. The PDR cloud surface component in OMC-1, with a mass density of 120–240 M⊙ pc−2, represents ~5% to ~10% of the total gas mass, however, it dominates the emitted line luminosity; the average CO J = 10–9 surface luminosity in the mapped region being ~35 times brighter than that of CO J = 2–1. These results provide insights into the source of submillimeter CH+ and mid-J CO emission from distant star-forming galaxies.

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“…Luhman et al (1997) provided the first combined IR/UV picture of an H 2 fluorescence cascade in a single object (a PDR). Deep near-infrared spectra of bright PDRs, taken with high resolution, enable us to detect many emission lines from ro-vibrationally excited molecular hydrogen that arise from transitions out of many upper ro-vibrational levels of the electronic ground state (e.g., Kaplan et al, 2017;Le et al, 2017, see Fig.5). Since atmospheric transmission in the K band is relatively good, H 2 lines in the near-IR have been searched for in relatively faint objects using large telescopes (such as disks, e.g.…”

Section: H 2 Transitions and Specific Diagnostic Powermentioning

confidence: 99%

H 2 formation on interstellar dust grains: The viewpoints of theory, experiments, models and observations

Wakelam

Bron

Cazaux

et al. 2017

Molecular Astrophysics

223

201

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a b s t r a c tMolecular hydrogen is the most abundant molecule in the universe. It is the first one to form and survive photo-dissociation in tenuous environments. Its formation involves catalytic reactions on the surface of interstellar grains. The micro-physics of the formation process has been investigated intensively in the last 20 years, in parallel of new astrophysical observational and modeling progresses. In the perspectives of the probable revolution brought by the future satellite JWST, this article has been written to present what we think we know about the H 2 formation in a variety of interstellar environments.

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Excitation of Molecular Hydrogen in the Orion Bar PhotodissociationRegion from a Deep Near-infrared IGRINS Spectrum

Cited by 38 publications

References 59 publications

Spatially resolved images of reactive ions in the Orion Bar

Spatially resolved images of reactive ions in the Orion Bar

Molecular tracers of radiative feedback in Orion (OMC-1)

H 2 formation on interstellar dust grains: The viewpoints of theory, experiments, models and observations

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