Chemistry and Dynamics in Pre‐protostellar Cores

We have compared molecular line emission to dust continuum emission andmodeled molecular lines using Monte Carlo simulations in order to study thedepletion of molecules and the ionization fraction in three preprotostellarcores, L1512, L1544, and L1689B. L1512 is much less dense than L1544 andL1689B, which have similar density structures. L1689B has a differentenvironment from those of L1512 and L1544. We used density and temperatureprofiles, calculated by modeling dust continuum emission in the submillimeter,for modeling molecular line profiles. In addition, we have used molecular lineprofiles and maps observed in several different molecules toward the threecores. We find a considerable diversity in chemical state among the threecores. The molecules include those sensitive to different timescales ofchemical evolution such as CCS, the isotopes of CO and HCO+, DCO+, and N2H+.The CO molecule is significantly depleted in L1512 and L1544, but not inL1689B. CCS may be in the second enhancement of its abundance in L1512 andL1544 because of the significant depletion of CO molecules. N2H+ might alreadystart to be depleted in L1512, but it traces very well the distribution of dustemission in L1544. On the other hand, L1689B may be so young that N2H+ has notreached its maximum yet. The ionization fraction has been calculated usingH13CO+ and DCO+. This study suggests that chemical evolution depends on theabsolute timescale during which a core stays in a given environment as well asits density structure.Comment: 33 pages, 12 figures, accepted to Ap