Abstract

This project studies the earliest stages of star formation in different environments by observing deuterated molecules and by measuring the deuterium fraction (i.e. the ratio between the column density of the species containing deuterium, in particular N2D+ and DCO+, and the column density of the same species containing hydrogen, N2H+ and H13CO+). The deuterium fraction is known to increase in pre-stellar cores, just before the formation of a protostar, and deuterated molecules are used to trace the kinematics and properties of pre-stellar cores. Deuterated molecules are important diagnostic tools for understanding the physical/chemical structure and kinematics of dense and cold gas in molecular clouds, i.e. to study the first steps in the process of star formation. In the PhD project, I focus my studies on two nearby low-mass star-forming regions: the Ophiuchus molecular cloud, the nearest cluster forming region, and the more quiescent Taurus molecular cloud. The project is based on three papers in which dense core chemistry and kinematics, as well as the substructure around pre-stellar cores, are discussed. In the first chapter, I study the deuterium fractionation in the dense cores of the L1688 clump in the Ophiuchus molecular cloud, one of the closest (∼120 pc) sites of star formation. A high deuterium fraction is one of the key features of pre-stellar cores, the dense cores on the verge of star formation. I study how the deuterium fraction depends on various physical conditions, such as gas density, temperature, turbulence, and depletion of CO. I show that regions of the same molecular cloud experience different dynamical, thermal, and chemical histories with consequences for the current star formation effciency and characteristics of future stellar systems. In the second chapter, I study the kinematics of dense cores in the L1495 filament in the Taurus molecular cloud. Interstellar filaments are common structures in molecular clouds and play an important role in the star-forming process. I use the N2H+(1-0) and N2D+(2-1) lines to trace the gas of the core centres where CO and other molecules are depleted, the H13CO+(1-0) and DCO+(2-1) lines to trace the core envelopes to search for any connections between core-scale and cloud-scale kinematics, and C18O(1-0) to reveal the kinematics of the filament gas. Unlike the L1688 cores in Ophiuchus, the L1495 cores show very similar properties - subsonic line widths, centroid velocities, velocity gradients, and specific angular momenta. I found that at the level of the cloud-core transition, the core's envelope is spinning up. At small scales the core material is slowing down implying a loss of specific angular momentum. The cloud material stays unaffected by the presence of rotating cores and protostars. In the third chapter, I study the substructure around a prototypical pre-stellar core, L1544, which is one of the isolated cores in the Taurus molecular cloud. The interferometric observations used reveal the structures of ∼700 au scale. The core shows a strong asymmetry in the distribution of methanol around the core. This asymmetry might be produced by asymmetric UV irradiation. The project gives a prospective to future work on the evolution of dense cores. As a part of the project, a number of observational proposals were submitted to single dish sub-mm telescopes and interferometers, for many of them the data already have been collected. These projects will continue the study of the evolution of dense cores, including their chemistry and kinematics. I will study the chemistry of the dense cores in L1495 (Taurus), L1688 (Ophiuchus), and B5 (Perseus), as well as the kinematics in L1688 and B5. I will compare the deuterium fractions of ions (N2D+/N2H+, DCO+/H13CO+) and neutrals (NH2D/NH3), as well as that of core centres (N2D+/N2H+) and envelopes (DCO+/H13CO+). I will study the small-scale structure of the selected cores with interferometric observations of high density tracers (NOEMA observations of para-NH2D towards the B213-10 core in Taurus). These observational data will be used in tandem with chemical models to unveil the chemical evolution of dense cores on the verge of star formation. Comparing the results from the different sets of dense cores embedded in Taurus, Perseus, and Ophiuchus, will allow us to quantify the environmental effects on the dynamical and chemical evolution of dense cores and the related star formation rate.

Abstract

Abstract