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2013:reasearch

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Scientific Research

Accretion Disks surrounding O-type Protostars Ciriaco Goddi (goddi@jive.nl)

High-mass stars are prominent in the ecology of the interstellar medium and the evolution of Galaxies. Though much needed, there is no general theory of high-mass star formation. The main issue for models is that the intense stellar radiation pressure and the thermal pressure from the surrounding ionized material, may reverse the accretion flow and prevent matter from reaching the star. Recent theoretical studies have however demonstrated that radiation pressure can be overcome if accretion occurs through a circumstellar disk. Despite the theoretical evidence, we have not yet conclusively identified a single high-mass protostar in the Galaxy surrounded by an accretion disk.

We observed NGC7538-IRS1 (at 2.7 kpc) with the European VLBI Newtwork (EVN) in the 6.7 GHz maser line of methanol, which is a typical signpost of high-mass star formation. Four observing epochs spanning eight years were used to measure positions, proper motions, line-of-sight velocities and accelerations of methanol masers. These measurements provided 3D kinematics and dynamics of circumstellar gas on scales from tens to a thousand AU.

We find compelling evidence that NGC7538-IRS1 is forming a multiple system of high-mass young-stars surrounded by disks. Data modeling enabled us to identify quasi-Keplerian rotation around a 25 solar mass star and a thick disk containing 16 solar masses around a nearby protostar. These measurements are critical to test theoretical models of accretion for massive stars. The results will be published in the following issue of A&A.

Figure 1: The NGC7538 Star Forming Region (2.7 kpc). (Left) Three color IRAC image in the mid-infrared from the Spitzer Space Observatory. (Right) 6.7 GHz methanol maser emission spots (filled circles) observed with the EVN overlaid on the the 22 GHz continuum map imaged with the VLA (black image and white contours). Colors denote line-of-sight velocities, with blue indicating blue-shifted emission and red indicating red-shifted emission. The sizes of the circles scale with the flux density. The ellipses represent the disk planes surrounding the two young stars IRS1a and IRS1b, resolved by the EVN, with the solid lines indicating the near-side and the dashed lines the far-side of the disks.

2013/reasearch.1415953885.txt.gz · Last modified: 2014/11/14 08:31 by paragi