Issue |
A&A
Volume 450, Number 2, May I 2006
|
|
---|---|---|
Page(s) | 607 - 616 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20053203 | |
Published online | 10 April 2006 |
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
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.
© ESO, 2006
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