EPoS Contribution
EPoS Contribution
The Draco Nebula: the Atomic-to-Molecular Transition in a Colliding Flow

Quentin Salome
IryA-UNAM, Morelia, MX
One of the key question related to star formation is how the gas transit from the warm and diffuse phase (n~0.5 cm-3; T~8000 K) to the denser phase (n~100 cm-3; T~40 K). It is now believed that the efficiency of this transition is one of the element that controls the star formation rate. Many numerical experiments have showed that this transition is favoured by the increase of pressure of the warm gas, in colliding flows for instance. In general, such conditions are difficult to identify in the interstellar medium (ISM), but there is one case that offers a clear view at this process: the infall of clouds from the Galactic halo. The Draco Nebula is one of the best candidates to understand this process and resulting physical conditions. Draco is a high Galactic latitude interstellar cloud observed at velocities corresponding to the intermediate velocity cloud regime (v~-25 km/s). It is located above the Galactic plane, at a height z=370 pc (distance~600 pc). Early on, based on the analysis of 21 cm data, it was suggested that the formation of Draco is the result of the collision of a cloud entering the disk of the Milky Way. I will first present recent Herschel-SPIRE data. These data revealed high-contrast small-scale structures. The analysis of these structures showed that high mass part of the clumps mass spectrum follows a power law similar to what is found for small scale fragments of molecular clouds. Most of the structures are not gravitationally bound and the volume density of the clumps follows a log-normal distribution. In addition, the PDF of column density is bimodal. These results indicate that the structure of the gas in Draco is the result of the interplay between the thermal instability and turbulence. The actual scenario is that the warm HI gas of the disc is compressed by the incoming cloud, sending the WNM in the thermally unstable regime. These conditions (i.e. a colliding flow) are known to facilitate the transition from WNM to CNM gas through the thermal instability. Due to the infall, the increased pressure in the front favours the formation of molecules. We detected and mapped strong 12CO emission in several targeted regions along the shock front with the IRAM 30m. The CO line mapping allows us to study the dynamics of the cold and dense gas, and compare it with the more diffuse WNM and CNM gas seen in HI (from GBT and DRAO; GHIGLS and DHIGLS surveys). I will present how the comparison of HI and CO data reveal the structure of the shock front and the physical conditions of the HI-H2 transition.
Caption: Multi-wavelength view of the Draco nebula. Left: Total column density map derived from the dust emission from Herschel-SPIRE at 250 microns (17.6" resolution). Middle: Cold HI column density map from DHIGLS (1' resolution). Right: 12CO(1-0) intensity of the regions observed with IRAM 30m (22.5" resolution).
Collaborators:
A. Marchal, IAS, FR
M.-A. Miville-Deschenes, IAS, FR
P.G. Martin, CITA, CA
G. Joncas, U Laval, CA
F.J. Lockman, NRAO-GBT, US
Key publication

Suggested Session: Molecular clouds