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The principal characteristics of the eclipsing Aurigæ star system can be reproduced by assuming that the optically unseen secondary is a disk of gas and dust which is in centrifugal balance in the radial direction and is hydrostatically supported perpendicular to its midplane. Several of the basic characteristics including ingress time, egress time, and the general flat shape of the minimum are well accounted for. The geometry of the eclipse implies that the disk midplane is tilted by < 3 from the line of sight.

The wavelength-independence of the observed eclipse depth in the visual and near-IR provides important constraints on the properties of the disk. This colorlessness implies that the particles responsible for the bulk of the opacity of the disk are m in radius. If, in contrast, an ISM particle distribution were assumed, the eclipse would be much deeper in the blue than in the red and the near-IR. This conclusion is stronger and more quantitative than that of Kopal (1971), who also noted that the grayness of the eclipse implies large particles. Unless particles of radius m are almost totally absent from the secondary, the scale height at the outer edge of the disk must be quite small, of the disk's radius. The low ratio of the scale height to the disk's radius lends some support to the high-mass model of the Aurigæ system.

Our analysis assumes that the gas and dusk in the disk are well mixed. If instead virtually all of the particles in the disk are settled into a significantly thinner dust zone, the secondary could present a sharp-edged profile that produces a colorless eclipse even with an ISM opacity law. However, even a small quantity of submicron-sized dust remaining well above the midplane would color the eclipse.

Our model can produce a significant mid-eclipse brightening only if the optical depth of the disk is small. A central hole can increase the magnitude of this mid-eclipse brightening. However, the duration of the model's mid-eclipse brightening is longer than that observed. In addition, a model disk optically thick enough to reproduce the mid-eclipse brightening in single-color data would only produce a ``gray'' eclipse if virtually all the particles in the disk are much larger than 5 m in radius.

Absorption lines seen during and immediately after the F star eclipse are probably produced by a thin layer of gas in the outer portion of the disk companion secondary which expands when it is heated by radiation from the F star and contracts when it is shielded.

The above analysis demonstrates that the Aurigæ disk is well-modeled by a gas/dust disk with a scale height at its outer edge that is 3% of the disk's radius. Such a disk contains particles characteristically much larger than interstellar dust. Our results will provide more valuable information on the process of planet formation when HIPPARCOS parallax measurements become available. Once the distance to the system is known, the mass and evolutionary state of the primary will be better understood (Webbink 1985). The masses of the components and the geometry of the Aurigæ system will then provide a lower bound on the maximum size of a (quasi-) stable circumstellar disk within a binary system of known dimensions. If the system turns out to be young (Carroll et al. 1991), then the disk properties that we have derived become directly applicable to models of protoplanetary disks. If the Aurigæ disk was produced by recent ( years) mass transfer (Eggleton and Pringle 1985), then the lack of small grains would imply that rapid grain growth can occur within circumstellar disks and pre-planetary processes may also occur in this context.


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Scott J. Wolk
Mon Nov 25 15:41:03 EST 1996