Constraining the Initial Conditions and Final Outcomes of Accretion Processes around Young Stars and Supermassive Black Holes
AuthorStone, Jordan Michael
AdvisorEisner, Joshua A.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractIn this thesis I discuss probes of small spatial scales around young stars and protostars and around the supermassive black hole at the galactic center. I begin by describing adaptive optics-fed infrared spectroscopic studies of nascent and newborn binary systems. Binary star formation is a significant mode of star formation that could be responsible for the production of a majority of the galactic stellar population. Better characterization of the binary formation mechanism is important for better understanding many facets of astronomy, from proper estimates of the content of unresolved populations, to stellar evolution and feedback, to planet formation. My work revealed episodic accretion onto the more massive component of the pre-main sequence binary system UY Aur. I also showed changes in the accretion onto the less massive component, revealing contradictory indications of the change in accretion rate when considering disk-based and shock-based tracers. I suggested two scenarios to explain the inconsistency. First, increased accretion should alter the disk structure, puffing it up. This change could obscure the accretion shock onto the central star if the disk is highly inclined. Second, if accretion through the disk is impeded before it makes it all the way onto the central star, then increased disk tracers of accretion would not be accompanied by increased shock tracers. In this case mass must be piling up at some radius in the disk, possibly supplying the material for planet formation or a future burst of accretion. My next project focused on characterizing the atmospheres of very low-mass companions to nearby young stars. Whether these objects form in an extension of the binary-star formation mechanism to very low masses or they form via a different process is an open question. Different accretion histories should result in different atmospheric composition, which can be constrained with spectroscopy. I showed that 3-4 μm spectra of a sample of these objects with effective temperatures greater than 1500 K are similar to the spectra of older more massive brown dwarfs at the same temperature, in contrast to objects at 1000 K that exhibit distinct L-band SEDs. The oldest object in my sample of young companions, 50 My old CD-35 2722 B, appears redder than field dwarfs with similar spectral type based on 1-2.5 μm spectra. This could indicate reduced cloud opacity compared to field dwarfs at the same temperature. I also present work to better understand the supermassive blackhole at the center of our Galaxy. Astrometric monitoring of stellar orbits about the blackhole have been used to sketch the gravitational potential, revealing 4 x 10⁶ M_⊙ within a radius of 40 AU. Further constraints on the gravitational potential, and the detection of post-Newtonian effects on the stellar orbits, will require improved astrometric precision. Currently confusion noise in the crowded central cluster limits astrometric precision. Increased spatial resolution can alleviate confusion noise. Dual field phase referencing on large-aperture infrared interferometers provides the sensitivity needed to observe the Galactic center, providing the fastest route to increased spatial resolution. I present simulations of dual-field phase referencing performance with the Keck Interferometer and with the VLTI GRAVITY instrument, to describe the potential contributions each could make to Galactic center stellar astrometry. I demonstrate that the near-future GRAVITY instrument at the VLTI will have a large impact on the ability to precisely track the paths of stars orbiting there, as long as a star with K-band apparent magnitude less than 20 exists within 70 milliarcseconds of the blackhole. Many of the stars orbiting the blackhole are in a post-main sequence wind phase. The wind from these stars is feeding an accretion flow falling onto the blackhole. This flow is radiatively inefficient, producing only 10⁻⁸ times the Eddington limit. Thus our relative proximity to the center of our own Galaxy, provides the opportunity to study a low-luminosity accretion mode that would be difficult or impossible to observe in more remote galaxies. Variable emission from the accretion flow arises from very deep within the flow and could be used to reveal the physics of the accretion process. Characterizing the variability is challenging because all wavelength regimes from radio through X-ray are affected by the process(es) that gives rise to the variations. I report observations of variability at wavelengths that are difficult or challenging to observe from the ground using the SPIRE instrument onboard the Herschel Space Observatory. My work provides the first constraints on the flux of the accretion flow at 250 μm. Variations at 500, 350, and 250 μm observed with Herschel exhibit typical amplitudes similar to the variations observed at 1300 μm from the ground, but the amplitude distribution of flux variations observe with Herschel does not exhibit a tail to large amplitudes that is seen at 1300 μm. This could suggest a connection between large-amplitude mm/submillimeter variations and X-ray activity, since no increased X-ray activity was observed during our Herschel monitoring.
Degree ProgramGraduate College