Physical Properties and Molecular Composition of
the Region of Massive Star Formation L379IRS1
S. V. Kalenskii and M. A. Shchurov
Astro Space Center, Moscow
DSS2 image of L379
Introduction
The region of massive star formation L379IRS1 (IRAS18265–1517) is located in a dark cloud
L379 at a distance of 2 kpc. Submillimeter observations showed that IRS1 consists of two
clumps, situated 0.2 pc north and 0.14 pc south of the IRAS position and connected by an
arc of submillimeter emission west of this position (Kelly and Macdonald, 1996). Each clump
probably contains a B0-5 protostar, which drives a bipolar outflow. Class I methanol masers,
detected at 36, 44, 132 GHz and some other frequencies are related to the northern outflow
(Slysh et al, 1999), while a group of Class II methanol masers have been found towards the
southern clump (Walsh et al., 1998).
We tried to study the molecular composition and the physical parameters of L379IRS1 in
more detail. For this purpose we observed with the Pico Veleta radio telescope a number of
positions in this object in seven spectral bands that include the frequencies of CS, SiO, N2H+,
CH3CN, CH3OH lines etc.
Results
Lines of 24 molecules, beginning with
simple diatomic or triatomic species, such
as SiO, CS, OCS, and ending with
complex eight-atom or nine-atom
chemical compounds, such as CH3OCHO,
CH3OCH3 were detected (Table 1).
The richest molecular composition was
found towards the −400, +2000 direction
(the position of the northern submm
peak). Lines of 21 molecules were
detected there.
Dimethyl ether
Table 1. Molecules detected in
L379IRS1
Diatomic
Triatomic
Four-atom
Five-atom
Six-atom
Seven-atom
Eight-atom
Nine-atom
CS, C33S, C34S, SiO
CCS?, DCN, H2S, N2H+,
OCS, SO2
H2CO, HDCS?, HNCO?
c-C3H2, HCCCN, HC13CCN,
HCOOH
CH3CN, CH3OH
HC5N, CH3CHO?
CH3OCHO, CH2OHCHO?
CH3OCH3
Methanol
I. Gas parameters determined from methanol lines
Rotation diagrams built from the
145 and 241 GHz lines (Type I
rotation diagrams) estimate density
rather than temperature (Kalenskii
& Kurtz, in preparation; see also
Leurini et al, 2004)
Type I rotational temperature about
7 − 8 K corresponds to a density of about
Fig. 1. Rotation diagrams, built from the 3K − 2K
106 cm−3.
For such a density rotational temperatures lines at 145 GHz, the 5K − 4K lines at 241 GHz, and
the J0 − J−1E lines at 157 GHz
derived from the 157 GHz lines (Type II
rotational temperatures) should be
multiplied by a factor of 1.5 − 2.
The gas density and temperature in L379IRS1 proved to be about 106 cm−3
and 40–50 K, respectively.
II. A and E-methanol abundance ratio
Methanol forms on dust grains at a
temperature 10–15 K (Watanabe et al.,
2004).
As the ground state of E-methanol lies
7.9 K above the ground state of A-methanol
the ratio of E-methanol to A-methanol
abundances should be about 0.7.
But our rotation diagrams show
Rotation diagrams, built from the observed
that the ratio of A to E-methanol
abundances in L379IRS1 is close to and model lines at 145 and 241 GHz. Filled
squares represent E-methanol, asterisks,
unity.
A-methanol.
Methyl cyanide
Rotation diagrams built from the 8K − 7K
lines of methyl cyanide shows the presence
of two components: warm component,
traced by the low-energy lines, and hot
component, traced by the high-energy lines.
The warm gas is the same as observed in
the methanol lines.
CH3CN abundance in the warm gas is
about 10−11, similar to that found by
Kalenskii et al (2000).
The high-energy lines suggest an existence of a hot core in the southern clump, which also
reveals itself in the 6.7 GHz maser emission. Those observed towards the northern core
may arise either in another hot core, related to the northern protostar, or in hot postshock
gas.
It is considered that the emission of complex
organic molecules (COM), such as dimethyl
ether (CH3OCH3) or methyl formate
(CH3OCHO) arises in hot cores or hot
postshock gas. But recently CH3OCH3
emission was detected in cold prestellar cores
and cold envelopes of low-mass protostars.
Can these molecules be observable in the
quiescent gas of dense cores?
Fig. 3. Rotation diagrams built from CH3OCH3 lines
observed towards the northern (left plot) and
southern (right plot) submillimeter peaks.
CH3OCH3 rotational temperature proved to be 44 K towards the southern peak and 33 K
towards the northern peak.
It is possible that:
quiescent gas emission dominates the observed low-energy lines.
our set of CH3OCH3 lines significantly underestimates the gas kinetic temperature, like
Type I methanol lines.
A careful mapping and a thorough analysis of dimethyl ether excitation is necessary in order
to choose between this possibilities.
Other molecules
Apart from the lines of methanol, methyl cyanide, and dimethyl ether, a number of single
lines of different molecules were detected. Each such line was used to determine the column
density of the relevant molecule.
Column densities of the most of molecules in L379IRS1 are nearly the same as those
obtained in the DR21(OH) by Kalenskii & Johansson (2010).
The only exception is sulfur dioxide. In L379IRS1, the derived column density proved to
be twenty times lower than that found by Kalenskii & Johansson in DR21(OH).
Abundance ratios [CS]/[SO2], [OCS]/[SO2], [H2S]/[SO2] show that L379IRS1 may be
younger than 105 years, according to modeling by Wakelam et al.(2004).
Conclusions
Spectral observations of a region of massive star formation L379IRS1 in
the 1, 2, and 3-mm wavelength ranges resulted in the detection of the
lines of different molecules, starting with simple diatomic or triatomic
species, such as SiO, CS, or OCS, and ending with complex eight- or
nine-atom chemical compounds, such as CH3OCHO or CH3OCH3.
Rotation diagrams showed that the temperature of the quiescent gas is
about 40–50 K, and the density is about 106 cm−3.
The abundances of A and E-methanol proved to be virtually the same, in
contradiction with the current views on methanol formation.
Apart from the quiescent gas, there is a hot component. One of the hot
regions is a compact hot core in the southern submillimeter clump. The
northern clump can host another hot core or/and and hot postshock
regions.
The rotational temperature for dimethyl ether proved to be about 40 K.
The [SO2]/[CS] and [SO2]/[H2S] abundance ratios show that the age of
L379IRS1 is less than 105 years.
The authors are grateful to the staff members off the Pico Veleta
observatory J.-F. Desmurs and S. Leon, who performed the service
observations. This research has made use of NASA’s Astrophysics Data
System.
References
P. Friberg, Å. Hjalmarson, S. C. Madden, and W. M. Irvine, A&A 195, 281 (1988).
S. Leurini, P. Schilke, K. M. Menten, ., A&A 422, 573 (2004).
S. V. Kalenskii, V. G. Promislov, A. V. Alakoz, et al., Astron. Rep. 44, 725 (2000).
S. V. Kalenskii and L. E. B. Johansson, Astron. Rep. 54, 295 (2010).
S. V. Kalenskii and S. Kurtz, Astron. Rep., in preparation (2015).
M. L. Kelly and G. H. Macdonald MNRAS 282, 401 (1996).
V. Wakelam, F. Hersant, and F. Herpin, A&A 529, 159 (2004).
A. J. Walsh, M. G. Burton, A. R. Hyland, and G. Robinson, MNRAS 301, 640 (1998).
E. S. Wirström, W. D. Geppert, Å. Hjalmarson, et al., A&A 533, A24 (2011).
.