Mass Assembly in Protoplanetary Disks: Polarimetry as a Probe of Dust Growth and Accretion

Harrison, Rachel Elizabeth

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https://hdl.handle.net/2142/117782 Copy

Description

Title

Mass Assembly in Protoplanetary Disks: Polarimetry as a Probe of Dust Growth and Accretion

Author(s)

Harrison, Rachel Elizabeth

Issue Date

2022-11-30

Director of Research (if dissertation) or Advisor (if thesis)

Looney, Leslie

Doctoral Committee Chair(s)

Looney, Leslie

Committee Member(s)

• Li, Zhi-Yun
• Gammie, Charles
• Fields, Brian

Department of Study

Astronomy

Discipline

Astronomy

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• protoplanetary disks
• star formation
• planet formation
• polarization
• interferometry

Abstract

Circumstellar disks are the formation sites of planets, and their physical and chemical characteristics affect the masses and compositions of planets, as well as planet formation timescales and protostellar accretion rates. Millimeter and submillimeter interferometry with instruments like the Atacama Large Millimeter/submillimeter Array (ALMA) gives us a detailed look at the gas and dust in disks, particularly dust near the midplane of the disk where planet formation takes place. There is now evidence that planet formation is well under way by the Class II phase of disk evolution, when the envelope has dissipated but a significant amount of gas and dust remains in the disk. In fact, there is now evidence that planet formation starts even earlier than Class II (Segura-Cox et al., 2020). Open questions remaining about propoplanetary disk evolution include the sizes of the dust grains in disks and the exact role of the magnetic field in accretion. Through observational studies of protoplanetary disks, this dissertation examines the roles of dust grain growth and magnetically-driven protostellar accretion in disk evolution. We first present a study of the polarized emission from four Class II protoplanetary disks at 3 mm. We find that otherwise similar disks can exhibit different polarization morphologies at the same wavelength, which provides evidence that multiple processes can produce polarized emission in protoplanetary disks at millimeter wavelengths. We then present a survey of disk polarization at 870 μm and 3 mm, which includes the sources observed in the previous chapter. We find that disks can exhibit different transitions between polarization morphologies at different wavelengths. We explore possible reasons for these differences, including differences in the disks’ dust populations and radiation environments. For two of the disks in the survey, we present models of the polarization that would be produced via self-scattering for several dust grain populations. Our modeling results indicate that optical depth effects and vertical dust settling should be taken into account when using polarization observations to constrain dust grain sizes. Then, we show the results of Zeeman splitting observations of nine hyperfine lines in the CN (N=2-1) in the Class II protoplanetary disk AS 209. These observations aimed to measure or set upper limits on the magnetic field strengths in these disks. Magnetic fields are thought to be crucial to the protostellar accretion process, and my constraining magnetic field strengths, we can constrain the mass accretion rate and the mechanisms driving accretion. We derive 3σ upper limits on the poloidal and vertical magnetic field strengths of ∼5-9 mG. If accretion in the disk is driven primarily by magnetic disk winds, then magnetic fields with strengths near these upper limits could drive the accretion rate derived from previous observations of AS 209. We also present the results of Zeeman splitting observations of several hyperfine lines in the CN (N=2-1) and (N=1-0) transitions in the Class II circumbinary disk V4046 Sgr. We find that the CN (2-1) observations provide stricter constraints on the magnetic field strength in the disk than the CN (1-0) observations (3σ upper limits on the line-of-sight magnetic field of ∼2 mG vs. ∼20 mG). From the upper limits on the field strengths provided by the (2-1) observations, we estimate that these magnetic field strengths could drive the accretion rate previously observed in V4046 Sgr if accretion is driven by the magnetorotational instability or magnetic disk winds. Finally, we detail improvements to the error calculation process in the SOFIA (Stratospheric Observatory for Infrared Astronomy) HAWC+ (High-resolution Airborne Wide-band Camera Plus) data reduction pipeline. We present polarimetric observations of the protostars HL Tau and L1527 to demonstrate the importance of these improvements when imaging and analyzing low signal-to-noise polarization data.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Rachel Harrison

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