VLBI study of maser kinematics in high-mass star-forming regions

Abstract

Aims. To study the high-mass star-forming process, we started a large project to unveil the gas kinematics close to young stellar objects (YSOs) through the Very Long Baseline Interferometry (VLBI) of maser associations. By comparing the high spatial resolution maser data that traces the inner kinematics of the (proto)stellar cocoon with interferometric thermal data that traces the large-scale environment of the hot molecular core (HMC) harboring the (proto)stars, we can investigate the nature and identify the sources of large-scale motions. The present paper focuses on the high-mass star-forming region G16.59-0.05. Methods. Using the VLBA and the EVN arrays, we conducted phase-referenced observations of the three most powerful maser species in G16.59-0.05: H 2 O at 22.2 GHz (4 epochs), CH 3 OH at 6.7 GHz (3 epochs), and OH at 1.665 GHz (1 epoch). In addition, we performed high-resolution (≥0. 1), high-sensitivity (<0.1 mJy) VLA observations of the radio continuum emission from the starforming region at 1.3 and 3.6 cm.Results. This is the first work to report accurate measurements of the relative proper motions of the 6.7 GHz CH 3 OH masers. The different spatial and 3-D velocity distributions clearly indicate that the 22 GHz water and 6.7 GHz methanol masers trace different kinematic environments. The bipolar distribution of 6.7 GHz maser line-of-sight velocities and the regular pattern of observed proper motions suggest that these masers are tracing rotation around a central mass of about 35 M . The flattened spatial distribution of the 6.7 GHz masers, oriented NW−SE, suggests that they can originate in a disk/toroid rotating around the massive YSO that drives the 12 CO (2−1) outflow, oriented NE−SW, observed on an arcsec scale. The extended, radio continuum source observed close to the 6.7 GHz masers could be excited by a wide-angle wind emitted from the YSO associated with the methanol masers, and such a wind has proven to be energetic enough to drive the NE−SW 12 CO (2−1) outflow. The H 2 O masers are distributed across a region offset about 0. 5 to the NW of the CH 3 OH masers, in the same area as where the emission of high-density molecular tracers, typical of HMCs, was detected. We postulate that a distinct YSO, possibly in an earlier evolutionary phase than what excites the methanol masers, is responsible for the excitation of the water masers and the HMC molecular lines.

Highly Variable Young Massive Stars in Atlasgal Clumps

“…This timescale and the nature of variability found here (eruptive, dipper, LPV) are very similar to the variations of the 6.7 GHz class II methanol maser emission in high-mass star-forming regions, both in its nature (Goedhart et al 2004) and in the timescales (Goedhart et al 2014). The class II methanol masers are thought to be pumped by infrared radiation (Cragg et al 2005) and found to originate in accretion disks in highmass star-forming regions (Sanna et al 2010;Sugiyama et al 2014). The similarity in the variability found in the methanol maser emission (MME) and the IR variability may therefore represent the frequency of the infrared luminosity variations of the warm disk from accretion events, which is also responsible for pumping the class II MME (Cragg et al 2005).…”

Parallaxes and proper motions of interstellar masers toward the Cygnus X star-forming complex

“…To obtain the averaged positions for a maser spot (procedure 3), we performed parallax fits on all the individual spots and removed their position offsets and proper motions, after which we averaged the positions of each epoch. The last approach has the advantage of reducing the random errors, introduced by small variations in the internal spot distribution for a maser feature (e.g., Sanna et al 2010), while leaving the systematic errors unaffected.…”

Infall and outflow within 400 AU from a high-mass protostar

“…This evidence favors the interpretation of the radio continuum as an ionized jet and suggests that water masers may originate in shocks produced by the interaction of the jet with the surrounding molecular environment. For optically thin emission, F d 2 = 10 3.5 (Ω/4π)Ṗ j (Sanna et al 2010), where F is the measured continuum flux in mJy,Ṗ j is the jet momentum rate in M yr −1 km s −1 , Ω is the jet solid angle in sr, and d is the source distance in kpc. Using a flux density of 0.24 mJy (0.17 mJy) measured at 8.4 GHz for the NW (SE) component and a distance of 1.8 kpc, we deriveṖ j = 2.5(1.7) × 10 −4 (Ω/4π) −1 M yr −1 km s −1 .…”