Ammonia cores in high mass star formation regions

¹ Astronomy Department, CAS-PKU Joint Beijing Astrophysics Center, Peking Univ., Beijing 100871, PR China e-mail: yfwu@bac.pku.edu.cn
² Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
³ Institute of Inorganische Chemistry, Uni. Bonn, Roemer St., 53117 Bonn, Germany
⁴ I, Physics Institut, Köln Univ., Zülpicher St., 50937 Köln, Germany
⁵ Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China

Received: 6 April 2005
Accepted: 3 January 2006

Abstract

We observed a sample of 35 water masers not coincident with known HII regions and/or low mass young stellar objects (YSOs) with the Effelsberg 100 m telescope in the  = (1, 1), (2, 2), (3, 3) and (4, 4) transitions. Sixteen sources were detected in the NH₃ emission. The detection rate is 46%. All these sixteen sources have NH₃ (1, 1) and (2, 2) emission, among which four sources have NH₃ (3, 3) emission. Comparing with the IRAS and the 2MASS data, we analyzed the relationship between the detection rate and the infrared color, the dust temperature and the source distance.
All the detected sources were mapped and 17 cores were obtained (one source IRAS 20215+3725 has two cores). From the detected sources five cores do not coincide with radio continuum or IRAS and MSX point sources. Excluding one core that has no MSX data available, the remaining eleven cores are coincident with IRAS or MSX point sources.
The typical size and mass of the cores are 1.6 pc and 1.5  10³ , respectively. The average line widths of the NH₃ (1, 1) and (2, 2) are 1.54 and 1.73 km s^-1. The average kinetic temperature of the gas is about 19 K. These values are much larger than those of low mass cores.
The NH₃ cores that coincide with IRAS sources (referred to as Group I) have slightly larger line widths (1.65 and 1.75 km s^-1 for the (1, 1) and (2, 2) lines, respectively) and larger masses ( ) than the mean values of the sample. For this type of core the kinetic temperature correlates with the line width. The line width appears to correlate with the bolometric luminosity and the core size. Despite the average luminosity of  , there is no detectable 6 cm emission. These are candidates for high mass protostars or precursors of UC HII regions.
The NH₃ cores with peaks offset from infrared sources (referred to as Group II) have an average size of 1.7 pc and an average line width of 1.50 km s^-1 for the (1, 1) line. The line width of the (1, 1) emission is smaller than that of the group I.
The average mass is  . One possible explanation for the deviation is that the NH₃ peak and the infrared source correspond to different clumps. These cores are potential high mass star formation sites and may be at an earlier evolutionary stage than those with IRAS point sources. This type of core is seen in mapping observations, and can be easily missed by single-spectrum observations toward the IRAS position.