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Observations show that, like light solar-mass stars, heavy stars also form through episodic disk-accretion; but faster, more energetic and emitting more light. Solar-mass stars form via disk-mediated accretion. Recent findings indicate that this process is probably episodic in the form of accretion bursts 1 , possibly caused by disk fragmentation 2 , 3 , 4 . Although it cannot be ruled out that high-mass young stellar objects arise from the coalescence of their low-mass brethren 5 , the latest results suggest that they more likely form via disks 6 , 7 , 8 , 9 . It follows that disk-mediated accretion bursts should occur 10 , 11 . Here we report on the discovery of the first disk-mediated accretion burst from a roughly twenty-solar-mass high-mass young stellar object 12 . Our near-infrared images show the brightening of the central source and its outflow cavities. Near-infrared spectroscopy reveals emission lines typical for accretion bursts in low-mass protostars, but orders of magnitude more luminous. Moreover, the released energy and the inferred mass-accretion rate are also orders of magnitude larger. Our results identify disk-accretion as the common mechanism of star formation across the entire stellar mass spectrum.

We report multi-epoch VLBI observations of molecular masers towards the high-mass star forming region AFGL 5142, leading to the determination of the 3D velocity field of circumstellar molecular gas at radii <0.″23 (or 400 AU) from the protostar MM–1. Our observations of CH3OH maser emission enabled, for the first time, a direct measurement of infall of a molecular envelope on to an intermediate-mass protostar (radius of 300 AU, velocity of 5 km s−1, and infall rate of 6 × 10−4n8M⊙ yr−1, where n8 is the ambient volume density in units of 108 cm−3). In addition, our measurements of H2O maser (and radio continuum) emission revealed a collimated bipolar molecular outflow (and ionized jet) from MM–1. The evidence of simultaneous accretion and outflow at small spatial scales, makes AFGL 5142 an extremely compelling target for high-angular resolution studies of high-mass star formation.

Due to their compactness, persistence and slow motion, Class II CH3OH masers are excellent targets for parallax and proper motion measurements for massive star-forming regions in the Galactic Disk. These measurements can be used to improve our understanding of the spiral structure and dynamics of the Milky Way. At the same time, Class II CH3OH masers can also be used to study gas kinematics close to the exciting star, tracing rotation, infall and/or outflow motions.

We investigated the high-mass star-forming region G28.87+0.07 by means of maser kinematics, including H2O, CH3OH, and OH, and radio to infrared, continuum observations. All observational evidence suggests that these masers are associated with the same young star of 20-30 M⊙, still in the main accretion phase and surrounded by a rich stellar cluster.

We have conducted multi-epoch EVN observations of the 22.2 GHz water masers towards the high-mass young stellar object in AFGL 5142. With four observing epochs, spanning a time of 1 year, 12 distinct maser features have been detected and accurate values of the proper motions are derived for those persistent over three or four epochs. On the basis of their spatial distribution, the observed maser features can be divided into two groups. A model fit to the positions and velocities of the maser features of Group I, detected in the same region (within 500 mas) where the massive YSO should be located, demonstrates that these might arise on the surface of a nearly edge-on Keplerian disk, rotating around a massive young stellar object. The maser features of Group II, found at large distances from the YSO (≥ 1''), have positions and line-of-sight velocities in agreement with the blue-shifted lobe of a large-scale molecular outflow, and might result from the interaction between the gas flowing away from the young stellar object and the ambient gas of the progenitor molecular core.[PUBLICATION ABSTRACT]

We have conducted phase-reference VLBI observations of H2O and CH3OH masers toward two high-mass star forming regions, Sh 2-255 IR and AFGL 5142. In Sh 2-255 infrared water masers are aligned along a direction close to the orientation of a large scale H2 jet, tracing possibly shocked material in a precessing jet, or, alternatively, the disk-wind emerging from the disk atmosphere. In AFGL 5142 water masers trace expansion at the base of a protostellar jet, whilst methanol masers are more probably tracing infalling than outflowing gas. Our results suggest that water and methanol masers trace different kinematic structures in the circumstellar gas.

This work presents the results of VLBI observations of 6.7 GHz methanol and 22.2 GHz water masers towards the mm core A in the massive star-forming region G24.78+0.08. Comparing the maser with previous millimeter interferometric and recent continuum VLA observations, the physical properties and the gas kinematics of the G24 A core on linear scales from ~100 AU to ~0.1 pc are determined.

We have measured the distance to the high-mass star-forming region G59.7+0.1 (IRAS 19410+2336) and W3OH. Their distances, 2.20 ± 0.11 kpc and 1.95 ± 0.04 kpc, respectively, were determined by triangulation using Very Long Baseline Array (VLBA) observations of 12.2 GHz methanol masers phase-referenced to compact extragalactic radio sources. In addition to the distances, we have also obtained their proper motions.

We report the first detection of a water megamaser in a radio-loud galaxy, 3C 403, and present a follow-up study using the VLA. 3C 403 has been observed as a part of a small sample of FR II galaxies with evidence of nuclear obscuration. The isotropic luminosity of the maser is 1200 L^sub ^. With a recessional velocity of cz 17680 km s^sup -1^ it is the most distant water maser so far reported. The line arises from the densest (> 10^sup 8^ cm^sup -3^) interstellar gas component ever observed in a radio-loud galaxy. Two spectral features are identified, likely bracketing the systemic velocity of the galaxy. Our interferometric data clearly indicate that these arise from a location within 0.1'' ([asymptotically =]110 pc) from the active galactic nucleus. We conclude that the maser spots are most likely associated with the tangentially seen parts of a nuclear accretion disk, while an association with dense warm gas interacting with the radio jets cannot yet be ruled out entirely.[PUBLICATION ABSTRACT]

Astrometric observations with the VLBA with accuracies approaching ~ 10 μas are being conducted in order to better understand the Galaxy. The location of Sgr A* on infrared images can be determined with an accuracy of a few mas, using stars with SiO maser emission as a calibration grid for infrared images. The apparent proper motion of Sgr A*, which is dominated by the effects of the orbit of the Sun around the Galactic center, has been measured with high accuracy. This measurement strongly constrains Θ0R0 and offers a dynamical definition of the Galactic plane with Sgr A* at its origin. The intrinsic motion of Sgr A* is very small and comparable to that expected for a supermassive black hole. When combined with infrared results, this provides overwhelming evidence that Sgr A* is a supermassive black hole. Finally, we are engaged in a large project to map the spiral structure and kinematics of the Galaxy. Preliminary trigonometric parallaxes, obtained with the VLBA, to eight massive star forming regions are presented.