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Session 12 - Binary Stars.
Display session, Monday, June 08
Atlas Ballroom,

[12.13] Brown Dwarf Detected in the CM Dra Binary System

E. F. Guinan (Villanova U.), D. H. Bradstreet (Eastern College), I. Ribas (U. Barcelona), M. Wolf (Charles Univ., Czech), G. P. McCook (Villanova U.)

We report on the detection of a probable brown dwarf or massive planet in the CM Dra (dM4.5+dM4.5) eclipsing binary system. Possible planetary transit eclipses (with depths of 0.08mag) have been reported previously for CM Dra (see IAUC 6423; IAUC 6425). In addition to searching for more planetary transit events, extensive photometry is being conducted to search for evidence of the "light-time effect" in the arrival times of the eclipses of CM Dra. The light-time effect occurs as the binary system moves around the barycenter of the binary-third body system. From extensive photometry obtained from March 1996 through March 1998, 17 eclipse timings have been obtained. Preliminary results indicate a light-time effect with an amplitude of delta t = 18 +/-2 sec and an orbital period of about P = 70.3 +/- 3.0 days. Adopting the systemic mass of CM Dra (M1+2 = 0.448 Msun) and assuming a circular orbit, the size of the 3rd body's orbit is about 0.27AU. The mass of the third body is about M3 = 64 +/- 4Mjupiter (0.061Msun) if its orbital plane is coplanar (or nearly so) with the eclipsing pair. This mass is near the upper limit for substellar bodies (brown dwarfs/massive planets) and below the lower mass limit of about 0.075Msun for hydrogen burning stars. The phasing of the light-time effect indicates that the object that produces the observed light-time effect is not the object(s) that produced the planetary transit dimming event in 1996. It is likely that CM Dra contains more than one substellar object.

The presence of a brown dwarf around an eclipsing binary is critically important because the object's physical properties (such as mass and radii) are determinable. We will discuss the updated analysis of our data including any additional eclipse timings. This research is supported by NSF/RUI Grants AST 93-15365, which we gratefully acknowledge.

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