Taming the Invisible Monster: System Parameter Constraints for є Aurigae from the Far-ultraviolet to the Mid-infrared

Creators: Hoard, D. W.; Howell, S. B.; Stencel, R. E.

Abstract

We have assembled new Spitzer Space Telescope IRAC observations of the mysterious binary star є Aurigae, along with archival far-ultraviolet to mid-infrared data, to form an unprecedented spectral energy distribution (SED) spanning 3 orders of magnitude in wavelength from 0.1 μm to 100 μm. The observed SED can be reproduced using a three-component model consisting of a 2.2^(+0.9)_(–0.8) M_☉ F-type post-asymptotic giant branch star, and a 5.9 ± 0.8 M_☉ B5±1 type main-sequence star that is surrounded by a geometrically thick, but partially transparent, disk of gas and dust. At the nominal HIPPARCOS parallax distance of 625 pc, the model normalization yields a radius of 135 ± 5 R_☉ for the F star, consistent with published interferometric observations. The dusty disk is constrained to be viewed at an inclination of i ≳87°, and has an effective temperature of 550 ± 50 K with an outer radius of 3.8 AU and a thickness of 0.95 AU. The dust content of the disk must be largely confined to grains larger than ~10 μm in order to produce the observed gray optical-infrared eclipses and the lack of broad dust emission features in the archival Spitzer mid-infrared spectra. The total mass of the disk, even considering a potential gaseous contribution in addition to the dust that produces the observed infrared excess, is « 1 M_☉. We discuss evolutionary scenarios for this system that could lead to the current status of the stellar components and suggest possibilities for its future evolution, as well as potential observational tests of our model.

Additional Information

© 2010 The American Astronomical Society. Received 2009 December 4; accepted 2010 March 18; published 2010 April 12. We thank Sally Seebode for her valuable assistance in assembling the data used in this work, and the anonymous referee, whose comments and suggestions improved the manuscript. D.W.H. thanks S. Carey and J. Surace of the IRAC Instrument Support Team for helpful discussions about the properties of the IRAC detector hardware. We give a special thanks to P. Kohler (Kohler 1992) for devising an appropriate epithet for Aur ("le monstre invisible"), which we have borrowed for the title of our paper. We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). Support for this work was provided by NASA. Some of this work was also supported by NASA through contract agreement 1275955 with the University of Denver, issued by the Jet Propulsion Laboratory, California Institute of Technology. R.E.S. is grateful to the estate of William Herschel Womble for support of astronomy at the University of Denver. This work makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/Caltech, funded by NASA and the NSF. Some of the data presented in this paper were obtained from the Multimission Archive at the Space Telescope Science Institute (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NAG5-7584 and by other grants and contracts. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France, and NASA's Astrophysics Data System. Facilities: FUSE, HST(GHRS), IUE, AAVSO, IRAS, Spitzer(IRAC, IRS, MIPS)

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