Observational Limits on Terrestrial‐sized Inner Planets around the CM Draconis System Using the Photometric Transit Method with a Matched‐Filter Algorithm

Abstract

A light curve of the eclipsing binary CM Draconis has been analyzed for the presence of transits of planets of size 2.5 Earth radii with periods of 60 days or less, and in coplanar orbits around the (R E ), binary system. About 400 million model light curves, representing transits from planets with periods ranging from 7 to 60 days, have been matched/correlated against these data. This process we call the ii transit detection algorithm ˇˇ or TDA. The resulting ii transit statistics ˇˇ for each planet candidate allow the quanti—cation of detection probabilities, and of false-alarm rates. Our current light curve of CM Dra has a coverage of 1014 hr with 26,043 individual points, at a photometric precision between 0.2% and 0.7%. Planets signi—cantly larger than would constitute a 3R E ii supranoise ˇˇ detection, and for periods of 60 days or less, they would have been detected with a prob- ability greater than 90%. ii Subnoise ˇˇ detections of smaller planets are more constrained. For example, planets with 10 day periods or less would have been detected with an 80% probability. The neces- 2.5R E sity for predicted observations is illustrated with the nine top planet candidates that emerged from our TDA analysis. They are the planet candidates with the highest transit statistics from the 1994¨1998 observing seasons, and for them transits for the 1999 observing season were predicted. Of the seven can- didates that were then observationally tested in 1999, all were ruled out except one, which needs further observational con—rmation. We conclude that the photometric transit method is a viable way to search for relatively small, inner extrasolar planets with moderate-sized telescopes using CCD photometry with a matching-—lter analysis. Subject headings: binaries: eclipsingmethods: statisticalplanetary systems ¨ stars: individual (CM Draconis) ¨ techniques: photometric