2015A&A...575A..27T

The Chamaeleon dark molecular clouds are excellent nearby targets for low-mass star formation studies. Even though they belong to the same cloud complex, Cha I and II are actively forming stars while Cha III shows no sign of ongoing star formation. We aim to determine the driving factors that have led to the very different levels of star formation activity in Cha I and III and examine the dynamical state and possible evolution of the starless cores within them. Observations were performed in various molecular transitions with the APEX and Mopra telescopes. We examine the kinematics of the starless cores in the clouds through a virial analysis, a search for contraction motions, and velocity gradients. The chemical differences in the two clouds are explored through their fractional molecular abundances, derived from a non-LTE analysis, and comparison to predictions of chemical models. Five cores are gravitationally bound in Cha I and one in Cha III. The so-called infall signature indicating contraction motions is seen toward 8-17 cores in Cha I and 2-5 cores in Cha III, which leads to a range of 13-28% of the cores in Cha I and 10-25% of the cores in Cha III that are contracting and may become prestellar. There is no significant difference in the turbulence level in the two clouds. Future dynamical interactions between the cores will not be dynamically significant in either Cha I or III, but the subregion Cha I North may experience collisions between cores within ∼0.7Myr. Turbulence dissipation in the cores of both clouds is seen in the high-density tracers N₂H⁺ 1-0 and HC₃N 10-9 which have lower non-thermal velocity dispersions compared to C¹⁷O 2-1, C¹⁸O 2-1, and C³⁴S 2-1. Evidence of depletion in the Cha I core interiors is seen in the abundance distributions of the latter three molecules. The median fractional abundance of C¹⁸O is lower in Cha III than Cha I by a factor of ∼2. The median abundances of most molecules (except methanol) in the Cha III cores lie at the lower end of the values found in the Cha I cores. A difference in chemistry is thus seen. Chemical models suitable for the Cha I and III cores are used to constrain the effectiveness of the HC₃N to N₂H⁺ abundance ratio as an evolutionary indicator. Both contraction and static chemical models indicate that this ratio is a good evolutionary indicator in the prestellar phase for both gravitationally bound and unbound cores. In the framework of these models, we find that the cores in Cha III and the southern part of Cha I are in a similar evolutionary stage and are less chemically evolved than the central region of Cha I. The measured HC₃N/N₂H⁺ abundance ratio and the evidence for contraction motions seen towards the Cha III starless cores suggest that Cha III is younger than Cha I Centre and that some of its cores may form stars in the future if contraction does not cease. The cores in Cha I South may on the other hand be transient structures.