Astrometry and radial velocities of the planet host M dwarf GJ 317: new trigonometric distance, metallicity and upper limit to the mass of GJ 317b

TL;DR

This study refines the distance and metallicity of M dwarf GJ 317, constrains the mass of its planet GJ 317b, and suggests the presence of additional long-period companions, highlighting the link between metallicity and gas giant formation.

Contribution

It provides the first precise astrometric distance for GJ 317, constrains the planet's mass using Bayesian analysis, and reports potential additional planets based on RV data.

Findings

Distance to GJ 317 is 15.3 pc, 65% farther than previous estimates.

The mass of GJ 317b is less than twice its minimum mass, most likely around 2.5 Jupiter masses.

Additional long-period planet candidates are suggested by RV data.

Abstract

We have obtained precision astrometry of the planet hosting M dwarf GJ 317 in the framework of the Carnegie Astrometric Planet Search project. The new astrometric measurements give a distance determination of 15.3 pc, 65% further than previous estimates. The resulting absolute magnitudes suggest it is metal rich and more massive than previously assumed. This result strengthens the correlation between high metallicity and the presence of gas giants around low mass stars. At 15.3 pc, the minimal astrometric amplitude for planet candidate GJ 317b is 0.3 milliarcseconds (edge-on orbit), just below our astrometric sensitivity. However, given the relatively large number of observations and good astrometric precision, a Bayesian Monte Carlo Markov Chain analysis indicates that the mass of planet b has to be smaller than twice the minimum mass with a 99% confidence level, with a most likely value of 2.5 Mjup. Additional RV measurements obtained with Keck by the Lick-Carnegie Planet search program confirm the presence of an additional very long period planet candidate, with a period of 20 years or more. Even though such an object will imprint a large astrometric wobble on the star, its curvature is yet not evident in the astrometry. Given high metallicity, and the trend indicating that multiple systems are rich in low mass companions, this system is likely to host additional low mass planets in its habitable zone that can be readily detected with state-of-the-art optical and near infrared RV measurements.