An unbiased deep search for small-area molecular structures

Institut für Physik und ihre Didaktik, Universität zu Köln, Gronewaldstraße 2, 50931 Köln, Germany e-mail: aheithau@uni-koeln.de

Received: 4 January 2006
Accepted: 26 January 2006

Abstract

Context.Small-area molecular structures (SAMS) resembling clumpuscules proposed as candidates for baryonic dark matter, have been detected in an area where the shielding is too low for them to survive for a long time.Aims.I present the results of an unbiased deep search for molecular clumpuscules to study the frequency of occurence of such structures km s^-1. Methods.The area surrounding these structures has been surveyed using the FCRAO 14m telescope in the CO transition. The field covered is 20' by 20'. The resulting rms of the data is only 0.04 K in a 0.127  wide channel. Additionally, high-angular resolution observations of the ¹³CO and C¹⁸O transitions were obtained with the IRAM Plateau-de-Bure Interferometer.Results.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. Their radial velocity is similar to that of 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 are caused by a large velocity difference between the clumps seen at highest angular resolution.
The non-detection of the structures in the high-angular resolution observations of the ¹³CO and C¹⁸O transitions shows that the ¹²CO transition must have a low optical depth. At an adopted distance of 100 pc the structures have masses of only Jupiter mass or below. Conclusions.The new observations show that SAMS might be an abundant phenomenon in the interstellar medium but 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.