Research Background

The gas and dust in interstellar clouds form the basic material from which future planetary systems like our own are built. In cold and dense clouds in which stars are formed, many of the heavy species will collide with the grains and freeze out, forming an icy mantle around the silicate grain cores. These ices have been observed directly by infrared absorption spectroscopy towards young stellar objects and dense clouds (see Boogert & Ehrenfreund 2004 in Astrophysics of Dust for review). In the densest cores, more than 90% of the heavy elements are frozen out as H₂O, CO and CO₂ ices, making these ices the second most abundant ingredient after H₂, even exceeding gaseous CO. Thus, quantifying how molecules freeze-out and desorb, and how they can be modified on the grains by (photo-)chemical processes, is of fundamental importance for understanding the evolution of star- and planet-forming regions.

A stage of special interest during star formation is the formation of a circumstellar disk. Circumstellar accretion disks transfer matter from molecular clouds to young stars and are the sites of planet formation. As such they form a pivotal link between the formation of stars and that of solar systems like our own. The chemistry in disks differs from that in normal molecular clouds in several ways due to high densities (> 10⁸ cm^-3) and enhanced UV radiation fields. The UV radiation has at least two important effects: it can bring molecules from the ices back into the gas via photodesorption, and it can create new, more complex molecules in the ices through photochemistry. If these processes are well constrained UV chemistry can also act as a tracer for the physical processes, such as vertical mixing, in disks.

The dark cloud B68

Circumstellar disks in the Orion nebula - the birthplace of planets

Measuring Photodesorption of Ices with CRYOPAD

Solid H₂O, CO and CO₂ are often the most abundant species in star forming regions, after H₂, and form the building blocks of a complex organic chemistry once desorbed into the gas phase. A long-standing problem is how these molecules can be maintained in the gas phase at the low temperatures found in star forming regions, where all molecules other than H₂ should stick on dust grains on timescales shorter than the cloud lifetimes. Yet dark clouds are detected in the sub-mm lines of gaseous CO. Similarly, abundant gas-phase CO and H₂O, which cannot be explained by thermal equilibrium chemistry, is observed in the cold mid-planes of protoplanetary disks. Photodesorption of ices has been suggested to explain these observations.I am currently conducting a laboratory study of UV (7-10.5 eV) photodesorption of pure ices (N₂ , CO, CO₂ and H₂O) under ultra high vacuum at astrophysically relevant temperatures (15-100 K) using CRYOPAD. CRYOPAD was espeically designed to measure astrophysically relevant photoprocesses in ices. The ice content is porbed by a refelction-absorption infrared spectrometer, while species in the gas phase are investigated with a quadropole mass spectrometer. In general I have found that UV photodesorption is highly molecule specific with rates differing orders of magnitude between different species investigated here, partly due to their different cross sections at 7-10.5 eV. For all ices, the experimental parameters have been varied to constrain the mechanisms involved in the desorption process. The determined photodesorption rates have been applied to astrophysical models to test the significance of photodesorption in space. For molecules that are found to have high photodesorption rates, such as CO, the role of photodesorption in preventing total removal of molecules in the gas has been strongly underestimated in previous models.

CRYOPAD - my UHV set-up used to quantify photodesorption and photochemical processes in interstellar ice equivalents

Infrared Spectroscopy of Ices

The observation of emission and absorption spectra of a large variety of (initially unknown) interstellar molecular in the millimetre, near and far infrared wavelength range, triggered laboratory astrophysics because it made clear the need of a database of known molecular spectra to use for identification of astrophysical spectral features. Spectroscopy of ices is carried out in the infrared. It is now generally known that many of these infrared ice features are temperature dependent and very sensitive to contamination. It is hence the main challenge for laboratory astrophysics to reduce the temperature and pressure to comparable levels to those commonly found in space.

In general, spectroscopy of molecules together with chemical understanding have proved a powerful tool in studying of densities, temperatures and turbulence in space. Spectra of ices contain additional information since the spectral profile of a molecule varies with its surroundings. Hence the spectral profiles can reveal the environment of certain ice molecules, giving clues both to its chemistry and desorption behavior.

Effects of CO₂ on H₂O band profiles and band strengths in mixed H₂O:CO₂ ices

H₂O is the most abundant component of astrophysical ices. In most lines of sight it is not possible to fit both the H₂O 3 μm stretching, the 6 μm bending and the 13 μm libration band intensities concurrently with a single pure H₂O spectrum. Recent Spitzer observations have revealed CO₂ ice in high abundances and it has been suggested that CO₂ mixed into H₂O ice can affect the relative band strengths of the 3 μm and 6 μm bands. In this study we investigated whether the discrepancy in intensity between H₂O bands in interstellar clouds and star forming regions can be explained by CO₂ mixed into the observed H₂O ice affecting the bands differently. To do this we employed laboratory infrared transmission spectroscopy is used to record spectra of H₂O:CO₂ ice mixtures at astrophysically relevant temperatures and composition ratios.We found that the H₂O peak profiles and band strengths are significantly different in H₂O:CO₂ ice mixtures compared to pure H₂O ice.

The Eagle Nebula - a star forming region

Observations of Ices in Star Forming Regions

Spitzer Survey of CH₄ Toward Low Mass Young Stellar Objects

CH₄ is proposed to be the starting point of a rich organic chemistry. Solid CH₄ abundances have previously been determined mostly toward high mass star forming regions through infrared observations of the 7.7 um absorption feature attributed to the bending mode of solid CH₄. Spitzer/IRS now provides a unique opportunity to probe solid CH₄ toward low mass star forming regions as well. We have recently used infrared spectra from Spitzer to determine the solid CH₄ abundances toward a large sample of low mass young stellar objects. 23 out of 46 ice sources in our sample - c2d (cores to disks) legacy and GTO data - have an absorption feature at 7.7 um. The solid CH₄/H₂O column density ratio is 2-15% toward all objects with no significant variation between the clouds except for unusually high abundances toward Ophiuchus. All sources with abundances above 10% have small total ice column densities, but beyond H₂O column densities of 2x10¹⁸ cm^-2 the CH₄ abundances (18 out of 23) are nearly constant around 5%. These abundances are equal or higher compared to what has been observed towards high mass objects. Correlation plots with solid H₂O, CH₃OH, CO₂ and CO column densities and abundances relative to H₂O reveal a closer relationship between solid CH₄ and CO₂ and H₂O than with solid CO and CH₃OH.

The Spitzer Space Telescope