High-mass star forming regions: An ALMA view

The advent of ALMA is bound to improve our knowledge of OB star formation dramatically. Here, we present an overview of this topic outlining how high angular resolution and sensitivity may contribute to shed light on the structure of high-mass star forming regions and hence on the process itself of massive star formation. The impact of this new generation instrument will range from establishing the mass function of pre-stellar cores inside IR-dark clouds, to investigating the kinematics of the gas from which OB stars are built up, to assessing or ruling out the existence of circumstellar accretion disks in these objects.     

Shock-heated NH3 in a Molecular Jet Associated with a High-Mass Young Star

We present the discovery of shock-excited NH3 in a well-collimated jet associated with the extremely young high-mass star IRAS 20126+4104. The NH3 (3, 3) and (4, 4) emission is dominated by three clumps along the SiO jet. At the end of the jet, there exists strong and broad (+/-10 km s-1) NH3 (3, 3) emission. With typical brightness temperatures greater than 500 K, the overall emission indicates a weakly inverted population and appears in an arc, consistent with the excitation by bow shocks. There are two bright spots in the NH3 (3, 3) emission with brightness temperatures of approximately 2000 K. The narrow line width (1.5 km s-1 FWHM), the small sizes (<0&farcs;3), and the unusually high brightness temperature of the features are indicative of maser emission. Our observations provide clear evidence that NH3 (3, 3) masers are excited in shock regions in molecular outflows.     

The massive hot core associated with G31.41+0.31

We have used the IRAM Plateau de Bure Interferometer to observe the ground state and vibrationally excited lines of Methyl Cyanide towards the UCHII region G31.41+0.31. We also obtained a map of the continuum emission at a frequency of 110 GHz. We detect a hot molecular core in the emission both of methyl cyanide and of the 110 GHz continuum. We estimate a temperature of 200 K and a mass of 1000 M for this compact massive region. We also detect a velocity gradient or shift across the methyl cyanide core whose origin could be due to rotation.     

Monitoring Water Masers in Star-Forming Regions

An overview is given of the analysis of more than a decade of H₂O maser data from our monitoring program. We find the maser emission to generally depend on the luminosity of the YSO as well as on the geometry of the SFR. There appears to be a threshold luminosity of a few times 10⁴L above and below which we find different maser characteristics.     

Outflow, Infall, and Rotation in High-Mass Star Forming Regions

According to theory, stars more massive than 8 M must form while still accreting material from the surrounding parental cloud: at this stage radiation pressure should reverse the infall thus preventing further growth of the stellar mass. After illustrating the two models proposed to solve this problem (accretion and coalescence), we review the observational evidence pro/contra such models, focusing on the kinematics of the molecular gas where the massive (proto)stars are embedded as the best tool to shed light on the formation mechanism. Special attention is devoted to the phenomena of infall, outflow, and rotation, concluding that the recent detection of rotating disks in massive young stellar objects is the best evidence so far in favour of the accretion model.