Dynamical Mass of the Exoplanet Host Star HR 8799

Aldo G Sepulveda,Brendan P Bowler

doi:10.3847/1538-3881/ac3bb5

Abstract

Abstract HR 8799 is a young A5/F0 star hosting four directly imaged giant planets at wide separations (∼16–78 au), which are undergoing orbital motion and have been continuously monitored with adaptive optics imaging since their discovery over a decade ago. We present a dynamical mass of HR 8799 using 130 epochs of relative astrometry of its planets, which include both published measurements and new medium-band 3.1 μm observations that we acquired with NIRC2 at Keck Observatory. For the purpose of measuring the host-star mass, each orbiting planet is treated as a massless particle and is fit with a Keplerian orbit using Markov chain Monte Carlo. We then use a Bayesian framework to combine each independent total mass measurement into a cumulative dynamical mass using all four planets. The dynamical mass of HR 8799 is 1.47 − 0.17 + 0.12 M ⊙ assuming a uniform stellar mass prior, or 1.46 − 0.15 + 0.11 M ⊙ with a weakly informative prior based on spectroscopy. There is a strong covariance between the planets’ eccentricities and the total system mass; when the constraint is limited to low-eccentricity solutions of e < 0.1, which are motivated by dynamical stability, our mass measurement improves to 1.43 − 0.07 + 0.06 M ⊙. Our dynamical mass and other fundamental measured parameters of HR 8799 together with Modules for Experiments in Stellar Astrophysics Isochrones and Stellar Tracks grids yields a bulk metallicity most consistent with [Fe/H] ∼ −0.25–0.00 dex and an age of 10–23 Myr for the system. This implies hot-start masses of 2.7–4.9 M Jup for HR 8799 b and 4.1–7.0 M Jup for HR 8799 c, d, and e, assuming they formed at the same time as the host star.

Highlights

HR 8799 is a A5/F0 V star (Gray & Kaye 1999) that is most well known for hosting four exoplanets that have been directly imaged within a cold extended debris disk and beyond a warmer interior dust belt (Marois et al 2008, 2010; Su et al 2009; Booth et al 2016; Wilner et al 2018; Faramaz et al 2021; see review by Konopacky & Barman 2018)
We present a dynamical mass of HR 8799 using 130 epochs of relative astrometry of its planets, which include both published measurements and new medium-band 3.1 μm observations that we acquired with NIRC2 at Keck Observatory
Sepulveda & Bowler strained the orbits of the companions (e.g., Soummer et al 2011; Currie et al 2012; Esposito et al 2013; Gozdziewski & Migaszewski 2014; Pueyo et al 2015; Maire et al 2015; Zurlo et al 2016; Konopacky et al 2016; Wertz et al 2017; Wang et al 2018b; Gravity Collaboration et al 2019), but the host star mass has been assumed to be fixed at ∼1.5 M in the majority of these analyses based on estimates from surface gravity measurements and high-mass evolutionary models (e.g., Gray & Kaye 1999; Moya et al 2010a; Baines et al 2012)

Summary

Introduction

HR 8799 is a A5/F0 V star (Gray & Kaye 1999) that is most well known for hosting four exoplanets that have been directly imaged within a cold extended debris disk and beyond a warmer interior dust belt (Marois et al 2008, 2010; Su et al 2009; Booth et al 2016; Wilner et al 2018; Faramaz et al 2021; see review by Konopacky & Barman 2018). Sepulveda & Bowler strained the orbits of the companions (e.g., Soummer et al 2011; Currie et al 2012; Esposito et al 2013; Gozdziewski & Migaszewski 2014; Pueyo et al 2015; Maire et al 2015; Zurlo et al 2016; Konopacky et al 2016; Wertz et al 2017; Wang et al 2018b; Gravity Collaboration et al 2019), but the host star mass has been assumed to be fixed at ∼1.5 M in the majority of these analyses based on estimates from surface gravity measurements and high-mass evolutionary models (e.g., Gray & Kaye 1999; Moya et al 2010a; Baines et al 2012). While host star dynamical masses have been measured in other systems with a directly imaged substellar companion (e.g., β Pic, Nielsen et al 2014; Wang et al 2016; Dupuy et al 2019; Gravity Collaboration et al 2020; Nielsen et al 2020; Lagrange et al 2020; Vandal et al 2020; Brandt et al 2021a), it is more often the case that the modest orbital coverage from long-period planets cannot admit a well constrained dynamical mass (e.g., Bowler et al 2020)

Results

Discussion

Conclusion