Circumstellar Disk Structure and Evolution Through Resolved Submillimeter Observations
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Circumstellar Disk Structure and Evolution through Resolved Submillimeter Observations A dissertation presented by Alanna Meredith Hughes to The Department of Astronomy in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the subject of Astronomy Harvard University Cambridge, Massachusetts May 2010 c 2010 — Alanna Meredith Hughes All rights reserved. iii Thesis Advisor: David J. Wilner Alanna Meredith Hughes Circumstellar Disk Structure and Evolution through Resolved Submillimeter Observations Abstract Circumstellar disks provide the reservoirs of raw material and determine conditions for the formation of nascent planetary systems. This thesis presents observations from millimeter-wavelength interferometers, particularly the Submillimeter Array, that address the following outstanding problems in the study of protoplanetary disks: (1) constraining the physical mechanisms driving the viscous transport of material through the disk, and (2) carrying out detailed studies of “transitional” objects between the gas-rich protoplanetary and tenuous, dusty debris disk phases to better understand how gas and dust are cleared from the system. We study accretion processes in three complementary ways: using spatially resolved observations of molecular gas lines at high spectral resolution to determine the magnitude and spatial distribution of turbulence in the disk; using polarimetry to constrain the magnetic properties of the outer disk in order to evaluate whether the MRI is a plausible origin for this turbulence; and investigating the gas and dust distribution at the outer disk edge in the context of self-similar models of accretion disk structure and evolution. The studies of transition disks use spatially resolved observations to study the detailed structure of the gas and dust in systems that are currently in the process of clearing material. We obtain snapshots of the inside-out clearing of gas and dust in several systems, and compare our observations with the theoretical predictions generated for different disk clearing mechanisms. Our observations are generally consistent with the characteristics predicted for viscous transport driven by the magnetorotational instability and disk clearing accomplished through the dual action of giant planet formation and photoevaporation by energetic radiation from the star. iv Contents Abstract..................................... iii Acknowledgments................................ x 1 Introduction 1 1.1 Why Millimeter Interferometry? . 2 1.2 Protoplanetary Disks as Accretion Disks . ... 4 1.3 DiskDissipation.............................. 6 2 An Inner Hole in the Disk around TW Hydrae Resolved in 7 mil- limeter Dust Emission 11 2.1 Introduction................................ 11 2.2 Observations................................ 13 2.3 Results................................... 13 2.3.1 7mmImage............................ 13 2.3.2 Radially Averaged 7 mm Visibilities . 14 2.4 Discussion................................. 15 2.4.1 ComparisonwithDiskModels . 19 2.4.2 DiskClearing ........................... 22 2.5 Conclusions ................................ 23 3 A Spatially Resolved Inner Hole in the Disk around GM Aurigae 25 3.1 Introduction................................ 26 v vi CONTENTS 3.2 ObservationsandDataReduction . 28 3.3 Results................................... 33 3.3.1 Millimeter Continuum Emission . 33 3.3.2 COChannelandMomentMaps . 34 3.4 DiskStructureModels .......................... 38 3.4.1 UpdatedSEDModel ....................... 38 3.4.2 ComparisonwithCOObservations . 43 3.5 Discussion................................. 44 3.5.1 InnerDiskClearing. .. .. 44 3.5.2 EvidenceforaWarp?. .. .. 48 3.6 Conclusions ................................ 50 4 A Resolved Molecular Gas Disk around the Nearby A Star 49 Ceti 53 4.1 Introduction................................ 54 4.2 Observations................................ 56 4.3 ResultsandAnalysis ........................... 57 4.4 DiskModeling............................... 59 4.4.1 GridofDiskModels ....................... 63 4.4.2 SpectralEnergyDistribution. 69 4.4.3 Best-FitDiskModel . .. .. 71 4.5 Discussion................................. 72 4.6 Conclusions ................................ 76 5 Structure and Composition of Two Transitional Circumstellar Disks in Corona Australis 77 5.1 Introduction................................ 78 5.2 ObservationsandDataReduction . 80 5.2.1 SMAObservations ........................ 80 CONTENTS vii 5.2.2 ASTEObservations. 81 5.3 Results................................... 82 5.3.1 Millimeter Continuum . 82 5.3.2 CO(2-1)andCO(3-2)LineObservations . 83 5.4 Analysis .................................. 84 5.4.1 Modeling the SED and Millimeter Visibilities . .. 84 5.4.2 RepresentativeModels . 87 5.4.3 Constraints on Molecular Gas Content . 88 5.5 Discussion and Conclusions . 91 6 Gas and Dust Emission at the Outer Edges of Protoplanetary Disks 95 6.1 Introduction................................ 96 6.2 DustContinuumandCOJ=3-2Data. 97 6.3 DiskModels................................ 98 6.3.1 TruncatedPowerLaw . .. .. 98 6.3.2 Similarity Solution from Accretion Disk Evolution . ..... 100 6.3.3 ModelComparison . 101 6.3.4 ModelFitting ........................... 102 6.4 ResultsandDiscussion . .. .. 106 6.5 SummaryandConclusions . 110 7 Stringent Limits on the Polarized Submillimeter Emission from Pro- toplanetary Disks 113 7.1 Introduction................................ 114 7.2 ObservationsandDataReduction . 117 7.3 Results................................... 119 7.4 AnalysisandDiscussion . 123 7.4.1 InitialModels ........................... 123 viii CONTENTS 7.4.2 ParameterExploration . 129 7.4.3 OtherEffects ........................... 137 7.5 SummaryandConclusions . 142 8 Empirical Constraints on Turbulence in Protoplanetary Accretion Disks 145 8.1 Introduction................................ 146 8.2 Observations................................ 148 8.3 Results................................... 149 8.4 Analysis .................................. 152 8.4.1 DescriptionofModels . 155 8.4.2 ModelingProcedure . 157 8.4.3 Best-fitModels .......................... 158 8.4.4 ParameterDegeneracies . 159 8.5 Discussion ................................. 165 8.5.1 ComparisonwithTheory. 165 8.5.2 Implications for Planet Formation . 169 8.5.3 FutureDirections . 170 8.6 SummaryandConclusions . 171 9 Conclusions and Future Directions 173 9.1 DiskDissipation.............................. 173 9.2 Protoplanetary Disks as Accretion Disks . ... 175 9.3 FutureDirections ............................. 177 A Protoplanetary Disk Visibility Functions 181 A.1 Power-LawDiskwithaCentralHole . 181 A.1.1 PositionoftheNull. 183 CONTENTS ix A.2 ThinWall ................................. 184 A.3 ApplicationtoTWHya . .. .. 184 B Supplementary Disk Polarimetry 187 B.1 Observations................................ 187 B.2 ResultsandAnalysis . .. .. 188 B.3 Discussion and Conclusions . 190 C High Spectral Resolution Channel Maps 195 References 201 Acknowledgments I can’t possibly say enough to thank David Wilner for the guidance and support that have made this thesis possible. I have never wanted for data, resources, opportunities, feedback, or attention as his student. I am grateful for his responsiveness in part because it has meant that my only limitations have been my own, which seems to be a rare and valuable experience in grad school. I am also thankful that fate and the Hubble fellowship brought Sean Andrews to the CfA two years after I arrived. He has made my experience here better in every way, as a valuable resource and a shining example to (try to) live up to, and by always being willing to talk about whatever’s on my mind. I am also grateful for Charlie Qi’s quiet support in helping me work with RATRAN and dealing with the weirder SMA issues we’ve encountered over the years. The contributions of many wonderful collaborators are represented in this thesis. I’m particularly grateful to Inga Kamp, Jungyeon Cho, and Michiel Hogerheijde for letting me mess with their wondrously complex codes, as well as Antonio Hales, Simon Casassus, and Michael Meyer for providing opportunities to work with new kinds of data. Dan Marrone and Ram Rao kindly taught me to use the SMA polarimeter and have helped me deal with its quirks. At the CfA, I’ve enjoyed rare but beneficial conversations with Ruth Murray-Clay, Alyssa Goodman, and Ramesh Narayan, and I thank Jim Moran for providing me with a thorough grounding in the fundamentals of radio astronomy. The mostly invisible work of the SMA staff also forms the backbone of this thesis, and I am particularly grateful to the schedulers, the operators, the TAC, and Taco for helping me to get such great data out of the telescope. Long nights at the summit were shortened by Shelbi’s wacky movies and Erin’s guitar. Life at the CfA has been immeasurably enriched by my fellow grad students. I’m not sure what I’d have done without the opportunity to talk through ideas and presentations, rant, or go for afternoon cookie or frisbee walks with Stephanie Bush, Joey Mu˜noz, and Ryan O’Leary. I also don’t think I could have worked back-to-back for five years with anyone other than Gurtina Besla. I’m grateful for the advice and encouragement of Antonella Fruscione, and to Jean Collins, Peg Herlihy, and Jennifer Barnett for cheerfully practicing their administrative magic. The observatory night bunch and the Friday afternoon EHI crew at the Museum of Science have provided a friendly atmosphere in which to