Protostellar Outflows at the EarliesT Stages (POETS)

Abstract

Context. 22 GHz water masers are the most intense and widespread masers in star-forming regions. They are commonly associated with protostellar winds and jets emerging from low- and high-mass young stellar objects (YSO). Aims. We wish to perform for the first time a statistical study of the location and motion of individual water maser cloudlets, characterized by typical sizes that are within a few au, with respect to the weak radio thermal emission from YSOs. Methods. For this purpose, we have been carrying out the Protostellar Outflows at the EarliesT Stages survey of a sample (38) of high-mass YSOs. The 22 GHz water maser positions and three-dimensional (3D) velocities were determined through multi-epoch Very Long Baseline Array observations with accuracies of a few milliarcsec (mas) and a few km s−1, respectively. The position of the ionized core of the protostellar wind, marking the YSO, was determined through sensitive radio continuum, multi-frequency Jansky Very Large Array observations with a typical error of ≈20 mas. Results. The statistic of the separation of the water masers from the radio continuum shows that 84% of the masers are found within 1000 au from the YSO and 45% of them are within 200 au. Therefore, we can conclude that the 22 GHz water masers are a reliable proxy for locating the position of the YSO. The distribution of maser luminosity is strongly peaked towards low values, indicating that about half of the maser population is still undetected with the current Very Long Baseline Interferometry detection thresholds of 50–100 mJy beam−1. Next-generation, sensitive (at the nJy level) radio interferometers will have the capability to exploit these weak masers for an improved sampling of the velocity and magnetic fields around the YSOs. The average direction of the water maser proper motions provides a statistically-significant estimate for the orientation of the jet emitted by the YSO: 55% of the maser proper motions are directed on the sky within an angle of 30° from the jet axis. Finally, we show that our measurements of 3D maser velocities statistically support models in which water maser emission arises from planar shocks with propagation direction close to the plane of the sky.

Water Masers as an Early Tracer of Star Formation

“…The same scale is found for water masers (Moscadelli et al 2019(Moscadelli et al , 2020. According to Moscadelli et al (2020), 84% of the 22 GHz water masers within the sample of 36 sources are found within 1000 au with respect to the radio continuum peak, which is a good proxy for the YSO position (Moscadelli et al 2016). On the other hand, the characteristic scale of the cIM maser distribution is about 50 times larger (e.g., Voronkov et al 2014).…”

“…cIIM masers at 6.7 GHz are pumped by infrared radiation and reside in the circumstellar disks and inner parts of the outflows of high-mass YSOs, which have sizes less than 1000au (Sanna et al 2015(Sanna et al , 2017. The same scale is found for water masers (Moscadelli et al 2019(Moscadelli et al , 2020. According to Moscadelli et al (2020), 84% of the 22 GHz water masers within the sample of 36 sources are found within 1000 au with respect to the radio continuum peak, which is a good proxy for the YSO position (Moscadelli et al 2016).…”

Multi-scale view of star formation in IRAS 21078+5211: from clump fragmentation to disk wind

“…The faint, thermal, and nonthermal emissions from the NE and SW radio lobes, respectively, and the water masers are all manifestations of different types of shocks produced by the jet. In the jet core, close to YSO-1, we observe free-free emission, which likely originates from relatively weak (C-type), internal shocks of the jet corresponding to changes in the mass loss or ejection velocity (Moscadelli et al 2016;Anglada et al 2018); the nonthermal emission is likely synchrotron emission from electrons accelerated at relativistic velocities in strong jet shocks through first-order Fermi acceleration (Bell 1978); finally, the fast water masers emerge from relatively strong (J-type) shocks, whereas the jet impinges on very dense (molecular hydrogen number density, n H 2 ∼ 10 8 cm −3 ) ambient material at high velocity (Moscadelli et al 2020).…”

Maser Tracers of Gas Dynamics near Young Stars New Perspectives

“…Since the last maser symposium, we have spent much efforts on a large maser project called POETS, which stands for the "Protostellar Outflows at the EarliesT Stages" survey. This project has been grounded on a survey of combined water masers and radio continuum observations at the highest angular resolution available towards a large number of young and massive stars, which we have presented in a series of papers between 2018 and 2020, with a pilot study in 2016 (Sanna et al 2018(Sanna et al , 2019bMoscadelli et al 2016Moscadelli et al , 2019Moscadelli et al , 2020.…”