The fragmentation of expanding shells,

by Kazunari Iwasaki (Nagoya University)

Room L269 (former D18, 2nd floor) at ENS, 24 rue Lhomond, 13:30 to 14:30

We investigate the gravitational fragmentation of expanding shells driven by HII regions by using the three-dimensional SPH.The ambient gas is assumed to be uniform. We find that perturbations begin to grow earlier than the prediction of the linear analysis under the thin-shell approximation. Moreover, the development of the gravitational instability is accompanied by the significant deformation of the contact discontinuity.

To understand these features, we perform a linear analysis considering the thickness of the shell which well describes the results of the SPH simulations. We derive useful fitting formulae of the epoch when the gravitational instability begins to grow and the fragment scale.

Formaldehyde in the Horsehead nebula

By Viviana Guzman (Laboratoire de Radioastronomie)

Room L269 (former D18, 2nd floor) at ENS, 24 rue Lhomond, 13:30 to 14:30

FUV photons determines much of the prevailing physics and chemistry of the gas and dust in molecular clouds. The Horsehead nebula is a well studied cold dark cloud. Because of its closeness (~400 pc) and favourable orientation it is well suited to study the physical conditions of molecular gas and to investigate the main chemical formation routes of molecules.

I will present observations of 7 transitions of formaldehyde (H2CO) toward two positions: the edge of the nebula exposed to the UV-field (PDR), and a colder region (cold core) shielded from the UV radiation. UV radiation. A non-LTE Montecarlo radiative transfer code is used to determine the H2CO abundance from the observed intensities and line profiles. We then compare the observed abundances with PDR models in order to study the main formation routes of H2CO. Pure gas-phase chemistry models fail to reproduce the observed abundance of H2CO. Grain-surface chemistry models are also tested. Finally, I will present the detection of HDCO and D2CO and the inferred abundances compared to H2CO.