Date of Award
Physics and Astronomy
Robert Stencel, Ph.D.
Binaries: spectroscopic, Line profiles, Polarization, Stars: atmospheres, Stellar evolution
The bright eclipsing binary system epsilon Aurigae 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. 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 considers radiative transfer of polarized light that contributes to diverse fields such as atmospheric studies of exoplanets.
My objective is to understand the evolutionary status of the eclipsing disk in the epsilon Aurigae system by exploring the distribution of gas and characteristics of dust grains in the disk. I evaluated spectral features and linear polarization in ESPaDOnS out-of-eclipse spectra in order to characterize line polarization observed in the system. It is important to understand the out-of-eclipse behavior in order to correctly differentiate signals seen in ESPaDOnS spectra taken during the most recent (2009--2011) eclipse. This thesis work tested the hypothesis that the F0 star in the system shows intrinsic broadband polarization and that the percent linear polarization (%p) is related to the brightness of the variable F0 star observed out--of--eclipse. Uncovering the position angle of intrinsic linear polarization also tested whether dust grains in the disk are small (< 1 micron) or large (>10 micron).
I evaluated spectral features and line polarization of select disk lines before, during and after eclipse in order to better resolve the distribution of gas in the disk. Significant (>4 sigma) linear polarization peaks are associated with spectral absorption features and the scattering geometry is revealed by an analysis of the dominant polarization position angles. I characterized the contribution of interstellar polarization using archived data from the HPOL polarimeter and from published broadband filter data and subtracted the interstellar component from broadband out--of--eclipse and in--eclipse observations to characterize the intrinsic continuum polarization of the system. Results of these efforts are itemized below.
First, linear polarization is persistent in out--of--eclipse observations, appears strongest in the core of each associated spectral line and is variable in time and strength. The polarization observed out--of--eclipse likely arises from an equatorial density enhancement in the gas in the outer layers of the F0 star.
Second, disk rotation signatures appear in eclipse polarization spectra in both low energy (Ell < 3eV) and high energy (Ell >3eV) atomic transitions just after mid--eclipse even when additional absorption does not appear to be present in the line. Linear polarization (%p) is bi-lobed and Stokes U is anti-symmetric through the line at these times. These polarimetric signatures are consistent with line polarization in models of optically thick rotating disks.
Third, an increase in line polarization precedes an increase in continuum polarization near eclipse 3rd contact. This is due to a density enhancement in the disk located on the side of the disk illuminated by the F0 star but offset from the direct line in the direction of disk rotation. I propose that scattering angles imposed by the system geometry are responsible for the phenomenon. Line polarization is stronger at egress than ingress supporting a model in which a temperature gradient is present in the disk, and the back side of the disk extends above the dusty opaque layer after mid-eclipse.
Finally, intrinsic continuum polarization observed out-of-eclipse (OOE) is sometimes wavelength-dependent. Changes to %p are not correlated with changes in F0 star brightness. I uncovered the underlying intrinsic polarization in published eclipse data to find an intrinsic linear polarization position angle of 90 degrees in the stellar/disk reference frame during mid--eclipse phases. Polarization during eclipse primarily arises from forward--scattering at large dust grains or from multiple scattering from optically thick dusty material. Optically thin scattering from small grains would yield position angles closer to 0 degrees in the stellar/disk reference frame. The presence of large dust grains suggests that the disk is an evolved debris disk rather than a young proto-planetary disk and that the F0 star in the system is an evolved star.
Geise, Kathleen M., "Spectropolarimetry of epsilon Aurigae: Probing Stellar and Disk Atmospheres" (2015). Electronic Theses and Dissertations. 235.
Received from ProQuest
Kathleen M. Geise