Distinct Rotational Evolution of Giant Planets and Brown Dwarf Companions

TL;DR

The paper tackles whether giant planets and brown dwarf companions exhibit distinct rotational evolution. It leverages Keck/KPIC high-resolution spectroscopy to perform a spin survey of 32 targets and combines these measurements with literature spins to build a larger benchmark sample. Through forward-modeling and cross-correlation techniques, the study derives spins and analyzes them in the context of angular momentum evolution, finding that giant planets tend to have higher fractional breakup velocities than low-mass brown dwarfs under plausible inclination assumptions, suggesting reduced disk-braking during formation. Additional results indicate brown dwarf companions rotate more slowly than isolated brown dwarfs, while planets and planetary-mass objects share similar spins, and an angular-momentum comparison across 221 substellar objects shows 5–40 M_Jup bodies retaining higher angular momenta than 40–100 M_Jup objects after 10 Myr, highlighting mass-dependent rotational histories relevant to formation models.

Abstract

We present a rotational velocity (vsini) survey of 32 stellar/substellar objects and giant planets using Keck/KPIC high-resolution spectroscopy, including 6 giant planets (2-7 M$_\mathrm{Jup}$) and 25 substellar/stellar companions (12-88 M$_\mathrm{Jup}$). Adding companions with spin measurements from the literature, we construct a curated spin sample for 43 benchmark stellar/substellar companions and giant planets and 54 free-floating brown dwarfs and planetary mass objects. We compare their spins, parameterized as fractional breakup velocities at 10 Myr, assuming constant angular momentum evolution. We find the first clear evidence that giant planets exhibit distinct spins versus low-mass brown dwarf companions (10 to 40 M$_\mathrm{Jup}$) at 4-4.5 $σ$ significance assuming inclinations aligned with their orbits, while under randomly oriented inclinations the significance is at 1.6-2.1 $σ$. Our findings hold when considering various assumptions about planets, and the mass ratio below 0.8% gives a clean cut for rotation between giant planets and brown dwarf companions. The higher fractional breakup velocities of planets can be interpreted as less angular momentum loss through circumplanetary disk braking during the planet formation phase. Brown dwarf companions exhibit evidence of slower rotation compared to isolated brown dwarfs, while planets and planetary mass objects show similar spins. Finally, our analysis of specific angular momentum versus age of 221 stellar/substellar objects below 0.1 MSun with spin measurements in the literature indicates that the substellar objects of 5-40 M$_\mathrm{Jup}$ retain much higher angular momenta compared to stellar and substellar objects of 40-100 M$_\mathrm{Jup}$ after 10 Myr, when their initial angular momenta were set.

Figures (2)

• Figure 1: Absolute $M_J$ versus $J-K$ color of our KPIC sample. In our KPIC sample, the directly imaged exoplanets are labeled in red circles, and the stellar/substellar companions are denoted in blue circles. The 100 pc M5--T9 dwarfs, compiled from the Ultracool Sheet Best:2024aa, are depicted in small dots and color-coded by their spectral types. Note that PDS 70 b is not shown here since it lacks $K$-band photometry in the literature. Directly imaged planets and a few young substellar companions occupy the redder colors in the plot, and have been pointed out in the literature.
• Figure 2: Our KPIC sample distributions. Top: Distribution of our KPIC targets as a function of mass (M$_\mathrm{Jup}$) and separation (au), color-coded by age (Myr). Hydrogen mass burning limit (78.6 M$_\mathrm{Jup}$; Chabrier:2023aa), deuterium mass burning limit (12.6 M$_\mathrm{Jup}$; Saumon:1996aaBurrows:1997aa) are labeled in the horizontal dashed and dashed-dotted lines, respectively, and typical disk size (192 au; Ansdell:2018aa) is depicted in the vertical dotted line. Bottom: Distribution of our KPIC targets as a function of mass (M$_\mathrm{Jup}$) and age (Myr), color-coded by separation (au). Note that the lack of brown dwarf companions at close separation is due to the so-called brown dwarf desert, reflecting the formation, not the observational bias.