Doctor of Philosophy, University of Toledo, 2022, Physics

We present a study of protostellar envelope and outflow evolution using HST images of 304 protostars in the Orion Molecular clouds. We present near-IR observations that resolve structures in the gaseous envelopes of forming protostars delineated by scattered light with 80 AU resolution. These observations complement the 1.2–870 micron spectral energy distributions obtained with the Herschel Orion Protostar Survey pro- gram (HOPS) and recent ALMA and ACA 870 micron continuum measurements. Based on their 1.6 micron morphologies, we classify the protostars into five categories: non-detections, point sources without nebulosity, bipolar cavity sources, unipolar cavity sources, and irregulars. We find point sources without associated nebulosity are the most numerous, and show through monochromatic Monte Carlo radiative transfer modeling that this morphology occurs when protostars are observed at low inclinations or have low envelope densities. We also find that the morphology is correlated with the SED-determined evolutionary class with Class 0 protostars more likely to be non-detections, Class I protostars to show cavities and flat-spectrum protostars to be point sources. We use an edge detection algorithm to trace the projected edges of the cavities, allowing us to measure shapes and the volume of their cavities. Previous work has explained the dissipation of envelopes by the progressive growth of outflow cavities. We find no evidence for the growth of outflow cavities as protostars evolve through the Class I protostar phase, in contradiction with previous studies of smaller samples. We conclude that the decline of mass infall with time cannot be explained by the progressive clearing of envelopes by growing outflow cavities. A persistent question in star formation is the issue of low efficiency of star forming cores, where only 30% to 40% of the gas is accreted onto the star. We find that the low star formation efficiency inferred for molecular cores cannot be explain (open full item for complete abstract)

Committee: S.Thomas Megeath (Advisor); John David Smith (Committee Member); Michael Cushing (Committee Member); Jacques Amar (Committee Member); Héctor Arce (Committee Member) Subjects: Astronomy; Astrophysics; Physics

Doctor of Philosophy, University of Toledo, 2019, Physics

To study transitions of embedded clusters into open clusters, we present visible light spectroscopic observations of 190 young stars in the Cepheus OB3b young cluster. These observations have enabled us to constrain the fate of Cep OB3b, the nearest example of a large young cluster in the late stages of gas dispersal. The Gaia DR2 data has resolved previous ambiguities in the distance to Cep OB3b with a distance of 819~pc. The cluster contains two distinct sub-clusters; the proper motions show the two sub-clusters are moving away from one another. Based on the velocity dispersion measured with the visible spectra, each sub-cluster will form a bound cluster of $\sim$300 stars each. To study the transition point between protostars and pre-main sequence stars, the 66 near-IR high resolution spectroscopy of Orion HOPS protostars is paving the way to observationally constrain the temperatures, luminosities, and radii of a large sample of late stage protostars at a common distance without the need to use theoretical models. We identify temperature sensitive features in the HOPS spectra spectral typing each target with a suite of spectral typing standards, simultaneously, lines without temperature sensitivity will be used to measure veiling. In parallel effort outside of this dissertation, hydrogen lines are currently being fit to measure the accretion luminosity in each HOPS protostar. The hydrogen line fits will be used to correct the intrinsic luminosity from the total luminosity. The luminosities and effective temperatures from spectral types will be used to determine the radii of the protostars and can be combined with masses from model tracks, as well as direct measurements of masses using the Keplerian disk motions resolved in future ALMA data. The combination of radii and masses for dozens of flat spectrum protostars will independently provide the starting point of contraction without need for theoretical models that assume an initial mass and radius. To measure th (open full item for complete abstract)

Committee: Samuel Megeath (Committee Chair); Steven Federman (Committee Member); Anne Medling (Committee Member); Nikolas Podraza (Committee Member); Jonathan Tobin (Committee Member) Subjects: Astrophysics

Doctor of Philosophy, University of Toledo, 2017, Physics

A central question in the study of star formation is: what sets the mass distribution of nascent stars? This distribution appears to vary little (especially at the low mass end) throughout our galaxy and others, despite variations in the heavy element abundance, interstellar radiation field, and amount of gas available. This work is a component of the Herschel Orion Protostar Survey (HOPS), a program to characterize hundreds of protostars in the Orion Molecular Clouds, largest site of on-going star formation with 0.5 kiloparsec. As part of the HOPS program, we have spectral energy distributions with a wavelength coverage of 1.2-870 µm, making this one of the most well characterized samples of nearby star formation. We extend this work with a large survey imaging 283 protostars in Orion with the Hubble Space Telescope, enabling us to observe with 80 AU resolution at 1.6 µm. At this wavelength, light from the central photosphere is scattered off the dust ubiquitous throughout the gas, resolving the structure of the protostellar envelope. We use the high angular resolution to perform a morphological study of the nearby protostellar gas (the envelope), finding that our morphologies are indicative of evolutionary trends. The high resolution of this data makes it uniquely suited for finding unambiguously edge-on protostars (of which we report thirteen at high confidence) and for tracing the structure of cavities cleared by protostellar outflows. We report on one of these unambiguously edge-on protostars, HOPS 171, which we've studied in detail with the Atacama Large Millimeter/submillimeter Array to find a complex structure. We use an edge detection routine to perform the analysis on tracing cavities caused by outflows, and report the first application of such a routine to study the role of feedback on the envelope. Using evolutionary diagnostics, we find no evidence for the growth of these outflow cavities as protostars evolve through the first 1 (open full item for complete abstract)

Committee: S. Thomas Megeath (Committee Chair); Rupali Chandar (Committee Member); Steve Federman (Committee Member); Stella Offner (Committee Member); Nikolas Podraza (Committee Member) Subjects: Astronomy; Astrophysics

Doctor of Philosophy, University of Toledo, 2012, Physics

Protostars are young stars in the process of accreting infalling envelopes of gas and dust which are transported from the diffuse interstellar medium through gravitational collapse. Although the envelopes are commonly thought to be comprised of cold, pristine material from the interstellar medium, recent space-based studies suggest that protostellar envelopes of low- and high-mass protostars contain thermally processed dust and ice. Unlike the envelope material from luminous, massive protostars, where dust and ice are subject to processing by direct stellar irradiation, thermally processed materials in low-mass protostars may be the consequence of accretion-driven outbursts, shocks in protostellar outflows, or transport of materials from the inner disk to the envelope by outflows and winds. We present an analysis of mid-infrared spectra of a large sample of protostars from the Orion Molecular Cloud complex, the most active region of star formation within the nearest 500 pc. The spectra, obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope, reveal strong silicate and solid molecular absorption bands. Using spectral decomposition analyses to determine the dust and ice composition toward the protostars, we find that the amorphous silicate composition is more dominated by amorphous pyroxene than dust in the Galactic diffuse interstellar medium, and that the mass fraction of amorphous pyroxene varies between protostars. Toward the perplexing protostar HOPS-68, we report the first unambiguous detection of (1) crystalline silicate absorption in a cold, infalling protostellar envelope and (2) highly processed carbon dioxide ice mantles. Moreover, we find evidence for crystalline silicate absorption towards two additional protostars. These results provide strong evidence that dust and ice delivered to planet-forming disks around low-mass stars in the protostellar phase may be processed by feedback from the central protostar.

Committee: S. Thomas Megeath Ph.D. (Committee Chair); Jon E. Bjorkman Ph.D. (Committee Member); Victor G. Karpov Ph.D. (Committee Member); John-David T. Smith Ph.D. (Committee Member); Dan M. Watson Ph.D. (Committee Member) Subjects: Astronomy; Astrophysics

Doctor of Philosophy, University of Toledo, 2011, Physics

Star formation occurs in a variety of environments, from massive star forming clouds like Orion, to low-mass clouds like Ophiuchus, and in more clustered and more distributed regions within clouds as well. It is not yet well understood how the environment (the density and temperature of the surrounding gas) in which a star forms a¿¿¿¿¿¿¿ects the properties i.e. mass, multiplicity, of the resulting star. This work investigates the youngest stars, which exist at or very near their birthplace, and examines the question, “How does Environment A¿¿¿¿¿¿¿ect Protostar Luminosity?” To assess this question, protostar candidates are identified in eleven tar-forming clouds, particularly clouds within 1 kpc of the sun including the relatively nearby regions of Serpens, Perseus, Ophiuchus, Chamaeleon, Lupus, Taurus, Orion, Cep OB3, and Mon R2, which combined host over 500 protostar candidates, and the massive star forming region Cygnus-X at a distance of 1.4kpc, which hosts over 2000 protostar candidates. Mid-infrared photometry from the Two-Micron All Sky Survey (2MASS) J, H, and Ks bands and Spitzer 3.6, 4.5, 5.8, 8.0, and 24 micron bands are used to identify the protostar candidates. In the nearby clouds (within 1 kpc) sources saturated at 24 micron are fit using a modified point-spread function (PSF) ¿¿¿¿¿¿¿ux extraction technique. The photometry of these sources is used to create spectral energy distributions (SEDs) from 1 - 24 microns. A new technique is developed to estimate the bolometric luminosities of protostars from their 1-24 micron photometry. Estimations of the bolometric luminosities for protostar candidates are combined to create luminosity functions for each cloud. Contamination due to edge-on disks, reddened Class II sources, and galaxies are considered and removed from luminosity functions. The luminosity functions of the clouds which form high-mass stars (Orion, Cep OB3, Mon R2, and Cygnus-X) peak near 1 Lsun and have a tail that extends to luminosities above (open full item for complete abstract)

Committee: S.T. Megeath PhD (Advisor); Rupali Chandar PhD (Committee Member); Bo Gao PhD (Committee Member); Lawrence Anderson-Huang PhD (Committee Chair); Judy Pipher PhD (Committee Member) Subjects: Astronomy