The University of Toledo The University of Toledo Digital Repository Theses and Dissertations 2013 A new era of observational capability at Ritter Observatory : spectropolarimetry from exoplanets to circumstellar disks and beyond James W. Davidson The University of Toledo Follow this and additional works at: http://utdr.edu/theses-dissertations Recommended Citation Davidson, James W., "A new era of observational capability at Ritter Observatory : spectropolarimetry from exoplanets to circumstellar disks and beyond" (2013). Theses and Dissertations. This Dissertation is brought to you for free and open access by The University of Toledo Digital Repository.
It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of The University of Toledo Digital Repository. For more information, please see the repository's About page. A Dissertation entitled A New Era of Observational Capability at Ritter Observatory: Spectropolarimetry from Exoplanets to Circumstellar Disks and Beyond by James W. Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics & Astronomy Dr.
Bjorkman, Committee Chair Dr. Collins, Committee Member Dr. Thomas Megeath, Committee Member Dr. Smith, Committee Member Dr.
Wisniewski, Committee Member Dr. Komuniecki, Dean College of Graduate Studies The University of Toledo May 2013 Copyright 2013, James W. This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.
An Abstract of A New Era of Observational Capability at Ritter Observatory: Spectropolarimetry from Exoplanets to Circumstellar Disks and Beyond by James W. Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics & Astronomy The University of Toledo May 2013 We undertook efforts to restore and relocate the University of Wisconsin’s Halfwave- Polarimeter (HPOL) spectropolarimeter to the University of Toledo’s Ritter Observa- tory. This process required fairly extensive work to the optical-mechanical alignment of the Ritter Observatory 1-meter telescope. Ultimately the restoration and reloca- tion efforts were successful, with first light at Ritter Observatory on March 11th, 2012.
Extensive observations of unpolarized standard stars were carried out in the first six months of observing time at Ritter Observatory. The results of this effort has shown the polarimetric stability in the observations is at the same level or better compared to almost 10 years of observations at Pine Bluff Observatory, after the upgrade to the detector. Aside from continued spectropolarimetric monitoring programs which were ini- tially focused on a selection of Be and Wolf-Rayet stars, we sought to investigate potential polarimetric variability in exoplanet systems. In particular we focused on the system HD189733, as there was previous observational work in the literature claiming both a detection and non-detection at different optical wavelengths.
The goal was to investigate this system with HPOL to provide simultaneous observations across the entire optical wavelength range to investigate the possibility of a wavelength dependent variable polarization with orbital phase. While the variability claimed in iii the literature is below the noise limits of our observations, an interesting and yet un- explained increase in the polarization level occurred in one of our observations. This higher than expected signal had vanished by the next observation, which was several nights later due to weather, and did not reappear in any of the observations there- after, implying such a higher than expected signal must be short lived, lasting less than a couple orbital periods. This increase is around an order of magnitude larger than the claimed detection in the literature from light scattering off the exoplanets atmosphere, and would seem to be caused by some other physical mechanism in the system.
Continued observations of HD189733 with HPOL in the next observing sea- son starting in July 2013 will help to identify if the increased polarization level is repeatable. iv To my family Acknowledgments I would like to thank my advisor, Karen Bjorkman, for all her assistance and support during this work; Jon Bjorkman for many great discussions and for all his guidance in collimating the Ritter Observatory 1-meter telescope; and my committee members for all their help, particularly John Wisniewski. I would like to thank those involved in the HPOL efforts: From Denver University, Jennifer Hoffman and Jamie Lomax; From the University of Wisconsin at Madison, HPOL creator Ken Nordsieck, Brian Babler for help with the calibration, Marilyn Meade for help with data reduction, Rick Williams for technical help with HPOL, and Richard Bonomo for help restoring the HPOL control computer; And from the University of Toledo, Rick Irving for computer assistance, Ashok Bhandary for helping testing HPOL in the lab, Mike Brown for help getting HPOL operating on the Ritter 1-meter telescope, Adolf Witt for all his help in regards to Ritter Observatory, and Scott Lee for generously loaning the optical equipment used to collimate the telescope. I would like to thank my office mates for the better part of six years, Charles Poteet and Blagoy Rangelov, for many great conversations.
I would also like to thank my family and friends for all their support over the years, in particular my soon to be wife, Kära Lindelof, who moved to Toledo with me and has been a tremendous source of support. Thank you, and I love you all. This research has made use of the SIMBAD database, operated at CDS, Stras- bourg, France. This work has been partially funded by a Small Research Grant from the AAS, and by the Scott E.
Smith Fund for Research at Ritter Observatory. vi Contents Abstract iii Acknowledgments vi Contents vii List of Tables xi List of Figures xii List of Abbreviations xvi 1 Introduction 1 1.1 Background on Polarization .2 Solar System Polarization .3 Polarization of Unresolved stars .4 Polarization of Circumstellar Material .5 Polarization of Exoplanet Systems .1 Scattered Light from an Exoplanet Atmosphere .2 Using Exoplanets to Probe Limb Polarization of Host Stars .3 Starspots as a Source of Variable Polarization. 16 2 The HPOL Spectropolarimeter 18 2.1 History of the Instrument .2 Description of the Instrument .1 Arc and Flat Lamp Assembly .3 Slit & Decker Assembly .3 The End of the PBO Era .1 Restoring HPOL to Operation .1 Control System Software Restoration .2 Slit Camera Replacements .2 Relocation to Ritter Observatory .3 New Layout of Equipment .2 Two Mirror Mis-alignments .3 Collimating the Ritter Observatory 1-meter Telescope .4 Development of a New Collimation Procedure .1 Optical Table Assembly .2 Defining the Telescope Optical Axis .3 Setting the Rotation of the Secondary Mirror Ring .4 Aligning the Secondary Mirror Housing Assembly and Spider with the Telescope Optical Axis .5 Adjusting the Tilt of the Secondary Mirrors .6 Adjusting the Tilt of the Primary Mirror .7 Final Adjustments On-Sky .5 Installing Monofilament Crosshairs .6 Identification and Solution to a Flexure Issue in Secondary Mir- ror Assembly .7 Extender for HPOL .2 Summary of Telescope Collimation .1 Observing Unpolarized Stars .2 Constructing a Calibration File .3 The Final Calibration File .4 Comparison With 10 Years of Previous PBO Calibration Work. 103 6 Polarimetric Observations of the Exoplanet System HD189733 107 6.4 Higher Than Expected Signal.
129 7 Future Work 130 ix 7.1 Evaluation of β-Cas as an Unpol Standard .2 Cutting Polarization Observations at 3200Å .2 Bringing Faint-Mode Back Online .3 The HD189733 System .4 Long Term Monitoring Programs. 140 References 143 x List of Tables 5.1 Unpolarized Standard Stars .2 Previous Observations of Unpolarized Standard Stars .3 List of Observations .4 Average polarization values for RO .5 Systematic error for RO .6 Systematic Errors from PBO and RO .1 Previous published parameters for the planet HD189733b .2 Previous published parameters for the star HD189733 .3 List of HD189733 Observations. 110 xi List of Figures 1-1 Polarization ellipse. 2 1-2 Examples of Stokes parameters.
5 1-4 Example QU plot of the polarized standard star HR6353. 6 1-5 QU plot illustration. 7 1-6 Centro-symmetric pattern of polarization of a star. 10 1-7 Summary of polarization observations for the exoplanet HD 189733 b.
14 1-9 Occultation polarization at the Ca I λ4227Å line. 15 1-10 Model of limb polarization for HD189733. 15 2-1 The Reticon photo-diode array detector used on HPOL up to 1995. 20 2-2 The HPOL spectropolarimeter optical design.
21 2-3 HPOL arc and flat lamp assembly. 22 2-4 Slit & Decker Assembly. 23 2-5 Halfwave plate schematic. 25 2-6 Wollaston prism schematic.
28 2-7 2-Dimension Wollaston prism. 28 2-8 Raw image examples of HPOL. 32 3-1 Air drying system. 36 4-1 Spherical aberration ray trace.
39 xii 4-2 Example image with spherical aberration. 40 4-3 Coma aberration ray trace. 41 4-4 Example image with coma. 41 4-5 Astigmatism ray trace.
42 4-6 Example image with astigmatism. 43 4-7 Types of mis-alignments. 44 4-8 Optical Table on Telescope. 47 4-9 Primary mirror baffle laser unit.
49 4-10 Primary mirror baffle mounting collar with crosshair. 50 4-11 Outside view of secondary rotation shaft. 51 4-12 Monofilament attached to secondary housing. 52 4-13 Outside view of secondary rotation shaft.
53 4-14 Inside view of the secondary rotation shaft. 55 4-15 Equipment used to define the secondary mirror assembly rotation axis. 56 4-16 Monofilament showing secondary mirror assembly rotation axis. 57 4-17 Custom bracket for holding half-silvered mirror in secondary mirror housing 59 4-18 Stop blocks for the secondary support ring.
61 4-19 Empty Coudé mirror cell with monofilament crosshair. 62 4-20 The f/8 secondary mirror mounted with monofilament crosshair. 65 4-21 Laser crosshairs on inside of dome. 68 4-22 Rotating image of the double cluster h and χ Persei,.
69 4-23 Image showing coma. 70 4-24 Defocused image showing astigmatism. 70 4-25 Optical table setup for crosshair installation. 72 4-26 Secondary support bracket.
75 5-1 Background subtraction example. 83 xiii 5-2 Background subtraction comparison in Stokes Q. 85 5-3 Background subtraction comparison in Stokes U. 86 5-4 Results when selectively turning off background subtraction.
87 5-5 UBVRI equivalent filter polarizations in Stokes Q with no instrumental calibration applied. 89 5-6 UBVRI equivalent filter polarizations in Stokes U with no instrumental calibration applied. 90 5-7 QU plot of blue observations in aperture 0. 91 5-8 QU plot of red observations in aperture 0.
92 5-9 QU plot of blue observations in aperture 1. 93 5-10 QU plot of red observations in aperture 1. 94 5-11 Average Stokes Q and U vs. wavelength in aperture 0.
95 5-12 Average Stokes Q and U vs. wavelength in aperture 1. 96 5-13 Rotated average Stokes Q and U vs. wavelength in aperture 0 with fit.
98 5-14 Rotated average Stokes Q and U vs. wavelength in aperture 1 with fit. 99 5-15 Calibrated UBVRI equivalent filter polarizations in Stokes Q. 100 5-16 Calibrated UBVRI equivalent filter polarizations in Stokes U.
101 5-17 UBVRI equivalent filter polarizations in Stokes Q with no instrumental calibration applied for PBO and Ritter Observatory. 104 5-18 UBVRI equivalent filter polarizations in Stokes U with no instrumental calibration applied for PBO and Ritter Observatory. 105 6-1 HD189733 Stokes Q results. 111 6-2 HD189733 Stokes U results.
112 6-3 HD189733 Stokes U breakout of observation from August 31st 2012. 114 6-4 β-Cas Stokes Q results. 115 6-5 β-Cas Stokes U results. 116 xiv 6-6 HD189733 aperture 0 Stokes U results.
117 6-7 HD189733 aperture 1 Stokes U results. 118 6-8 HD189733 aperture 0 Stokes Q results. 119 6-9 HD189733 aperture 1 Stokes Q results. 120 6-10 HD189733 Q/σ for aperture 0.
121 6-11 HD189733 Q/σ for aperture 1. 122 6-12 HD189733 U/σ for aperture 0. 123 6-13 HD189733 U/σ for aperture 1. 126 6-16 HD189733 spectral comparison.
128 7-1 φ-Per results from PBO .