A study on subarcsecond scales of the ammonia and continuum emission toward the G16.59−0.05 high-mass star-forming region

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: mosca@arcetri.astro.it
² Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
³ The University of Tokushima Minami Jousanjima-machi 1-1, Tokushima, Tokushima 770-8502, Japan
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ INAF – Istituto Fisica Spazio Interplanetario, via Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 24 May 2013
Accepted: 15 September 2013

Abstract

Aims. We wish to investigate the structure, velocity field, and stellar content of the G16.59−0.05 high-mass star-forming region, where previous studies have established the presence of two almost perpendicular (NE–SW and SE–NW), massive outflows, and a rotating disk traced by methanol maser emission.

Methods. We performed Very Large Array observations of the radio continuum and ammonia line emission, complemented by COMICS/Subaru and Hi-GAL/Herschel images in the mid- and far-infrared.

Results. Our centimeter continuum maps reveal a collimated radio jet that is oriented E–W and centered on the methanol maser disk, placed at the SE border of a compact molecular core. The spectral index of the jet is negative, indicating non-thermal emission over most of the jet, except the peak close to the maser disk, where thermal free-free emission is observed. We find that the ammonia emission presents a bipolar structure consistent (on a smaller scale) in direction and velocity with that of the NE–SW bipolar outflow detected in previous CO observations. After analyzing our previous N₂H⁺(1–0) observations again, we conclude that two scenarios are possible. In one case both the radio jet and the ammonia emission would trace the root of the large-scale CO bipolar outflow. The different orientation of the jet and the ammonia flow could be explained by precession and/or a non-isotropic density distribution around the star. In the other case, the N₂H⁺(1–0) and ammonia bipolarity is interpreted as two overlapping clumps moving with different velocities along the line of sight. The ammonia gas also seems to undergo rotation consistent with the maser disk. Our infrared images complemented by archival data allow us to derive a bolometric luminosity of ~10⁴  L_⊙ and to conclude that most of the luminosity is due to the young stellar object associated with the maser disk.

Conclusions. The new data suggest a scenario where the luminosity and the outflow activity of the whole region could be dominated by two massive young stellar objects: 1) a B-type star of ~12 M_⊙ at the center of the maser/ammonia disk; 2) a massive young stellar object (so far undetected), very likely in an earlier stage of evolution than the B-type star, which might be embedded inside the compact molecular core and power the massive, SE–NW outflow.