Extending the limits of globule detection - ISOPHOT Serendipity Survey observations of interstellar clouds II

Small isolated clouds were discovered by the ISOPHOT 170 m Serendipity Survey (ISOSS) as faint I170 < 13 MJy sr 1 FIR sources. One of them is ISOSS J20246+6540, a cold (Td 14:5 K) pointlike ISOSS source without an IRAS pointsource counterpart. In the Digitized Sky Survey B band it is seen as a 3 0 size bipolar nebulosity with an average excess DSS blue band surface brightness of 8% of the background's photographical density. The CO column density distribution determined by multi-isotopic, multi-level CO measurements with the IRAM-30 m telescope agrees well with the optical appearance. An av- erage hydrogen column density of10 21 cm 2 was derived from both the FIR and CO data. Using a kinematic distance estimate of 400 pc the NLTE modelling of the CO, HCO + , and CS measurements gives a peak density of10 4 cm 3 . The multiwave- length data characterise ISOSS 20246+6540 as a representative of a class of globules which has not been discovered so far due to their small angular size and low 100m brightness. A significant overabundance of 13 CO is found X( 13 CO) 150X(C 18 O).