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Astrophysics Luncheon Seminar

Gas-ice Chemical Interplay in Interstellar Clouds
Presented by Seyit Hocuk
Max-Planck-Institute for Extraterrestrial Physics (MPE)

Monday, July 6, 2015
12:00 noon in 169-336

Abstract
When diffuse clouds evolve into translucent clouds then into molecular and finally into dense cores, the gas-ice chemical interplay plays an important role on the cloud dynamics. Not only the chemical abundances, but also the thermodynamic and the density evolution are systematically affected. In this work, spread over a few years, we follow the evolution of a gravitationally bound diffuse cloud all the way to the formation of stars. We fully considered the gas-dust interplay by including the details of freeze-out, thermal and non-thermal desorption, as well as the the most important photo-processes on grain surfaces. For this purpose, we used time-dependent rate equations to calculate the molecular abundances in the gas phase and on solid surfaces and perform 3-d hydrodynamical simulations with the adaptive mesh code FLASH. In Milky Way like conditions, our findings show that while the dust grains are still bare, water formation is enhanced by grain surface chemistry, enriching the molecular medium. The CO molecules tend to gradually freeze out on bare grains. This causes CO to be well mixed and strongly present within the first ice layer. Once one monolayer of water ice has formed a strong depletion of gas-phase water and CO molecules occurs. Since gaseous CO is a major coolant during the translucent and molecular cloud phases, depletion by way of freeze-out affects the thermal balance, increasing the gas temperature. This affects the equation of state of the cloud, to which fragmentation is sensitive. However, non-thermal desorption processes at these cold temperatures can release enough coolants back into the gas phase which diminish the overall effect. In the end, when forming stars, the IMF seems to be inaffected if enough CO remains in the gas phase during the critical stages.

JPL Contact: Umut Yildiz (4-4909)


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