SIMBAD references

Optical, infrared, and millimetre data of five interstellar cirrus clouds at high galactic latitude are used to study their physics and chemistry. Blue deep Schmidt plates were taken with the ESO 1m Schmidt telescope and calibrated after digitization. The clouds have an average visual extinction A_V<1mag. Using deep IRAS co-added images we studied the dust in these clouds. The observed average 100µm brightness of the clouds of our sample is nearly constant: (2.0-2.4)10^–8Wm^–2/sr, while the average blue surface brightness varies between (1.0-4.9)10^–8Wm^–2/sr. This behaviour can be explained by projection effects of the backscattered radiation. We detected ¹²CO (J=1-0) and ¹³CO (J=1-0) emission in all clouds with the 15m Swedish ESO Submillimetre Telescope. The CO observations were carried out along cuts from the centre of a cloud towards its edge with a spacing of 1 arcmin. The cuts were selected to pass across clumps recognizable on the optical and infrared images; two of the clouds have been fully mapped in the ¹²CO line. The observed ¹²CO/¹³CO intensity ratio varies considerably within the individual clouds as well as between the clouds, and reaches values between 3 and 60. These limiting values are in agreement with values found in giant molecular clouds and diffuse clouds, respectively. The variation is due to a change of the ¹²CO J=1-0 millimetre line from optically thick to optically thin, and to ¹³CO selective processes: the carbon-ion exchange reaction and photodissociation. These processes are very sensitive to the temperature and the radiation field in diffuse cirrus clouds where the ¹²CO just starts to become stable against photodissociation. We find very good agreement between the structure seen on the optical images and the distribution of the CO emission. The optical images are an excellent tool to distinguish column density variations from space density variations. Chemical models are used to interpret the observed CO emission and to predict the intensities of the far-infrared/submm lines of carbon and oxygen.