Submillimeter Studies of Low-Mass Star Forming Regions

Abstract

The nearby molecular clouds Chamaeleon I and III (Cha I, Cha III) constitute excellent targets for low-mass star formation studies. Their large starless core population presents the opportunity to explore the earliest phases of star formation, prior to the formation of the protostellar object. The prestellar phase can offer valuable constraints for the initial conditions necessary for star formation to occur. The apparent similarity between the stellar initial mass function and the core mass distribution suggests that the prestellar core fragmentation plays a determining role in the subsequent evolution of the self-gravitating dense cores to stars. A deep understanding of the physical processes taking place during the prestellar phase and the dynamical evolution of these objects is therefore essential in order to constrain the multiple core collapse models and obtain a complete picture of star formation.<br /> The first part of the thesis focusses on an object in the Cha I molecular cloud, Cha- MMS1. Cha-MMS1 is very likely to be in the theoretically predicted intermediate evolutionary phase between the prestellar and protostellar phases, the first hydrostatic core. The dynamical state of this object is examined through molecular line observations obtained with the APEX and Mopra telescopes. The molecular emission is modelled using a radiative transfer code in order to derive constraints on the kinematics of the envelope, which are then compared to MHD simulations for the first core phase. Both the derived internal luminosity of Cha-MMS1 and the constrained infall velocity structure of the envelope are consistent with predictions of MHD simulations for the first core phase. Excess emission in high-density tracers additionally suggests the possible presence of a compact, slow outflow driven by Cha-MMS1, which is the main predicted observational signature of first cores. Overall, Cha-MMS1 does not belong to the prestellar phase. The kinematics of its envelope are consistent with a first hydrostatic core candidate, but it cannot be ruled out that this object might also be a Class 0 protostar. A future detection of a slow, compact outflow with ALMA would serve as definite proof that Cha-MMS1 is indeed a first hydrostatic core. <br /> The second part of the thesis presents a molecular line survey that was conducted with the APEX and Mopra telescopes toward the starless core populations of the Cha I and III molecular clouds. Cha I is an actively star-forming cloud, whereas Cha III shows no sign of ongoing star formation. The main goal of this work is to determine the driving factors that have led to the strikingly different star formation activities in Cha I and III and to deduce the future dynamical evolution of the clouds. The kinematics of the starless cores are examined through a virial analysis and a search for infall motions. The chemical differences between Cha I and III are investigated through the observed fractional molecular abundances of the cores and by a comparison to predictions of chemical models. It is observationally derived that 15–30 % of the Cha I cores and 10–25 % of the Cha III cores will likely become prestellar and therefore form stars in the future. Interactions between the starless cores will not be dynamically significant in either cloud, thus eliminating competitive accretion as a likely process in these clouds. The analysis of the kinematics in the cores shows that turbulence has likely not affected the different star formation activities in Cha I and III. Nevertheless, a difference in chemistry between Cha I and III is seen in the fractional abundances of C¹⁸O and CH₃OH. The HC₃N to N₂H⁺ abundance ratio is then examined as an evolutionary indicator in the prestellar phase through comparison to predictions of collapse and static chemical models. In the framework of these models, this abundance ratio is proven to be a good evolutionary tracer. An evolutionary “gradient” was thus seen within Cha I, with the cores in its southern part being younger than the cores in the central region of Cha I. The suggested interpretation is that the southern Cha I cores will undertake the same evolutionary path as the central Cha I cores, given that they belong to the same cloud. Interestingly, the Cha III cores have similar HC₃N to N₂H⁺ abundance ratios as the southern Cha I cores. Therefore, both the measured HC₃N to N₂H⁺ abundance ratio and the detected infall signatures indicate that Cha III is younger than Cha I, and therefore on the verge of forming stars. This conclusion points to the existence of an evolutionary sequence in the Chamaeleon complex, with the youngest Cha III cloud to the eldest Cha I cloud, and with Cha II likely at an intermediate evolutionary state. The dynamical state of Cha II is therefore worth investigating in the future through both an unbiased, continuum survey as well as molecular line observations.