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: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: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: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: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" } ]