High-spatial-resolution CN and CS observation of two regions of massive star formation

Molecular line CN, CS and mm continuum observations of two intermediate- to high-mass star-forming regions - IRAS20293+3952 and IRAS19410+2336 - obtained with the Plateau de Bure Interferometer at high spatial resolution reveal interesting characteristics of the gas and dust emission. In spite of the expectation that the CN and CS morphology might closely follow the dense gas traced by the dust continuum, both molecules avoid the most central cores. Comparing the relative line strengths of various CN hyperfine components, this appears not to be an opacity effect but to be due to chemical and physical effects. The CN data also indicate enhanced emission toward the different molecular outflows in the region. Regarding CS, avoiding the central cores can be due to high optical depth, but the data also show that the CS emission is nearly always associated with the outflows of the region. Therefore, neither CS nor CN appear well suited for dense gas and disk studies in these two sources, and we recommend the use of different molecules for future massive disk studies. An analysis of the 1 and 3mm continuum fluxes toward IRAS20293+3952 reveals that the dust opacity index beta is lower than the canonical value of 2. Tentatively, we identify a decreasing gradient of beta from the edge of the core to the core center. This could be due to increasing optical depth toward the core center and/or grain growth within the densest cores and potential central disks. We detect 3mm continuum emission toward the collimated outflow emanating from IRAS20293+3952. The spectral index of alpha ~ 0.8 in this region is consistent with standard models for collimated ionized winds.Comment: 5 pages, 2 tables, 9 figures, accepted for Ap

Water masers in the massive protostar IRAS 20126+4104: ejection and deceleration

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.Comment: 11 pages, 8 figures. Accepted for publication in A&

Infall of gas as the formation mechanism of stars up to 20 times more massive than the Sun

Theory predicts and observations confirm that low-mass stars (like the Sun) in their early life grow by accreting gas from the surrounding material. But for stars ~ 10 times more massive than the Sun (~10 M_sun), the powerful stellar radiation is expected to inhibit accretion and thus limit the growth of their mass. Clearly, stars with masses >10 M_sun exist, so there must be a way for them to form. The problem may be solved by non-spherical accretion, which allows some of the stellar photons to escape along the symmetry axis where the density is lower. The recent detection of rotating disks and toroids around very young massive stars has lent support to the idea that high-mass (> 8 M_sun) stars could form in this way. Here we report observations of an ammonia line towards a high-mass star forming region. We conclude from the data that the gas is falling inwards towards a very young star of ~20 M_sun, in line with theoretical predictions of non-spherical accretion.Comment: 11 pages, 2 figure

Dissecting a hot molecular core: The case of G31.41+0.31

We made a detailed observational analysis of a well known hot molecular core lying in the high-mass star-forming region G31.41+0.31. This core is believed to contain deeply embedded massive stars and presents a velocity gradient that has been interpreted either as rotation or as expansion, depending on the authors. Our aim was to shed light on this question and possibly prepare the ground for higher resolution ALMA observations which could directly detect circumstellar disks around the embedded massive stars. Observations at sub-arcsecond resolution were performed with the Submillimeter Array in methyl cyanide, a typical hot molecular core tracer, and 12CO and 13CO, well known outflow tracers. We also obtained sensitive continuum maps at 1.3 mm. Our findings confirm the existence of a sharp velocity gradient across the core, but cannot confirm the existence of a bipolar outflow perpendicular to it. The improved angular resolution and sampling of the uv plane allow us to attain higher quality channel maps of the CH3CN lines with respect to previous studies and thus significantly improve our knowledge of the structure and kinematics of the hot molecular core. While no conclusive argument can rule out any of the two interpretations (rotation or expansion) proposed to explain the velocity gradient observed in the core, in our opinion the observational evidence collected so far indicates the rotating toroid as the most likely scenario. The outflow hypothesis appears less plausible, because the dynamical time scale is too short compared to that needed to form species such as CH3CN, and the mass loss and momentum rates estimated from our measurements appear too high.Comment: Astronomy and Astrophysics, in pres

Interferometric mapping of Magnetic fields: G30.79 FIR 10

We present polarization maps of G30.79 FIR 10 (in W43) from thermal dust emission at 1.3 mm and from CO J=$2 \to 1$ line emission. The observations were obtained using the Berkeley-Illinois-Maryland Association array in the period 2002-2004. The G30.79 FIR 10 region shows an ordered polarization pattern in dust emission, which suggests an hourglass shape for the magnetic field. Only marginal detections for line polarization were made from this region. Application of the Chandrashkar-Fermi method yielded $B_{pos} \approx 1.7$ mG and a statistically corrected mass to magnetic flux ratio $\lambda_{C} \approx 0.9$, or essentially critical.Comment: 11 pages, 2 Figures, Published in Ap

The structure of molecular clumps around high-mass young stellar objects

We have used the IRAM 30-m and FCRAO 14-m telescopes to observe the molecular clumps associated with 12 ultracompact (UC) HII regions in the J=6-5, 8-7 and 13-12 rotational transitions of methyl-acetylene (CH3C2H). Under the assumption of LTE and optically thin emission, we have derived temperature estimates ranging from 30 to 56 K. We estimate that the clumps have diameters of 0.2-1.6 pc, H_2 densities of 10^5-10^6 {cm^{-3}}, and masses of 10^2-2 10^4 M_\odot. We compare these values with those obtained by other authors from different molecular tracers and find that the H_2 density and the temperature inside the clumps vary respectively like n_{H_2} ~ R^{-2.6} and T ~ R^{-0.5}, with R distance from the centre. We also find that the virial masses of the clumps are ~3 times less than those derived from the CH3C2H column densities: we show that a plausible explanation is that magnetic fields play an important role to stabilise the clumps, which are on the verge of gravitational collapse. Finally, we show that the CH3C2H line width increases for decreasing distance from the clump centre: this effect is consistent with infall in the inner regions of the clumps. We conclude that the clumps around UC HII regions are likely to be transient (~10^(5) yr) entities, remnants of isothermal spheres currently undergoing gravitational collapse: the high mass accretion rates (~10^{-2} M_\odot yr^{-1}) lead to massive star formation at the centre of such clumps.Comment: 15 pages, 11 figures, A & A in pres

Kinematics of a hot massive accretion disk candidate

Characterizing rotation, infall and accretion disks around high-mass protostars is an important topic in massive star formation research. With the Australia Telescope Compact Array and the Very Large Array we studied a massive disk candidate at high angular resolution in ammonia (NH3(4,4) & (5,5)) tracing the warm disk but not the envelope. The observations resolved at ~0.4'' resolution (corresponding to ~1400AU) a velocity gradient indicative of rotation perpendicular to the molecular outflow. Assuming a Keplerian accretion disk, the estimated protostar-disk mass would be high, similar to the protostellar mass. Furthermore, the position-velocity diagram exhibits additional deviation from a Keplerian rotation profile which may be caused by infalling gas and/or a self-gravitating disk. Moreover, a large fraction of the rotating gas is at temperatures >100K, markedly different to typical low-mass accretion disks. In addition, we resolve a central double-lobe cm continuum structure perpendicular to the rotation. We identify this with an ionized, optically thick jet.Comment: 5 pages, 3 figures, accepted for Astrophysical Journal Letters, a high-resolution version of the draft can be found at http://www.mpia.de/homes/beuther/papers.htm

A High-Mass Protobinary System in the Hot Core W3(H2O)

We have observed a high-mass protobinary system in the hot core W3(H2O) with the BIMA Array. Our continuum maps at wavelengths of 1.4mm and 2.8mm both achieve sub-arcsecond angular resolutions and show a double-peaked morphology. The angular separation of the two sources is 1.19" corresponding to 2.43X10^3 AU at the source distance of 2.04 kpc. The flux densities of the two sources at 1.4mm and 2.8mm have a spectral index of 3, translating to an opacity law of kappa ~ nu. The small spectral indices suggest that grain growth has begun in the hot core. We have also observed 5 K components of the CH3CN (12-11) transitions. A radial velocity difference of 2.81 km/s is found towards the two continuum peaks. Interpreting these two sources as binary components in orbit about one another, we find a minimum mass of 22 Msun for the system. Radiative transfer models are constructed to explain both the continuum and methyl cyanide line observations of each source. Power-law distributions of both density and temperature are derived. Density distributions close to the free-fall value, r^-1.5, are found for both components, suggesting continuing accretion. The derived luminosities suggest the two sources have equivalent zero-age main sequence (ZAMS) spectral type B0.5 - B0. The nebular masses derived from the continuum observations are about 5 Msun for source A and 4 Msun for source C. A velocity gradient previously detected may be explained by unresolved binary rotation with a small velocity difference.Comment: 38 pages, 9 figures, accepted by The Astrophysical Journa