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A 215 to 265 GHz spectral line survey of the
Sgr B2 molecular cloud
By A l b e r t Numme l i n
e­mail: albert@bele.oso.chalmers.se
Onsala Space Observatory, S­439 92 Onsala, SWEDEN
The spectrum of the Sgr B2 molecular cloud has been surveyed between 215 and 265 GHz towards
3 positions, the dense cores Sgr B2(N)and Sgr B2(M), and the quiescent core Sgr B2(NW). The
observational part of the survey is completed and data analysis has begun. Increased SO and
SO2 abundances indicate shock chemistry in the M core, which could contain an outflow.
1. General
The Sgr B2 molecular cloud, located approximately 100 pc from the Galactic Centre
and with a mass of 5 \Gamma 10 \Theta 10 6 M fi , is one of the most extreme cases among star
formation sites (Lis & Goldsmith (1991)). This region contains many tracers of recent
massive star formation: ? 20 resolved ultra­compact Hii regions, numerous H 2 O­ and
OH­masers, large far infrared luminosity, all of which indicate active formation of O­type
stars (Gordon et al. (1993)). Another prominent characteristic of the Sgr B2 cloud is its
extremely rich chemistry.
The two strongest sources of millimetre and submillimetre emission, each with a total
luminosity of ъ 30 \Theta that of Orion A, are denoted Sgr B2(N) (``North'') and Sgr B2(M)
(``Middle'') respectively and are currently being surveyed in the 215--265 GHz region
using the SEST in order to learn about the physical and chemical conditions in this very
active star forming region. Both of these positions correspond to compact clusters of Hii
regions and contain very dense and warm gas.
A third position, denoted Sgr B2(NW) (``North­West''), towards a quiescent part of
the cloud, is also being surveyed in order to compare physics and chemistry before and
after processing in protostellar cores.
Sgr B2 has previously been surveyed at wavelengths around 3 mm (Cummins et al.
(1986), Turner (1989), Turner (1991)) and 0.8 mm (Sutton et al. (1991)).
2. Preliminary results
The observational part of the survey has recently been completed but analysis has only
just begun. Of the 3 observed positions Sgr B2(N) has the strongest line emission in most
molecular species with a few exceptions. The linewidths are very high (ъ 20 km s \Gamma1 )
and the number of lines per GHz is also very high (ъ 25 \Gamma 35). With such broad lines
and high line density blending and confusion is a very severe problem in the analysis.
Sgr B2(M) is the stronger of the 3 sources in a few species, the most important of
which are SO and SO 2 . The abundances of some Sulphur species are predicted to be
increased in regions where the gas is shocked (Pineau des For“ets et al. (1993)), which
suggests that M might contain outflows. Otherwise, M has more moderate linewidths,
ъ 15 \Gamma 20 km s \Gamma1 , and fewer lines per GHz (ъ 15 \Gamma 20) than does N, which makes the
data from this source easier to reduce and analyze. A preliminary line identification
contains some 600 identified and unidentified lines towards M.
1

2 Albert Nummelin: Spectral line survey of the Sgr B2 molecular cloud
Figure 1. Two sample spectra towards Sgr B2(M) and Sgr B2(N). Because of the width and
density of the spectral lines towards N, a ``line forest'' can easily be mistaken for a baseline
offset.
I would like to acknowledge my gratitude for being able to work with Dr Per Bergman
and Dr љ Ake Hjalmarson. Thanks also to the SEST team.
REFERENCES
Cummins S.E., Linke R.A., Thaddeus P., 1986, ApJS, 60, 819.
Gordon M.A., Berkermann U., Mezger P.G., Zylka R., Haslam, C.G.T., Kreysa E., Sievers A.,
Lemke R., 1993, A&A, 280, 208.

Albert Nummelin: Spectral line survey of the Sgr B2 molecular cloud 3
Lis D.C., Goldsmith P.F., 1991, ApJ, 369, 157.
Pineau des For“ets G., Roueff E., Schilke P., Flower D.R., 1993, MNRAS, 262, 915.
Sutton E.C., Jaminet P.A., Danchi W.C., Blake G.A., 1991, ApJS, 77, 255.
Turner B.E., 1989, ApJS, 70, 539.
Turner B.E., 1991, ApJS, 76, 617.