Monitoring of the polarized H2O maser emission around the massive protostars W75N(B)-VLA 1 and W75N(B)-VLA 2

SURCIS, G.; VLEMMINGS, W.H.T.; GODDI, C.; TORRELLES, J.M.; GÓMEZ, J.F.; RODRÍGUEZ-KAMENETZKY, A.; CARRASCO-GONZÁLEZ, C.; CURIEL, S.; KIM, S.-W.; KIM, J.-S.; VAN LANGEVELDE, H.J.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2023

0004-6361

Context. Several radio sources have been detected in the high-mass star-forming region W75N(B), among them the massive young stellar objects VLA 1 and VLA 2 are of great interest. These are thought to be in different evolutionary stages. In particular, VLA 1 is at the early stage of the photoionization and it is driving a thermal radio jet, while VLA 2 is a thermal, collimated ionized wind surrounded by a dusty disk or envelope. In both sources 22 GHz H2O masers have been detected in the past. Those around VLA1 show a persistent linear distribution along the thermal radio jet and those around VLA 2 have instead traced the evolution from a non-collimated to a collimated outflow over a period of ∼20 years. The magnetic field inferred from the H2O masers showed an orientation rotation following the direction of the major-axis of the shell around VLA 2, while it is immutable around VLA 1.Aims. By monitoring the polarized emission of the 22 GHz H2O masers around both VLA1 and VLA2 over a period of six years, we aim to determine whether the H2O maser distributions show any variation over time and whether the magnetic field behaves accordingly.Methods. The European VLBI Network was used in full polarization and phase-reference mode in order to determine the absolute positions of the 22 GHz H2O masers with a beam size of ∼ 1 mas and to determine both the orientation and the strength of the magnetic field. We observed four epochs separated by two years from 2014 to 2020.Results. We detected polarized emission from the H2O masers around both VLA1 and VLA2 in all the epochs. By comparingthe H2O masers detected in the four epochs, we find that the masers around VLA1 are tracing a nondissociative shock originating from the expansion of the thermal radio jet, while the masers around VLA 2 are tracing an asymmetric expansion of the gas that is halted in the northeast where the gas likely encounters a very dense medium. We also found that the magnetic field inferred from the H2O masers in each epoch can be considered as a portion of a quasi-static magnetic field estimated in that location rather than in that time. This allowed us to study locally the morphology of the magnetic field around both VLA 1 and VLA 2 in a larger area by considering the vectors estimated in all the epochs as a whole. We find that the magnetic field in VLA 1 is along the jet axis and bends toward north and south at the northeast and southwest ends of the jet, respectively, reconnecting with the large-scale magnetic field. The magnetic field in VLA 2 is perpendicular to the expansion directions till it encounters the denser matter in the northeast, here the magnetic field is parallel to the expansion direction and agrees with the large-scale magnetic field. We also measured the magnetic field strength along the line of sight in three of the four epochs, whose values are −764 mG < BVLA 1 < −676 mG and ||−355mG