Different evolutionary stages in massive star formation

Centimeter continuum and H₂O maser emission with ATCA^⋆,⋆⋆

¹ Osservatorio Astrofisico di Arcetri, INAF, Largo Enrico Fermi 5, 50125 Firenze, Italy
e-mail: asanchez@arcetri.astro.it
² Istituto di Radioastronomia & Italian ALMA Regional Centre, via P. Gobetti 101, 40129 Bologna, Italy
³ Istituto di Fisica dello Spazio Interplanetario, INAF, Area di Recerca di Tor Vergata, Via Fosso Cavaliere 100, 00133 Roma, Italy
⁴ ESO, Karl Schwarzschild str. 2, 85748 Garching bei Munchen, Germany
⁵ School of Physics, University of New South Wales, NSW 2052, Australia

Received: 26 June 2012
Accepted: 30 October 2012

Abstract

Aims. We present Australia Telescope Compact Array (ATCA) observations of the H₂O maser line and radio continuum at 18.0 GHz and 22.8 GHz toward a sample of 192 massive star-forming regions containing several clumps already imaged at 1.2 mm. The main aim of this study is to investigate the water maser and centimeter continuum emission (that likely traces thermal free-free emission) in sources at different evolutionary stages, using evolutionary classifications previously published.

Methods. We used the recently comissioned Compact Array Broadband Backend (CABB) at ATCA that obtains images with  ~20′′ resolution in the 1.3 cm continuum and H₂O maser emission in all targets. For the evolutionary analysis of the sources we used millimeter continuum emission from the literature and the infrared emission from the MSX Point Source Catalog.

Results. We detect centimeter continuum emission in 88% of the observed fields with a typical rms noise level of 0.45 mJy   beam^-1. Most of the fields show a single radio continuum source, while in 20% of them we identify multiple components. A total of 214 cm continuum sources have been identified, that likely trace optically thin H ii regions, with physical parameters typical of both extended and compact H ii regions. Water maser emission was detected in 41% of the regions, resulting in a total of 85 distinct components. The low angular (~20′′) and spectral (~14 km s^-1) resolutions do not allow a proper analysis of the water maser emission, but suffice to investigate its association with the continuum sources. We have also studied the detection rate of H ii regions in the two types of IRAS sources defined in the literature on the basis of the IRAS colors: High and Low. No significant differences are found, with high detection rates (>90%) for both High and Low sources.

Conclusions. We classify the millimeter and infrared sources in our fields in three evolutionary stages following the scheme presented previously: (Type 1) millimeter-only sources, (Type 2) millimeter plus infrared sources, (Type 3) infrared-only sources. We find that H ii regions are mainly associated with Type 2 and Type 3 objects, confirming that these are more evolved than Type 1 sources. The H ii regions associated with Type 3 sources are slightly less dense and larger in size than those associated with Type 2 sources, as expected if the H ii region expands as it evolves, and Type 3 objects are older than Type 2 objects. The maser emission is mostly found to be associated with Type 1 and Type 2 sources, with a higher detection rate toward Type 2, consistent with the results of the literature. Finally, our results on H ii region and H₂O maser association with different evolutionary types confirm the evolutionary classification proposed previously.