Massive star-formation in G24.78+0.08 explored through VLBI maser observations^*

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: mosca@arcetri.astro.it
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Av. Diagonal 647, 08028, Barcelona, Catalunya, Spain
⁴ Subaru Telescope, National Astronomical Observatory of Japan, 650 North A'ohoku Place, Hilo, HI 96720, USA

Received: 9 May 2007
Accepted: 10 July 2007

Abstract

Context.Previous interferometric observations have demonstrated that, across a distance of a few 0.1 pc, the high-mass star forming region (SFR) G24.78+0.08 contains at least four distinct centers of massive star formation, possibly in different evolutionary stages.

Aims.This study aims to provide a detailed picture of the physical environment and the gas kinematics in a cluster of high-mass YSOs.

Methods.Using EVN (single epoch) and VLBA (four epochs) phase-referenced observations, we have derived the absolute positions and velocities for 6.7 GHz methanol and 22.2 GHz water masers, respectively. Using the BIMA and VLA interferometers, positions and line of sight velocities of 95 GHz and 44 GHz methanol masers, are also obtained. The derived interferometric and VLBI maser maps are compared with previous sub-arcsecond maps of the G24.78+0.08 region, observed in thermal continuum and molecular line tracers.

Results.In the hot molecular cores G24 A1 and G24 A2, 6.7 GHz methanol and 22.2 GHz water masers are emerging at similar positions and line of sight velocities, which suggests that in both cores a same YSO is responsible for the excitation of the two types of maser emission. At the center of the G24 A1 core, water masers distribute along an arc at the border of a hyper-compact HII region (of size ≈1000 AU) and expand away from the center of the HII region with high velocities (≈40 km s^-1). We think that such a fast expansion is driven by a strong stellar wind emitted by the star exciting the hyper-compact HII region. This outflowing motion might dominate the gas kinematics of G24 A1 also at larger (≈0.1 pc) scale, where a well defined velocity gradient in the CH₃CN line and 6.7 GHz masers is observed. Alternatively, water masers might mark the border of the dynamical interaction between the evolving star and harboring core, and the gas surrounding the hyper-compact HII region, not yet affected by the expansion of the ionized gas, might be still rotating and, perhaps, slightly contracting. In the G24 C core, water maser spots show very fast (100–200 km s^-1) and nearly parallel proper motions, which might indicate that the water maser emission is tracing a collimated jet.