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Research background

The space between the stars is not empty but filled with a very dilute gas. The colder (T=10-100 K) and denser (10² - 10⁶ cm-3) concentrations are called `interstellar clouds' and can be seen on optical pictures as dark patches blocking the light from background stars. These clouds contain a surprisingly rich variety of molecules and small solid particles called grains, and form a unique chemical laboratory in which processes occur that are not normally found in a laboratory on Earth. Moreover, dark clouds are the birthplaces of new stars and planets, and molecular lines are often the only means to penetrate and study these stellar nurseries. The gas and dust in clouds provide the raw building blocks from which future planetary systems are made, so that tracing their chemical evolution is an essential step in establishing our origins. These two themes - the basic chemistry in space and the study of star- and planet formation - form the main thrust of our research.

More details about molecular astrophysics in Leiden:

• Physics and chemistry during star- and planet formation
• Laboratory astrophysics
• Theoretical chemistry

Close collaborations exist with other groups within NOVA Network 2

dark cloud 1018 cm		circumstellar disk around young star 1015 cm		planet 109 cm

Physics and Chemistry during Star- and Planet Formation

How do clouds collapse to form stars? What are the conditions in circumstellar disks around young stars where planets are formed? How is the raw material from the clouds chemically modified before incorporation into new solar systems? These questions are addressed through observations at submillimeter and infrared wavelengths, combined with models of the excitation and abundance of molecules. Our group uses primarily data obtained with Herschel, Spitzer, JCMT, SubMillimeter Array and ESO-VLT.

Molecular lines are excellent diagnostics of the physical conditions in star-forming regions. From the relative strength of lines information about temperature and density can be obtained, whereas the absolute strengths give the abundances. The line profiles provide kinematical and dynamical data. To extract this information, sophisticated radiative transfer models are required, some of which have been developed in our group. The abundances are subsequently compared with results of pure gas-phase and gas-grain chemistry models.

Our research focuses on a systematic study of the physical and chemical structure of young stellar objects at different stages of evolution, including low-mass protostars, massive stars in the embedded phase, and circumstellar disks around young stars.

Important chemical diagnostics at different stages of star- and planet-formation. From: van Dishoeck and Blake 1998, ARAA 36, 317	Methanol helps to date early evolution of massive stars. From: van der Tak, van Dishoeck and Caselli, 2000, A&A, 361, 327.

Laboratory for Astrophysics at Leiden Observatory

In the cold and dark interstellar clouds, gas-phase molecules collide with the grains at least once every 100,000 years and freeze-out, forming icy mantles around the silicate cores (typically 0.1 micrometer in size). The ices consist largely of simple molecules such as H₂O, CO, CO₂, CH₄ and NH₃, together with more complex species like CH₃OH (methanol). In the Laboratory for Astrophysics, led by Prof. dr. H. Linnartz, the physical and chemical processes that take place in these ices are simulated.

Recent research has focussed on the analysis and interpretation of ISO and Spitzer data (see the ISO spectra here and here). Through careful comparison between laboratory and astronomical data, the ice components have been identified and clues about their thermal history have been obtained. Basic grain-surface reactions (SURFRESIDE) and reactions under influence of UV radiation (CRYOPAD) are being simulated in ultra high vacuum set-ups. For more information, see the laboratory astrophysics home page.

Photo of the new SURFRESIDE ultra-high vacuum set-up (left) used to simulate basic gas-grain processes in space (right, dark cloud B68: ESO-VLT).

History

Theoretical chemistry

Because the temperatures and densities are low, radicals, ions and atoms have long lifetimes in interstellar space and can exist in high abundances. Such species are very reactive under Earth-like conditions, however, and it is difficult to simulate their reactions in a laboratory. For these molecules, modern ab initio quantum chemical techniques combined with dynamical calculations can provide accurate information on the basic chemical processes. In collaboration with Marc van Hemert and Geert-Jan Kroes from the UL chemistry department, fundamental processes such as photodissociation (e.g., CH₂, H₂O), collisional excitation (e.g., OH + H₂), neutral-neutral reactions (e.g., C + H₂ -> CH + H) and reactions on icy surfaces (e.g. CO + OH -> CO₂) are investigated. The results from these calculations are incorporated into interstellar chemistry networks and models of the abundance and excitation of molecules in different astrophysical regions.

Photodissociation cross sections of CH2, obtained in three-dimensional quantum dynamical calculations by Bearda, Kroes et al.