An unbiased deep search for small-area molecular structures

Abstract

Small-area molecular structures (SAMS) resembling those clumpuscules, proposed by Pfenniger and Combes (1994) as candidate for baryonic dark matter, have recently been detected (Heithausen 2002, 2004) in an area where the shielding is too low for them to survive for a long time. To study the frequency of occurence of such structures I present the results of an unbiased deep search for molecular clumpuscules. The area surrounding these structures has been surveyed using the FCRAO 14m telescope in the CO J=1-0 transition. The field covered is 20' by 20'. The resulting rms of the data is only 0.04K in a 0.127 km/s wide channel. Also, high-angular resolution observations of the 13CO and C18O (J=1-0) transitions were obtained with the IRAM Plateau-de-Bure Interferometer. 3 new SAMS have been detected. The structures have very low intensities which makes it impossible to detect them in large scale CO surveys conducted to map the molecular gas of the Milky way. They move with a similar radial velocity as the surrounding HI gas. The clouds follow the same size-linewidth relation as found for giant molecular clouds or Galactic cirrus clouds. The observations clearly show that most of the large linewidths observed at low angular resolution is caused by a large velocity difference among the clumps seen at highest angular resolution. At an adopted distance of 100pc the structures have masses of only Jupiter mass or below. My observations show that SAMS might be an abundant phenomenon in the interstellar medium however not recognized as such due to their small size. If they are made of ordinary interstellar matter with solar metallicity they likely contribute only little to the total interstellar mass.