Detection of HD in emission towards Sagittarius B2

The giant molecular cloud Sagittarius B2, located near the Galactic Centre, has been observed in the far-infrared by the ISO Long Wavelength Spectrometer. Wavelengths in the range 47{196 mum were covered with the high resolution Fabry-Perot spectrometer, giving a spectral resolution of 30-40 km s(-1). The J = 1 --> 0 and J = 2 --> 1 rotational transitions of HD fall within this range at 112 mum and 56 mum. A probable detection was made of the ground state J = 1 --> 0 line in emission but the J = 2 --> 1 line was not detected above the noise. This allowed us to calculate an upper limit on the temperature in the emitting region of approximately 80 K and a value for the deuterium abundance in the Sgr B2 envelope of D/H = (0.2-11) x 10(-6).