PhD defense by Elizabeth Artur de la Villarmois

The physics and chemistry of envelopes and disks around young protostars

Studies of low-mass protostars in different evolutionary stages provide critical insights into the origin of stars and planets and the formation of our own Solar System. Recent evidence suggests that the first steps toward planet formation take place in circumstellar disks while they are still young, yet very little is known about the physical and chemical structures of these disks during these early stages. Observational studies of the formation and evolution of disks are challenging since they are embedded in their parental cloud core and have small sizes. However, (sub)millimeter wavelength interferometers such as the Atacama Large Millimeter/submillimeter Array (ALMA) and Submillimeter Array (SMA) provide unique opportunities to observe these disks at long wavelengths with high sensitivity and angular resolution, making it possible to constrain the physical and chemical processes that are at the play in the earliest stages of the protoplanetary disks.

The goal of this thesis is to perform a systematic study of the physics and chemistry at play at small scales towards a sample of low-mass “Class I” protostars with ALMA, and follow those up by establishing the full chemical inventory of individual sources using the SMA. By studying the dust emission and gas kinematics on small scales, we find a linear correlation between the bolometric luminosities and the mass accretion rates of the protostars, with an average mass accretion rate of 2.4 x 10^-7 Msun/year. This value is lower than the expected if the accretion is constant in time and supports a scenario where accretion occurs in bursts. The molecular line signatures also vary significantly and demonstrate the important interplay between the physics and chemistry in these pivotal stages. For example, a number of sources show compact emission from warm SO2 lines while no lines of the simplest “complex” organic CH3OH are seen. The SO2 emission is consistent with the presence of accretion shocks produced at the interface between the inner envelope and the disk surface. The absence of CH3OH, in contrast, suggests that the presence of the disks cause the desorption of complex-organic molecules to be inefficient. 

These and other results demonstrate that Class I sources provide a physical and chemical link between deeply embedded Class 0 sources that are often rich in organics and the more evolved Class II sources where the envelope is completely dissipated. 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. These results allow us to make the first statements about how the chemistry may be preserved from the early Class 0 stages to the protoplanetary disks.