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

Abstract

Context. The role of disks in the formation of high-mass stars is still a matter of debate but the detection of circumstellar disks around O-type stars would have a profound impact on high-mass star formation theories. Aims. 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. Methods. Observations at sub-arcsecond resolution were performed with the Submillimeter Array in methyl cyanide, a typical hot molecular core tracer, and 12 CO and 13 CO, well known outflow tracers. We also obtained sensitive continuum maps at 1.3 mm. Results. 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 CH 3 CN lines with respect to previous studies and thus significantly improve our knowledge of the structure and kinematics of the hot molecular core. Conclusions. 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 CH 3 CN, and the mass loss and momentum rates estimated from our measurements appear too high.

A double-jet system in the G31.41 + 0.31 hot molecular core

“…That suggests that the water masers in "J2" trace the root of a collimated outflow which has propagated throughout the HMC accelerating the core gas up to 20 km s −1 (along the line-of-sight) at separations of ∼10 4 AU from the powering MYSO. That the observed bipolar (east-west) distribution of the high-velocity 12 CO(2-1) emission could mark a collimated outflow has been also discussed by Cesaroni et al (2011). Using the CH 13 3 CN(12-11) lines, which show a well defined V LSR gradient (at PA = 68 • ) consistent in orientation and amplitude with that seen in 12 CO(2-1) and, in addition, being optically thin, permit a reliable estimate of the whole flow mass, these authors derive a value for the outflow momentum rate oḟ P = 0.3 M yr −1 km s −1 .…”

“…In an attempt to search for outflows emerging from the HMC in G31.41+0.31, Cesaroni et al (2011) have used the SMA to map the emission of the CO(2-1) line with an angular resolution of ≈0. 8.…”

“…8. The high-velocity (10 km s −1 ≤ |V LSR − V sys | ≤ 20 km s −1 ) 12 CO(2-1) emission presents a bipolar structure, with red-and blue-shifted emission peaks offset to the east and the west of the HMC, respectively (see Cesaroni et al 2011, Fig. 11).…”

“…The HMC in G31.41+0.31 (at a systemic LSR velocity, V sys = 97.0 km s −1 ) was originally imaged with the VLA in the high-excitation (4, 4) inversion transition of ammonia by Cesaroni et al (1994a), and successively mapped by Beltrán et al (2004) and Cesaroni et al (2011), using the PdBI and SMA interferometers, respectively, in the (12-11) rotational transitions of methyl cyanide (CH 3 CN). These latter observations, achieving an angular resolution of 0.…”

“…8, reveal a well-defined velocity gradient across the HMC, oriented in the NE-SW direction. Considering the large size (≈0.1 pc) and mass (several 100 M ) of this structure, Beltrán et al (2004) and Cesaroni et al (2011) interpret the velocity gradient in terms of a toroid in pseudo-Keplerian rotation around a stellar cluster of ≈10 3 M , tightly packed at the center of the HMC. In contrast to this interpretation, Araya et al (2008) explain the velocity gradient in terms of a compact outflow on the basis of the NE-SW bipolar morphology of thermal methanol (at 44 GHz), observed with the VLA at an angular resolution of ≈0.…”