Study of the molecular and ionized gas in a possible precursor of an ultra-compact HII region

ORTEGA, M. E.; PARON, S.; GIACANI, E.; CELIS PEÑA, M.; RUBIO, M.; PETRIELLA, A.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2017

0004-6361

Aims. We study the molecular and the ionized gas in a possible precursor of an ultra-compact Hii region to contribute to the understandingof how high-mass stars build-up their masses once they have reached the zero-age main secuence.Methods. We carried out molecular observations towards the position of the RedMSX source G052.9221-00.4892, using the AtacamaSubmillimeter Telescope Experiment (ASTE; Chile) in the 12CO J=3-2, 13CO J=3-2, C18O J=3-2, and HCO+ J=4-3 lines with anangular resolution of about 22" . We also present radio continuum observations at 6 GHz carried out with the Jansky Very LargeArray (JVLA; USA) interferometer with a synthesized beam of 4.8"x 4.1". The molecular data were used to study the distribution andkinematics of the molecular gas, while the radio continuum data were used to characterize the ionized gas in the region. Combiningthese observations with public infrared data allowed us to inquire about the nature of the source.Results. The analysis of the molecular observations reveals the presence of a kinetic temperature and H2 column density gradientsacross the molecular clump in which the Red MSX source G052.9221-00.4892 is embedded, with the hotter and less dense gas in theinner region. The 12CO J=3−2 emission shows evidence of misaligned massive molecular outflows, with the blue lobe in positionalcoincidence with a jet-like feature seen at 8 μm. The radio continuum emission shows a slightly elongated compact radio source, witha flux density of about 0.9 mJy, in positional coincidence with the Red MSX source. The polar-like morphology of this compact radiosource perfectly matches the hourglass-like morphology exhibited by the source in the Ks-band. Moreover, the axes of symmetry ofthe radio source and the near-infrared nebula are perfectly aligned. Thus, based on the presence of molecular outflows, the slightlyelongated morphology of the compact radio source matching the hourglass-like morphology of the source at the Ks−band, and thelack of evidence of collimated jets in the near-infrared spectrum, one interpretation for the nature of the source, is that the Red MSXsource G052.9221-00.4892 could be transiting a hyper-compact Hii region phase, in which the young central star emits winds andionizing radiation through the poles.By the other hand, according to a comparison between the Br  intensity and the radio flux density at 6 GHz, the source would be in amore evolved evolutionary stage of an optically thin UC HII region in photoionization equilibrium. If this is the case, from the radiocontinuum emission, we can conjecture upon the spectral type of its exciting star which would be a B0.5V.