High-latitude molecular clouds or galactic cirrus clouds are commonly found to be gravitationally unbound. Their kinematics is largely dominated by turbulence and their virial masses are 1-2 orders of magnitudes larger than their observed masses (Magnani et al. 1987; de Vries et al. 1987; Heithausen 1996). Some of these cirrus clouds harbour dense cores (Mebold et al. 1987) with abundances similar to those dark clouds in star-forming regions (e.g. Grossmann et al. 1990). The dense cores found in some cirrus clouds appear to be much closer to virial equilibrium (Großmann & Heithausen 1992) compared to the whole cloud.
It has been found that the star-formation efficiency in high-latitude cirrus clouds must be very low (e.g. Hearty et al. 1999); no low mass T Tauri stars associated with cirrus clouds have been found so far (Martin & Kun 1996). Searches for mm- or submm-sources with low dust temperatures or kinematic signatures of infall motions in galactic cirrus clouds are missing. Detection of such signatures of beginning star-formation may elucidate the mechanisms that lead to low-mass star-formation.
In this paper I present evidence for inward motion in the dense molecular cloud MCLD 123.5+24.9 located in the Polaris Flare, a huge molecular cirrus cloud in the direction of the north celestial pole (Heithausen & Thaddeus 1990). Its distance is between 130 pc and 240 pc (cf. Heithausen et al. 1993); throughout this paper a distance of 150 pc is adopted. First molecules detected towards MCLD 123.5+24.9 were CO, 13CO, H2CO, and OH (Großmann et al. 1990), these authors found an increased OH abundance for this cloud. Follow-up observations led to the detection of dense condensations seen in NH3, HCO+, HCN, and HNC, with abundances consistent with those for dark clouds (Großmann & Heithausen 1992). The kinetic temperature derived from these observations is between 6 K and 15 K. The mass of the core is 1.2 (Großmann & Heithausen 1992, rescaled from 200 pc to 150 pc distance).
© European Southern Observatory (ESO) 1999
Online publication: September 2, 1999