The Core of a Quiescent Cloud, L183

Extract

Simultaneous observation of the J,K=1,1 and 2,2 inversion transitions of ammonia (NH₃) with high spatial resolution (≲1 arc min) offers a powerful method of probing the core region of interstellar clouds for evidence of molecular clumping and of prevailing physical conditions which could lead to star formation. We have therefore used the Effelsberg 100-m radiotelescope to make an extensive study of the central region of the nearby dark dust cloud L183 (also known as L134N) in the NH₃ (1,1) transition; the spatial resolution was 40 arcsec. The core region as mapped in the NH₃ (1,1) transition with a velocity resolution of 0.08 km s^-1 consists of two elongated condensations separated by about 2 arcmin in north-south direction (see Fig. 1). The central part of the NH₃ cloud has an approximate dimension of 6′ (N-S) by 2′ (E-W) corresponding to a linear extent of 0.17 × 0.06 pc at an assumed distance of 100 pc. The measured velocity structure of the NH₃ cloud seems to reflect the double peaked nature of the cloud in that it increases from 2.30 km s^-1 in the south to about 2.5 km s^-1 at the northern end of the southern NH₃ peak, and then decreases again to 2.3 km s^-1 towards the north. The intrinsic linewidths of NH₃ (corrected for hyperfine blending) do not vary significantly with position and are between 0.2 and 0.3 km s^-1. The two ammonia peaks are part of a central molecular ridge from which we have observed NH₃ (2,2) emission at 9 positions (see Fig. 1). The rotation temperature T₂₁ as determined from the optical depths of the (1,1) and (2,2) transitions is ∼9K for all positions, and hence the kinetic temperature T_kin seems close to this value as well, i.e. ∼10K throughout the central part of L183. A more detailed account is being publsihed elsewhere (Ungerechts, Walmsley and Winnewisser).