15 March 2018

 

Steffen Kjær Jacobsen

A thesis for the degree of Doctor of Philosophy defended July, 2018.

The PhD School of Science, Faculty of Science, Centre for Star and Planet Formation,Astrophysics and Planetary Science, Niels Bohr Institute, University of Copenhagen

Supervisor:
Jes Kristian Jørgensen

3D Radiative Transfer Modeling of Embedded Protostellar Regions

The formation of stars and planetary systems is a physical as well as a chemical process. Through decades of research, our understanding of star formation has greatly improved, though many aspects are still unknown or heavily debated. Stable, rotationally supported disks of gas and dust around protostars are routinely observed in the intermediate and later stages of star formation. However, the exact details of how early and how they form, are still open questions. Concurrently with these issues, Complex Organic Molecules (COMs), and even prebiotic molecules such as simple sugars, have been observed in the gas phase of early, protostellar cores. Their exact formation route and the physical origin of their arrival to the gas phase are heavily debated. It is not known if the observed COMs are linked to the warm disk atmosphere of settled protoplanetary disks, or if they predominantly exist in the gas phase in the warmest, inner regions of protostellar cores, before the emergence of a protoplanetary disk. These questions are important, as the protostellar chemistry acts as a chemical precursor for the composition of the material that ends up in planet formation, and may explain the rich chemistry we see in our own Solar System. Using high angular resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA), together with complex 3D radiative transfer codes, we can investigate the innermost environment of young protostars, and hopefully answer some of the questions stated above.

In this thesis I present research focusing on the two low-mass protostellar cores, IRAS 16293-2422 and L483, in terms of their dust and gas density, temperature structures, and chemistries. Two of the three research papers presented in this thesis concentrate on IRAS 16293-2422, with one focusing on the 3D modeling of the envelope, the bridge of dust and gas conjoining the two protostars, and the disk-like emission around each protostar, as well as their individual luminosities. A 3D dust density model is presented, which morphologically matches the 868 μm continuum emission and explains the observed C17O emission through a jump abundance model. This model emulates freeze-out of molecules upon the dust grains, when the temperature drops beneath the sublimation temperature of CO on the dust ice-mantles. The individual luminosities of the deeply embedded protostars in IRAS 16293-2422 are found to be LA > 18 L_sun and LB  < 3 L_sun, for radiation source A and B, respectively, which presents the first estimation of the individual luminosities.

The second research paper on IRAS 16293-2422 focuses on the outflows and the kinematics of the observed gas line emission from the different molecules in the gas phase, and their relation to the bridge of dust and gas. Molecular gas line emission of CO, H2CO, HCN, CS, SiO and C2H reveal that only the dust continuum and C17O emission have a physical origin in the bridge of dust and gas, while all other molecular transitions are found to be related to the outflows emanating from radiation source A. The lack of outflow activity from radiation source B leads us to conclude that it is likely on a lower evolutionary stage than radiation source A.

The last research paper describes ~ 0.1" observations of L483 with ALMA, which reveal that the COMs observed towards L483 reside in the innermost hot region of the envelope, within 40–60 au of the central protostar, and arise from thermal sublimation of the icy mantle around the dust grains. By analyzing the kinematics of the H13CN J = 4–3 and CS J =7–6 gas line emission, the presence of a Keplerian disk is excluded down to at least a 15 au radius. This means that the observed COMs cannot come from an abrupt transition region between the collapsing envelope and a Keplerian disk, as hypothesized by an earlier research team, or from a warm disk atmosphere. Within 15 au, a small Kepler disk could hypothetically reside.

Suggestions are made for future research projects targeting IRAS 16293-2422 and L483, to further constrain the spatial distribution of COMs (which constrains their formation routes), and the timeline for the emergence of protoplanetary disks.

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