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LARGE CIRCUMBINARY DUST GRAINS AROUND EVOLVED GIANTS? M. Jura, R. A. Webb (UCLA) & C. Kahane (Obs. de Grenoble) INTRODUCTION The growth of solids into planetesimals in circumstellar disks is a major unsolved astrophysical problem. Almost all investigations of dust disks have concentrated on pre-main sequence stars, but there are some post-main sequence binary stars which possess orbiting circumbinary dust disks. We are studying disks in evolved systems with the goal of learning more about particle growth. Accretion disks around a mass-receiving star in mass-exchange binary systems such as cataclysmic variables and symbiotics stars have been studied for many years. In an accretion disk, the lifetime of an individual grain may be short even if the system is long-lived. In contrast, in a circumbinary disk, the particles orbit both stars and are not being continuously destroyed. As a result, in circumbinary disks, particles may survive enough orbits to grow by coagulation to sizes as large as 1 mm. With only a few known examples of this phenomenon such as the Red Rectangle and AC Her (see Waters et al. 1993, Jura & Kahane 1999, Jura, Chen & Werner 2000, Molster et al. 2000), we hope to identify more such systems. Because small spherical grains do not emit efficiently at wavelengths much larger than their size, one way to identify ``big" particles is to observe at relatively low frequencies. Therefore, we have obtained mm and sub-mm observations of SS Lep (mV = 5.0 mag, period = 260 days, A1 + M4; Cowley 1967, Welty & Wade 1995) and 3 Pup (mV = 4.0 mag, period = 161 days, A2I + ?; Plets et al. 1995) because they are A-type stars in the Yale Bright Star Catalog with anomalously and uniquely high IRAS fluxes (Jura & Kleinmann 1990) and we suspected that they may have circumbinary disks. As part of this program, we also obtained data for BM Gem, a highly luminous carbon star with oxygen-rich circumstellar matter which also may possess a long-lived orbiting disk (Kahane et al. 1998). There are many previous infrared studies of evolved binary stars (for example, Friedemann, Gurtler & Lowe 1996). A system that might have some properties in common with SS Lep and 3 Pup is the binary containing the post-main sequence luminous A-type star, epsilon Aur (Mv <= -6.0 mag) which has a companion of unknown type that is surrounded by a dust disk containing grains larger than 5 um (Lissauer et al. 1996). Here, we consider circumbinary environments where the grains may be more than an order of magnitude larger in size and thus 103 times more massive than those found around the companion to epsilon Aur. In contrast to their infrared properties, little is known about submillimeter dust emission from evolved binary stars. Symbiotic stars have been detected at wavelengths > 100 um (Seaquist & Taylor 1992, Ivison et al. 1995, Corradi et al. 1999), but this emission is probably produced by ionized gas and not dust. In the systems discussed here, thermal emission by dust probably dominates at wavelengths <= 1350 um.
CONCLUSIONS We have detected mm and sub-mm continuum emission from two evolved binaries, SS Lep and 3 Pup, and also possibly from BM Gem. 1. This continuum is probably produced by emission from dust colder than ~ 70 K lying within 6" of the star and can be explained if the dust particles are at least as large as 0.1 mm in radius. 2. We propose that there are circumbinary orbiting disks of at least 5 x 1028 g and that the ``large" particles have grown by coagulation in this disk. These disks may have winds with mass loss rates of ~ 5 x 1017 g s-1 and lifetimes >= 2000 yr. A preprint of this article is available here. BIBLIOGRAPHY Corradi, L. M., Brandi, E., Ferrer, O. E., & Schwarz, H. E. 1999,
AstronAstroph, 343, 841
jura@clotho.astro.ucla.edu
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