In recent decades, it has become clear that solar systems such as our own form within the circumstellar disks of gas and dust that surround young stars. Thus, to understand how these solar systems come to be, it is necessary to study the conditions within these disks. Until recently, such studies have required a focus on intrinsically brighter and younger disks that are easier to observe. However, a full picture of planet formation requires the characterization of older disks to determine how these systems change over time. The unprecedented capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA) now present the opportunity to study populations of these disks in detail for the first time. In this thesis, I present a study of over 100 such disks in the 5-11 Myr old Upper Scorpius OB Association (Upper Sco) using ALMA, as well as Keck Observatory, with the aim of comparing the properties of these systems to younger disks in order to shed new light on disk evolution.
\r\n\r\n\r\nFollowing background discussion on disks and their evolution, ALMA measurements of the continuum and CO line fluxes of these disks at 0.88 mm are reported in Chapter 3. The continuum fluxes are used to show that the majority of these systems contain less than 1 M\u2295 of dust. It is then shown that dust masses around these stars are on average a factor of \u223c4.5 lower than their younger counterparts in the Taurus star-forming region, placing important constraints on the mass evolution of these systems. Finally, constraints on depletion of gas in these disks are discussed using the CO measurements.
\r\n\r\n\r\nThe spatial distributions of the gas and dust within these Upper Sco disks are modeled in Chapter 4. The radial extents of gas and dust are measured and compared, with several systems showing evidence of the gas being more extended. The sizes of the dust disks are compared to younger systems, showing that these disks shrink by a factor of approximately three as they age. These results suggest that dust evolves from the outside-in within disks, perhaps through radial drift. Despite this evolution, dust disks in Upper Sco fall on the same correlation between size and millimeter luminosity as their younger counterparts. This implies a link between the radial structures of disks of different ages, perhaps indicating that these systems are composed of optically-thick dust substructure.
\r\n\r\n\r\nOf course, an understanding of planetary system formation would be incomplete without accounting for the presence of stellar companions, which are common around young stars and are expected to shorten disk lifetimes by truncating their sizes. As such, Chapter 5 presents a search for stellar companions in the Upper Sco disk sample analyzed in Chapters 3 and 4. Using adaptive optics imaging and aperture masking observations with the NIRC2 instrument on the Keck II telescope, stellar companions are identified in 27 of 112 systems. It is then shown that the companion fraction of systems with disks is lower than those without, confirming the harmful effects of stellar companions on disks seen in younger systems. However, the fraction of disk systems in Upper Sco with a close companion is shown to match that of younger disks in Taurus. This indicates that these effects occur within the first \u223cMyr of disk evolution, after which stellar companions have little to no effect. Additionally, while the millimeter luminosities of disks with stellar companions are observed to be lower than those around single stars in Taurus, there exists no such difference in Upper Sco. This provides further support for outside-in dust evolution, as the shrinking of disks around single stars would cause them by the 5-11 Myr age of Upper Sco to match the sizes and brightnesses of truncated disks in binary systems.
\r\n\r\n\r\nTaken together, the results presented in this thesis show the masses and radial extents of the dust-component of circumstellar disks decrease with age. This thesis concludes by summarizing these results and discussing their link within a scenario of outside-in dust evolution involving radial drift and dust substructure. To close, potential avenues are presented to continue the study of disk evolution with ALMA.
", "doi": "10.7907/5DK0-H549", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:9604", "collection": "thesis", "collection_id": "9604", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03042016-144127692", "primary_object_url": { "basename": "Swarnima-Manohar-thesis-2016.pdf", "content": "final", "filesize": 8264476, "license": "other", "mime_type": "application/pdf", "url": "/9604/1/Swarnima-Manohar-thesis-2016.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Lurking in ULIRGs: Molecular Gas in local Merging Galaxies", "author": [ { "family_name": "Manohar", "given_name": "Swarnima", "orcid": "0000-0002-3506-566X", "clpid": "Manohar-Swarnima" } ], "thesis_advisor": [ { "family_name": "Scoville", "given_name": "Nicholas Zabriskie", "orcid": "0000-0002-0438-3323", "clpid": "Scoville-N-Z" } ], "thesis_committee": [ { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" }, { "family_name": "Readhead", "given_name": "Anthony C. S.", "orcid": "0000-0001-9152-961X", "clpid": "Readhead-A-C-S" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Mawet", "given_name": "Dimitri", "orcid": "0000-0002-8895-4735", "clpid": "Mawet-D" }, { "family_name": "Scoville", "given_name": "Nicholas Zabriskie", "orcid": "0000-0002-0438-3323", "clpid": "Scoville-N-Z" } ], "local_group": [ { "literal": "Owens Valley Radio Observatory (OVRO)" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Mergers and interacting galaxies are pivotal to the evolution of galaxies in the universe. They are the sites of prodigious star formation and key to understanding the starburst processes: the physical and chemical properties and the dynamics of the molecular gas. ULIRGs or Ultraluminous Infrared Galaxies are a result of many of these mergers. They host extreme starbursts, AGNs, and mergers. They are the perfect laboratory to probe the connection between starbursts, black hole accretion and mergers and to further our understanding of star formation and merging.
\r\n\r\nNGC 6240 and Arp 220 can be considered the founding members of this very active class of objects. They are in different stages of merging and hence are excellent case studies to further our understanding about the merging process. We have imaged the dense star-forming regions of these galaxies at sub-arcsec resolution with CARMA C and B Configurations (2\" and 0.5 - 0.8\"). Multi-band imaging allows excitation analysis of HCN, HCO+, HNC, and CS along with CO transitions to constrain the properties of the gas. Our dataset is unique in that we have observed these lines at similar resolutions and high sensitivity which can be used to derive line ratios of faint high excitation lines.
\r\n\r\nArp 220 has not had confirmed X-ray AGN detections for either nuclei. However, our observations indicate HCN/HNC ratios consistent with the chemistry of X-ray Dominated Regions (XDRs) -- a likely symptom of AGN. We calculated the molecular Hydrogen densities using each of the molecular species and conclude that assuming abundances of HNC and HCO+ similar to those in galactic sources are incorrect in the case of ULIRGs. The physical conditions in the dense molecular gas in ULIRGs alter these abundances. The derived H2 volume densities are ~ 5 x 104 cm-3 in both Arp 220 nuclei and ~ 104 cm-3 in NGC 6240.
", "doi": "10.7907/Z9T72FDK", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:7782", "collection": "thesis", "collection_id": "7782", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302013-223659579", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 11805855, "license": "other", "mime_type": "application/pdf", "url": "/7782/1/thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Grain Growth in Protoplanetary Disks", "author": [ { "family_name": "P\u00e9rez Mu\u00f1oz", "given_name": "Laura Mar\u00eda", "clpid": "Perez-Munoz-Laura-Mar\u00eda" } ], "thesis_advisor": [ { "family_name": "Carpenter", "given_name": "John M.", "orcid": "0000-0003-2251-0602", "clpid": "Carpenter-J-M" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" } ], "thesis_committee": [ { "family_name": "Hirata", "given_name": "Christopher M.", "orcid": "0000-0002-2951-4932", "clpid": "Hirata-C-M" }, { "family_name": "Carpenter", "given_name": "John M.", "orcid": "0000-0003-2251-0602", "clpid": "Carpenter-J-M" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" }, { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" }, { "family_name": "Johnson", "given_name": "John A.", "clpid": "Johnson-J-A" }, { "family_name": "Kulkarni", "given_name": "Shrinivas R.", "orcid": "0000-0001-5390-8563", "clpid": "Kulkarni-S-R" } ], "local_group": [ { "literal": "Owens Valley Radio Observatory (OVRO)" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "The majority of young, low-mass stars are surrounded by optically thick accretion disks. These circumstellar disks provide large reservoirs of gas and dust that will eventually be transformed into planetary systems. Theory and observations suggest that the earliest stage toward planet formation in a protoplanetary disk is the growth of particles, from sub-micron-sized grains to centimeter- sized pebbles. Theory indicates that small interstellar grains are well coupled into the gas and are incorporated to the disk during the proto-stellar collapse. These dust particles settle toward the disk mid-plane and simultaneously grow through collisional coagulation in a very short timescale. Observationally, grain growth can be inferred by measuring the spectral energy distribution at long wavelengths, which traces the continuum dust emission spectrum and hence the dust opacity. Several observational studies have indicated that the dust component in protoplanetary disks has evolved as compared to interstellar medium dust particles, suggesting at least 4 orders of magnitude in particle- size growth. However, the limited angular resolution and poor sensitivity of previous observations has not allowed for further exploration of this astrophysical process.
\r\n\r\nAs part of my thesis, I embarked in an observational program to search for evidence of radial variations in the dust properties across a protoplanetary disk, which may be indicative of grain growth. By making use of high angular resolution observations obtained with CARMA, VLA, and SMA, I searched for radial variations in the dust opacity inside protoplanetary disks. These observations span more than an order of magnitude in wavelength (from sub-millimeter to centimeter wavelengths) and attain spatial resolutions down to 20 AU. I characterized the radial distribution of the circumstellar material and constrained radial variations of the dust opacity spectral index, which may originate from particle growth in these circumstellar disks. Furthermore, I compared these observational constraints with simple physical models of grain evolution that include collisional coagulation, fragmentation, and the interaction of these grains with the gaseous disk (the radial drift problem). For the parameters explored, these observational constraints are in agreement with a population of grains limited in size by radial drift. Finally, I also discuss future endeavors with forthcoming ALMA observations.
", "doi": "10.7907/1BJC-4G54", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:4146", "collection": "thesis", "collection_id": "4146", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10172008-054754", "primary_object_url": { "basename": "master.2.pdf", "content": "final", "filesize": 12864721, "license": "other", "mime_type": "application/pdf", "url": "/4146/1/master.2.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Optimizing Image Fidelity with Arrays", "author": [ { "family_name": "Corder", "given_name": "Stuartt Allan", "clpid": "Corder-Stuartt-Allan" } ], "thesis_advisor": [ { "family_name": "Sargent", "given_name": "Anneila Isabel", "clpid": "Sargent-A-I" } ], "thesis_committee": [ { "family_name": "Blake", "given_name": "Geoffrey A.", "clpid": "Blake-G-A" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "clpid": "Sargent-A-I" }, { "family_name": "Phinney", "given_name": "E. Sterl", "clpid": "Phinney-E-S" }, { "family_name": "Carpenter", "given_name": "John M.", "clpid": "Carpenter-J-M" }, { "family_name": "Wright", "given_name": "Melvyn", "clpid": "Wright-M" }, { "family_name": "Kulkarni", "given_name": "Shrinivas R.", "clpid": "Kulkarni-S-R" } ], "local_group": [ { "literal": "Caltech Submillimeter Observatory" }, { "literal": "Owens Valley Radio Observatory (OVRO)" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Through simulations, I have investigated the limitations imposed upon the image fidelity of interferometric observations by primary beam errors. Significant antenna surface and pointing errors lead to the greatest reduction in fidelity for most cases, but, when present, imaginary beam components dominate the degradation. Beam errors were addressed by optimizing the antenna surfaces and aligning the optics and then determining baseline based primary beams. Methods for applying these measured patterns to actual data were discussed. Pointing errors were reduced by improving the fit to the pointing model. Further reduction was achieved by integrating the use of optical pointing observations into standard radio observing. The greatest benefit was seen during daytime observations, but general reduction in pointing error was seen.
\r\n\r\nThe dense uv-coverage of the Combined Array for Research in Millimeter-wave Astronomy (CARMA) coupled with the techniques described above make it an ideal instrument for imaging extended regions with high fidelity. The NGC 7538 star-forming cloud contains dense peaks, many high-mass stars and associated accretion disks, and multiple outflows. I obtained CARMA images at the requisite fidelity, employing the above techniques. These mosaiced, spectral-line, and 3-mm band continuum observations provide a clearer picture of the bulk morphology of the region and the fine-scale structures within it than has hitherto been possible. For the first time in the region, infall signatures were found towards two sources, allowing comparison of the infall and outflow mass and verifying that significant accretion (>10\u22124M\u2299 yr\u22121) continues well into the stage where a massive protostar has formed. One of the sources, NGC 7538IRS1, shows one of the few definitive signatures of an inverse PCygni profile towards a massive protostar. Three outflows were found centered on sources that are separated by 10,000-20,000 AU in projection. The calculated energy injection rate provides constraints for models of outflow feedback. The NGC 7538 results demonstrate clearly the capability of CARMA to provide high quality images over wide-fields and the benefits of the techniques I developed. While work to improve CARMA image fidelity continues, the program described here lays the groundwork and should help guide further enhancements of image fidelity at CARMA and at other radio facilities.
\r\n", "doi": "10.7907/Q7H2-2P58", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:2779", "collection": "thesis", "collection_id": "2779", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06292007-134316", "primary_object_url": { "basename": "mle_thesis.pdf", "content": "final", "filesize": 9101360, "license": "other", "mime_type": "application/pdf", "url": "/2779/10/mle_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Molecular Clouds and Star Formation: A Multiwavelength Study of Perseus, Serpens, and Ophiuchus", "author": [ { "family_name": "Enoch", "given_name": "Melissa Lanae", "clpid": "Enoch-Melissa-Lanae" } ], "thesis_advisor": [ { "family_name": "Carpenter", "given_name": "John M.", "clpid": "Carpenter-J-M" } ], "thesis_committee": [ { "family_name": "Scoville", "given_name": "Nicholas Zabriskie", "clpid": "Scoville-N-Z" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "clpid": "Sargent-A-I" }, { "family_name": "Carpenter", "given_name": "John M.", "clpid": "Carpenter-J-M" }, { "family_name": "Sari", "given_name": "Re'em", "clpid": "Sari-R" }, { "family_name": "Golwala", "given_name": "Sunil", "clpid": "Golwala-S-R" } ], "local_group": [ { "literal": "Caltech Submillimeter Observatory" }, { "literal": "Owens Valley Radio Observatory (OVRO)" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "In this thesis I utilize large-scale millimeter and mid- to far-infrared surveys to address a number of outstanding questions regarding the formation of low mass stars in molecular clouds. Continuum \u03bb = 1.1 mm maps completed with Bolocam at a resolution of 31\" cover the largest areas observed to date at millimeter or submillimeter wavelengths in three molecular clouds: 7.5 deg\u00b2 in Perseus (140 pc\u00b2 at the adopted distance of d = 250 pc), 10.8 deg\u00b2 (50 pc\u00b2 at d = 125 pc) in Ophiuchus, and 1.5 deg\u00b2 (30 pc\u00b2 at d = 125 pc) in Serpens. These surveys are sensitive to dense substructures with mean density n \u2273 2 - 3 x 10\u2074 cm\u207b\u00b3. A total of 122 cores are detected in Perseus, 44 in Ophiuchus, and 35 in Serpens above mass detection limits of 0.1 - 0.2 Msun. Combining with Spitzer mid- and far-infrared maps from the c2d Legacy program provides wavelength coverage from \u03bb = 1.25-1100 \u03bcm, and enables the assembly of an unbiased, complete sample of the youngest star forming objects in three environments. This sample includes 108 prestellar cores, 43 Class 0 sources and 94 Class I sources.
\r\n\r\nThe approximately equal number of starless cores and embedded protostars in each cloud implies a starless core lifetime of 2 - 4 x 10\u2075 yr, only a few free-fall timescales. This timescale, considerably shorter than the timescale predicted by the classic scenario of magnetic field support in which core evolution is moderated by ambipolar diffusion, suggests that turbulence is the dominant process controlling the formation and evolution of dense cores. However, dense cores in all three clouds are found only at high cloud column densities, where AV\u2273 7 mag, and the fraction of cloud mass in these cores is less than 10%, indicating that magnetic fields must play some role as well. Measured angular deconvolved sizes of the majority of starless cores are consistent with relatively flattened radial density profiles, or with Bonnor-Ebert spheres. The prestellar core mass distribution (CMD) has a slope of \u03b1 = -2.5 \u00b1 0.2 for M > 0.8 Msun, remarkably similar to recent measurements of the slope of the stellar initial mass function: \u03b1 = -2.3 to -2.8. While this result does not rule out the importance of feedback or competitive accretion, it provides support for the hypothesis that stellar masses are determined during the core formation process.
\r\n\r\nThe lifetime of the Class 0 phase is estimated to be 1 - 2 x 10\u2075 yr in Perseus and Serpens, or approximately half that of the Class I phase, arguing against a very rapid early accretion phase. In Ophiuchus the fraction of Class 0 sources is much smaller, consistent with previous measurements of a short (~ 10\u2074 yr) Class 0 phase in that cloud. A large population of low luminosity Class I sources that cannot be explained by constant or monotonically decreasing accretion rates is observed in each cloud. This result strongly suggest that accretion during the Class I phase is episodic, with sources spending approximately 25% of the Class I lifetime in a quiescent state.
\r\n\r\nFinally, I investigate the environmental dependence of star formation by comparing the dense core populations of the three clouds. Cores are found at considerably higher cloud column densities in Ophiuchus than in Perseus or Serpens; more than 75% of cores occur at visual extinctions of AV\u2273 8 mag in Perseus, AV\u2273 15 mag in Serpens, and AV\u2273 20 - 23 mag in Ophiuchus. Cloud CMDs are well characterized by power-law fits above their empirically derived 50% completeness limits, resulting in slopes of \u03b1 = -2.1 \u00b1 0.1 in Perseus, \u03b1 = -2.1 \u00b1 0.3 in Ophiuchus, and \u03b1 = -1.6 \u00b1 0.2 in Serpens. Measured slopes for Perseus and Ophiuchus broadly agree with turbulent fragmentation, but the relative shapes of the observed cloud CMDs are inconsistent with detailed simulations of the dependence of CMD shape on Mach number.
", "doi": "10.7907/25QK-5K72", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:3090", "collection": "thesis", "collection_id": "3090", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-08102007-161741", "primary_object_url": { "basename": "fullthesis.pdf", "content": "final", "filesize": 42720980, "license": "other", "mime_type": "application/pdf", "url": "/3090/13/fullthesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "1-10 Myr-Old Low Mass Stars and Brown Dwarfs in Nearby Star Forming Regions", "author": [ { "family_name": "Slesnick", "given_name": "Catherine Louise", "clpid": "Slesnick-Catherine-Louise" } ], "thesis_advisor": [ { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" } ], "thesis_committee": [ { "family_name": "Brown", "given_name": "Michael E.", "orcid": "0000-0002-8255-0545", "clpid": "Brown-M-E" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" }, { "family_name": "Carpenter", "given_name": "John M.", "orcid": "0000-0003-2251-0602", "clpid": "Carpenter-J-M" }, { "family_name": "Cohen", "given_name": "Judith G.", "orcid": "0000-0002-8039-4673", "clpid": "Cohen-J-G" }, { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "My thesis research has focused primarily on identifying and characterizing pre-main sequence stars in nearby star-forming regions. To this end, I carried out wide-field (~150-250 deg\u00b2) optical photometric and spectroscopic surveys in and near the star forming regions of Taurus and Upper Scorpius.
\r\n\r\nIn Taurus, the aim of my optical photometric/spectroscopic survey was specifically to probe for a population of intermediate-age pre-main sequence stars outside of the young subclusters that are known to contain most of the young Taurus population. From this work I found tens of young (~1-3 Myr) and intermediate-age (~5-10 Myr) stars both near the known Taurus population and to the east, but relatively few pre-main sequence stars of any age to the west. I argued that the new pre-main sequence stars identified far from Taurus can not have originated from the vicinity of the 1-2 Myr-old subclusters and I proposed instead that they comprise a new, previously undiscovered region of recent star formation.
\r\n\r\nIn USco, the aim of my optical photometric/spectroscopic survey was to probe beyond previously explored regions to identify large numbers of 5 Myr-old low mass stars. From the spectroscopic data I have discovered 145 low mass members of USco. Using Monte Carlo simulations I showed that, taking into account known observational errors, the observed age dispersion for the low mass population in USco is consistent with all stars forming in a single burst ~5 Myr ago. I also derived the first spectroscopic mass function for USco that extends into the substellar regime and compared results to similar mass functions derived for stars in four other young clusters and associations.
", "doi": "10.7907/D7XM-E815", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:5055", "collection": "thesis", "collection_id": "5055", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12182007-120046", "primary_object_url": { "basename": "jmb_thesis.pdf", "content": "final", "filesize": 4892114, "license": "other", "mime_type": "application/pdf", "url": "/5055/1/jmb_thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Childhood to Adolescence: Dust and Gas Clearing in Protoplanetary Disks", "author": [ { "family_name": "Brown", "given_name": "Joanna Margaret", "clpid": "Brown-Joanna-Margaret" } ], "thesis_advisor": [ { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" } ], "thesis_committee": [ { "family_name": "Brown", "given_name": "Michael E.", "orcid": "0000-0002-8255-0545", "clpid": "Brown-M-E" }, { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Kamionkowski", "given_name": "Marc P.", "orcid": "0000-0001-7018-2055", "clpid": "Kamionkowski-M-P" } ], "local_group": [ { "literal": "Caltech Submillimeter Observatory" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Disks are ubiquitous around young stars. Over time, disks dissipate, revealing planets that formed hidden by their natal dust. Since direct detection of young planets at small orbital radii is currently impossible, other tracers of planet formation must be found. One sign of disk evolution, potentially linked to planet formation, is the opening of a gap or inner hole in the disk. In this thesis, I have identified and characterized several cold disks with large inner gaps but retaining massive primordial outer disks. While cold disks are not common, with ~5% of disks showing signs of inner gaps, they provide proof that at least some disks evolve from the inside-out. These large gaps are equivalent to dust clearing from inside the Earth's orbit to Neptune's orbit or even the inner Kuiper belt. Unlike more evolved systems like our own, the central star is often still accreting and a large outer disk remains.
\r\n\r\nI identified four cold disks in Spitzer 5-40 \u00b5m spectra and modeled these disks using a 2-D radiative transfer code to determine the gap properties. Outer gap radii of 20-45 AU were derived. However, spectrophotometric identification is indirect and model-dependent. To validate this interpretation, I observed three disks with a submillimeter interferometer and obtained the first direct images of the central holes. The images agree well with the gap sizes derived from the spectrophotometry. One system, LkH&alpha 330, has a very steep outer gap edge which seems more consistent with gravitational perturbation rather than gradual processes, such as grain growth and settling. Roughly 70% of cold disks show CO v=1&rarr 0 gas emission from the inner 1 AU and therefore are unlikely to have evolved due to photoevaporation. The derived rotation temperatures are significantly lower for the cold disks than disks without gaps. Unresolved (sub)millimeter photometry shows that cold disks have steeper colors, indicating that they are optically thin at these wavelengths, unlike their classical T Tauri star counterparts. The gaps are cleared of most ~100 \u00b5m sized grains as well as the ~10 \u00b5m sized grains visible in the mid-infrared as silicate emission features.
\r\n", "doi": "10.7907/560K-TZ76", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:3380", "collection": "thesis", "collection_id": "3380", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09082005-113029", "primary_object_url": { "basename": "main.pdf", "content": "final", "filesize": 14019184, "license": "other", "mime_type": "application/pdf", "url": "/3380/1/main.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "The Properties of Star-Forming Galaxies at z ~ 2: Kinematics, Stellar Populations, and Metallicities", "author": [ { "family_name": "Erb", "given_name": "Dawn Karuna", "orcid": "0000-0001-9714-2758", "clpid": "Erb-Dawn-Karuna" } ], "thesis_advisor": [ { "family_name": "Steidel", "given_name": "Charles C.", "clpid": "Steidel-C-C" } ], "thesis_committee": [ { "family_name": "Ellis", "given_name": "Richard S.", "clpid": "Ellis-R-S" }, { "family_name": "Blain", "given_name": "Andrew W.", "clpid": "Blain-A-W" }, { "family_name": "Kamionkowski", "given_name": "Marc P.", "clpid": "Kamionkowski-M-P" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "clpid": "Sargent-A-I" }, { "family_name": "Steidel", "given_name": "Charles C.", "clpid": "Steidel-C-C" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "We study the properties of star-forming galaxies at redshift z ~ 2, an era in which a substantial fraction of the stellar mass in the universe formed. Using 114 near-IR spectra of the H\u03b1 and [N II] emission lines and model spectral energy distributions fit to rest-frame UV through IR photometry, we examine the galaxies' star formation properties, dynamical masses and velocity dispersions, spatially resolved kinematics, outflow properties, and metallicities as a function of stellar mass and age. While the stellar masses of the galaxies in our sample vary by a factor of ~ 500, dynamical masses from H\u03b1 velocity dispersions and indirect estimates of gas masses imply that the variation of stellar mass is due as much to the evolution of the stellar population and the conversion of gas into stars as to intrinsic differences in the total masses of the galaxies. About 10% of the galaxies are apparently young starbursts with high gas fractions, caught just as they have begun to convert large amounts of gas into stars. Using the [N II]/H\u03b1 ratio of composite spectra to estimate the average oxygen abundance, we find a monotonic increase in metallicity with stellar mass. From the estimated gas fractions, we conclude that the increase in metallicity as gas is converted to stars is more likely to explain the observed mass-metallicity relation than the loss of metals through outflows. The picture that emerges is of galaxies with a broad range in stellar population properties, from young galaxies with ages of a few tens of Myr, stellar masses M* ~ 10\u2079 M\u2299, and metallicities Z ~ 1/3 Z\u2299, to massive objects with M* ~ 10\u00b9\u00b9 M\u2299, Z ~ Z\u2299, and ages as old as the universe allows. All, however, are rapidly star-forming, power galactic-scale outflows, and have masses in gas and stars of at least ~ 10\u00b9\u2070 M\u2299, in keeping with their likely role as the progenitors of elliptical galaxies seen today.
", "doi": "10.7907/C5D0-7789", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:2095", "collection": "thesis", "collection_id": "2095", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05262005-141109", "type": "thesis", "title": "High Angular Resolution Studies of the Structure and Evolution of Protoplanetary Disks", "author": [ { "family_name": "Eisner", "given_name": "Joshua Aaron", "clpid": "Eisner-Joshua-Aaron" } ], "thesis_advisor": [ { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" } ], "thesis_committee": [ { "family_name": "Sari", "given_name": "Re'em", "orcid": "0000-0002-1084-3656", "clpid": "Sari-R" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" }, { "family_name": "Carpenter", "given_name": "John M.", "orcid": "0000-0003-2251-0602", "clpid": "Carpenter-J-M" }, { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" }, { "family_name": "Kulkarni", "given_name": "Shrinivas R.", "orcid": "0000-0001-5390-8563", "clpid": "Kulkarni-S-R" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Young stars are surrounded by massive, rotating disks of dust and gas, which supply a reservoir of material that may be incorporated into planets or accreted onto the central star. In this dissertation, I use high angular resolution observations at a range of wavelengths to understand the structure, ubiquity, and evolutionary timescales of protoplanetary disks.
\r\n\r\nFirst, I describe a study of Class I protostars, objects believed to be at an evolutionary stage between collapsing spherical clouds and fully-assembled young stars surrounded by protoplanetary disks. I use a Monte Carlo radiative transfer code to model new 0.9 micron scattered light images, 1.3 mm continuum images, and broadband spectral energy distributions. This modeling shows that Class I sources are probably surrounded by massive protoplanetary disks embedded in massive infalling envelopes. For the best-fitting models of the circumstellar dust distributions, I determine several important properties, including envelope and disk masses, mass infall rates, and system inclinations, and I use these results to constrain the evolutionary stage of these objects.
\r\n\r\nSecond, I discuss observations of the innermost regions of more evolved disks around T Tauri and Herbig Ae/Be stars, obtained with the Palomar Testbed and Keck Interferometers. I constrain the spatial and temperature structure of the circumstellar material at sub-AU radii, and demonstrate that lower-mass stars are surrounded by inclined disks with puffed-up inner edges 0.1-1 AU from the star. In contrast, the truncated inner disks around more massive stars may not puff-up, indicating that disk structure depends on stellar properties. I discuss the implications of these results for disk accretion, terrestrial planet formation and giant planet migration.
\r\n\r\nFinally, I put these detailed studies of disk structure into a broader context by constraining the mass distribution and evolutionary timescales of circumstellar disks. Using the Owens Valley Millimeter Array, I mapped the millimeter continuum emission toward >300 low-mass stars in the NGC 2024 and Orion Nebula clusters. These observations demonstrate that the average disk mass in each cluster is comparable to the \"minimum-mass protosolar nebula\", and that there may be disk evolution on one million year timescales.
", "doi": "10.7907/61Y5-F650", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:4691", "collection": "thesis", "collection_id": "4691", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-11302001-000213", "primary_object_url": { "basename": "qch_thesis.pdf", "content": "final", "filesize": 6850519, "license": "other", "mime_type": "application/pdf", "url": "/4691/1/qch_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Aperture Synthesis Studies of the Chemical Composition of Protoplanetary Disks and Comets", "author": [ { "family_name": "Qi", "given_name": "Chunhua", "clpid": "Qi-Chunhua" } ], "thesis_advisor": [ { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" } ], "thesis_committee": [ { "family_name": "Brown", "given_name": "Michael E.", "orcid": "0000-0002-8255-0545", "clpid": "Brown-M-E" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" }, { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" }, { "family_name": "Stevenson", "given_name": "David John", "orcid": "0000-0001-9432-7159", "clpid": "Stevenson-D-J" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_gps" } ], "abstract": "Improvements in observational methods have now made it possible to directly observe planet-forming environments around young stars and to better characterize the most primitive relics of planetary growth in our own solar system. This thesis describes one such method, aperture synthesis imaging using the Owens Valley Radio Observatory (OVRO) Millimeter Array, and its application to the chemical composition of circumstellar accretion disks and comets.
\r\n\r\nThe observations presented in this thesis concentrate on [lambda]~3 mm transitions of HCN/HCO+ and {13}CO/CN in LkCa 15, GM Aur, MWC 480, and HD 163296. These disks were chosen based on their large spatial extent, Keplerian kinematic patterns, and strong CO emission.
\r\n \r\nEven at a resolution of 2\" (or a linear scale of ~300 AU at the distance of Taurus and Ophiuchus), the OVRO observations show that the chemistry in circumstellar disks is sensitive to both the central stellar luminosity and the degree of dust settling toward the disk midplane. Abundance ratios such as CN/HCN and HCO+/CO serve as unique probes of pivotal processes such as photoevaporation or cosmic ray induced ionization. The observed lower limit to the fractional ionization of 10{-10} is sufficient to support magnetorotational instabilities that are likely to dominate the transport mechanisms in the outer reaches of protoplanetary disks. CN/HCN gradients in the T Tauri and Herbig Ae star disks appear to be correlated with the local UV radiation field and with the degree of dust settling as judged by recent fits to their spectral energy distributions, illustrating the important role of photochemistry at large disk radii.
\r\n\r\nThe disk emission toward LkCa 15 is particularly intense, with many molecules being detected, including HCN/HCO+ and their {13}C-isotopomers, DCN, CN, HC_3N, CH_3OH, CS, {13}CO, and C{18}O. The overall abundance patterns are consistent with recent models of photon-dominated chemistry in the near surface regions of flaring circumstellar disks that also provide a natural explanation for the mid- and far-infrared properties of the disk spectral energy distribution.
\r\n\r\nDirect ties between accretion disks and the formation of planetary systems can be tested by examining primitive solar system bodies such as comets. Comet Hale-Bopp was observed at OVRO from 1997 March 29 to April 2 in a variety of spectroscopic settings between 3.4 and 1.2 mm. The resulting aperture synthesis millimeter-wave continuum and molecular line images reveal in great detail the inner coma. The millimeter-wave continuum brightness of Hale-Bopp is dominated by emission from dust grains in the coma. By subtracting a spherically symmetric coma model from the continuum visibilities, the millimeter-wave flux from the nucleus has been isolated, and leads to an estimated radius of 19-22.5 km.
\r\n\r\nThe large size of comet Hale-Bopp accounts for its extraordinary outgassing rates, which permitted the aperture synthesis observations of over 18 millimeter transitions of HCN, DCN, HDO, HC_3N, HNC, HNCO, CS, H_2S}, SO, OCS, CO, CH_3OH and CH_3OCH_3. The OVRO Millimeter Array was able to image, for the first time, molecular analogs of the dust jets commonly observed at optical and infrared wavelengths. This is particularly significant for investigating the true composition of comets, since jets are known to lift off large, icy grains from which direct sublimation can occur as they are exposed to the Sun. The production rates derived from the aperture synthesis images are similar to those found by other researchers, and reveal a marked similarity between the composition of Hale-Bopp and that derived for dense molecular clouds, in particular the hot cores observed near massive young stars. In addition, quite substantial D/H fractionations, comparable to the OVRO DCN/HCN measurement in LkCa 15, are found in the jets. While this clearly suggests an evolutionary history in which cometary materials remain at very low temperatures throughout their assemblage and for the bulk of their lives, the complex, kinetically controlled chemistry revealed in the OVRO images of the cold, outer regions of disks around young stars means that it is difficult to characterize cometary volatiles as being primarily \"interstellar\" or \"nebular\" in origin.
", "doi": "10.7907/VNV1-8B86", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:3670", "collection": "thesis", "collection_id": "3670", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-09202008-105312", "primary_object_url": { "basename": "Kuchner_mj_2001.pdf", "content": "final", "filesize": 7392327, "license": "other", "mime_type": "application/pdf", "url": "/3670/1/Kuchner_mj_2001.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Exozodiacal Dust", "author": [ { "family_name": "Kuchner", "given_name": "Marc Jason", "clpid": "Kuchner-Marc-Jason" } ], "thesis_advisor": [ { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" } ], "thesis_committee": [ { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" }, { "family_name": "Goldreich", "given_name": "Peter Martin", "clpid": "Goldreich-P-M" }, { "family_name": "Scoville", "given_name": "Nicholas Zabriskie", "orcid": "0000-0002-0438-3323", "clpid": "Scoville-N-Z" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Besides the sun, the most luminous feature of the solar system is a cloud of \"zodiacal\" dust released by asteroids and comets that pervades the region interior to the asteroid belt. Similar clouds of dust around other stars\u2014exozodiacal clouds\u2014may be the best tracers of the habitable zones of extra-solar planetary systems. This thesis discusses three searches for exozodiacal dust:
\r\n\r\nWe also discuss the interaction of dust with planets. We used the COBE DIRBE Sky and Zodi Atlas and the IRAS Sky Survey Atlas to search for dynamical signatures of three different planets in the solar system dust complex:
\r\n\r\nThis thesis studies disks in three astrophysical contexts: (1) protoplanetary disks; (2) the Edgeworth-Kuiper Belt; and (3) planetary rings.
\r\n\r\nWe derive hydrostatic, radiative equilibrium models of passive protoplanetary disks surrounding T Tauri and Herbig Ae/Be stars. Each disk is encased by an optically thin layer of superheated dust grains. This layer is responsible for up to ~70% of the disk luminosity at wavelengths between ~5 and 60 \u03bcm. The heated disk flares and absorbs more stellar radiation at a given stellocentric distance than a flat disk would. Spectral energy distributions are computed and found to compare favorably with the observed flattish infrared excesses of several young stellar objects. Spectral features from dust grains in the superheated layer appear in emission if the disk is viewed nearly face-on.
\r\n\r\nWe present the results of a pencil-beam survey of the Kuiper Belt using the Keck 10-m telescope. Two new objects are discovered. Data from all surveys are pooled to construct the luminosity function from mR = 20 to 27. The cumulative number of objects per square degree, \u2211(< mR, is such that log10\u2211(< mR = 0.52(mR - 23.5). The luminosity function is consistent with a power-law size distribution for which the smallest objects possess most of the surface area but the largest bodies contain most of the mass. To order-of-magnitude, 0.2M\u2d32 and 1 x 1010 comet progenitors lie between 30 and 50 AU. The classical Kuiper Belt appears truncated at a distance of 50 AU.
\r\n\r\nWe propose that rigid precession of narrow eccentric planetary rings surrounding Uranus and Saturn is maintained by a balance of forces due to ring self-gravity, planetary oblateness, and interparticle collisions. Collisional impulses play an especially dramatic role near ring edges. Pressure-induced accelerations are maximal near edges because there (1) velocity dispersions are enhanced by resonant satellite perturbations, and (2) the surface density declines steeply. Remarkably, collisional forces felt by material in the last ~100 m of a ~10 km wide ring can increase equilibrium masses up to a factor of ~100. New ring surface densities are derived which accord with Voyager radio measurements.
\r\n", "doi": "10.7907/8B2W-8V06", "publication_date": "2000", "thesis_type": "phd", "thesis_year": "2000" }, { "id": "thesis:4347", "collection": "thesis", "collection_id": "4347", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10312007-162008", "primary_object_url": { "basename": "Koerner_dw_1995.pdf", "content": "final", "filesize": 4981212, "license": "other", "mime_type": "application/pdf", "url": "/4347/1/Koerner_dw_1995.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "The kinematics of circumstellar disks around T Tauri stars", "author": [ { "family_name": "Koerner", "given_name": "David", "clpid": "Koerner-David" } ], "thesis_advisor": [ { "family_name": "Sargent", "given_name": "Anneila Isabel", "clpid": "Sargent-A-I" } ], "thesis_committee": [ { "family_name": "Sargent", "given_name": "Anneila Isabel", "clpid": "Sargent-A-I" }, { "family_name": "Stevenson", "given_name": "David John", "clpid": "Stevenson-D-J" }, { "family_name": "Beckwith", "given_name": "Steve V. W.", "clpid": "Beckwith-S-V-W" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_gps" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nAperture synthesis images of molecular gas around the T Tauri stars, GM Aurigae, RY Tauri, DL Tauri, DO Tauri, and AS 209 are interpreted with the aid of a kinematic model of a circumstellar disk. The velocity structure and morphology of the gas strongly suggest that circumstellar disks with radii of a few hundred AU are present around all five stars. Explicit identification of kinematic patterns in the spectral line maps is achieved for the two largest disks, GM Aur and DO Tau.\r\n\r\nMaps of the relatively old T Tauri star, GM Aur, in the [...] line at 4\" resolution reveal compact gas associated with the stellar position and at the core of a larger rotating gaseous disk, 950 x 530 AU in extent. The mean velocity gradient across the disk, which is oriented along [...], is consistent with rotation about an axis at [...]. The structure observed in [...] aperture synthesis maps agrees well with synthetic maps of the gas emission generated from a model. For a disk that is inclined [...] from face on, in Keplerian rotation, a 0.80 [...] central mass (star + disk), a systemic velocity, [...], of 15.38 km [...] and a mass, 0.1 [...] is derived.\r\n\r\nA survey of T Tauri stars for circumstellar molecular gas also yields evidence for rotating disks. RY Tau, DL Tau, DO Tau, and AS 209 are detected in CO[...] emission elongated along [...], respectively, with deconvolved half-maximum radii of 110, 250, 350, and 290 AU in aperture synthesis images at [...] resolution. Three of these, RY Tau, DL Tau, and AS 209, exhibit velocity gradients parallel to this direction in first moment velocity maps, suggesting rotation of the circumstellar gas. Spectra of RY Tau and AS 209 exhibit a characteristic double-peaked shape, but that of DO Tau is dominated by the presence of high-velocity blue-shifted gas. DL Tau's spectrum has a linewidth that is too narrow [...] to be resolved into double peaks. Position-Velocity Diagrams (PVDs) constructed parallel and perpendicular to the axis of elongation imply rotation and infall or outflow, respectively. Those for RY Tau, DL Tau, and AS 209 show predominantly rotation, but some evidence of an orthogonal gradient (infall or outflow) is seen for RY Tau and AS 209. In contrast, the PVDs for DO tau suggest that its molecular emission originates predominantly from outflow or infall.\r\n\r\nRecent evidence of infall in circumstellar gas around T Tauri stars has motivated construction of a kinematic model of molecular emission from a disk of rotating and infalling gas. The velocity structure of the model disk assumes that infalling gas obeys angular momentum conservation on ballistic trajectories until it reaches the radial value, [...], where the magnitude of the rotational velocity component reaches the Keplerian value. For [...], gas is assumed to be in circular Keplerian orbits. Synthetic spectral line maps are generated by the model and cross-correlated with aperture synthesis maps for a range of the free parameters, including [...]. The best-fit value of [...] for RY Tau, DL Tau, and AS 209, approximately matches that of the outer radius of their emission, indicating these disks are predominantly in Keplerian rotation. However, emission from these stars is marginally resolved and uncertainties in the best-fit parameters are correspondingly high. In contrast, kinematic patterns are well-resolved in emission from DO Tau found in maps within a narrow, near-systemic velocity range. These are best matched by simulations from a model disk with [...] = 350 AU and an outer radius of 500 AU. This result strongly suggests that, in this case, infall from the molecular cloud onto a rotating disk is still in progress.\r\n", "doi": "10.7907/css3-2v20", "publication_date": "1995", "thesis_type": "phd", "thesis_year": "1995" }, { "id": "thesis:4346", "collection": "thesis", "collection_id": "4346", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10312007-094538", "primary_object_url": { "basename": "McMuldroch_s_1995.pdf", "content": "final", "filesize": 6352429, "license": "other", "mime_type": "application/pdf", "url": "/4346/1/McMuldroch_s_1995.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "The Circumstellar Environments of FU Orionis Stars", "author": [ { "family_name": "McMuldroch", "given_name": "Stuart", "clpid": "McMuldroch-Stuart" } ], "thesis_advisor": [ { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" }, { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" } ], "thesis_committee": [ { "family_name": "Westphal", "given_name": "James A.", "clpid": "Westphal-J-A" }, { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" }, { "family_name": "Burnett", "given_name": "Donald S.", "clpid": "Burnett-D-S" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" }, { "family_name": "Muhleman", "given_name": "Duane Owen", "clpid": "Muhleman-D-O" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_gps" } ], "abstract": "Extensive observations were made of six FU Orionis objects (RNO 1B/1C, V1057 Cygni, Elias 1-12, V1515 Cygni and FU Orionis) and one pre-outburst candidate (V1331 Cygni) using the Owens Valley millimeter-wave array and the Caltech Submillimeter Observatory (CSO). Aperture synthesis maps of CO (1\u21920), \u00b9\u00b3CO (1\u21920), \u00b9\u00b3CO (2\u21921), \u00b9\u2078CO (1\u21920), and \u00b3CO (2\u21921) molecular lines and associated dust continuum emission trace the masses, kinematics and morphology of FU Orionis disks, envelopes, and outflows. Maps from the CSO delineate outflowing gas at larger spatial scales while line strengths, when input into radiative transfer models, yield column densities and fractional chemical abundances.
\r\n\r\nUnresolved 1.3 mm continuum emission from Vl331 Cygni and V1057 Cygni reveal massive circumstellar disks of 0.5 and 0.09 M\u2609, respectively. Maps of the 2.6 and 3.1 mm continuum emission reveal that RNO 1C is surrounded by a flattened dusty envelope, 5000 AU in size, with mass \u2a7e 1.1 M\u2609. No evidence is seen for multiple systems with orbital periods \u2273 4 x 10\u2074 years years.\r\n\r\nAll sources, with the exception of RNO 1B/1C are surrounded by large molecular gas envelopes between 2000-7500 AU in size, with masses ranging from 2 x 10\u207b\u00b3 to 0.36 M\u2609. Aperture synthesis maps suggest the envelopes are asymmetrically distributed. Gas kinematics around V1057 Cygni and Elias 1-12 suggest, but do not demand, that this material is rotating and possibly infalling.
\r\n\r\nOf the seven sources observed, all but FU Orionis show signs of outflowing molecular gas. No high velocity clumps or \"bullets\" are seen towards any of the sources. V1331 Cygni, V1057 Cygni, and V1515 Cygni possess arc or ring-like outflow morphologies, while emission from RNO 1B/1C and Elias 1-12 delineates filled outflow shells. All emission patterns are consistent with outflow shells being viewed at different angles. Although based on the statistics of small numbers, observations suggest shells are seen more frequently around FUors than T Tauri stars. Shell formation may therefore be caused by time-dependent events in the outflow. Cross-cutting arcs within the shell structure, seen towards RNO 1B/1C and Elias 1-12, are probably ridges of gas swept-up by the most recent outbursts, confirming the repetitive nature of FUor outbursts. Estimates of the dynamical ages of the arcs suggest that the interval between outbursts is ~5 x 10\u00b3 and 1.3 x 10\u2074 years for RNO 1B/1C and Elias 1-12 respectively, consistent with previous estimates of FUor cycling times.
\r\n\r\nComparable envelopes and shell-like outflow structure are seen towards embedded sources while envelopes surrounding T Tauri stars are smaller and less massive. This suggests FU Orionis objects are transition sources between deeply embedded and optically visible stars. The strength of the molecular outflow emission is correlated with the mass of the extended envelope; the outflow has evacuated molecular gas leaving less to be swept-up by subsequent outbursts, while envelope masses are smaller since less material is available for accretion.
\r\n\r\nChemically, fractional abundances of SiO and methanol are enhanced towards RNO 1B/1C by over an order of magnitude. Methanol is enhanced relative to HCN and H2CO towards Elias 1-12. Such large changes in fractional abundances must be caused by chemical processing. The SiO enrichments may be produced by sputtering or evaporation of dust grains in regions of directly shocked material or by chemical reactions of SiH4 after sublimation from grain surfaces. Methanol enrichments are difficult to produce in strong shocks, and may be caused by low velocity grain-grain collisions. For RNO 1B/1C, evaporation of the entire methanol rich grain mantles seems to be required; while for Elias 1-12 mantle phase changes from an amorphous ice to a clathrate hydrate can be invoked, expelling the methanol, with smaller molecules remaining trapped in the clathrate. Such low velocity collisions probably occur in the turbulent shear zones surrounding the outflowing gas.
", "doi": "10.7907/14NE-ZW09", "publication_date": "1995", "thesis_type": "phd", "thesis_year": "1995" }, { "id": "thesis:3971", "collection": "thesis", "collection_id": "3971", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-10072004-143305", "type": "thesis", "title": "The Multiplicity of T Tauri Stars in the Star Forming Regions Taurus-Auriga and Ophiucus-Scorpius: A 2.2 \u00b5m Speckle Imaging Survey", "author": [ { "family_name": "Ghez", "given_name": "Andrea Mia", "orcid": "0000-0003-3230-5055", "clpid": "Ghez-Andrea-Mia" } ], "thesis_advisor": [ { "family_name": "Neugebauer", "given_name": "Gerry", "clpid": "Neugebauer-G" } ], "thesis_committee": [ { "family_name": "Neugebauer", "given_name": "Gerry", "clpid": "Neugebauer-G" }, { "family_name": "Frautschi", "given_name": "Steven C.", "clpid": "Frautschi-S-C" }, { "family_name": "Prince", "given_name": "Thomas A.", "clpid": "Prince-T-A" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "clpid": "Sargent-A-I" }, { "family_name": "Soifer", "given_name": "B. Thomas", "clpid": "Soifer-B-T" } ], "local_group": [ { "literal": "Nobel Prize" }, { "literal": "Astronomy Department" }, { "literal": "Caltech Distinguished Alumni Award" }, { "literal": "div_pma" } ], "abstract": "We present the results of a magnitude limited (K \u2264 8.5 mag) multiplicity survey of T Tauri stars (TTS) in two nearby star forming regions (SFR), Taurus-Auriga and Ophiuchus-Scorpius. Each of the 69 stars in the sample was observed at K(2.2 \u00b5m) with an infrared array camera on the Hale 5-m Telescope at Palomar Observatory and imaged using two-dimensional speckle interferometric techniques.
\r\n\r\nThirty three companion stars were found, of which 15 were new detections. A subset of 64 observations was sensitive to all companion stars in the projected linear range 14 to 225 AU and the magnitude difference range 0.0 to 2.0 mag. We used this subset and region to study the multiplicity of TTS; the frequency of companion stars within this region is 34 \u00b1 9%, independent of SFR. We discovered a distinction between the classical TTS (CTTS) and the weak-lined TTS (WTTS) based on the binary star frequency as a function of separation; the WITS dominate the binary star distribution at the closer separations and the CTTS populate the wider separations. The cross over occurred near 100 AU, the size typically quoted for a circumstellar disk. We suggest that all TTS begin as CTTS and become WTTS when accretion has ceased, and that the nearby companion stars act to shorten the accretion timescale in multiple star systems.
\r\n\r\nIntegrated over all magnitude differences the binary star frequency in the projected linear separation range 14 to 225 AU for TTS (59 \u00b1 16%) is a factor of 3.5 greater than that of the solar-type main sequence stars (17 \u00b1 3%). Given the limited angular separation range that we are sensitive to, i.e., both the spectroscopic and wide binaries are missed, the rate at which binaries are detected suggests that most, if not all, TTS stars have companions. We propose that the observed overabundance of companions to TTS with respect to their older counterparts on the main sequence is an evolutionary effect; in this scheme triple and higher order TTS, which are observed at higher frequencies than for the solar-type main sequence stars, are disrupted by close encounters with another star or system of stars.
", "doi": "10.7907/AD5X-DA38", "publication_date": "1993", "thesis_type": "phd", "thesis_year": "1993" }, { "id": "thesis:2640", "collection": "thesis", "collection_id": "2640", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06182007-104442", "type": "thesis", "title": "Molecular Gas and Star Formation in the Central Regions of Virgo Spiral Galaxies", "author": [ { "family_name": "Canzian", "given_name": "Blaise J.", "clpid": "Canzian-Blaise-J" } ], "thesis_advisor": [ { "family_name": "Scoville", "given_name": "Nicholas Zabriskie", "clpid": "Scoville-N-Z" } ], "thesis_committee": [ { "family_name": "Scoville", "given_name": "Nicholas Zabriskie", "orcid": "0000-0002-0438-3323", "clpid": "Scoville-N-Z" }, { "family_name": "Kulkarni", "given_name": "Shrinivas R.", "orcid": "0000-0001-5390-8563", "clpid": "Kulkarni-S-R" }, { "family_name": "Mould", "given_name": "Jeremy R.", "clpid": "Mould-J-R" }, { "family_name": "Phinney", "given_name": "E. Sterl", "orcid": "0000-0002-9656-4032", "clpid": "Phinney-E-S" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "The central regions of eight normal spiral galaxies in the Virgo Cluster spanning a wide range of morphological types have been observed in CO (J = 1 \u2192 0) emission using the Owens Valley mm-wave interferometer. Broadband optical and H\u03b1 CCD photometry was obtained using the Palomar 60-inch telescope.
\r\n\r\nThe H\u2082 masses in the central five kiloparsecs of the spirals are large, typically several 10\u2078 solar masses, far exceeding the atomic hydrogen masses. H\u2082 densities in the cloud complexes are comparable to those of Galactic center complexes such as Sgr B2. Molecular gas comprises a substantial fraction of the dynamical mass of the central regions. The gas mass fraction, typically 5-15% at 2.5 kiloparsec radius, increases to 15-30% at the center. Molecular gas in the interferometer maps is concentrated in structures with scale lengths of only 400-750 parsecs. Such massive gas disks are, nonetheless, dynamically stable. The H\u2082 masses may be overestimated in some spirals because of cloud heating. The main source of heating is probably dissipation of turbulence, which is probably greater at the centers of spirals based on CO line widths.
\r\n\r\nCurrent central star formation rates are about a few solar masses per year. The star formation rate in the central regions of most of the spirals has been relatively constant over a Hubble time, although some galaxies are currently undergoing star formation bursts. Several mechanisms are suggested that could supply enough gas to maintain current star formation rates. The current rate of star formation is well correlated with the mean square molecular gas surface density computed from interferometer maps. Such a correlation is expected if cloud-cloud collisions or another stimulation mechanism acting between clouds is the principle means to produce high-mass stars where the concentration of clouds is greatest. The B luminosity is better correlated with the total gas mass surface density, which supports a stochastic or independent mode of forming intermediate-mass stars in the general cloud population.
", "doi": "10.7907/pcga-sm96", "publication_date": "1990", "thesis_type": "phd", "thesis_year": "1990" } ]