The Gemini Planet Imager Exoplanet Survey: Dynamical Mass of the Exoplanet β Pictoris b from Combined Direct Imaging and Astrometry

Abstract

We present new observations of the planet β Pictoris b from 2018 with GPI, the first GPI observations following conjunction. Based on these new measurements, we perform a joint orbit fit to the available relative astrometry from ground-based imaging, the Hipparcos Intermediate Astrometric Data (IAD), and the Gaia DR2 position, and demonstrate how to incorporate the IAD into direct imaging orbit fits. We find a mass consistent with predictions of hot-start evolutionary models and previous works following similar methods, though with larger uncertainties: 12.8 +5.3 −3.2 M Jup . Our eccentricity determination of 0.12 +0.04 −0.03 disfavors circular orbits. We consider orbit fits to several different imaging datasets, and find generally similar posteriors on the mass for each combination of imaging data. Our analysis underscores the importance of performing joint fits to the absolute and relative astrometry simultaneously, given the strong covariance between orbital elements. Time of conjunction is well constrained within 2.8 days of 2017 September 13, with the star behind the planet's Hill sphere between 2017 April 11 and 2018 February 16 (± 18 days). Following the recent radial velocity detection of a second planet in the system, β Pic c, we perform additional two-planet fits combining relative astrometry, absolute astrometry, and stellar radial velocities. These joint fits find a significantly smaller mass for the imaged planet β Pic b, of 8.0 ± 2.6 M Jup , in a somewhat more circular orbit. We expect future ground-based observations to further constrain the visual orbit and mass of the planet in advance of the release of Gaia DR4.

Constraining the Orbit and Mass of epsilon Eridani b with Radial Velocities, Hipparcos IAD-Gaia DR2 Astrometry, and Multiepoch Vortex Coronagraphy Upper Limits

“…We use data from van Leeuwen (2007) that consists of the IAD residuals from the fit to the data (van Leeuwen 2007), the scan direction, epochs, and the errors associated with the original data. Using the fit from van Leeuwen (2007) and the residuals combined with the scan direction data, we reconstruct the one-dimensional scan measurements following the method used in Nielsen et al (2020). The available data consists of 78 data points; Table 6 lists the complete set of data points used.…”

“…For the astrometric model, we use a similar approach to Rosa et al (2019a) and Nielsen et al (2020) while also fitting for the parallax and M tot . This framework consists on deriving from the model a displacement of the photocenter from the barycenter induced by the presence of a companion.…”

“…We compare this values to the corresponding α IAD and δ IAD that we calculate from the 1D scans from IAD. We compute c H 2 in the same way as in Nielsen et al (2020).…”

Multiwavelength Mitigation of Stellar Activity in Astrometric Planet Detection

“…As the host star and planet orbit their barycenter, astrometric measurements of the host star should oscillate with the orbital period. While astrometry has not yet directly found any exoplanets on its own, such measurements have already helped constrain orbital parameters of planets, such as in the case of 51 Eridani b (Rosa et al 2015) and β Pictoris b (Nielsen et al 2020).…”

Constraints on the nearby exoplanet ϵ Indi Ab from deep near- and mid-infrared imaging limits

“…In two cases so far, directly imaged planets have also been observed astrometrically. The planet β Pic b (Lagrange et al 2010) causes an acceleration of its host star between the Hipparcos (Perryman et al 1997) and Gaia (Gaia Collaboration et al 2018) astrometric epochs (Snellen & Brown 2018;Nielsen et al 2020;Brandt et al 2020). In combination with direct monitoring of the planet motion, this yields a dynamical mass estimate of 11±2 M J .…”