Dynamical Mass of the Exoplanet Host Star HR 8799

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.

MADYS: the Manifold Age Determination for Young Stars

“…Several age estimates have been derived for the system in the recent literature: 10 − 23 Myr from Sepulveda & Bowler (2022), 33 +7 −13 Myr from Baines et al (2012) or 42 +6 −4 Myr from Bell et al (2015); we notice that the last estimate, based on the lithiumdepletion boundary for the Colomba association which the star appears to be a member of, is independently indicated by madys when inspecting the kinematic neighborhood of the star. Indeed, we identified three stars 13 with projected separation < 3 pc and tangential velocity difference < 3 km s −1 : all of them have a bestfit mass ∈ [0.7, 1]M and age ∼ 40 Myr.…”

Orbital and dynamical analysis of the system around HR 8799

“…Regarding the stellar mass, we considered two fixed values of m = 1.52 M determined by Baines et al (2012) as m = 1.516 +0.038 −0.024 M for the age of 33 Myr, and m = 1.47 +0.12 −0.17 M , following the most recent dynamical estimate by Sepulveda & Bowler (2022). The same value was used by Wang et al (2018) in their earlier work.…”

“…8), 600 Myr (Model 5, the middle column), and just 200 Myr (Model 6, the right column). Despite this, for the initial few hundred million years, extending safely beyond most approximations of the stellar age, which range between 30-160 Myr (Baines et al 2012;Sepulveda & Bowler 2022), the systems are bounded and locked in the resonance. Simultaneously, Models 3 and 4 yield masses of the inner planet in the 10 M Jup range, and Model 4 is especially interesting given its low χ 2 compared to the initial starting PO configuration.…”

Retrieving C and O Abundance of HR 8799 c by Combining High- and Low-Resolution Data

“…In addition to the progress of spectral analysis, recent observational progress has been made on the study of the HR 8799 planetary system since Wang et al (2020). Doelman et al (2022) used LMIRCam/ALES observations to further constrain the planetary properties and cloud conditions, while Sepulveda & Bowler (2022) and Brandt et al (2021) further pinned down dynamical masses for HR 8799 bcde using Gaia data. Ruffio et al (2021) employed Keck/OSIRIS data to obtain consistent C/O ratios for planets b,c, and d.…”