Letter to the Editor

Star formation triggered by the Galactic H II region RCW 120^*

First results from the Herschel Space Observatory

¹ Laboratoire d'Astrophysique de Marseille (UMR 6110 CNRS & Université de Provence), 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France e-mail: annie.zavagno@oamp.fr
² Laboratoire AIM, CEA/IRFU CNRS/INSU Université Paris Diderot, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France e-mail: fmotte@cea.fr
³ Laboratoire d'Astrophysique de Bordeaux, CNRS/INSU Université de Bordeaux, BP 89, 33271 Floirac cedex, France
⁴ Institut d'Astrophysique Spatiale, CNRS/Université Paris-Sud 11, 91405 Orsay, France
⁵ Department of Physics & Astronomy, The Open University, UK
⁶ Space Science Department, Rutherford Appleton Laboratory, Chilton, UK

Received: 31 March 2010
Accepted: 12 April 2010

Abstract

Context. By means of different physical mechanisms, the expansion of H ii regions can promote the formation of new stars of all masses. RCW 120 is a nearby Galactic H ii region where triggered star formation occurs. This region is well-studied – there being a wealth of existing data – and is nearby. However, it is surrounded by dense regions for which far infrared data is essential to obtain an unbiased view of the star formation process and in particular to establish whether very young protostars are present.

Aims. We attempt to identify all young stellar objects (YSOs), especially those previously undetected at shorter wavelengths, to derive their physical properties and obtain insight into the star formation history in this region.

Methods. We use Herschel-PACS and -SPIRE images to determine the distribution of YSOs observed in the field. We use a spectral energy distribution fitting tool to derive the YSOs physical properties.

Results. Herschel-PACS and -SPIRE images confirm the existence of a young source and allow us to determine its nature as a high-mass (8–10 ) Class 0 object (whose emission is dominated by a massive envelope M_env 10³ ) towards the massive condensation 1 observed at (sub)-millimeter wavelengths. This source was not detected at 24 μm and only barely seen in the MISPGAL 70 μm data. Several other red sources are detected at Herschel wavelengths and coincide with the peaks of the millimeter condensations. SED fitting results for the brightest Herschel sources indicate that, apart from the massive Class 0 that forms in condensation 1, low mass (0.8–4 ) stars are forming around RCW 120 with ages younger than 5 × 10⁴ years. This indicates that YSOs observed on the borders of RCW 120 are younger than its ionizing star, which has an age of about 2.5 Myr.

Conclusions. Herschel images allow us to detect new YSOs that are too young and embedded to be detected at shorter wavelengths (25 of the 49 Herschel sources are new detections). This offers a new and more complete view of the star formation in this region. PACS and SPIRE fluxes were obtained for the brightest YSOs and allow us to strongly constrain both their spectral energy distribution and their physical properties through SED fitting. A more accurate determination of their properties allows us, for the first time, to discuss the star formation history in this region by comparing similar sources at different evolutionary stages.