Astrometric and Light-Travel Time Orbits to Detect Low-Mass Companions: A Case Study of the Eclipsing System R Canis Majoris
Résumé
We discuss a method to determine orbital properties and masses of low-mass bodies orbiting eclipsing binaries. The analysis combines long-term eclipse timing modulations (the light-travel time [LTT] effect) with short-term, high-accuracy astrometry. As an illustration of the method, the results of a comprehensive study of Hipparcos astrometry and over 100 years of eclipse timings of the Algol-type eclipsing binary R Canis Majoris are presented. A simultaneous solution of the astrometry and the LTTs yields an orbital period of P 12 ¼ 92:8 AE 1:3 yr, an LTT semiamplitude of 2574 AE 57 s, an angular semimajor axis of a 12 ¼ 117 AE 5 mas, and values of the orbital eccentricity and inclination of e 12 ¼ 0:49 AE 0:05 and i 12 ¼ 91=7 AE 4=7, respectively. Adopting the total mass of R CMa of M 12 ¼ 1:24 AE 0:05 M , the mass of the third body is M 3 ¼ 0:34 AE 0:02 M , and the semimajor axis of its orbit is a 3 ¼ 18:7 AE 1:7 AU. From its mass, the third body is either a dM3-4 star or, more unlikely, a white dwarf. With the upcoming microarcsecond-level astrometric missions, the technique that we discuss can be successfully applied to detect and characterize long-period planetary-size objects and brown dwarfs around eclipsing binaries. Possibilities for extending the method to pulsating variables or stars with transiting planets are briefly addressed.
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