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Register a new userWater masers in the massive protostar IRAS 20126+4104: ejection and deceleration
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L. Moscadelli INAF
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We report on the first multi-epoch, phase referenced VLBI observations of the water maser emission in a high-mass protostar associated with a disk-jet system. The source under study, IRAS 20126+4104, has been extensively investigated in a large variety of tracers, including water maser VLBA data acquired by us three years before the present observations. The new findings fully confirm the interpretation proposed in our previous study, namely that the maser spots are expanding from a common origin coincident with the protostar. We also demonstrate that the observed 3-D velocities of the maser spots can be fitted with a model assuming that the spots are moving along the surface of a conical jet, with speed increasing for increasing distance from the cone vertex. We also present the results of single-dish monitoring of the water maser spectra in IRAS 20126+4104. These reveal that the peak velocity of some maser lines decreases linearly with time. We speculate that such a deceleration could be due to braking of the shocks from which the maser emission originates, due to mass loading at the shock front or dissipation of the shock energy.
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We present new spectral line observations of the CH3CN molecule in the accretion disk around the massive protostar IRAS 20126+4104 with the Submillimeter Array that for the first time measure the disk density, temperature, and rotational velocity with sufficient resolution (0.37, equivalent to ~600 AU) to assess the gravitational stability of the disk through the Toomre-Q parameter. Our observations resolve the central 2000 AU region that shows steeper velocity gradients with increasing upper state energy, indicating an increase in the rotational velocity of the hotter gas nearer the star. Such spin-up motions are characteristics of an accretion flow in a rotationally supported disk. We compare the observed data with synthetic image cubes produced by three-dimensional radiative transfer models describing a thin flared disk in Keplerian motion enveloped within the centrifugal radius of an angular-momentum-conserving accretion flow. Given a luminosity of 1.3x10^4 Lsun, the optimized model gives a disk mass of 1.5 Msun and a radius of 858 AU rotating about a 12.0 Msun protostar with a disk mass accretion rate of 3.9x10^{-5} Msun/yr. Our study finds that, in contrast to some theoretical expectations, the disk is hot and stable to fragmentation with Q > 2.8 at all radii which permits a smooth accretion flow. These results put forward the first constraints on gravitational instabilities in massive protostellar disks, which are closely connected to the formation of companion stars and planetary systems by fragmentation.
We measured polarized dust emission at 350um towards the high-mass star forming massive dense clump IRAS 20126+4104 using the SHARC II Polarimeter, SHARP, at the Caltech Submillimeter Observatory. Most of the observed magnetic field vectors agree well with magnetic field vectors obtained from a numerical simulation for the case when the global magnetic field lines are inclined with respect to the rotation axis of the dense clump. The results of the numerical simulation show that rotation plays an important role on the evolution of the massive dense clump and its magnetic field. The direction of the cold CO 1-0 bipolar outflow is parallel to the observed magnetic field within the dense clump as well as the global magnetic field, as inferred from optical polarimetry data, indicating that the magnetic field also plays a critical role in an early stage of massive star formation. The large-scale Keplerian disk of the massive (proto)star rotates in almost opposite sense to the clumps envelope. The observed magnetic field morphology and the counter-rotating feature of the massive dense clump system provide hints to constrain the role of magnetic fields in the process of high mass star formation.
We present results of Chandra ACIS-I and Karl G. Jansky Very Large Array (VLA) 6 cm continuum observations of the IRAS 20126+4104 massive star forming region. We detect 150 X-ray sources within the 17 arcmin x 17 arcmin ACIS-I field, and a total of 13 radio sources within the 9.2 primary beam at 4.9 GHz. Among these are the first 6 cm detections of the central sources reported by Hofner et al. (2007), namely I20N1, I20S, and I20var. A new variable radio sources is also reported. Searching the 2MASS archive we identified 88 NIR counterparts to the X-ray sources. Only 4 of the X-ray sources had 6 cm counterparts. Based on an NIR color-color analysis, and on the Besancon simulation of Galactic stellar populations (Robin et al. 2003), we estimate that about 90 X-ray sources are associated with this massive star forming region. We detect an increasing surface density of X-ray sources toward the massive protostar and infer the presence of a cluster of at least 46 YSOs within a distance of 1.2 pc from the massive protostar.
We present Submillimeter Array observations of the massive star-forming region IRAS 20126+4104 in the millimeter continuum and in several molecular line transitions. With the SMA data, we have detected nine molecular transitions, including DCN, CH3OH, H2CO, and HC3N molecules, and imaged each molecular line. From the 1.3 mm continuum emission a compact millimeter source is revealed, which is also associated with H2O, OH, and CH3OH masers. Using a rotation temperature diagram (RTD), we derive that the rotational temperature and the column density of CH3OH are 200 K and 3.7times 1017 cm-2, respectively. The calculated results and analysis further indicate that a hot core coincides with IRAS 20126+4104. The position-velocity diagrams of H2CO 3(0,3)-2(0,2) and HC3N 25-24 clearly present Keplerian rotation. Moreover, H2CO 3(0,3)-2(0,2) is found to trace the disk rotation for the first time.
We report the direct detection of a binary/disk system towards the high-mass (proto)stellar object IRAS20126+4104 at infrared wavengths. The presence of a multiple system had been indicated by the precession of the outflow and the double jet system detected earlier at cm-wavelengths. Our new K, L & M band infrared images obtained with the UKIRT under exceptional seeing conditions on Mauna Kea are able to resolve the central source for the first time, and we identify two objects separated by ~ 0.5 (850 AU). The K and L images also uncover features characteristic of a nearly edge-on disk, similar to many low mass protostars with disks: two emission regions oriented along an outflow axis and separated by a dark lane. The peaks of the L & M band and mm-wavelength emission are on the dark lane, presumably locating the primary young star. The thickness of the disk is measured to be ~ 850 AU for radii < 1000 AU. Approximate limits on the NIR magnitudes of the two young stars indicate a high-mass system, although with much uncertainty. These results are a demonstration of the high-mass nature of the system, and the similarities of the star-formation process in the low-mass and high-mass regimes viz. the presence of a disk-accretion stage. The companion is located along the dark lane, consistent with it being in the equatorial/disk plane, indicating a disk-accretion setting for massive, multiple, star-formation.
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