Search

The present paper focuses on the high-mass star-forming region G23.01-0.41. Methods: Using the VLBA and the EVN arrays, we conducted phase-referenced observations of the three most powerful maser species in G23.01-0.41: H2O at 22.2 GHz (4 epochs), CH3OH 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 HMC at 1.3 and 3.6 cm. Results: We have detected H2O, CH3OH, and OH maser emission clustered within 2000 AU from the center of a flattened HMC, oriented SE-NW, from which emerges a massive 12CO outflow, elongated NE-SW, extended up to the pc-scale. Although the three maser species show a clearly different spatial and velocity distribution and sample distinct environments around the massive YSO, the spatial symmetry and velocity field of each maser specie can be explained in terms of expansion from a common center, which possibly denotes the position of the YSO driving the maser motion. Water masers trace both a fast shock (up to 50 km/s) closer to the YSO, powered by a wide-angle wind, and a slower (20 km/s) bipolar jet, at the base of the large-scale outflow. Since the compact free-free emission is found offset from the putative location of the YSO along a direction consistent with that of the maser jet axis, we interpret the radio continuum in terms of a thermal jet. The velocity field of methanol masers can be explained in terms of a composition of slow (4 km/s in amplitude) motions of radial expansion and rotation about an axis approximately parallel to the maser jet. Finally, the distribution of line of sight velocities of the hydroxyl masers suggests that they can trace gas less dense (n(H2) < 10^6 cm^-3) and more distant from the YSO than that traced by the water and methanol masers, which is expanding toward the observer. (Abridged), 23 pages, 8 figures, 4 tables, accepted by Astronomy and Astrophysics

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: H2O at 22.2 GHz (4 epochs), CH3OH 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 star-forming 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 CH3OH masers. The different spatial and 3-D velocity distribution clearly indicate that the 22 GHz water and 6.7 GHz methanol masers are tracing different kinematic environments. The bipolar distribution of 6.7 GHz maser l.o.s. velocities and the regular pattern of observed proper motions suggest that these masers are tracing rotation around a central mass of about 35 solar masses. 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 which drives the 12CO(2-1) outflow, oriented NE-SW, observed on 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 is proven to be sufficiently energetic to drive the NE-SW 12CO(2-1) outflow. The H2O masers distribute across a region offset about 0".5 to the NW of the CH3OH masers, in the same area where emission of high-density molecular tracers, typical of HMCs, was detected. We postulate that a distinct YSO, possibly in an earlier evolutionary phase than that exciting the methanol masers, is responsible for the excitation of the water masers and the HMC molecular lines. (Abridged), 20 pages, 8 figures, 3 tables, accepted by Astronomy and Astrophysics

We present evidence of infall in a circumstellar rotating toroid enshrouding a luminous star in the massive star-forming region G24.78+0.08. Besides being one of the rare direct detections of infall in a young high-mass star, our finding stands unique for the simultaneous presence of three elements in the same massive object: a rotating, collapsing toroid, a bipolar outflow, ejected along the rotation axis, and a hypercompact ionized H II region. The large accretion rate and the existence of a hypercompact H II region confirm that the accretion cannot be spherically symmetric and must occur in a circumstellar disk.

This article reports first results of a long-term observational program aimed to study the earliest evolution of jet/disk systems in low-mass YSOs by means of VLBI observations of the 22.2 GHz water masers. Up to now we collected data for the cluster of low-mass YSOs in the Serpens molecular core and for the single object RNO 15-FIR. Towards Serpens SMM1, the most luminous submm source of the Serpens cluster, the water maser emission comes from two small (le 5 AU in size) cluster of features separated by ≈25 AU, having line of sight velocities strongly red-shifted (by more than 10 km s-1) respect to the LSR velocity of the molecular cloud. The two maser clusters are oriented on the sky along a direction that is approximately perpendicular to the axis of the radio jet observed with the VLA towards SMM1. The spatial and velocity distribution of the maser features let us favour the interpretation that the maser emission can be excited by interaction of the receding lobe of the jet with dense gas in the accretion disk surrounding the YSO in SMM1. Towards RNO 15-FIR, the few detected maser features have both positions and (absolute) velocities aligned along a direction which is parallel to the axis of the molecular outflow observed on much larger angular scales. In this case the maser emission likely emerges from dense, shocked molecular clumps displaced along the axis of the jet emerging from the YSO. The protostar in Serpens SMM1 is more massive than the one in RNO 15-FIR. We discuss the case where an higher mass ejection rate can generate jets sufficiently powerful to evacuate along the direction of motion, close to the YSO, the densent portions of circumstellar gas. In that case, the excitation conditions for water masers might preferably occur at the interface between the jet and the accretion disk, rather than along the jet axis.