Efficient ortho-para conversion of H2 on interstellar grain surfaces

Abstract

Context: Fast surface conversion between ortho- and para-H2 has been observed in laboratory studies, and this mechanism has been proposed to play a role in the control of the ortho-para ratio in the interstellar medium. Observations of rotational lines of H2 in Photo-Dissociation Regions (PDRs) have indeed found significantly lower ortho-para ratios than expected at equilibrium. The mechanisms controlling the balance of the ortho-para ratio in the interstellar medium thus remain incompletely understood, while this ratio can affect the thermodynamical properties of the gas (equation of state, cooling function). Aims: We aim to build an accurate model of ortho-para conversion on dust surfaces based on the most recent experimental and theoretical results, and to validate it by comparison to observations of H2 rotational lines in PDRs. Methods: We propose a statistical model of ortho-para conversion on dust grains with fluctuating dust temperatures, based on a master equation approach. This computation is then coupled to full PDR models and compared to PDR observations. Results: We show that the observations of rotational H2 lines indicate a high conversion efficiency on dust grains, and that this high efficiency can be accounted for if taking dust temperature fluctuations into account with our statistical model of surface conversion. Simpler models neglecting the dust temperature fluctuations do not reach the high efficiency deduced from the observations. Moreover, this high efficiency induced by dust temperature fluctuations is quite insensitive to the values of microphysical parameters of the model. Conclusions: Ortho-para conversion on grains is thus an efficient mechanism in most astrophysical conditions that can play a significant role in controlling the ortho-para ratio.

Accepted in Astronomy & Astrophysics

References in corpus (14)

• The importance of the ortho:para H2 ratio for the deuteration of molecules during pre-protostellar collapse
• Spitzer observations of HH54 and HH7-11: mapping the H2 ortho-to-para ratio in shocked molecular gas
• Surface chemistry in the Interstellar Medium II. ${\mathrm{H}}_2$ formation on dust with random temperature fluctuations
• Chemical probes of turbulence in the diffuse medium: the TDR model
• The penetration of FUV radiation into molecular clouds
• A Spitzer Infrared Spectrograph Survey of Warm Molecular Hydrogen in Ultra-luminous Infrared Galaxies
• Excitation mechanisms in newly discovered H2-bearing Damped Lyman-alpha clouds: systems with low molecular fractions
• The Ionization Fraction of Barnard 68: Implications for Star and Planet Formation
• Spitzer Mapping of PAHs and H2 in Photodissociation Regions
• Warm molecular gas temperature distribution in six local infrared bright Seyfert galaxies
• An HST/COS survey of molecular hydrogen in DLAs & sub-DLAs at z < 1: Molecular fraction and excitation temperature
• Ortho-para transition in molecular hydrogen
• Molecular Hydrogen Formation on Low Temperature Surfaces in Temperature Programmed Desorption Experiments
• Molecular hydrogen emission in the interstellar medium of the Large Magellanic Cloud

Cited by in corpus (20)

• Photodissociation and X-Ray Dominated Regions
• Structure of photodissociation fronts in star-forming regions revealed by observations of high-J CO emission lines with Herschel
• PDRs4All: A JWST Early Release Science Program on radiative feedback from massive stars
• SOLIS IV. Hydrocarbons in the OMC-2 FIR 4 region, a probe of energetic particle irradiation of the region
• Constraining physical conditions for the PDR of Trumpler 14 in the Carina Nebula
• JWST study of the DG Tau B disk wind candidate: I -- Overview and Nested H${}_2$/CO outflows
• Herschel survey and modelling of externally-illuminated photoevaporating protoplanetary disks
• Adsorption of H${}_2$ on Amorphous Solid Water Studied with Molecular Dynamics Simulations
• Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud. II. 30 Doradus
• On the Inference of the Cosmic Ray Ionization Rate $\zeta$ from the HCO${}^{+}$-to-DCO${}^{+}$ Abundance Ratio: The Effect of Nuclear Spin
• H${}_2$ ortho-to-para conversion on grains: A route to fast deuterium fractionation in dense cloud cores?
• On the H${}_2$ abundance and ortho-to-para ratio in Titan's troposphere
• H${}_2$ ortho-para spin conversion on inhomogeneous grain surfaces
• Measurements of Ortho-to-para Nuclear Spin Conversion of H2 on Low-temperature Carbonaceous Grain Analogues: Diamond-like Carbon and Graphite
• C-type shock modelling -- the effect of new H${}_2$-H collisional rate coefficients
• The ortho-to-para ratio of interstellar NH${}_2$: Quasi-classical trajectory calculations and new simulations
• The sulfur plume in the Horsehead nebula: New detections of S${}_2$H, SH${}^{+}$, and CO${}^{+}$
• A Tentative Detection of Molecular Hydrogen (H${}_2$) Emission Lines at Cosmic Dawn
• Vacuum ultraviolet photolysis of hydrogenated amorphous carbons. III. Diffusion of photo-produced H2 as a function of temperature
• SOFIA/upGREAT far-infrared spectroscopy of bright rimmed pillars in IC 1848