PhD Thesis defense by Rajeeb Sharma
Title: Unveiling the Early Stages of Star and Planet Formation: Observational Insights From Studying Class 0 and I Protostars
Abstract: It is now well established that the formation of low-mass stars, like our Sun, begins with the gravitational collapse of cold, dense cores within large interstellar clouds of cold dust and gas. This collapse initiates the birth of a protostar, which gradually grows in mass by accreting gas and dust from its surroundings. To achieve a comprehensive understanding of the star formation process, it is necessary to observe and study the various physical and chemical processes that govern the protostellar systems at each stage of their evolution. Especially in the earliest stages (the so-called Class 0 and I stages), this is challenging, as the young stars are so deeply enveloped in the surrounding dust that they can only be observed at infrared radio wavelengths.
Due to these challenges, previous observations have left many questions unanswered about the physical evolution of such young stars. With the introduction of modern instruments and interferometers like the Atacama Large Millimeter/Submillimeter Array (ALMA), it is now possible to observe the dust and gas around such young stars with high resolution and study, for example, their planet-forming disks or the material falling towards them. This provides a unique opportunity to finally illuminate the processes occurring around young stars in their earliest stages. The purpose of this thesis is to utilize these opportunities to observe and characterize the physical and chemical properties of young protostars and thereby contribute to our understanding of formation of stars and planets.
The foundation for this thesis is ALMA observations at high-resolution (~0.04" - 0.1"; ~6 - 15 au) of continuum and molecular lines towards 19 young protostars from nearby star-forming regions (d < 200 pc). These observations were carried out as part of a large ALMA program, “Early Planet Formation in Embedded Disks (eDisk)”, which primarily aims to investigate whether rotating disks around very young protostars have substructures similar to those seen in more evolved systems.
Chapter 4 presents a comprehensive analysis of the Class 0 protostar R CrA IRS5N from the eDisk program. The continuum emission from the dust shows a smooth, flattened, disk-like structure without obvious substructures. An analysis of the velocity field in the disk reveals the presence of Keplerian rotation around a central star with a mass of ~0.3 M☉. In addition to IRS5N itself, the observations also show emission from the nearby binary system IRS5a/b. This binary system is considerably smaller than IRS5N and the line emission from it shows that material from IRS5b streams into a disk-like structure around IRS5a.
Chapter 5 presents a systematic investigation of the spatial distribution of the various molecules observed towards all 19 protostars from the eDisk program. While some molecular lines clearly show the presence of extended emission from winds and jets, others are systematically concentrated towards the inner regions around the protostars and show clear signs of rotation. Some molecules are only seen towards the younger protostars which are typically more deeply embedded in dust and gas. Finally, there are a number of molecules that show very large variation from source to source. This suggests that multiple formation and excitation mechanisms are active in these protostars, which further emphasizes the importance of our understanding of the chemistry during the star formation process so that molecules can be used to study different parts of it.
In Chapter 6, I present an analysis of infrared measurements from the Stratospheric Observatory for Infrared Astronomy (SOFIA) of highly excited CO lines for the protostars SVS13-A and SVS13-C as well as the Herbig-Haro object HH-7 in the NGC 1333 cluster. These CO lines are sensitive to the warm part of the gas in these sources, which provides insight into the mechanisms which contributes to the overall cooling budget in these systems.
Supervisor: Jes Kristian Jørgensen, NBI
Evaluation committee members: Audrey Coutens, Institut de Recherche en Astrophysique et Planetologie, University of Toulouse; Michiel Hogerheijde, Leiden Observatory, Leiden University; Troels Haugbølle, NBI