Download RESEARCH STATEMENT Chromospheres and winds

RESEARCH STATEMENT
Chromospheres and winds of giant stars
Kathy Geise ([email protected])
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Summary
My research centers around mass loss from evolved stars and the evolution of circumstellar
material. An upcoming eclipse of ζ Aurigae offers a relatively rare opportunity to observe the
extended atmosphere of an eclipsing binary system in order to identify the physics contributing
to the formation of chromospheres in evolved giant stars. I will collaborate with Elizabeth
Griffin to look for evidence of magnetic field contribution to chromospheric emission lines in
spectra taken during egress and to model the results. Archival data, including Dr. Griffin’s
numerous series of unpublished monitoring of past chromospheric eclipses of the system, as well
as other data available through the DAO and CADC, are fundamental to the interpretation of
transient phenomena in the system. This project makes excellent use of both archival data and
the instruments employed in the long observing history of this object. I am well–qualified to
undertake the proposed research project because of my prior experience analyzing spectra and
polarimetric data taken of the eclipsing binary system, ε Aurigae.
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Background
Mass loss from late–type evolved stars contributes material to the next generation of stars and
planets and plays an important role in both stellar and galactic evolution. The mechanisms that
drive winds from cool (.8000K) evolved stars such as K and early-M stars are not well known.
Unlike hot stars, radiative acceleration of gaseous material in K stars is not large enough to drive
a wind. There is insufficient dusty material to contribute to dust–driven winds and coherent
pulsations strong enough to deliver sufficient acoustic energy to the outer layers of cool stars
are also not evident. Magnetic processes such as Alfvén waves are likely a major contributor
to winds of dust–free, non–pulsating K and M giant and supergiant stars. However, current
magnetic models do not agree well with observed spectra. Observations continue to provide new
insights into mass-loss driven by magnetic processes.
An understanding of the dynamics and thermodynamics of the atmospheres of late-type
evolved stars is essential to constrain mass–loss mechanisms. Eclipsing binary systems offer a
unique opportunity to study stellar atmospheres, especially when the geometry is edge–on or
nearly edge–on. ζ Aurigae binary systems are an important class of interacting binaries with
both a hot and a cool component. Classical ζ Aurigae systems contain a primary supergiant star
of spectral type G or K and secondary star of spectral type B. The supergiant’s extended atmosphere may be studied using eclipse spectra because the primary star’s atmosphere is probed
by the light of hot secondary star during eclipse, revealing the height dependence of thermodynamic parameters such as density, temperature, ionization and the extent and motion of the
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chromosphere.
The eponymous ζ Aurigae (K4 Ib + B5 V) is an eclipsing binary system with a period of
972 days and an eclipse duration of about 38 days [1]. Atmospheric eclipse phenomena have
been observed in the optical using ground–based instruments [2] and in the ultraviolet using
the International Ultraviolet Explorer (IUE) [3]. ζ Aurigae has been observed at the Dominion Astronomical Observatory (DAO) over many eclipse cycles with the 1.2 m telescope. The
chromospheric Ca ii K absorption line in high resolution spectra taken using the McKellar spectrograph have sometimes been observed to split into two components. One possible explanation
for the resulting line profile is that a central emission overlays the absorption. Such emission
could be an indication of a transient magnetic phenomenon visible at certain heights in the
stellar atmosphere. Chromospheric modeling could establish whether the narrow profiles of the
split lines are independent absorptions or a combined absorption overlain by emission.
Two other ζ Aurigae systems, 31 Cyg (K4 Ib + B3–4) and 32 Cyg (K4–5 Ib + B6–7) have
also been observed by Dr. Griffin over many eclipse cycles. The chromospheres of these stars
differ from each other and vary between eclipse cycles. Comparisons between the supergiants in
ζ Aurigae, 31 Cyg and 32 Cyg will be important to our understanding of evolved stars because
their chromospheric features behave quite differently even though nominally the supergiants
are very similar in both size and luminosity. The spectra for this study have been prepared
using advanced processes to isolate the contribution to the light from each component to create
untangled spectra [4].
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Prior Work
My current research project was part of an international campaign to observe ε Aurigae during
the recent 2009–2011 eclipse. Epsilon Aurigae (HD 31964) is a single-lined spectroscopic binary
system consisting of a spectral type F0 star and a binary companion embedded in an opaque
dusty disk. The system eclipses once every 27 years for a duration of two years, the longest
period of totality of known eclipsing systems [5]. The ε Aurigae system offered 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.
Spectropolarimetric observations of the ε Aurigae system also revealed evidently persistent,
irregularly variable, intrinsic line polarization of the F0 star in the system [6]. Epsilon Aurigae was observed by ESPaDOnS during late 2012, well out of eclipse, to solidify our growing
understanding of the underlying physics contributing to variability of the F0 star (PI: Nadine
Manset, CFHT observing request 12BD001).
The long history of observations of ε Aurigae paint a picture rich with features that may
inform other similar studies. Energy propagation through the outer regions of the stellar atmosphere of the F0 star in the ε Aurigae system may be inferred from observations. For example,
ε Aurigae exhibits highly variable spectral line absorption features, many of which appear to
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be persistently polarized, which may indicate the presence of large turbulent cells in the stellar atmosphere. Underlying emission has been observed in hydrogen Balmer lines and is most
pronounced in Hα; however, the source of the emission is not well–defined. Lastly, Griffin and
Stencel [7] used current and digitized archival spectra to discover a mass transfer stream visible
between specific eclipse egress phases.
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Proposed Work
I propose to study the chromospheric spectra of ζ Aurigae giant stars in collaboration with
Elizabeth Griffin using spectra from the DAO collected over many eclipse cycles. We will monitor upcoming eclipse cycles for transient chromospheric phenomena. An upcoming eclipse of
ζ Aurigae begins in 2014 with centrality on 2014 July 21 and egress about August 9. 32 Cyg
may also be observed during an upcoming eclipse on 2015 October 26. My contribution to the
collaboration will include spectral analysis and support for chromospheric modeling.
I also propose that we look for collaborations at DAO to facilitate acquisition of polarimetric
and spectropolarimetric data of the ζ Aurigae system to look for magnetic effects using Stokes V
observations. Linear Stokes observations may also help identify anisotropies in the stellar atmosphere or asymmetries associated with granulation, turbulence or other flows contributing
energy to the outer stellar atmosphere. These studies may help constrain parameters for chromospheric models of the system. CADC archives do not include ESPaDOnS observations of
ζ Aurigae although ε Aurigae was observed with great success over its recent eclipse cycle.
References
[1] Philip D Bennett, Graham M Harper, Alexander Brown, and Christian A Hummel. The Masses and Radii
of the Eclipsing Binary zeta Aurigae. Astrophysical Journal v.471, 471:454, November 1996.
[2] R E M Griffin, R F Griffin, K P Schroeder, and D Reimers. Optical spectra of Zeta Aurigae binary systems.
I - The 1987 eclipse of Zeta Aurigae. Astronomy and Astrophysics (ISSN 0004-6361), 234:284–298, August
1990.
[3] R E Stencel and R D Chapman. The 1979-1980 eclipse of Zeta Aurigae. II - The emission spectrum.
Astrophysical Journal, 251:597–603, December 1981.
[4] R E M Griffin, R F Griffin, and K P Schroder. The Technique of Spectral Subtraction - Optical Spectra of
the Chromosphere of Zeta-Aurigae. Cambridge Workshop on Cool Stars, 9:249, January 1990a.
[5] R E Stencel. epsilon Aurigae - an Overview of the 2009-2011 Eclipse Campaign Results. The Journal of the
American Association of Variable Star Observers, 40:618, June 2012.
[6] K Geise, R E Stencel, N Manset, D Harrington, and J Kuhn. Eclipse Spectropolarimetry of the ε Aurigae
System. The Journal of the American Association of Variable Star Observers, 40:767, November 2012.
[7] R Elizabeth Griffin and Robert E Stencel. Merging Recent and Historic Spectra of Aurigae: Properties of
the System’s Components, and Discovery of a Mass Transfer Stream. Publications of the Astronomical Society
of the Pacific, 125(929):775–792, July 2013.
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