Date of Award

8-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Committee Member

Sean D Brittain, Committee Chair

Committee Member

Máté Ádámkovics

Committee Member

Dieter Hartmann

Committee Member

Joan Marler

Abstract

One of the most open-ended philosophical questions a person can ask is ``Where did we come from?" This question can be interpreted and approached from a multitude of angles. The work presented here will be investigating this question from the astronomical/physical perspective by studying the astronomical objects that are thought to be the immediate precursors to solar systems like our own. In order to determine how we came to be, we must first learn how our home was created.

My research area focuses on the study of the circumstellar disks around intermediate mass (2 - 8 Mʘ) young stellar objects, known as Herbig Ae/Be stars. These stars have been observed to harbor large disks of gas and dust left over from their parent molecular cloud. It is within these disks where planet formation is believed to be on-going. Our current models of planet formation are unable to fully depict the process of creating planets from the dusty, gaseous disk. Therefore, in order to understand how these disks create the planetary systems we now know to be ubiquitous, we must first improve our understanding of the composition of and physical processes occurring within the circumstellar disk.

Observing the molecular emission originating in the inner 10 au of these circumstellar disks allows us to understand the composition and physical environment where terrestrial planet formation is believed to occur. Molecular emission can provide a wealth of information on the inner disk. Observing multiple rovibrational transitions can give insight on the temperature of the gas. The emission line profile can also provide information on the location of the gas within the disk. The primary molecular species discussed in this research are OH, H2O, and CO. These are the three most abundant molecules present in the circumstellar disk behind H2, but all are more easily studied in the near-infrared wavelength regime.

In this dissertation, I present my findings on HD~101412. This source presented the first direct detection of water vapor emission in the near-infrared. Previous observations of Herbig Ae/Be stars have yielded detections of OH, yet water has been more elusive. Due to the strong far-ultraviolet radiation fields produced by the A and B-type central star, it is believed that most of the water vapor within the disk is photodissociated. This would increase the amount of OH within the upper layers of the disk atmosphere that are irradiated most directly. HD~101412 is also unique in that we observe it nearly edge-on and the gas-to-dust ratio is high compared to other Herbig Ae/Be sources. These factors combine to allow us to observe a larger column of water vapor in the disk which allows the emission to be observable above the continuum.

I also observed OH emission in the V380~Ori multiple star system to determine the variability of asymmetrical emission lines previously reported. The observation of multiple OH ν = 1 → 0 rovibrational transitions allow for the gas temperature to be determined. This, coupled with archival CO data, present information that point to a vertical temperature gradient within the V380~Ori circumstellar disk. Both the OH and CO emission line profiles show CO and OH to be co-spatially located, however, the OH gas appears to be hotter, thus higher in the disk atmosphere, than the CO emission. This is consistent with models that show OH should be more abundant in the upper atmosphere due to the irradiation from the far-ultraviolet radiation from the central star. Our observations are unable to reproduce the asymmetry previously reported in the OH emission. High-resolution CO emission also lacks the observed asymmetry. Due to the timescales on which these data were acquired, it can rule out the previously reported asymmetry being caused by an eccentric inner disk due to the presence of a stellar companion located within the molecular emission.

A survey on OH and H2O emission in Herbig Ae/Be disks, expanding on previous studies, is also presented here. Two more detections of water vapor emission are presented, along with five more detections of OH P4.5 (1+,1−). This increases the sample of OH detections in Herbig Ae/Be disks to 15 out of 31 (48.4\%) of sources with observations in the near-infrared. The relative strength of the OH and water emission is compared to different stellar and disk parameters of Herbig Ae/Be and T Tauri sources. This provides insight as to what could be the cause of the lack of water vapor emission observed around Herbig Ae/Be stars.

Finally, a survey of hydrogen recombination emission lines is presented in an effort to understand the physical origins within the Herbig Ae/Be system. Integral field spectroscopy is combined with spectro-astrometry to study the disk-star interface of Herbig Ae/Be stars. Two \ion{H}{I} emission lines are studied (Paβ and Brγ) to determine if they are produced directly by accretion onto the star, or if they originate from more extended regions of the system. Initially attempts are made to remove artifacts in the spectro-astrometric analysis via the use of models. Ultimately, it is concluded that these artifacts are not removed entirely. Later data is acquired such that artifacts can be removed organically via the combination of parallel and anti-parallel observing position angles. The emission lines observed in each observing run can provide some insights based on the overall line profile. Only the spectro-astrometric information from data acquired with parallel and anti-parallel observations can be relied upon to provide information on milliarcsecond scales.

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