02 Octobre – Thesis defense - Lars Bonne

14 h30 Room Univers - LAB / Building B18N (Pessac)

The formation of dense gas in low- and high-mass star forming regions.

To understand how stars can form in the interstellar medium (ISM), it has to be understood how cold (~ 10 K) and dense gas (> 10^{4} cm^{-3}) can emerge during the evolution of the ISM. With the Herschel telescope it was found that most of this dense star forming gas is organised in filamentary structures.
To understand how this dense filamentary gas forms, multiple CO transitions were observed towards the Musca filament, which can form low-mass stars, using the APEX telescope. These observations were complemented with [CII] and [OI] observations by the SOFIA telescope. The non-detection of [CII] demonstrates that the Musca cloud is embedded in a weak FUV field (< 1 G0). However, the observed CO(4-3) line with APEX demonstrates the presence of warm (> 50 K) CO gas around the Musca filament which cannot be explained with heating by the FUV radiation field. A comparison of the observed CO(4-3) emission with shock models shows that the emission can be the result of a low-velocity (< 4 km/s) J-type shock. Further analysis of this emission demonstrates that this shock emission resembles the signature of a shock responsible for mass accretion on a filament. This suggests that a low-velocity shock as a result of continuous mass accretion is responsible for the formation of cold and dense gas that can form stars in the Musca filament.
The accretion scenario for Musca is further analysed with low-J CO observations from APEX and NANTEN2 to study the large scale gas kinematics. These observations unveil a velocity gradient over the Musca filament crest which is correlated with the velocity field of the nearby ambient gas. This suggests that the velocity gradient is the result of mass accretion from the ambient cloud. Analysing the full Musca cloud demonstrates a spatial and kinematic asymmetry from low- to high-density gas. This asymmetry is seen as a V-shape in the position-velocity (PV) diagram perpendicular to the Musca filament. Including atomic hydrogen (HI) observations in the analysis first of all confirms that Musca is part of a larger HI cloud, the Chamaeleon-Musca complex. It also demontrates that the kinematic asymmetry is seen from the HI cloud down to the filament crest. Furthermore, the CO-HI asymmetry is found for basically all dense regions (Cha I, Cha II, Cha III and Musca) with archival data of Chamaeleon-Musca, while HI shows indications of more than one velocity component. This asymmetric accretion scenario is predicted by magnetised cloud-cloud collision simulations, where the bending of the magnetic field is responsible the observed asymmetric accretion scenario. The filament formation in Musca is thus the result of two intersecting converging flows which are driven by the magnetic field bending due to a large-scale colliding HI flow that triggered the observed star formation in the full Chamaeleon-Musca complex.
Finally, the kinematics of the high-mass star forming ridge DR21 and its surrounding gas are studied to compare low- and high-mass star formation. This shows a similar spatial and kinematic asymmetry as in Musca, which suggests that DR21 is formed by a giant molecular cloud (GMC) collision. However, it is also found for high-mass star formation in the DR21 cloud that gravity plays an important role on large scales (> 1 pc) while for Musca gravity only starts to dominate locally (r < 0.1-0.2 pc). So, due to the high density in the DR21 cloud after the GMC collision, gravity eventually drives the evolution of the compressed cloud for high-mass star forming regions. Kinematic observations of the full Cygnus-X north region show further indications of two interacting velocity components over the entire region, which indicates that a high-velocity (> 10 km/s) GMC collision can result in the formation of an OB association similar to OB2. These OB stars then form in gravitationally collapsing hubs and ridges due to the compression by the GMC collision.

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