Survey of intermediate/high mass star-forming regions at centimeter and millimeter wavelengths

¹ Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain e-mail: asanchez@am.ub.es
² Laboratorio de Astrofísica Espacial y Física Fundamental, INTA, Apartado 78, 28691 Villanueva de la Cañada, Madrid, Spain
³ Institut de Ciències de l'Espai (CSIC-IEEC), Campus UAB – Facultat de Ciències, Torre C5 – parell 2, 08193 Bellaterra, Catalunya, Spain

Received: 3 August 2007
Accepted: 20 February 2008

Abstract

Aims. The goal of this work is to characterize the millimeter and centimeter properties of intermediate/high mass young stellar objects (YSOs) to search for any evolutionary trends.

Methods. We carried out observations at 1.2 mm with the IRAM 30 m telescope, and at 3.6 and 1.3 cm with the VLA toward a sample of 11 luminous (> 10³ ) IRAS sources classified as high mass protostellar object candidates. The most promising regions additionally were observed at 7 mm with the VLA.

Results. The 1.2 mm emission, tracing the dust core in which the massive YSO is forming, shows a clear peak surrounded by some substructure in most cases, while in others it is very extended and weak. The masses from the 1.2 mm data range from 10 to 140 . For all (but one) of the sources, we detected centimeter emission associated with the IRAS source, with spectral indices between 3.6 and 1.3 cm typical of optically thin emission, and deconvolved sizes from < 0.01 to 0.3 pc, suggesting that the emission comes from compact or ultra-compact (UC) H II regions. The physical parameters of the UCH II regions indicate that the ionizing stars are early B-type. The 7 mm emission is partially resolved for the four regions observed at this wavelength, and we estimated the contribution of the dust emission to the 7 mm flux density, ranging from negligible to 45%. By combining our data with infrared surveys, we built the spectral energy distribution and fitted a modified blackbody law. We found dust temperatures between 25 and 35 K, dust emissivity indices between 1.5 and 2.2, and masses similar to the masses derived from the 1.2 mm continuum emission. In addition, we found a correlation between the degree of disruption of the natal cloud, estimated from the fraction of dust emission associated with the centimeter source relative to the total amount of dust in its surroundings, and the size of the centimeter source.

Conclusions. From the correlation found, we established an evolutionary sequence in which sources with compact millimeter emission clearly associated with compact centimeter emission are younger than sources with the millimeter emission dispersed and with the centimeter emission extended. Such a sequence is consistent with the evolutionary stage expected from maser/outflow/dense gas emission reported in the literature, and with the infrared excess of the 2MASS sources associated with the centimeter source.