Presented by Kathy Geise DU Graduate Colloquium May 14, 2014 HPOL out-of-eclipse observations of epsilon Aurigae Advisors: Robert Stencel Jennifer Hoffman

Epsilon Aurigae Research focus and motivation Background HPOL observations Polarization Preliminary results Outline 2 Image credit: NASA/JPL-Caltech

Eclipse Long period 27 years Long duration 2 years Single-lined spectroscopic binary system Spectral type F star Opaque disk Hidden B star companion Epsilon Aurigae system 3 Image credit: NASA/JPL-Caltech

Research focus and motivation  The mass and evolutionary status of the system are uncertain  Hypothesis: the visible star in the system is an evolved F supergiant star  Method: differential analysis  Motivation:  Stellar evolution in the context of binary systems  Disk formation and evolution 4

5  Medium resolution spectropolarimeter  Spectral range 3200A to 7750A  Spectral resolution of 25A  Previously at the Pine Bluff Observatory, University of Wisconsin  Relocated to Ritter Observatory, University of Toledo  6 observations of epsilon Aurigae (OOE)  1990 – 1996  Broadband artificial filter HPOL

6 X Orbital solution by Stefanik et al. (2010) Approx. eclipse phases Epsilon Aurigae orbit schematic (not to scale)

Bjorkman, K. (2012) Polarization and asymmetries 7

Sources of broadband polarization 8 ScattererSizeObserved polarization Electron---Pure Thomson scattering Wavelength independent DustSmall (x<<1) Wavelength dependent DustLarge (x>1) Wavelength independent “gray”

9 U B V Filter R

10 U B V Filter R

11 (same scale as above) 25% 0% 10% 0% Δ%p

Summary  Wavelength dependent polarization  Change in polarized flux larger than change in flux for epochs of greater %p  Suggests an increase in number of scatterers  Next steps: remove stellar contribution from eclipse broadband observations 12

13 Acknowledgements Thank you to Mike Wolf for the original observations of epsilon Aurigae; James Davidson, University of Toledo, for recent data reduction; and Marilyn Meade and Brian Babler at the University of Wisconsin. Thank you

References (1/2) Bjorkman, K, “Polarimetry of Binary Stars and Exoplanets”, Proceedings IAU Symposium No Coyne, G. V “Preliminary Observations of Variable Pulsation in Epsilon Aurigae.” Ricerche Astronomiche 8: 311–18. Stefanik, Robert P, Guillermo Torres, Justin Lovegrove, Vivian E Pera, David W Latham, Joseph Zajac, and Tsevi Mazeh “Epsilon Aurigae: an Improved Spectroscopic Orbital Solution.” The Astronomical Journal 139: doi: / /139/3/

References (2/2) HPOL in the IR photo credit Wendy Mukluk (2002) – Artist's concept image credit. Data from Spitzer Space Telescope. Spitzer's infra-red vision revealed the size of the dusty disk that swirls around the companion object. When astronomers plugged this data into a model of the system, they were able to rule out the theory that the main bright star is a supergiant. Instead, it is a bright star with a lot less mass. The new model also holds that the companion object is a so- called "B star" circled by a dusty disk. 3 February 2010 –

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Polarized flux  %p = P/I * 100  Need calibrated intensity 18 Bessell (1998)

19 AAVSO

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21 HPOL Observed Flux

22 AAVSO

Calculate flux from magnitudes  Bessel et al. (1998) 23

How are flux and polarized flux changing?  Measure with respect to one epoch  Epoch of smallest observed flux ( ) 24

Stokes frame to stellar reference frame 25

26 Stefanik (2010) Spectroscopic orbital solution

27 Coyne (1972) Polarization observations of epsilon Aurigae

28 Sample

Pre-eclipse, Hα 29

Pre-eclipse, Hα 30 α Per F5Ib

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Phase = [ JD - 2,454,515] / 9896d Stefanik et al. (2010) 32 Henson (1982)) Cole (2009)

Phase = [ JD - 2,454,515] / 9896d Stefanik et al. (2010) 33 Henson (1982) Cole (2009)

34 Phase function Polarization θ 

35 Phase function Polarization axial θ 

Research The epsilon Aurigae system offers a unique opportunity to uncover physical mechanisms contributing to disk formation and evolution and to explore the relationship between photospheric anisotropies, stellar pulsation and mass loss. The distance to the system is ill-defined, making the mass and evolutionary status of the components of the system uncertain. I will show that the system consists of an evolved F supergiant star, its B star binary companion(s) and a disk of gas anddust that originated from the F star. This research contributes to our understanding of stellar evolution in the context of binary stars. The research also offers the opportunity to investigate disk formation and evolution significant to our understanding of protoplanetary disks now seen in many star systems. Lastly, the project will consider radiative transfer of polarized light which contributes to diverse fields such as atmospheric studies of exoplanets. 36

Abstract Context: Epsilon Aurigae is a single-lined spectroscopic binary system that consists of a variable F0 supergiant star and an occulting disk surrounding an unseen object, probably a B star. The eclipse occurs once every 27 years and lasts for almost 2 years. Aims: We tested the hypothesis that the F0 star in the system shows intrinsic broadband polarization and that the degree of polarization is related to the brightness of the variable F0 star. Method: We characterized the contribution of interstellar polarization using archive data from the HPOL polarimeter and subtracted the interstellar component from broadband observations to characterize the intrinsic polarization of the system. Results: We determined that continuum polarization out-of-eclipse was sometimes wavelength dependent. The most likely scattering mechanism out-of-eclipse is pure electron scattering because the regime is too hot for dust formation. One possible mechanism is wavelength dependent absorption along the line of sight superposed over wavelength dependent Thomson scattering. 37

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