TESS Hunt for Young and Maturing Exoplanets (THYME) XIV: A Comoving-Based Age Constraint for KELT-20

Abstract

Young stellar moving groups offer unique opportunities to investigate the early evolution of stellar and planetary systems. In continuation of an ongoing effort to age-date compelling planetary systems, we provide an in-depth age analysis of KELT-20, a young A-type star that hosts a well-aligned ultra-hot Jupiter. This system poses a useful case study to investigate migration mechanisms at early stages of evolution. Using Gaia DR3 data, we identify 77 stars with proper motions consistent with KELT-20, including 19 with measured radial velocities that enable full 3D kinematic confirmation. Using isochronal analyses, gyrochronology, photometric variability, and stellar activity indicators, we converge on an age of $58 \pm 5\,$Myr. This constraint provides critical insights into the dynamical processes shaping hot Jupiter formation.

Figures (8)

• Figure 1: Comparison of FriendFinder results obtained using the Gaia DR3 radial velocity (left) and the Lund2017 radial velocity (right) for KELT-20. Plotted are the differences between predicted ($V_\mathrm{r,pred}$) and observed ($V_\mathrm{r,obs}$) radial velocities as a function of absolute Gaia magnitude. Blue circles mark stars with RUWE $<1.25$. The Lund2017 RV yields a closer alignment with the overdensity, supporting its adoption in this analysis.
• Figure 2: Color–magnitude diagram of comoving targets to KELT-20. 3D comoving stars are shown as blue circles, while non-3D PM targets are shown as white circles. KELT-20 (coral star) is highlighted, along with a solid line showing the best-fit PARSEC isochrone derived from the isochronal analysis, corresponding to the association's median age. The dashed line shows a solar-age, solar-metallicity isochrone for comparison. Targets whose radial velocity differed by more than 3 km s$^{-1}$ and thus not included in our analysis are shown as an 'x.' Shaded background panels denote spectral type color regions based on Harre2021.
• Figure 3: Top Panel: rotation sequence for the comoving targets of KELT-20. The target with Kepler photometry is marked with a $+$. For comparison, we also show the 80 Myr $\alpha$ Per association (purple triangles; Boyle2023), 120 Myr Pleiades (yellow pentagons; Rebull2016), the 120 Myr Blanco-1 (yellow hexagons; Gillen2020),the 300 Myr Group X (salmon squares; Messina2022), the 300 Myr NGC 3532 (salmon diamonds; Fritzewski2021), and the 670 Myr Praesepe cluster (pink inverted triangles; Rampalli2021). The corresponding gyrochrones for each association are also shown Bouma2023. Bottom Panel: Our gyro-interp results for the comoving targets displayed in the rotation sequence. The lower and upper errors are the 16th and 84th percentiles of the resulting probability distribution function. Also shown for comparison are the age of the Theia 427 moving group (teal band) and our isochronal age estimate (pink band), with shaded regions indicating the $1\sigma$ uncertainties.
• Figure 4: Gaia DR3 activity index as a function of effective temperature. Shown are PM comoving targets (white circles), 3D comoving targets (blue circles), $\alpha$ Per (purple inverted triangles; Boyle2023), the Pleiades (yellow triangles; Rebull2016), and Hyades (pink diamonds; Oh2020). Theia 427 KounkelCovey2020 is also shown as black x's.
• Figure 5: Summary of age estimates for KELT-20. Isochrone fitting, gyrochronology, and variability-based ages are derived from the comoving ensemble.