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

Abstract

Small isolated clouds were discovered by the ISOPHOT 170 mu m Serendipity Survey (ISOSS) as faint I₁₇₀<13 MJy sr^-1 FIR sources. One of them is ISOSS J20246+6540, a cold (T_d~ 14.5 K) pointlike ISOSS source without an IRAS pointsource counterpart. In the Digitized Sky Survey B band it is seen as a 3 arcmin 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 average hydrogen column density of ~10²¹ 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 of ~10⁴ cm^-3. The multiwavelength 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 100 mu m brightness. A significant overabundance of ¹³CO is found X(¹³CO) >=150x X(C¹⁸O). This is likely due to isotope selective chemical processes. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA.