## Graduate Theses and Dissertations

Dissertation

2018

#### Degree Name

Doctor of Philosophy

#### Department

Physics and Astronomy

Astrophysics

#### First Advisor

Charles R. Kerton

#### Abstract

This work presents four studies on the environments where massive stars evolve and how these stars influence their surroundings as they age. The first two studies investigate properties associated with high-mass stars themselves and the signatures of their own formation. The theory of mass accretion via a circumstellar disk is a leading hypothesis for the formation of high-mass stars. We used infrared surveys to identify massive stars and spectroscopic observations of our sample to search for remnant accretion disks. We also study the Lyman excess phenomenon in ultra-compact \hydro regions to determine if these anomalies can be explained via astrophysical processes or if there is an inherent shortcoming of the modeling of high-mass stars in these compact environments. The final two studies observed how high-mass stars influence environments from a distance. We explore regions in which high-mass stars may be the progenitor of future generations of stars in both a low-mass, isolated cometary cloud and as an integral part an enormous molecular cloud. We used young stellar object populations to quantify how well these molecular clouds turn their material into stars by determining star formation efficiencies in these regions. By studying these stars at different timescales of their evolution, we can achieve a more complete picture of the evolutionary process of OB stars.

In our first study, we collected spectra of a selection of candidate OB stars believed to be the stars powering infrared bubble/\hydro regions. We obtained optical and infrared spectra using the \emph{Wyoming Infrared Observatory} and the \emph{Gemini Near-Infrared Spectrometer}, respectively. Chief among our findings was that the O8.5~V type star VES~735, shows Br$\gamma$ emission and double peaked H$\alpha$ emission that has been sustained for more than 15 years. This long-lived emission of H$\alpha$ suggests that this may come from the outer layers of a circumstellar disk, rather than being due to episodic mass-loss events, as seen in $\zeta$ Oph. \emph{A portion of this work has been submitted to the Research Notes of the American Astronomical Society.}

The second study is an investigation of a large number (67) of compact/ultra-compact H{\scriptsize II} regions recently identified in the CORNISH catalog. These regions were determined to be powered by a Lyman continuum flux in excess of what was expected given their corresponding luminosity. We attempted to reasonably explain away this Lyman excess phenomenon in as many of the 67 H{\scriptsize II} regions as possible through a variety of observational and astrophysical means. We calculated new luminosities, performed new \textit{Herschel} photometry, determined new distances, used different models for dust and ionized gas covering factors, and used different stellar calibrations. This phenomenon has been observed before; however, the objects shown to exhibit this behavior have decidedly different physical properties than the regions in our sample, and thus the origin of the excess is not the same. Though the exact mechanism producing the excess is still uncertain, we found that a scaled up magnetospheric accretion model, often used to explain similar emission from T Tauri stars, is unable to match our observations. We found that the excess can be reproduced using OB stellar atmosphere models that have been slightly modified in the extreme ultraviolet. Our results suggest that the Lyman excess may be associated with younger \hydro regions and that it is more commonly found in early B-type stars. Our refined sample of 24 Lyman excess \hydro regions provides an ideal sample for comparative studies with regular H{\scriptsize II} regions and can act as the basis for the further detailed study of individual regions. \emph{This work was accepted for publication in The Monthly Notices of the Royal Astronomical Society in May, 2018.}

The third study reports the first high-resolution \co and \coiso observations of the bright outer Galaxy \hydro region CTB~102 (KR 1), using the Taeduk Radio Astronomy Observatory (TRAO). CTB~102 is one of the largest H{\scriptsize II} regions/bubble structures in the Milky Way, but had not been previously observed in the two main high-resolution FCRAO CO surveys: the Outer Galaxy Survey (OGS) and the Galactic Ring Survey (GRS). With the first look at the molecular gas along this line of sight, we determined the sites of ongoing star formation associated with CTB~102 using \WISE and \emph{2MASS} archival data. Using the high-resolution CO observations, we also determined the molecular cloud mass using multiple techniques, concluding that it is between $\sim10^{4.8} - 10^{5.0}$ M\textsubscript{$\odot$}. Applying color-color cuts, SED fitting, and spectral index classification, we found that the YSO population seems to be grouped into separate pockets of star formation within the giant cloud. \WISE sensitivities prevent us from observing YSOs at masses $\lesssim 1-2$ M\textsubscript{$\odot$}, but, by adopting a Salpeter IMF, we estimated star cluster masses for the pockets of star formation as well as for the entire cloud. We found an interesting region within the molecular cloud that has an unusually high star formation efficiency, but the cloud as a whole is only slightly higher than an average GMC. \emph{This study has been submitted to the Astrophysical Journal.}

We conclude with one final study on how high-mass stars influence their environment by examining a cometary molecular/atomic cloud in Cepheus. This cloud is believed to have had its appearance sculpted by the presence of the Cep~OB3 association, located $\sim$70 pc from the head of the cloud. We were able to make mass estimates based on \co observations and determined that the mass of the head of the molecular cloud is between 200 -- 300 M$_{\odot}$. This molecular cloud is much closer than CTB~102, only 750 pc away, meaning that \WISE is sensitive down to sub-solar YSOs in this case. We then used the properties of the entire sample of identified YSOs, as determined by SED fitting, to approximate the stellar cluster mass, and determined that the star formation efficiency is $\sim2.75-4$\%. We also explore the possibility that the star formation activity was triggered by an ionization/shock front from Cep~OB3, causing the cloud to implode. \emph{This study is in preparation for submission to The Monthly Notices of the Royal Astronomical Society.}

Brandon Marshall

en

application/pdf

205 pages

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