Gas-phase Astrochemistry

Text: Ewine van Dishoeck, 16 oct 2009

Topic 1: Photodissociation processes

In any region in which UV photons can penetrate, photodissociation is the major destruction mechanism of (neutral) gas-phase molecules. This includes diffuse and translucent clouds, dense clouds exposed to intense UV from nearby young stars (so-called Photodissociation or Photon-Dominated Regions, PDRs) in our own and other galaxies, the envelopes around evolved stars, and the surface layers of protoplanetary disks. Even deep inside dense clouds, the UV photons produced by the interaction of cosmic rays with H₂ provide a weak field which can dissociate both gas-phase and solid-state molecules (ices). Photodissociation processes may well be responsible for the isotopic anomalies observed in meteorites in our own solar system: how and where did this happen? While data on a number of molecules are available from the older literature (see www.strw.leidenuniv.nl/~ewine/photo for overview), information on several key rates and processes is lacking.

In this topic, unique Dutch theoretical and experimental expertise on photodissociation processes will be brought together to address a number of outstanding questions in (astro)chemistry:

what are the photodissociation rates and mechanisms of key small molecules in astrochemistry, in particular N₂ and CO? How does this vary among the isotopologues and what is the importance of self- and mutual shielding?
what are the branching ratios of the photodissociation products, especially for > triatomic molecules? For example, does CH₃OH dissociate primarily to CH₃ + OH, CH₂OH + H, or CH₂ + H₂O, etc.? How does this depend on photon energy?
do the branching ratios change between molecules in the gas and ices? (link with "Solid-state astrochemistry" theme)
how stable are complex organic molecules (non-PAHs) against UV radiation? Where does the cross-over from very efficient dissociation as found for small molecules to highly stable because of internal conversion as found for large molecules occur? How does this depend on the structure of the molecule?

Topic 2: Inelastic collisions

Because of their rich energy level structure, molecules are excellent probes of the physical conditions in which they reside. Their abundances can also serve as chemical diagnostics of, for example, the evolutionary state of the sources. In order to extract any information about temperatures, densities and abundances, the state-to-state collisional rate coefficients need to be known over a range of temperatures (typically 10 K - few hundred K). These can only be determined by a combination of experiments and theory. Experimental determinations of individual cross sections at specific energies are crucial to benchmark the theoretical efforts.

Over the last decades, there have been significant efforts in computing and measuring collisional rate coefficients for astrophysically relevant molecules with the main collision partners H₂ and H. This, in turn, also provides new challenges for basic chemical physics on treating open shell molecules (e.g., OH) and both para- and ortho- H₂ properly. A summary up to a few years ago is given by Schoeier et al. 2005 (Astronomy & Astrophysics) and the relevant data are summarized in an easy-to-access form at www.strw.leidenuniv.nl/~moldata. The Molecular Universe EU network has been very active in the last few years in providing new data on a number of species, most importantly H₂O, crucial for interpretation of upcoming Herschel data.

Most recently, observations using the Spitzer Space Telescope combined with ground-based 8-m class telescopes have detected a wealth of mid-infrared (3-30 micron = 333-3333 cm^-1) lines of small molecules like CO, HCN, C₂H₂, CO₂, H₂O and OH. These are mostly the vibration-rotation lines (but in the case of H₂O and OH also very highly excited pure rotational lines). Very little is known about collisional rate coefficients for vibration rotation transitions; even for "simple" systems like CO-H or CO-H₂ uncertainties are still an order of magnitude. The goal of this topic would be to define a number of astrophysically relevant systems to study in a combined theory-experimental effort. For both topic 1 and 2, the molecular data would subsequently be incorporated into state-of-the-art molecular excitation and radiative transfer models and applied to interpretations of observations of molecules in star-forming regions, protoplanetary disks and extragalactic regions.

Photodissociation and excitation of molecules in protoplanetary disks

Ewine van Dishoeck, 19 oct 2009

N₂ photodissociation (and CO)

N₂ is the dominant nitrogen-bearing molecule in dense clouds, but can only be observed indirectly through tracers like N₂H⁺. Recent observations and models have suggested that not all nitrogen is converted to molecular form even inside a dense cloud but that a significant fraction is still atomic (e.g., Maret et al. 2006, Nature 442, 425). Similarly, models of the surface layers of protoplanetary disks suggest that whether a star has sufficient UV radiation or not to photodissociate N₂ affects the nitrogen chemistry (Pascucci et al. 2009, ApJ 696, 143). Finally, large 15N/14N ratios have been observed in some meteoritic material (e.g., Alexander et al. 1998, Meteor. Planet. Sci. 33, 603), interplanetary dust particles (e.g., Messenger et al. 2003, Spa. Sci. Rev. 106, 155) and comets (e.g., Arpigny et al. 2003, Science 301, 1522), which are thought to originate in the interstellar cloud. One possible explanation is isotope selective photodissociation, along the same lines as invoked and demonstrated for CO to explain ¹²C/¹³C and ¹⁶O/¹⁸O/¹⁷O anomalies in clouds and meteorites (Visser et al. 2009, A&A).

In contrast with CO, the N₂ photodissociation in interstellar clouds has never been treated properly. This is mostly because of lack of accurate molecular data for the very highly excited (Rydberg) electronic states through which the photodissociation occurs. Thanks to the detailed experimental and theoretical work of Glenn Stark, Brenton Lewis and collaborators, much of this data is now available. The first step would be to collect all the relevant (J-dependent) line positions, oscillator strengths and predissociation rates for ¹⁴N₂ and ¹⁴N¹⁵N and to compute the photodissociation rates with depth into a simple cloud model and check where self-shielding and mutual shielding by H₂ and CO could occur. This study will also reveal where there is still a lack of data (or need for higher accuracy), which could then be obtained either through additional experiments at Amsterdam and/or theory at Nijmegen.

The second step would be to consider both ¹⁴N₂ and ¹⁴N¹⁵N and compute the isotope selective photodissociation rates into realistic cloud models with and without grain growth. The final step would be to include the processes into a protoplanetary disk model and investigate the influence on the chemistry and isotope selective photodissociation.

This project will take at least 1 year work of the PhD student, with support from postdoc Ruud Visser and myself.

An extension of this project would be to update the CO photodissociation model by Visser et al. as new molecular data become available. Glenn Stark is starting a new set of measurements, in collaboration with Brenton Lewis for the analysis. This may be an area for Gerrit (and perhaps Wim) to get involved in as well; the predissociation processes are not well understood for CO. As a result, the J-dependent oscillator strengths and predissociation rates are not at all well known for the rarer CO isotopologues.

Photodissociation of larger molecules

The plan in Leiden would be to

a) search the literature on experimental data on photodissociation of large organic molecules of astrophysical interest like CH₃OCH₃, HCOOCH₃, C₂H₅CN,
b) use (modified) RRKM theory (see also papers M. Gruebele) to compute their stability of molecules and compare - where possible - with experiments.

Such calculations have been done for PAHs (see Visser et al. 2007, A&A for recent results) but never for other large complex organic molecules. Although they won't be accurate to better than an order of magnitude, they may give some (qualitative) information on photodissociation efficiencies. I would appreciate comments from experts on whether this is at all a feasible approach.

The second important question is the products of photodissociation. In my view, this can only be approached by experiments such as done in Nijmegen. In Leiden, we could then incorporate the results into astrochemical models, investigate effects on abundances and compare with observations. We would also update the photo website so that the astrochemical community has easy access to the results.

Water photodissociation

Now that OH has been detected in high rotational states consistent with photodissociation of H₂O through the B state, there may be further need for model results on the OH excitation. To be discussed with Marc, cf. the van Harrevelt & van Hemert results.

Molecular excitation

In order to interpret the infrared data on CO, H₂O, CO₂, HCN, C₂H₂, ..., the vibration-rotation inelastic cross sections need to be known. Most urgent is CO + H₂ and CO + H. As far as I know, the rate coefficients for these systems are still highly uncertain, see summary in Gonzalez-Alfonso et al. (2002, A&A 386, 1074, section 4.1.2) and Najita et al. (1996, ApJ 462, 919, Appendix B). For the case of CO₂, see the Appendix of Boonman et al. (2003, A&A 399, 1047), and more generally David Flower's book on inelastic collisions.

I know that Laurent Wiesenfeld has been working on H₂O-H₂ vibration-rotation transitions and perhaps some other species, so you may want to discuss this when he is visiting Nijmegen (this week?). The Leiden component would be to incorporate these rate coefficients into molecular excitation and radiative transfer models to simulate the infrared emission spectra from protoplanetary disks. Groningen is also interested in this (e.g., Inga Kamp's models), and in making the rate coefficients available through the LAMDA database and applying them to other astrophysical situations (e.g., high-mass YSOs, Floris).

Last updated: October 20, 2009, by Gerrit C. Groenenboom,