Vorschau

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Abstract

With the goal of constraining the initial physical and chemical conditions of low-mass star formation, the thermal dust emission of a sample of prestellar cores has been observed with the Herschel Space Observatory. From these observations, the most accurate maps of the dust temperature and density structures in prestellar cores existing today have been derived using a ray-tracing technique. Based on this new information on the physical conditions in the prestellar cores I model the chemical evolution of the associated gas. Comparison of the models to molecular line observations reveals that CO freezes out strongly in the core centers and that even the high density tracer N2H+ is affected by depletion. I derive a chemical age of the gas in all cores on the order of 10^5 yr which is comparable to the free-fall time of the cores. Furthermore, I calculate the thermal equilibrium distributions of the prestellar cores between the two methods confirming the reliability of the ray-tracing technique. It is also shown that the agreement is good for a large range of dust models. Finally, I present ammonia observations of three prestellar cores and use them as a gas temperature probe. Comparison of gas and dust temperatures shows that both agree in the inner parts of two cores traced by ammonia while the gas is slightly warmer than the dust in the third object; maybe due to a reduced collisional coupling between gas and dust because of coagulation of the dust grains.