Observations of 6.7 GHz methanol masers with East-Asian VLBI Network. I. VLBI images of the first epoch of observations

Very-long-baseline interferometry (VLBI) monitoring of the 6.7 GHz methanol maser allows us to measure the internal proper motion of maser spots and therefore study the gas motion around high-mass young stellar objects. To this end, we have begun monitoring observations with the East-Asian VLBI Network. In this paper we present the results of the first epoch observation for 36 sources, including 35 VLBI images of the methanol maser. Since two independent sources were found in three images, images of 38 sources were obtained. In 34 sources, 10 or more spots were detected. The observed spatial scale of the maser distribution was from 9 to 4900 astronomical units, and the following morphological categories were observed: elliptical, arched, linear, paired, and complex. The position of the maser spot was determined with an accuracy of approximately 0.1 mas, which is sufficiently high to measure the internal proper motion from two years of monitoring observations. The VLBI observation, however, detected only approximately 20% of all maser emissions, suggesting that the remaining 80% of the total flux was spread into an undetectable extended distribution. Therefore, in addition to high-resolution observations, it is important to observe the whole structure of the maser emission including extended low-brightness structures, in order to reveal the associated site of the maser and gas motion

Observations of 6.7 GHz methanol masers with East-Asian VLBI Network. II. Internal proper motion measurement in G006.79-00.25

We detected internal proper motions of the methanol maser features at 6.7 GHz in a high-mass star-forming region G006.79-00.25 with the East-Asian VLBI Network. The spatial distribution of the maser features shows an elliptical morphology. The internal proper motions of 17 methanol maser features relative to the barycenter of the features were measured. The amplitude of the internal motions ranged from 1.30 to 10.25 kms(-1). Most of the internal proper motions of the maser features seem to point counter-clockwise along the elliptical morphology of the maser features. We applied the disk model, which includes both rotating and expanding components, to the observed positions, i.o.s. velocities, and proper motions. The derived rotation, expansion, and systemic velocities are +3(-2)(+2), +6(-2)(+2), and +21(-2)(+2) kms(-1), respectively, at the radius of 1260 au on the disk with a position angle of the semi-major axis of -140 degrees. and an inclination of 60 degrees. The derived rotating motion suggests that the methanol maser emissions showing the elliptical spatial morphology possibly trace the rotating disk. The derived expanding motion might be caused by the magnetic-centrifugal wind on the disk, which was estimated on the basis of the typical magnetic field strength at emitting zones of a methanol maser