2013:reasearch
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====== Scientific Research ====== | ====== Scientific Research ====== | ||
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+ | 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 | ||
+ | if accretion occurs through a circumstellar disk. | ||
+ | | ||
+ | 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& | ||
+ | |||
+ | |||
+ | 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. | ||
+ | |||
+ | {{: | ||
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+ | Massive Star Forming Regions and Magnetic Fields | ||
+ | |||
+ | Gabriele Surcis, Huib Jan van Langevelde, and Ciriaco Goddi together with colleagues from Sweden, Spain, Mexico, | ||
+ | and South Korea observed the polarized emission of 22 GHz water masers around the massive young stellar objects (YSOs) | ||
+ | W75N-VLA2 and W75N-VLA1. The two massive YSOs are separated by just 1300 astronomical units and are at two | ||
+ | different evolutionary stages, i.e., VLA1 is the most evolved and VLA2 the least evolved. The interferometric | ||
+ | observations showed at 1~au scale that while the water masers distribution and the magnetic field around VLA1 have | ||
+ | not changed since 2005, the shell structure of the masers around VLA2 is still expanding and increasing its | ||
+ | ellipticity. Furthermore, | ||
+ | direction of the major-axis of the shell-like structure and it is now aligned with the magnetic field in VLA1. | ||
+ | This confirm that the water masers around VLA2 are tracing the evolution from a non-collimated to a collimated | ||
+ | outflow. | ||
+ | |||
+ | Gabriele Surcis and Huib Jan van Langevelde together with Wouter Vlemmings (Chalmers University of Technology), | ||
+ | Busaba Hutawarakorn Kramer (MPIfR), and Anna Bartkiewicz (Nicolaus Copernicus University) | ||
+ | continued their work on measuring magnetic fields morphology close to massive YSOs (au scale) by using 6.7 GHz | ||
+ | methanol masers. The group measured the magnetic field morphology towards eight new massive YSOs. These sources | ||
+ | are part of a large sample of massive YSOs, the so-called flux-limited sample, that is composed of 31 sources. | ||
+ | A preliminary statistical analysis of the results obtained from the first 19 sources reveals evidences that the | ||
+ | magnetic fields around massive YSOs are preferentially oriented along the molecular outflow. | ||
+ | Furthermore, | ||
+ | morphology of the magnetic field at au scale around the massive YSO IRAS20126+4104 by observing the polarized | ||
+ | emission of the 22 GHz water masers and the 6.7 GHz methanol masers. The orientation of the magnetic field | ||
+ | derived from the masers agrees with the S-shaped morphology on larger scale by using the dust-polarized emission | ||
+ | at 350~\mu~m. | ||
+ | |||
+ | --------------------------- | ||
+ | Figures captures. | ||
+ | |||
+ | figure_1a.eps, | ||
+ | and VLA2 (right panel). The reference position is $\alpha_{2000}=20^{\rm{h}}38^{\rm{m}}36^{\rm{s}}\!.435$ and | ||
+ | $\delta_{2000}=42^{\circ}37' | ||
+ | according to their peak flux density. The linear polarization vectors, scaled logarithmically according to | ||
+ | polarization fraction, are overplotted. In the bottom-right corner of both panels the error-weighted orientation | ||
+ | of the magnetic field ($\Phi_{\rm{B}}$) is also reported; the two dashed segments indicate the uncertainties. The | ||
+ | ellipse drawn in the right panel is the result of the best fit of the water masers. The estimated values of the | ||
+ | magnetic field strength are also shown in both panels next to the corresponding water maser. | ||
+ | |||
+ | figure_2a.eps, | ||
+ | the polarized dust emission at 350~$\mu$m, whose continuum emission is in the background. The white box indicates | ||
+ | the position of the right panel. | ||
+ | Right panel: methanol (triangles), | ||
+ | | ||
+ | $\delta_{2000}=41^{\circ}13' | ||
+ | $\sim$1000~au ($\rm{PA_{disk}}=53^{\circ}\pm7$\d. The red and blue lines indicate the red- and blue-shifted lobes | ||
+ | of the jet, respectively. The thick green segments represent the magnetic field direction determined from the | ||
+ | polarized methanol and water maser emissions. The green dashed segments represent the magnetic field direction | ||
+ | determined from the linearly polarized emission of OH masers (Edris et al. 2005, A&A, 434, 213). | ||
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+ | {{: | ||
+ | {{: | ||
+ | {{: | ||
+ | {{: | ||
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2013/reasearch.1390313937.txt.gz · Last modified: 2014/01/21 14:18 by poll