A multiwavelength investigation of G24.78+0.08 A2 using observations from VLA and VLT-VISIR ^*,**

¹ INAF - Osservatorio Astrofisico de Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: sarita@arcetri.astro.it
² ESO, Karl Schwarzschild str. 2, 85748 Garching, Germany
³ Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain
⁴ INAF - Istituto di Radioastronomia, Sezione di Firenze, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 18 January 2008
Accepted: 13 June 2008

Abstract

Context. G24.78+0.08 is a massive star-forming region where three large massive disk-like rotating structures (toroids), namely A1, A2 and C, have been found around massive (proto)stars. In particular, while G24 A1 has been extensively investigated through multiwavelength surveys, G24 A2 has been hitherto less studied.

Aims. We carry out a detailed investigation of G24 A2 and speculate the scenario of the environment in the vicinity of the central exciting object(s).

Methods. We have carried out mid-infrared imaging observations of the G24.78+0.08 region at 11.9 and 18.7 μm using the VISIR located on the VLT. In addition, the radio continuum VLA observations at 1.3 cm as well as the NH₃ (2, 2) line emission have been investigated.

Results. Compact mid-infrared emission is observed only from one source in the G24.78+0.08 region and this is likely to be associated with G24 A2. The radio continuum measurements show a faint compact source which is resolved out at higher angular resolutions (60 mas); the diameter D of the radio emitting region is estimated to be 1000 AU < D < 2000 AU. The spectral index (~1) points towards the possibility of this emission being from an ionised jet or an H II region with a density gradient. The NH₃ (2, 2) emission from the main component is optically thick and appears self-absorbed at blue-shifted velocities. The velocity distributions of the NH₃ lines can be explained by a motion which is a combination of rotation and expansion. Based on these observations, we speculate that the expansion is due to the outflow (seen in CO) with A2 being the likely origin of this outflow. The comparison of various properties of G24 A1 and G24 A2 like rotation, mass of cores, infrared emission, excitation temperature, suggests that G24 A2 and G24 A1 possibly harbour similar young stellar objects, but the toroid associated with G24 A2 is more inclined than that of G24 A1.