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E. J. Rivera (Carnegie Institution), J. J. Lissauer (NASA/Ames)
We present two planet fits to the radial velocity measurements taken at the Keck observatory of the star GJ 876 that account for the mutual perturbations between the planets. Additionally, the guessed parameters that are used to start the iterative procedure to determine the fits are constrained by recent astrometric measurements of the star performed by Benedict et al. (2002; ApJ 581, L115). For some fits, it was also assumed that the planets were nearly coplanar; specifically, the inclinations were within 6\circ of each other, centered around 25\circ, and the difference in the nodes was < 4\circ. With fixed inclinations, the \sqrt{\chi\nu2} lies in the range 1.62 to 1.66. When the inclinations (and one of the nodes) are fitted, the fitting procedure converged to the same set of parameters for about 80% of the (initial guessed) parameter space surveyed, with a \sqrt{\chi\nu2} of 1.6552. In all cases, the mutual inclination was < 20\circ and the planetary masses were close to their nominal values. Thus, with the astrometric constraints and the assumption that the system is nearly coplanar, these dynamical fits can place strong constraints on the system parameters. Without the astrometric constraints, the fitting algorithm can find stable systems with large mutual inclinations or with coplanar, but significantly more massive planets.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.