Radio interferometric planet search. II. Constraints on sub-jupiter-mass companions to GJ896A

We present results from the Radio Interferometric Planet search for companions to the nearby star GJ896A. We present 11 observations over 4.9yr. Fitting astrometric parameters to the data reveals a residual with peak-to-peak amplitude of 3mas in right ascension. This residual is well fit by an acceleration term of 0.458 0.032masyr^-2. The parallax is fit to an accuracy of 0.2mas and the proper motion terms are fit to accuracies of 0.01masyr^-1. After fitting astrometric and acceleration terms, residuals are 0.26mas in each coordinate, demonstrating that stellar jitter does not limit the ability to carry out radio astrometric planet detection and characterization. The acceleration term originates in part from the companion GJ896B, but the amplitude of the acceleration in declination is not accurately predicted by the orbital model. The acceleration sets a mass upper limit of 0.15 M_J at a semimajor axis of 2AU for a planetary companion to GJ896A. For semimajor axes between 0.3 and 2AU upper limits are determined by the maximum angular separation; the upper limits scale from the minimum value in proportion to the inverse of the radius. Upper limits at larger radii are set by the acceleration and scale as the radius squared. An improved solution for the stellar binary system could improve the exoplanet mass sensitivity by an order of magnitude.