Extending the limits of globule detection ^*

ISOPHOT Serendipity Survey observations of interstellar clouds II

¹ Helsinki University Observatory, PO Box 14, 00014 University of Helsinki, Finland
² Department of Astronomy, Loránd Eötvös University, Pázmány Péter sétány 1/a, 1117 Budapest, Hungary
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Konkoly Observatory, Budapest, PO Box 67, 1525, Hungary
⁵ IRAM, Avenida Divina Pastora 7, N.C., 18012 Granada, Spain

Corresponding author: L. V. Tóth, lvtoth@mpia-hd.mpg.de

Received: 1 March 2002
Accepted: 2 September 2002

Abstract

Small isolated clouds were discovered by the ISOPHOT 170 μm Serendipity Survey (ISOSS) as faint  MJy sr^-1 FIR sources. One of them is ISOSS J20246+6540, a cold ( K) pointlike ISOSS source without an IRAS pointsource counterpart. In the Digitized Sky Survey B band it is seen as a 3´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 μm brightness. A significant overabundance of ¹³CO is found COC¹⁸O. This is likely due to isotope selective chemical processes.