Elizabeth Artur de la Villarmois
A thesis for the degree of Doctor of Philosophy defended June 2019.
The PhD School of Science, Faculty of Science, Physics of Ice Climate and Earth, Niels Bohr Institute, University of Copenhagen
Jes K. Jørgensen
Lars E. Kristensen
The physics and chemistry of envelopes and disks around young protostars
Important clues about the formation of our own Solar System are revealed by studying other young sources that are currently forming. The study of low-mass protostars in dierent evolutionary stages is, therefore, essential to link their physical and chemical evolution to what we know about the Solar System. In particular, low-mass protostars are associated with disks, where planets form. Very little is known about the initial physical and chemical conditions for planet formation and the relationship between the physical and chemical structures of embedded disks. Therefore, the study of the physics and chemistry at small scales may provide important clues about how the material falls from the envelope to the disk and accretes from the disk into the protostar and what is the physico-chemical link between deeply embedded stages and the onset of planet formation.
The goal of this thesis is to study the physics and chemistry at play at small scales (disk scales) towards low-mass protostars in order to constrain the mass flow and identify the physical and chemical processes that dominate at these scales. The study of the formation and evolution of disks is challenging since they are embedded in the parental cloud and are relatively small in size (.700 AU), requiring observations with high sensitivity and angular resolution. For this, we observe a sample of Class I protostars using the Atacama Large Millimeter/submillimeter Array (ALMA) and the Submillimeter Array (SMA) to characterise their physical and chemical structures. Class I sources have been chosen since disks are expected to have been formed at this stage and because they serve as a link between the deeply embedded Class 0 sources and the emergence of protoplanetary disks.
The results from these studies are presented in three papers (published in or submitted to international journals). We find an empirical linear correlation between the bolometric luminosity and the mass accretion rate, suggesting that more massive protostars accrete material with a higher accretion rate. The mean mass accretion estimated for a sample of 13 Class I sources is 2.4 107 M yr1. This value is lower than the expected if the accretion is constant in time and rather points to a scenario of accretion occurring in bursts. In addition, this low mass accretion rate provides observational evidence that a typical protostar will spend most of its lifetime in a quiescent state of accretion.
The formation and evolution of the disk is reflected on the chemical structure of the envelope, from large to small scales. The disk shields material beyond its extent where cold temperature tracers are detected, such as DCO+. Furthermore, the non detection of CH3OH suggests that material from the inner envelope follows the flattened structure of the disk and, since less material is exposed to high temperatures, desorption of complex-organic molecules is not ecient. In addition, compact emission and large line widths of warm SO2 emission are consistent with the presence of accretion shocks produced at the interface between the inner envelope and the disk surface.
Class I sources show a physical and chemical link between deeply embedded Class 0 sources and more evolved Class II sources. The gas column density decreases as the system evolves, which is reflected on the emission of high density tracers such as CO isotopologues. The formation and evolution of the disk, together with the increase of the outflow-opening angle as the system evolves, allow the UV radiation from the central protostar to reach the surface layers of the disk, promoting the photodissociation of molecules and enhancing the abundance of others, for example, CN. On the other hand, the chemistry of Class 0 sources can be preserved, to some extent, in Class II sources mainly towards the disk midplane and beyond its extension, where the shielding is ecient.