Doppler Data and Analysis for the Three Planets Orbiting GJ 876 From Rivera et al. 2005, submitted to The Astrophysical Journal.

The Keck-HIRES echelle spectrometer was upgraded with a new triple-CCD mosaic (2K x 4K, Lincoln-Lab CCD devices, 15 micron pixels) and new optics, all optimized for precise Doppler planet hunting by the California & Carnegie Planet Search Team. Team member Steven Vogt was PI of the HIRES upgrade, completed in August 2004 .

Doppler velocity versus time for standard M dwarfs spanning 7 years. The Keck-HIRES system achieves precision of 3 - 5 meters/sec for M dwarfs brighter than Vmag = 11.

Velocity vs Time for the star, GJ 876. Top: Observed Doppler velocities (points) with model radial velocity curve (solid line) generated from the best-fit, two-planet model, for orbital planet inclination, i = 90 deg (edge-on). The two-planet model was described in Marcy et al. 2001. The two-planet model fit is poor, and is not significantly improved for other inclinations. Bottom: Residuals to the orbital fit.

Periodogram of residuals to the two-planet, inc = 90 deg, co-planar fit. Note the strong power at 1.94 days.

Top: Velocity vs Time from Keck, 1996 - 2005 for GJ 876. Observed Doppler measurements (points) are shown with the best-fit model radial velocities (solid line) generated from the self-consistent, three-planet fit for GJ 876 (coplanar, inc = 50 deg). Bottom: Residuals to the orbital fit. The fit is far superior to that which invokes only 2 planets, as expected from the periodicity seen in the residuals to the 2-planet fit.

Velocity Residuals to the Two-Planet (only) Fit vs Phase. Residuals to a two-planet model versus phase. Model radial velocities were generated from a two-planet fit (inc = 90 deg) to the observed radial velocities for GJ 876. The residuals are periodic, and can be fit using a one-planet (Keplerian) model, as shown here. The solid (red) line is for a fit with eccentricity of the inner planet, ecc = 0, and the dashed (blue) line is for a fit with eccentricity, ecc = 0.22. The residual velocities are phased with a period of 1.9379 days and are shown as small solid points with vertical error bars corresponding to the uncertainties. The model corresponds to a planet with Msini = 5.9 M_Earth, P = 1.94 d, and a = 0.021 AU.