Luận Văn A New Era Of Observational Capability At Ritter Observatory - Spectropolarimetry From Exoplanets To Circumstellar Disks And Beyond.pdf

A new era of observational capability at Ritter Observatory spectropolarimetry from exoplanets to circumstellar disks and beyond The University of Toledo The University of Toledo Digital Repository Th[.]

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

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 Paper 59.

A Dissertationentitled

A New Era of Observational Capability at Ritter Observatory:

Spectropolarimetry from Exoplanets to Circumstellar Disks and Beyond

byJames W Davidson Jr

Submitted to the Graduate Faculty as partial fulfillment of the requirements for the

Doctor of Philosophy Degree in Physics & Astronomy

Dr Karen S Bjorkman, Committee Chair

Dr Robert W Collins, Committee Member

Dr S Thomas Megeath, Committee Member

Dr J.D Smith, Committee Member

Dr John P Wisniewski, Committee Member

Dr Patricia R Komuniecki, DeanCollege of Graduate Studies

The University of Toledo

May 2013

Copyright 2013, James W Davidson Jr.

This document is copyrighted material Under copyright law, no parts of this

An Abstract of

A New Era of Observational Capability at Ritter Observatory:

Spectropolarimetry from Exoplanets to Circumstellar Disks and Beyond

byJames W Davidson Jr

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 Polarimeter (HPOL) spectropolarimeter to the University of Toledo’s Ritter Observa-tory This process required fairly extensive work to the optical-mechanical alignment

Halfwave-of the Ritter Observatory 1-meter telescope Ultimately the restoration and 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 sixmonths of observing time at Ritter Observatory The results of this effort has shownthe polarimetric stability in the observations is at the same level or better compared

reloca-to almost 10 years of observations at Pine Bluff Observareloca-tory, after the upgrade reloca-to thedetector

Aside from continued spectropolarimetric monitoring programs which were tially focused on a selection of Be and Wolf-Rayet stars, we sought to investigatepotential polarimetric variability in exoplanet systems In particular we focused onthe system HD189733, as there was previous observational work in the literatureclaiming both a detection and non-detection at different optical wavelengths Thegoal was to investigate this system with HPOL to provide simultaneous observationsacross the entire optical wavelength range to investigate the possibility of a wavelengthdependent variable polarization with orbital phase While the variability claimed in

ini-the literature is below ini-the noise limits of our observations, an interesting and yet explained increase in the polarization level occurred in one of our observations Thishigher than expected signal had vanished by the next observation, which was severalnights 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 lessthan a couple orbital periods This increase is around an order of magnitude largerthan the claimed detection in the literature from light scattering off the exoplanetsatmosphere, and would seem to be caused by some other physical mechanism in thesystem 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 isrepeatable.

un-To my family

I would like to thank my office mates for the better part of six years, CharlesPoteet and Blagoy Rangelov, for many great conversations.

I would also like to thank my family and friends for all their support over theyears, in particular my soon to be wife, K¨ara Lindelof, who moved to Toledo with meand 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, bourg, France This work has been partially funded by a Small Research Grant fromthe AAS, and by the Scott E Smith Fund for Research at Ritter Observatory

1.1 Background on Polarization 1

1.2 Solar System Polarization 8

1.3 Polarization of Unresolved stars 9

1.4 Polarization of Circumstellar Material 11

1.5 Polarization of Exoplanet Systems 11

1.5.1 Scattered Light from an Exoplanet Atmosphere 12

1.5.2 Using Exoplanets to Probe Limb Polarization of Host Stars 13

1.5.3 Starspots as a Source of Variable Polarization 16

2 The HPOL Spectropolarimeter 18 2.1 History of the Instrument 18

2.2 Description of the Instrument 19

2.2.1 Arc and Flat Lamp Assembly 19

2.2.2 Polarizing Prism 22

2.2.3 Slit & Decker Assembly 23

2.2.4 Filters 24

2.2.5 Halfwave Plate 24

2.2.6 Cylindrical Lens 25

2.2.7 Wollaston Prism 26

2.2.8 Gratings 29

2.2.9 Camera 30

2.2.10 Detector 30

2.3 The End of the PBO Era 31

3 Restoration & Relocation 33 3.1 Restoring HPOL to Operation 33

3.1.1 Control System Software Restoration 33

3.1.2 Slit Camera Replacements 34

3.2 Relocation to Ritter Observatory 34

3.3 New Layout of Equipment 35

4 Ritter Observatory 37 4.1 Telescope Collimation 38

4.1.1 Optical Aberrations 38

4.1.1.1 Spherical 38

4.1.1.2 Coma 39

4.1.1.3 Astigmatism 40

4.1.2 Two Mirror Mis-alignments 43

4.1.3 Collimating the Ritter Observatory 1-meter Telescope 45

4.1.4 Development of a New Collimation Procedure 45

4.1.4.1 Optical Table Assembly 46

4.1.4.2 Defining the Telescope Optical Axis 47

4.1.4.3 Setting the Rotation of the Secondary Mirror Ring 60 4.1.4.4 Aligning the Secondary Mirror Housing Assembly and Spider with the Telescope Optical Axis 62

4.1.4.5 Adjusting the Tilt of the Secondary Mirrors 65

4.1.4.6 Adjusting the Tilt of the Primary Mirror 67

4.1.4.7 Final Adjustments On-Sky 68

4.1.5 Installing Monofilament Crosshairs 72

4.1.6 Identification and Solution to a Flexure Issue in Secondary Mir-ror Assembly 73

4.1.7 Extender for HPOL 74

4.2 Summary of Telescope Collimation 75

5 Polarization Calibration 77 5.1 Observing Unpolarized Stars 79

5.2 Constructing a Calibration File 80

5.3 The Final Calibration File 97

5.4 Comparison With 10 Years of Previous PBO Calibration Work 103

6 Polarimetric Observations of the Exoplanet System HD189733 107 6.1 Observations 108

6.2 Data Reduction 109

6.3 Results 111

6.4 Higher Than Expected Signal 113

6.5 Conclusions 129

7.1 Calibration Improvements 130

7.1.1 Evaluation of β-Cas as an Unpol Standard 130

7.1.2 Cutting Polarization Observations at 3200˚A 131

7.2 Bringing Faint-Mode Back Online 131

7.3 The HD189733 System 132

7.4 Long Term Monitoring Programs 133

7.5 Conclusions 140

List of Tables

5.1 Unpolarized Standard Stars 79

5.2 Previous Observations of Unpolarized Standard Stars 80

5.3 List of Observations 84

5.4 Average polarization values for RO 102

5.5 Systematic error for RO 103

5.6 Systematic Errors from PBO and RO 106

6.1 Previous published parameters for the planet HD189733b 108

6.2 Previous published parameters for the star HD189733 109

6.3 List of HD189733 Observations 110

List of Figures

1-1 Polarization ellipse 2

1-2 Examples of Stokes parameters 4

1-3 Poincar´e Sphere 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

1-8 Transit geometry 14

1-9 Occultation polarization at the Ca I λ4227˚A 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

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 Coud´e 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

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

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 1 124

6-14 HD189733 Q/σ 125

6-15 HD189733 U/σ 126

6-16 HD189733 spectral comparison 128

7-1 φ-Per results from PBO 134

7-2 φ-Per results from Ritter 135

7-3 ψ-Per results from PBO 136

7-4 ψ-Per results from Ritter 137

7-5 π-Aqr results from PBO 138

7-6 π-Aqr results from Ritter 139

7-7 ζ-Tau results from PBO 140

7-8 ζ-Tau results from Ritter 141

List of Abbreviations

CCD Charge-coupled deviceHPOL Halfwave-PolarimeterISM interstellar mediumPBO Pine Bluff Observatory

Figure 1-1 Polarization ellipse showing the ellipse traced out by the electric

field, ~E, in the x-y plane for polarized light traveling in the

z-direction, into the page

as

Ex = I1/2(cos(β) cos(θ) cos(2πνt) + sin(β) sin(θ) sin(2πνt))

Ey = I1/2(cos(β) sin(θ) cos(2πνt) − sin(β) cos(θ) sin(2πνt))

(1.2)

where I is the intensity of the beam and the angle β is defined such that tan(β) =B/A, the ratio of the semi-minor to semi-major axes as seen in Figure 1-1 Relatingequations 1.1 and 1.2 gives

Ex o(cos(2πνt) cos(δx) − sin(2πνt) sin(δx)) =

I1/2(cos(β) cos(θ) cos(2πνt) + sin(β) sin(θ) sin(2πνt))

Eyo(cos(2πνt) cos(δy) − sin(2πνt) sin(δy)) =

I1/2(cos(β) sin(θ) cos(2πνt) − sin(β) cos(θ) sin(2πνt))

Equating common coefficients then gives

Ex ocos(δx) = I1/2cos(β) cos(θ)

Ex osin(δx) = −I1/2sin(β) sin(θ)

Ey ocos(δy) = I1/2cos(β) sin(θ)

Ey osin(δy) = I1/2sin(β) cos(θ)

Algebraic manipulation of these equations provides the four definitions,

Icos= 2ExoEyocos(δy − δx) = I cos(2β) sin(2θ)

Isin = 2ExoEyosin(δy− δx) = I sin(2β)

These are the Stokes equations, which are more commonly written as

where I is the total intensity, p is the fraction of polarized light, Q and U are used

to describe the linear polarization, and V describes the rotation of the polarizationvector for circular and elliptical polarization A more detailed derivation of the Stokesparameters can be found, for example, in Clarke (2010) Figure 1-2 shows examples

of 100% polarization in each of the Q, U and V Stokes values As the Stokes equationsare expressed in terms of the two angles, 2β and 2θ, the polarization can be plotted

in polar coordinates in what is referred to as a Poincar´e sphere (see Figure 1-3) Inthis work only linear polarization (I,Q,U) is measured, and circular polarization has

Figure 1-2 Examples of Stokes parameters In all cases the light,

repre-sented by the red line, is 100% polarized in the x-y plane asshown

Figure 1-3 Poincar´e sphere showing the parameterization of the Stokes

val-ues Q, U and V

been mentioned only for completeness Q and U represent orthogonal states of linearpolarization, where the total amplitude of linear polarization is

P = pQ2+ U2

For linear polarization, the ellipse from Figure 1-1 becomes a line, where the angle

β = 0 In this case the three dimensional Poincar´e sphere is now just the dimensional Q-U plane, which can simply be plotted in the common QU-plot Anexample of a QU-plot for the polarized standard star HD6353 is seen in Figure 1-4

two-In a QU-plot the polarization is represented by a vector originating at the origin,where the length of the vector is the amplitude of polarization, and the angle fromthe positive Q-axis is 2θ, or twice the angle the polarized electric field ~E makes withthe positive x-axis In this vector space individual sources of polarization, such asinstrumental, line of sight polarization, and the intrinsic polarization of the source,are added together The line formed by the data points in Figure 1-4 is due to the

Figure 1-4 Example QU plot of the polarized standard star HR6353.

wavelength dependence of polarization from the ISM along the light of sight Figure1-5 shows an illustration of a QU-plot for a given wavelength value demonstrating howthe intrinsic polarization of a source, blue dashed vector, and the ISM polarization,red dashed vector, add together to produce the polarization value which is measured,black vector

To measure Q, or U, calcite is used to split the incoming light into its ordinaryand extraordinary components The details of this will be discussed in Section 2.2.7.The flux difference between the two components is the measure of Q, or U depending

on the orientation with respect to the sky The Stokes I parameter is obtained bycombining the two components of polarized light, the ordinary and extraordinary,and hence for each observation the total intensity of a source is also measured.Most people have experience with polarization whether or not they are familiarwith Stokes parameters; for example, through wearing a pair of polarized sunglasses,

Figure 1-5 QU plot illustration The blue dashed vector represents the

intrinsic polarization of a source The red dashed vector

rep-resents the ISM polarization along the line of sight The black

vector represents the measured polarization

or more recently with 3D movie and television technology Most 3D video works byprojecting two images through separate polarizers on the same screen, though thesesystems typically make use of circular polarization The 3D movie glasses containtwo different polarizers as lenses, allowing only one of the images to pass througheach lens Each eye sees a different image, providing the 3D effect While humansneed polarized glasses to view polarized light, bees (for example) can see polarizedlight directly, and use the polarization of the sky as a means of navigation (Rossel

& Wehner 1986) Other animals, such as the octopus, are also sensitive to polarizedlight and use this ability for everything from navigation to locating prey (e.g., Shashar

& Cronin 1996 and references therein)

Polarized light can be produced in a variety of scattering mechanisms, such asRayleigh, Thompson or Mie scattering In astronomy, polarized light has been used

to study a wide range of objects; everything from bodies in our own solar system,

to circumstellar material, to the cosmic microwave background (a key area listed in

the 2010 Decadal Survey, New Worlds, New Horizons), and everything in between.

Polarimetry has been conducted in a wide range of wavelengths Most importantly,polarimetry probes a region of parameter space which would otherwise be unavail-able by other techniques, providing information about the properties of the materialcausing the scattering as well as the orientation of the scatterers by measuring theStokes parameters.

The scattering of sunlight off planet atmospheres in our solar system has beenstudied using polarimetry ever since Lyot (1924) first measured the polarization ofVenus Uranus and Neptune, for example, show strong limb polarization (Schmid

et al 2006) The radial change in the polarization starts around zero in the center,and increases to ∼2.2% and ∼1% at 5500 ˚A for Uranus and Neptune respectively(Joos & Schmid 2007) The polarization of Jupiter, first observed by Lyot (1929),

is as high as 60% at the poles, measured from Pioneer 10 and 11, compared to theequatorial region which is <10% (Smith & Tomasko 1984) Similar results are alsoseen in Saturn (Tomasko & Doose 1984), and even Titan shows ∼50% polarizationfor the integrated disk (Tomasko & Smith 1982) The mechanisms for polarization inall these cases are a combination of Rayleigh scattering (usually molecular) and Miescattering by small particles and aerosols The wavelength and phase angle depen-

dence of the polarization (c.f Coffeen & Gehrels 1969) gives information on the size,

nature and chemical composition of scatterers (more precisely, the real and complexindices of refraction) For example, such studies were instrumental in discovering thepresence of a haze layer of sulfuric acid droplets in the atmosphere of Venus (Hansen

& Hovenier 1974) Mars was first observed in polarized light by Lyot (1929), and thefirst UV spectropolarimetry used to measure the surface pressure was performed byFox et al (1997) The Moon, again first observed by Lyot (1929), was first observed

spectropolarimetrically in the UV by Fox et al (1998) who showed the albedo mum in the UV was consistent with previous work (Wagner et al., 1987; Lucke et al.,1976) Polarimetry has been utilized to study the surface of the Moon and lunar re-golith particles (for a review see Shkuratov et al (2007, 2011)) Most recently Sterzik

mini-et al (2012) observed Earthshine using spectropolarimmini-etry to investigate atmosphericand surface composition and biosignatures from Earth

Our Sun also shows limb polarization in what is now referred to as the second solarspectrum (Ivanov 1991; Stenflo & Keller 1997) This polarization is due to Thompsonscattering, from free electrons, and Rayleigh scattering Studies of magnetic fieldshave been conducted by observing the Zeeman effect and the Hanle effect, a process

by which lines become depolarized and are thereby hidden to Zeeman studies alone

A center-to-limb variation can reach ∼1% in the limb (e.g Leroy 1977) However,certain resonance lines can exhibit much higher levels of polarization, such as the

Ca I λ4227˚A line, which has a 16.5% polarization (Bianda et al 1999) While thesepolarization levels can be high, it is strictly because the Sun is resolved, and so thepolarization is measured over small regions rather than over the disk as a whole.The integrated solar disk has been measured by Kemp et al (1987) to have a linearpolarization of less than 10− 7 in V band

Polarimetry provides information about a star, even when that star is unresolved.The low, close to zero, level of polarization of the integrated stellar disk of the Sun

is due to a centro-symmetric pattern of the polarization (see Figure 1-6) The larization at any given point along a stellar limb is essentially canceled out by thepolarization at a point on the limb 90 degrees away, assuming a spherical shape tothe star However, if part of the stellar limb of an unresolved star were to be oc-

po-Figure 1-6 The centro-symmetric pattern of the polarization of a star being

oc-culted by an exoplanet The black polarization vectors are visible

while the white vector is being blocked by the transiting planet

Po-larization vectors are larger towards the limb and zero at the center,

symbolizing the increased polarization level near the limb The gray

dashed line represents the orbital path of the planet

culted, the net result would be a measure of the occulted star’s limb polarization,which Chandrasekhar (1950) predicted could be as high as 11% Figure 1-6 showsschematically the centro-symmetric pattern of polarization vectors, which are zero atthe center of the star, and increasing in magnitude towards the limb Averaged overthe whole disk, the sum of all these vectors would be zero However, when part ofthe disk is blocked, as illustrated by the black dot, the white vector will be blockedand the total integrated polarization will have a net result orthogonal to the vectorbeing blocked Despite the original prediction of limb polarization by Chandrasekhar(1946), it was not observed until 1983, when Kemp et al (1983) measured a variationaround primary eclipse of Algol, a binary system This is still the only measurement

of the limb polarization of a star other than the Sun, despite the predictions of thiseffect being around for over 65 years

1.4 Polarization of Circumstellar Material

Polarimetry can also be used to study details about both the star itself and thecircumstellar environment, such as in Herbig Ae/Be stars, T Tauri stars, Classical Bestars, B[e] stars, and massive stars Polarization can provide information on the ori-entation of circumstellar disks, as well as on the densities and types of scatterers Forexample, Herbig Ae/Be stars are dominated by dust scattering (Waters & Waelkens,1998), producing an increase in the polarization versus wavelength from the optical

to infrared (Coyne & Vrba, 1976) The circumstellar disks of classical Be stars onthe other hand are gaseous (Porter & Rivinius, 2003), giving rise to a saw-toothlike wavelength dependence of the polarization, with a Balmer jump, sloping Paschen

continuum, and Paschen jump (e.g., Bjorkman 2006; Clarke & Bjorkman 1998)

Clas-sical Be stars are also typically less polarized, ∼0.5-1%, compared to Herbig Ae/Bestars, with an average polarization around 3% (and maximum observed polarization

∼14.5%; Yudin 2000) Poeckert & Marlborough (1976) observed 48 Be stars anddiscovered that the intrinsic polarization depends strongly on v sin(i), where v is theequatorial velocity of the star and i is the inclination between the star’s rotation axisand the observer’s line of sight For a review of Be stars using polarimetry, see Coyne

& McLean (1982)

Since the first discovery of an exoplanet by Mayor & Queloz (1995), the number

of known exoplanets has steadily grown to now over 800, and increases almost daily.Observations of these systems using a variety of techniques across the electromagneticspectrum provide insight into how planets form and evolve Polarimetry is a techniquewhich to this point has been underutilized in the area of exoplanets, and theoreticallycan be used to study aspects of exoplanet system other techniques cannot, as discussed

in the following sections.

1.5.1 Scattered Light from an Exoplanet Atmosphere

It has already been shown that planetary atmospheres can do a fairly efficient job

of polarizing reflected light, so it should be anticipated that exoplanet atmosphereswill do the same thing This idea prompted theoretical investigations to determinethe level of polarization expected from such system (e.g Seager et al 2000) However,the early results of these works predicted the level of polarization to be 10− 5 for aperfectly reflecting lambertian sphere, and more realistically would have an upperlimit of 10− 6

Hough et al (2006) built the imaging polarimeter PlanetPol, which can measurepolarization down to the 10− 6 level, to try and detect these low polarization lev-els Observations of the systems 55Cnc and τ Boo yielded only upper limits, andsupported the theory that such signals were very low (Lucas et al 2009) However,Berdyugina et al (2008) observed HD189733 and claimed to observe polarization vari-ability at the level of 2 × 10− 4, well above the theoretical predictions This result hassparked discussion in the community as to whether or not this detection is real (e.g.,Seager & Deming 2010) Wiktorowicz (2009), using the instrument POLISH, which

is designed to be similar to the PlanetPol instrument, claimed a non-detection for thesame system with a >99.99% confidence down to a level of 10− 5, and similarly Lucas

et al (2009) also find inconsistent results with the Berdyugina et al (2008) work.Fluri & Berdyugina (2010) proposed that the reason for the detection by Berdyugina

et al (2008), and non-detection by Wiktorowicz (2009), is the wavelength dependence

of Rayleigh scattering, since the observations by Berdyugina et al (2008) were in the

U and B bands, and the POLISH and PlanetPol instruments operate in the green andred respectively Follow-up observations by Berdyugina et al (2011) were conducted

their V band observations to be consistent with the non-detection of Wiktorowicz(2009) (see Figure 1-7) These results suggest that simultaneous multi-band obser-vations would be ideal for evaluating the possible variability, and spectropolarimetryprovides a means by which to explore this.

1.5.2 Using Exoplanets to Probe Limb Polarization of Host

Stars

The occultation of a host star by an exoplanet provides a unique opportunity formeasuring the limb polarization of the host stars, and has been explored theoretically

by Carciofi & Magalh˜aes (2005) Independently, David Clarke also discusses this

technique in his recent book Stellar Polarimetry (Clarke 2010) Carciofi & Magalh˜aes

(2005) show that for a star with a limb polarization of 1% being occulted by a hotJupiter, the expected polarization signal would be of order 10− 4, reaching as high as

10− 3if molecular Rayleigh scattering dominates in a high-albedo cool star atmosphere.Figure 1-8 depicts the transits of exoplanets with radii of 1 RJupand 2 RJup around aSun-like star, while Figure 1-9 shows the expected polarization for the Ca I λ4227˚Aresonance line These hot Jupiter systems provide an excellent testing ground forthis technique as they are already well studied photometrically and have very preciseorbital periods, typically on the order of days More recently, Kostogryz et al (2011)have used the work by Carciofi & Magalh˜aes (2005), using known system parameters,

to calculate the predicted values for HD189733 (see Figure 1-10)

As mentioned previously the Sun has certain spectral lines which are more highlypolarized, such as Ca I λ4227˚A, and which have been reviewed by Landi Degl’Innocenti(2002) and Stenflo (2002) In addition to the Ca I λ4227˚A line, other notable linesinclude Sr I 4607 and Na I D2, as well as several others (e.g., in the 4884-4888 ˚A, 4931-

Figure 1-7 Figure from Berdyugina et al (2011) showing the purported

de-tection of polarimetric variations due to scattered light from theatmosphere of exoplanet HD189733b in and U and B bands, while

explaining the non-detection in V band Top: U B V

measure-ments for stokes q (left) and stokes u (right) U and V-Band datahave been shifted vertically by ±4 × 10− 4 Open squares repre-sent the binned data from Berdyugina et al (2011), open circlesare the binned data from Berdyugina et al (2008), and crosses

are measurements from Wiktorowicz (2009) Bottom: Combined

U and B-band data from Berdyugina et al (2008) and ina et al (2011) for stokes q (left) and stokes u (right) (Figurefrom Berdyugina et al 2011)

Berdyug-Figure 1-8 Geometry of the transit of a 1RJup (small solid circle) and a

2RJup (dashed circle) exoplanet in front of a Sun-like star forthree inclinations (from Carciofi & Magalh˜aes 2005)

Figure 1-9 Occultation polarization at the Ca I λ4227˚A line for an exoplanet

transiting a Sun-like star Results shown are for two planet radii,

(dashed lines) and

(dotted lines) (from Carciofi & Magalh˜aes (2005))

Figure 1-10 Modeling of the B-band flux and polarization (Stokes q=Q/I,

u=U/I and polarization magnitude) for the planetary transitfor HD 189733 (from Kostogryz et al (2011))

4936 ˚A and 5682-5687 ˚A regions)(Stenflo et al 1983a,b, 1998) For certain transitingexoplanets, Doppler shifts within lines have already been measured via the Rossiter-McLaughlin (R-M) effect The R-M effect can be used to measure the orbital planeinclination with respect to the spin axis of the host star due to Doppler shifts in thestellar absorption lines during occultation (e.g Winn 2011) So far the R-M effect hasbeen measured for around 50 exoplanet systems (a database for systems known to datecan be found at: http://www.aip.de/People/rheller/content/main spinorbit.html),making these systems well suited for study since line effects have already been mea-sured Polarization observations of such systems will allow for the determination ofthe orbital plane orientation on the sky, which can otherwise not be obtained.

1.5.3 Starspots as a Source of Variable Polarization

Polarization signatures from exoplanets occulting the stellar limb, or from tering in the exoplanet atmosphere, are not the only variable sources of polarization

scat-in these systems, and it is crucial to understand other possible sources of contributscat-ingpolarization Another possible contributor to the polarization signal is from starspots.Sunspots have been studied extensively in polarized light to study the Zeeman effect,which produces a large amount of circular polarization; however the linear polar-ization produced by the transverse Zeeman effect can be comparable to the limbpolarization, again in the resolved case (for a review see Lin 2005) Starspots havealready been observed in polarized light for other stars (e.g., Alekseev & Kozlova2003; Mekkaden et al 2007) When starspots are near the stellar limb they willbreak the centro-symmetric pattern since there will be less flux to scatter compared

to a position 90 degrees away on the stellar limb However, unlike the case of anexoplanet which blocks a section of the stellar limb, the starspots are still emitting,and so would need to be of significant size compared to the star to produce the same

red dwarfs with linear polarization and found no polarimetric signal from starspots.These systems have a much higher filling factor in terms of starspot coverage com-pared to G type stars, and hence for this case the signal level should be even lower.While photometric observations of the K type star HD189733 have shown that thissystem is chromospherically active, only ∼1% of the surface has starspots at anygiven time (Pont et al 2007; Winn et al 2007) Additionally, the starspots probe therotational period of the host star, which is typically quite different from the orbitalperiod of exoplanets, and much longer than the occultation timescales For exampleHD189733 has a photometric transit time of 1.8 hours (Winn et al 2007), an orbitalperiod of 2.22 days (Winn et al 2007), and a rotational period of almost 12 days(Henry & Winn 2008) Similarly the HD 209458 system has a photometric transittime of ∼3 hours (Knutson et al 2007), an orbital period of 3.52 days (Knutson et

al 2007), and references therein), and a rotational period anywhere from 11.4 to 15days (Silva-Valio 2008 and references therein) Thus for long-term observations ofsuch systems, the periodic behavior of starspots should be distinguishable

Chapter 2

The HPOL Spectropolarimeter

Instrumentation efforts related to the HPOL spectropolarimeter constitute a jor portion of this dissertation work This chapter focuses on a brief history of theinstrument and a detailed description The restoration and relocation efforts arediscussed in Chapter 3, the extensive work performed at Ritter Observatory to in-corporate HPOL is discussed in Chapter 4, and the instrumental calibration work isdiscussed in Chapter 5

The University of Wisconsin’s (UW) Halfwave Polarimeter (HPOL) was installed

as a dedicated instrument at Pine Bluff Observatory (PBO) located outside of son, WI, in January 1989 HPOL was a ground-based support instrument for the Wis-consin Ultraviolet Photo-Polarimeter Experiment (WUPPE), one of the telescopesflown as part of NASA’s Astro package aboard the Space Shuttle in 1990 (STS-35,Astro-1) and 1995 (STS-67, Astro-2) Over the years at PBO, HPOL underwent anumber of upgrades and modifications The most important of these was in 1995when the detector was changed from a Reticon dual channel photo-diode array, de-picted in Figure 2-1, to a Reticon 400x1200 CCD HPOL also was taken on several

Madi-it was mounted to the WIYN 3.5m telescope Numerous spectropolarimetric studies

of different types of astronomical objects have been conducted with HPOL over manyyears including: classical Be stars, B[e] stars, pre-main-sequence stars, luminous redvariable stars, novae and supernovae (for a list of publications resulting from HPOLdata, see the instrument web page at http://www.sal.wisc.edu/HPOL/ )

The HPOL spectropolarimeter is a standard Boller & Chivens spectrograph, model

26767, that has been extensively modified There are three main sources for tions of HPOL, Wolff et al (1996), Nook (1990) and Nordsieck & Harris (1996).However, none of these are complete, particularly given the detector upgrade to theCCD in 1995 In this section a detailed description of HPOL is presented While parts

descrip-of this description can be found in the other sources, the purpose here is to compile

a complete description of HPOL in its current configuration at Ritter Observatorywhich will prove a useful reference moving forward Figure 2-2 shows schematicallythe layout of HPOL with most of the components labeled The individual componentsare discussed in the following sections in order from telescope to detector, along theoptical path

2.2.1 Arc and Flat Lamp Assembly

Mounted to the underside of the instrument mounting plate is a slide assemblywhich houses the arc (wavelength calibration) and flat lamps Figure 2-3 shows theunderside of the mounting plate, when it was off the instrument, with the lampassembly connected This assembly is remotely activated via the instrument controlsoftware and moved into the optical path by a small stepper motor A potentiometermeasures the positioning of this assembly to ensure the lamps are within a specified

Quartz Window S-20 Photocathode Proximity-focus Micro Channel Plate Electron Multiplier Aluminum Anti-Feedback Layer P-1 Phosphor Fiberoptic Backplate Fiberoptic Conduit RETICON

Single Photon

Primary Photo-Electron

Many Photons

Micro Channel Electron Multiplier The "gain" across the top to bottom

of the micro channel tube determines the velocity of the cascading electrons The velocity of the the number of secondary electrons kicked off at each strike Low gain will produce 1/4 the number of secondary electrons as high gain nth order

Secondary Electrons

Single Photo-Electron

Many Secondary Electrons

Actual Size

A B

Figure 2-1 The Reticon photo-diode array detector used on HPOL up to

1995 (Credit: K Nordsieck)

UV Glan-Thompson calibration prism Instrument focal plane

All-Refractive

UV Achromatic Camera Plane Grating

HPOL Spectropolarimeter

IR block

Figure 2-2 The HPOL spectropolarimeter optical design (Credit: K

Nord-sieck)

Figure 2-3 Underside of the HPOL mounting plate The arc and flat lamp

assembly, seen toward the top of the image, is mounted to a

threaded rod on the left and a guide rod on the right A small

stepper motor seen in the bottom left rotates the threaded rod

to move the assembly into and out of the optical path The large

central hole in the middle of the plate is the entrance aperture

to HPOL In the position shown the lamp assembly is moved out

of the optical path

range before allowing the lamps to be turned on The flat lamp is a Tungsten lamp,used for flat fielding A HgAr Oriel pencil style calibration lamp, often referred to as

a ‘Penray’ lamp, is used for wavelength calibration (Oriel part number 6035) Whennot in use, the assembly is moved along the track out of the optical path such thatall parts are outside the entrance aperture of the instrument

2.2.2 Polarizing Prism

The first optical component inside HPOL is a UV Glan-Thompson calibrationprism This prism can be manually inserted into the optical path to produce 100%polarized light across the entire wavelength coverage of the instrument

Figure 2-4 The slit (left) and decker (right) assemblies for HPOL with their

corresponding numbers and sizes in arcseconds labeled Sizes

in arcseconds for Ritter Observatory have been added in red

(Credit: K Nordsieck)

2.2.3 Slit & Decker Assembly

A slit and decker assembly sits at the focal plane of this instrument Consisting oftwo surface aluminized Dow Fotoform glass plates, there are five possible slit sizes andeight possible decker options (see Figure 2-4) Most observations are carried out withslit #3 and decker #5, which creates two apertures with a size of ∼5.4‚Ö10.8‚when

on the Ritter Observatory 1-meter telescope (sizes in arcseconds when used at PBOand WIYN are listed in Figure 2-4, along with Ritter Observatory sizes listed in red)

On the underside of the slit and decker mounting bracket is a Uniblitz VS25 shutterwhich is connected to the Photometrics control box for the detector This shuttertakes 3.0msec to open and features a 25mm aperture, large enough for all slit anddecker combinations