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Title:Mapping the interstellar magnetic field: Observations of the Goldreich-Kylafis effect towards late-type evolved stars and star-forming regions
Author(s):Huang, Ko-Yun
Director of Research:Kemball, Athol J
Doctoral Committee Chair(s):Kemball, Athol J
Doctoral Committee Member(s):Looney, Leslie W; Wong, Tony; Ricker, Paul M.
Department / Program:Astronomy
Discipline:Astronomy
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):Stellar magnetic fields
polarimetry
asymptotic giant branch stars
star formation
Abstract:Mapping magnetic fields is the key to resolving the yet unclear physical picture of circumstellar magnetic fields in both late-type evolved stars and star-forming regions. Observations of linearly polarized molecular line transitions arising from the Goldreich-Kylafis (G-K) effect provide valuable insight into the magnetic field geometry in these sources. Through observing with CARMA, the VLA, and ALMA, this dissertation addresses the questions concerning the dynamical role of magnetic fields in the environment of both the young and the late-type evolved stars. In the circumstellar envelope (CSE) of AGB stars, the uncertainties concerning both the morphology and magnitude of circumstellar magnetic fields and the relative dynamical influence of magnetic fields in shaping AGB mass-loss outflows relative to other proposed mechanisms such as wind interaction models and binarity remain important open astrophysical questions. We examined the magnetic field morphology in the CSE of two Thermal Pulsating (TP-) AGB stars, R Crt and R Leo, revealed by the detected linear polarization of both thermal and maser lines with CARMA and VLA observations to understand the field morphology on multiple scales in the CSE. We detected both thermal (CO J=2-1) and maser (SiO v=1, J=5-4 and SiO v=0,1,2; J=1-0) line polarizations in these two TP-AGB stars. The observed fractional linear polarization due to the G-K effect in the CO emission is measured as $m_{\ell,p}$ ∼ 3% and $m_{\ell,p}$ ∼ 9% for R Crt and R Leo, respectively. We utilize a model of the CSE to estimate both circumstellar envelope (CSE) temperature and density profile to yield an estimated linear polarization from G-K modeling (Yang and Lai, 2010) and compare it with the detected CO J = 2 − 1 linear polarization signal. The observed thermal line polarization level is consistent with the predicted results from the model in the case of R Crt, and the missing flux density due to spatial filtering by the interferometer may explain the higher fractional linear polarization in R Leo if the polarized emission originates from a region of smaller spatial extent than the Stokes I emission. The compiled comparison between the inferred magnetic field orientation from our G-K mapping using the polarized J = 2 − 1 CO transition and other intrinsic alignments published for these sources in the literature suggested that there is a more confined magnetic field geometry in R Crt. In star-forming regions, both the magnetic field and turbulence are considered the main agents that support the cloud from collapsing against self gravity (Krumholz and Tan, 2007). We studied the massive star-forming region G10.6-0.4 in ALMA Band 3 polarization observation. G10.6-0.4 is one of the most luminous HII regions with embedded OB stars and thus is a good template for investigating massive star formation processes. In this dissertation, the observed dust continuum polarization mapping and G-K polarization mapping of the CO J = 1−0 transition are presented. The detected dust continuum polarization is at $m_{\ell,p}$ ∼ 1.52% with a fairly concentrated spatial structure and with an error-weighted PA ∼ 7.1◦. The peak fractional linear polarization measured in the J = 1 − 0 CO line ($m_{\ell,p}$ ∼ 2.28%) is broadly consistent with the predicted signal strength from detailed G-K modeling. We also found a detailed spatial structure of emission and absorption features across this massive star-forming region, where the absorption region tends to concentrate at the phase center, with the emission originating prominently in the outer region. We discuss future avenues of investigation arising out of the work in this dissertation.
Issue Date:2020-09-08
Type:Thesis
URI:http://hdl.handle.net/2142/109560
Rights Information:Copyright 2020 Ko-Yun Huang
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12


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