Determining the dynamics of the ultracompact HII region (UCHII) Monoceros R2 

By Acension Fuente (OAN - IGN, Spain)

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

Ultracompact (UC) HII regions constitute one of the earliest phases in the formation of a massive star and are characterized  by extreme physical and chemical conditions (G>10^5 in units of  Habing field and n>10^6 cm^-3). Their understanding is important for distinguishing the different processes in massive  star formation (UV radiation vs shocks) and because they can be used as a template for other extreme photon-dominated regions (PDRs) such as the surface layers of circumstellar disks and the nuclei of starburst galaxies. The UC HII Mon R2 is the only one that can be spatially resolved with single-dish telescopes,  including Herschel. It has been extensively studied at millimeter and submillimeter wavelengths and all evidences support a chemistry driven by the strong UV field while shocks play only a minor role. Its physical and kinematical structure is well known  and current observations are well explained by an expanding  envelope with v_e=0.5x(r/R_{out})^{-1} km/s triggered by the inner HII region. These observations, however, only probe the cold phase of the molecular gas (Tk < 100 K) which is clearly insufficient for the full understanding of the massive star formation process.
Mon R2 has been observed with Herschel within the Guarantee Time Key Program "Warm and Dense Interestellar Medium" (PI:Volker Ossenkopf). These new HIFI observations probe the innermost part of the dense PDR around the HII region and the external diffuse cloud envelope and provide valuable insights into the chemistry and dynamics of this massive star forming region.