The Orbital Eccentricities of Directly Imaged Companions Using Observable-based Priors: Implications for Population-level Distributions

Abstract

The eccentricity of a substellar companion is an important tracer of its formation history. Directly imaged companions often present poorly constrained eccentricities. A recently developed prior framework for orbit fitting called “observable-based priors” has the advantage of improving biases in derived orbit parameters for objects with minimal phase coverage, which is the case for the majority of directly imaged companions. We use observable-based priors to fit the orbits of 21 exoplanets and brown dwarfs in an effort to obtain the eccentricity distributions with minimized biases. We present the objects’ individual posteriors compared to their previously derived distributions, showing in many cases a shift toward lower eccentricities. We analyze the companions’ eccentricity distribution at a population level, and compare this to the distributions obtained with the traditional uniform priors. We fit a Beta distribution to our posteriors using observable-based priors, obtaining shape parameters α = 1.09 − 0.22 + 0.30 and β = 1.42 − 0.25 + 0.33 . This represents an approximately flat distribution of eccentricities. The derived α and β parameters are consistent with the values obtained using uniform priors, though uniform priors lead to a tail at high eccentricities. We find that separating the population into high- and low-mass companions yields different distributions depending on the classification of intermediate-mass objects. We also determine via simulation that the minimal orbit coverage needed to give meaningful posteriors under the assumptions made for directly imaged planets is ≈15% of the inferred period of the orbit.

VLTI/GRAVITY Provides Evidence the Young, Substellar Companion HD 136164 Ab Formed Like a “Failed Star”

“…HD 136164 Ab's high eccentricity and mass ratio of 0.02 could therefore indicate a core fragmentation formation pathway, if eccentricities are expected to be damped during formation in a disk, but not during core fragmentation. In the context of population-level studies of substellar companions, this eccentricity also empirically indicates more of a "brown dwarf/failed star" nature, as directly imaged planets appear to follow a distinct eccentricity distribution with generally lower eccentricities (as their eccentricities are expected to be damped by their parent disks), whereas substellar objects produced by fragmenting cores tend to have a distribution of on average higher eccentricities (Bowler et al 2020;Doó et al 2023;Nagpal et al 2023). Parker & Daffern-Powell (2022) use Nbody simulations of star-forming regions to show that earlytype stars can capture single brown dwarfs or even steal planets from other lower-mass systems within the first 10 Myr of an association's lifetime, so it is a reasonable assumption that, even if the brown dwarf did not form directly into a binary configuration with HD 136164 A, it could have formed nearby and been subsequently captured onto a binary orbit.…”

“…With a low mass ratio and close separation, Wagner et al (2020) posit that the companion could have formed via disk fragmentation (e.g., Boss 1997;Kratter et al 2010;Forgan et al 2018), or even the very high end of a "planetary" coreaccretion formation channel (e.g., Pollack et al 1996;Mordasini et al 2012) although the possibility (e.g., Emsenhuber et al 2021) and occurrence (e.g., Schlaufman 2018) of such high-mass core-accretion planets is still heavily debated. Placing the companion's orbit in context could provide insight into this question, as recent population-level studies of substellar companions indicate two populations in eccentricity (Bowler et al 2020;Doó et al 2023;Nagpal et al 2023). It appears that low-mass objects (that likely formed within a protoplanetary disk, whose gas damped their initial eccentricity) exhibit a distribution of eccentricities that is on average lower than the distribution for higher-mass objects (that likely formed via core fragmentation along with the stellar population, and formed within or became captured into binary orbits).…”