Astrochemistry Research

COol or what? Unravelling the mysteries of star formation with laboratory surface science

Astrochemistry is the study of molecules in space – where are they? how did they get there? what role they play in controlling or influencing astrophysical processes? It's also the focus of Dr. Helen Fraser’s work – and the latest addition to the research portfolio of the Department of Physics in Strathclyde. It turns out that chemistry, and interstellar molecules in particular, are one of modern astronomy’s best tools for probing the processes of star and planet formation (see Fig. 1). Through a combination of observational spectroscopy and imaging, theoretical modelling and controlled laboratory studies, we are beginning to unlock the secrets of the cosmic chemical cauldron…

It turns out that of the 140 or so molecules that have been identified to date in interstellar space, many, including the simplest molecules such as H2, H2O, CH3OH and CO2, must be formed at the surfaces of interstellar dust, in reactions involving atoms and molecules that adsorb or ‘freeze-out’ from the gas phase. Although we can observe the infrared spectra of both the dust grains and their icy mantles, almost nothing is yet understood about the processes involved in the build up of ices in interstellar regions, nor the chemical processes that subsequently lead to the formation of the more complex organic species that are often observed in the gas phase as the ices desorb. However, with inputs from surface chemistry and physics, as well as the constraints placed on the scenarios by astronomical observations, we have been able to postulate the processes involved in the solid-state chemical evolution of star forming regions (see Fig. 2). It is these process that we are now trying to elucidate under controlled pseudo-interstellar conditions in the laboratory, feeding back our knowledge to the astronomy community, and pulling back a little further the molecular veil that currently obscures our complete understanding of star and planet formation.

Fig. 2 – A schematic representation of the life cycle of an interstellar dust grain in star forming regions. (Image credit: adapted from Fraser et al. Rev. Sci. Inst., 73, 2161 (2002)).

Together with her colleagues at the Raymond and Beverly Sackler Laboratory at Leiden Observatory, Netherlands, and the Kapyten Institute at The University of Groningen, Helen has recently published a paper which suggests not only that gases can interact directly with the bare grain surfaces of interstellar dust without freezing out, but even hints at the chemical composition of the surfaces of the dust grains. For the first time, she and her colleagues have observed CO adsorbed directly to cationic sites on interstellar dust grains, and used laboratory experiments to confirm their assignment, (Fraser et al. MNRAS, 356, 1283 (2005) (pdf, 580kb)).

For some time it has been known that interstellar dust is composed from a predominantly silicate (or possibly carbonaceous) core, most closely resembling amorphous silicates, such as Olivine or Pyroxene – but in reality the IR bands at around 9-11 um, in interstellar spectra, arise from the ‘bulk’ of the dust grains. In a recent survey, using the ISAAC spectrometer on the ESO (European Southern Observatory) VLT (Very Large Telescope) at Paranal, Chile, of over 50 low-mass and intermediate mass star-forming regions in the southern hemisphere skies, they observed (amongst other things), a previously undetected band at 2175 cm-1, around the region where gas and solid-phase CO spectra are found (Pontoppidan et al. 408, 981, (2003), van Broekhuizen et al. A&A (2005) submitted, Thi et al. A&A (2005) submitted). Following extensive literature searches, it seemed that the most likely carrier of the band would be CO gas – adsorbed at the surface of the dust grains themselves. Together with her (then Masters) student, Suzanne Bisschop, (who is now doing a PhD in Leiden, co-supervised by Helen), Helen undertook a series of experiments where CO was adsorbed to the clean surface of a naturally occurring Zeolite, Clinoptilolite, under high-vacuum and low-temperature conditions. Using transmission FTIR measurements, spectra of the adsorbed CO were taken, down to temperatures of 100 K. The spectra were then compared directly with the astronomical spectra, and used to show that the 2175cm-1 band is related to CO gas adsorbed at the surfaces of interstellar dust grains (see Fig. 3). Absorption bands were detected at 2177 and 2168 cm-1 (corresponding to CO chemisorbed at the Zeolite surface), and 2130 cm-1 (corresponding to CO physisorbed at the Zeolite surface), (see Fig. 3(a) and (b)), and gave an excellent match to the observational data (see example in Fig. 3 (c)). This result provides the first direct evidence that gas-surface interactions do not have to result in the formation of ice mantles on interstellar dust.

Fig 3: The assignment of the 2175 cm-1 band in interstellar spectra to CO gas adsorbed directly on cationic sites at interstellar dust grain surfaces, using laboratory studies of CO adsorption on ‘model’ grains. (a) The laboratory spectrum of CO chemisorbed and physisorbed on Clinoptilolite, a naturally occurring Zeolite, showing the Levenberg-Marquet non-linear least squares fitting of the spectrum, used to identify and subsequently assign the underlying components of the spectrum. (b) Illustration of the CO binding sites on the surface of the Zeolite. (c) Comparison between the observational spectrum of one low-mass object (black line), with a computed fit to the observation (solid grey line), convolved from the laboratory CO-zeolite spectrum (grey dashed line), and a contribution from OCN- ions (dotted grey line -see van Broekhuizen et al A&A (2005) submitted).

Zeolites are unlikely to be found in interstellar space, but are found in meteorites, as are GEMS, glasses with embedded metals and sulphides – i.e. amorphous silicates with cations and sulphur bearing species encapsulated in them. These are thought to be representative of the pristine pre-solar nebular material, i.e. a ‘fossil’ of interstellar dust. So in these experiments, the Zeolite represents a proof of concept, used because it was readily available for laboratory work to disseminate the observational data – it can be shown from the literature that the when adsorbed at the surface of cation-doped amorphous silicates, CO stretching vibrations occur at identical frequencies to those in Zeolites.

Consequently this result has far-reaching implications for laboratory astrochemistry. So far, studies have concentrated on the chemistry and physics occurring on-top of, in the bulk of, and involving the icy mantles of interstellar dust grains. Using the band-strength of the CO-adsorbate band (estimated to be ~ 4*10^-19 cm/molecule), the abundance of CO adsorbed at bare grain surfaces ranges from 0.06 to 0.16 relative to H2O ice. These findings imply that interstellar grains have a large (catalytically-active) surface area, providing a refuge for interstellar species. Consequently the potential exists for heterogeneous chemistry to occur involving CO molecules in unique surface chemistry pathways not currently considered in gas-grain models or laboratory studies of the interstellar chemistry. Heterogeneous catalysis is a potentially powerful route to forming many simple and complex molecules found in interstellar space, and is one of the new research avenues that Helen will be working on in Strathclyde.

If you would like more information on this work or astrochemistry in general, Helen can be found in room JA 6.24 (x3420, h.fraser phys.strath.ac.uk). She will also be giving two talks on March 23rd – one at 9am, “Its life Jim (West) but not as we know it (yet)” in JA 8.11 for the technical and administrative staff, and one highlighting her recent scientific results, in the general departmental seminar series, at 4pm the same day, in JA 3.14 “COol or what? Probing the mysteries of star formation using surface science”.