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

Wakelam, Valentine; Bron, Emeric; Cazaux, S.; Dulieu, F.; Gry, C.; Guillard, P.; Habart, E.; Hornekær, Liv; Morisset, S.; Nyman, Gunnar; Pirronello, V.; Price, S. D.; Valdivia, Valeska; Vidali, Gianfranco; Watanabe, Naoki

doi:10.1016/j.molap.2017.11.001

Cited by 210 publications

(201 citation statements)

References 414 publications

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“…Since molecular hydrogen formation occurs on the surface of dust grains (e.g., Wakelam et al 2017), dust is expected to be very well mixed with molecular hydrogen gas. Moreover, measurements of the dust can provide more reliable cloud mass and structural information than those of molecular lines since for GMCs in the Milky Way the dust accounts for ∼1% of total cloud masses (∼ 30 × that of molecular lines), and measurements of it are not hampered by considerations of excitation, chemistry (e.g., depletion) and opacity that severely hinder molecular-line observations.…”

Section: Introductionmentioning

confidence: 99%

First Resolved Dust Continuum Measurements of Individual Giant Molecular Clouds in the Andromeda Galaxy

Forbrich

Lada

Viaene

et al. 2020

ApJ

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In our local Galactic neighborhood, molecular clouds are best studied using a combination of dust measurements, to determine robust masses, sizes and internal structures of the clouds, and molecularline observations to determine cloud kinematics and chemistry. We present here the first results of a program designed to extend such studies to nearby galaxies beyond the Magellanic Clouds. Utilizing the wideband upgrade of the Submillimeter Array (SMA) at 230 GHz we have obtained the first continuum detections of the thermal dust emission on sub-GMC scales (∼ 15 pc) within the Andromeda galaxy (M 31). These include the first resolved continuum detections of dust emission from individual GMCs beyond the Magellanic Clouds. Utilizing a powerful capability of the SMA, we simultaneously recorded CO(2-1) emission with identical (u, v) coverage, astrometry and calibration, enabling the first measurements of the CO conversion factor, α CO(2−1) , toward individual GMCs across an external galaxy. Our direct measurement yields an average CO-to-dust mass conversion factor of α CO−dust = 0.042 ± 0.018 M (K km s −1 pc 2 ) −1 for the J = 2 − 1 transition. This value does not appear to vary with galactocentric radius. Assuming a constant gas-to-dust ratio of 136, the resulting α CO = 5.7 ± 2.4 M (K km s −1 pc 2 ) −1 for the 2-1 transition is in excellent agreement with that of Milky Way GMCs, given the uncertainties. Finally, using the same analysis techniques, we compare our results with observations of the local Orion molecular clouds, placed at the distance of M 31 and simulated to appear as they would if observed by the SMA.

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Section: Introductionmentioning

confidence: 99%

First Resolved Dust Continuum Measurements of Individual Giant Molecular Clouds in the Andromeda Galaxy

Forbrich

Lada

Viaene

et al. 2020

ApJ

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“…One is HI absorption at the same velocity as 13 CO emission. This is likely due to cold HI gas within the dense CO clumps, which may still have a relatively high HI column density due to high total column density in a CO clump combined with modest fractional abundance of HI due to relatively young age and incomplete conversion to H 2 (Wakelam et al 2017, and reference therein). The absorption feature associated with 13 CO intensity peak towards position 3 suggests that there are cold HI gas within the molecular clumps.…”

Section: Channel Maps Of [C Ii] 158 µM Co 1-0 Hi 21cm and H92αmentioning

confidence: 99%

Probing ISM Structure in Trumpler 14 and Carina I Using the Stratospheric Terahertz Observatory 2

Seo

Goldsmith

Walker

et al. 2019

ApJ

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We present observations of the Trumpler 14/Carina I region carried out using the Stratospheric Terahertz Observatory 2 (STO2). The Trumpler 14/Carina I region is in the west part of the Carina Nebula Complex, which is one of the most extreme star-forming regions in the Milky Way. We observed Trumpler 14/Carina I in the 158 µm transition of [C ii] with a spatial resolution of 48 and a velocity resolution of 0.17 km s −1 . The observations cover a 0.25 • by 0.28 • area with central position l = 297.34 • , b = -0.60 • . The kinematics show that bright [C ii] structures are spatially and spectrally correlated with the surfaces of CO clouds, tracing the photodissociation region and ionization front of each molecular cloud. Along 7 lines of sight that traverse Tr 14 into the dark ridge to the southwest, we find that the [C ii] luminosity from the HII region is 3.7 times that from the PDR. In same los we find in the PDRs an average ratio of 1:4.1:5.6 for the mass in atomic gas:dark-CO gas: molecular gas traced by CO. Comparing multiple gas tracers including HI 21cm, [C ii], CO, and radio recombination lines, we find that the HII regions of the Carina Nebula Complex are well-described as HII regions with one-side freely expanding towards us, consistent with the champagne model of ionized gas evolution. The dispersal of the GMC in this region is dominated by EUV photoevaporation; the dispersal timescale is 20-30 Myr. Effective Resolution a 55 Pointing Accuracy <15 Spectral Resolution 1 MHz Velocity Resolution 0.17 km s −1 a Angular resolution of the regridded map.This study contains extensive analysis with many new findings. The followings are the highlights of this study and can be found in the discussion and conclusions sections. Comparing our [C ii] spectral map to the Mopra CO map, we found that bright [C ii] emission in the CNC is closely related to the CO clumps in position-position-velocity space, suggesting that bright [C ii] emission likely arises from PDR and ionization fronts. We also found large absorption cavities in HI 21 cm emission and that those cavities are in a good agreement with the CO clouds/clumps and the Keyhole Nebula, a CO-dark molecular cloud, in position-position space. On the other hand, velocities of the absorption cavities are ±5-10 km s −1 shifted from the CO velocity centroids, suggesting that the cavities may follow cold HI gas photoevaporating or stripped from cloud surfaces. Through detailed PDR modeling of 10 different regions representing various ISM structures, we found a mass proportion of 1:4.1:5.6 for the atomic:dark:molecular(CO) gas and that six out of ten regions are dominated by [C ii] emission from HII regions rather than PDRs. Finally, combining kinematics and modelings, we found that the three-dimensional morphology of the CNC is consistent with one side of numerous blister HII regions expanding freely toward us, similar to a champagne flow, with a lifetime of CO clouds exposed to HII regions being 20-30 Myr.We describe details of observations using STO2 and data reduction ...

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“…The rate of H 2 formation in a dense interstellar gas is poorly known. In diffuse clouds, H 2 forms with a rate of R H 2 = (3 − 5) × 10 −17 cm 3 s −1 (Wakelam et al 2017). Here, we assume that H 2 molecule quickly leaves dust grain after the formation, and that one-third of the binding energy of H 2 is deposited statistically as internal excitation of the newly formed H 2 molecule (Black & van Dishoeck 1987).…”

Section: Appendix A: Chemistrymentioning

confidence: 99%

C-type shock modelling – the effect of new H2–H collisional rate coefficients

Nesterenok

Bossion

Scribano

et al. 2019

Monthly Notices of the Royal Astronomical Society

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We consider collisional excitation of H 2 molecules in C-type shocks propagating in dense molecular clouds. New data on collisional rate coefficients for (de-)excitation of H 2 molecule in collisions with H atoms and new H 2 dissociation rates are used. The new H 2 -H collisional data are state of the art and are based on the most accurate H 3 potential energy surface. We re-examine the excitation of rotational levels of H 2 molecule, the para-to-ortho-H 2 conversion, and H 2 dissociation by H 2 -H collisions. At cosmic ray ionization rates ζ 10 −16 s −1 and at moderate shock speeds, the H/H 2 ratio at the shock front is mainly determined by the cosmic ray ionization rate. The H 2 -H collisions play the main role in the para-to-ortho-H 2 conversion and, at ζ 10 −15 s −1 , in the excitation of vibrationally excited states of H 2 molecule in the shock. The H 2 ortho-to-para ratio (OPR) of the shocked gas and column densities of rotational levels of vibrationally excited states of H 2 are found to depend strongly on the cosmic ray ionization rate. We discuss the applicability of the presented results to interpretation of observations of H 2 emission in supernova remnants.

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

Cited by 210 publications

References 414 publications

First Resolved Dust Continuum Measurements of Individual Giant Molecular Clouds in the Andromeda Galaxy

First Resolved Dust Continuum Measurements of Individual Giant Molecular Clouds in the Andromeda Galaxy

Probing ISM Structure in Trumpler 14 and Carina I Using the Stratospheric Terahertz Observatory 2

C-type shock modelling – the effect of new H2–H collisional rate coefficients

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