Star forming cores in L 1251: Maps and molecular abundances

Abstract

We have mapped the dense parts of the cometary-shaped, star-forming dark cloud L 1251 in the rotational lines of HCN, HNC, HCO + and CS at 3 mm, and observed selected positions in SO, CH 3 CCH and rare isotopomers of the mapped molecules. Using the CS line we detected 15 cores with sizes of ∼0.1-0.3 pc. New estimates of the fraction of dense gas in the cores yield a revised average SFE of ∼10%. Although 3 times lower than the previous estimate, this high SFE still points to externally triggered star formation in the cloud. Around IRAS 22343+7501, the source proposed to drive a previously detected extended CO outflow, our data suggest the existence of either a rotating HCO + disk or a dense outflow with a dynamical age of ∼2 x 10 4 years. A stability check seems to rule out the disk interpretation. We suggest that both continuum sources of Beltran et al. are protostars each driving its own outflow. Using methyl acetylene as a thermometer we find indications that at lower temperatures the A and E species are defined by different partition functions. A gradient was found in the cloud, with the highest temperature detected in the region. The column density ratios derived from these observations and the previously published NH 3 data show in general little variations, but for two exceptional locations. One of these is in the tip of the head with high relative SO and NH 3 abundances, and the other is in the tail with low CO and HCO + column densities with respect to HNC, HCN and NH 3 . In the first case the abundance ratios can probably be explained by an advanced stage of chemical evolution assisted by an elevated temperature. The second location is likely to be an example of CO and HCO + depletion, and the implication is that also HNC and HCN belong to the molecules which are more resistant against freezing-out than CO and HCO + .