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

A faint $I_{170}=4$ MJysr${}^{-1}$ bipolar globule was discovered with the ISOPHOT 170 $\mu$m Serendipity Survey (ISOSS). ISOSS J 20246+6541 is a cold ($T_{\mathrm{d}}\approx 14.5$ K) FIR 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 surface brightness of $\approx 26$ mag/$\mathrm{\square }$\arcsec . The CO column density distribution determined by multi-isotopic, multi-level CO measurements with the IRAM-30m telescope agrees well with the optical appearance. An average hydrogen column density of $\approx {10}^{21}$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 of $\approx {10}^4$cm${}^{-3}$. The multiwavelength data characterise ISOSS 20246+6541 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 ${}^{13}$CO is found $X{(}^{13}CO)\ge 150\times X(C^{18}O)$. This is likely due to isotope selective chemical processes.