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A 215 to 265 GHz spectral line survey of the Sgr B2 molecular cloud

Albert Nummelin
e-mail: albert@bele.oso.chalmers.se

Onsala Space Observatory, S-439 92 Onsala, SWEDEN

Abstract:

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 and abundances indicate shock chemistry in the M core, which could contain an outflow.

Contents

1. General

The Sgr B2 molecular cloud, located approximately 100 pc from the Galactic Centre and with a mass of , is one of the most extreme cases among star formation sites (Lis & Goldsmith (1991)). This region contains many tracers of recent massive star formation: resolved ultra-compact HII regions, numerous - and OH-masers, large far infrared luminosity, all of which indicate active formation of O-type 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 30 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) (``North-West''), 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

 
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.

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 () and the number of lines per GHz is also very high (). 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 . The abundances of some Sulphur species are predicted to be increased in regions where the gas is shocked (Pineau des Forêts et al. (1993)), which suggests that M might contain outflows. Otherwise, M has more moderate linewidths, , and fewer lines per GHz () 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.

Acknowledgments

I would like to acknowledge my gratitude for being able to work with Dr Per Bergman and Dr Åke Hjalmarson. Thanks also to the SEST team.

References



YERAC 94 Account
Wed Feb 22 20:46:57 GMT 1995