Abstract

Planetary nebulae constitute the near-end stages of low-to-intermediate-mass stars, when their ejected envelopes of gas and dust become ionized by their unveiled stellar cores. As essential components of our understanding of late-stage stellar evolution and the enrichment of the interstellar medium in the products of stellar nucleosynthesis, planetary nebulae serve as laboratories for the study of plasma physics and shock processes in astrophysical environments. While known mainly for their optical emission lines and high ionization states due to high-energy radiation from their hot central stars, certain planetary nebulae also contain large masses of molecular gas and dust that surround or even lie embedded within their ionized interiors. The resulting regions of photoionization and photodissociation thus represent geometrically straightforward analogues for highly complex or poorly resolved systems, such as protoplanetary disks, cold cloud cores, and active galactic nuclei. The abundance of emission features in the radio-wavelength spectra of planetary nebulae enables state-of-the-art single-dish radio telescopes and interferometers to obtain high-quality measurements of the molecular chemistry present in the ejected gas. Ultimately, the stellar ejecta that constitute planetary nebulae are incorporated into the next generation of stars and planetary systems such that, by studying this material, we strive to understand the origins of our solar system and even life itself.

Through radio molecular line observations carried out with the 30 m telescope and interferometer operated by the Institut de Radioastronomie Millimétrique (IRAM), as well as the Atacama Large Millimeter Array (ALMA), we seek to explore and identify the role that UV and X-ray irradiation plays in driving the molecular chemistry within the envelopes of planetary nebulae. In our 30 m survey of nine nearby planetary nebulae, we have made new detections of molecular species in four objects, and established an anticorrelation between the HNC/HCN line intensity ratio and central star UV luminosity that suggests this high-energy radiation continues to drive the chemistry within PNe as they age. Our detailed studies of the nearby, bright NGC 7293 and NGC 7027 probe deeper into the relationships between central star high-energy radiation fields and molecular chemistry. Radiative transfer codes have been applied to model the dense globule structures in NGC 7293, and comparisons of these models to our IRAM 30 m and APEX 12 m observations suggest that, in addition to gradients in UV irradiation, gradients in gas pressure and density may be required to explain the observed variations in HNC/HCN. We further investigate the spatial gradients of this and other emission line diagnostics within individual parcels of molecular gas with arcsecond-resolution ALMA spectral line mapping of two prominent globules in NGC 7293. Finally, we present NOEMA interferometric maps of NGC 7027 in the molecular ions CO+, which is imaged for the first time in any planetary nebula, and HCO+. Via comparison with archival near-IR H2 imaging, we diagnose the chemical pathways, and in particular the dependence on UV and X-ray irradiation, that drives production of CO+ and HCO+ in NGC 7027.

Library of Congress Subject Headings

Planetary nebulae--Observations; Molecular clouds; Stars--Evolution; Radio telescopes

Publication Date

8-2020

Document Type

Dissertation

Student Type

Graduate

Degree Name

Astrophysical Sciences and Technology (Ph.D.)

Department, Program, or Center

School of Physics and Astronomy (COS)

Advisor

Anthony Vodacek

Advisor/Committee Member

Andrew Robinson

Advisor/Committee Member

Adam Frank

Campus

RIT – Main Campus

Plan Codes

ASTP-PHD

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