The true age of multi-planet host star HD 110067: Or: Don't be fooled by K0V-type stars

Abstract

HD 110067 is a near (d = 32.22 pc, V = 8.43 mag) K0V star in Coma Ber that was recently discovered to host a six-planet system in stable resonances. The star has a very old age of 8+/‑4 Gyr. However, by the nature of the respective evolution tracks (i.e. with masses of 0.78...0.85 M_⊙), which run parallel to the zero-age main sequence for ≈8 Gyr, such age estimates are impossible unless the mass and luminosity are independently known to very high precision. We demonstrate this point using physical parameters derived from two different spectroscopic methods. As an alternative age indicator, we looked at the emission in Ca II H&K using TIGRE/HEROS spectra (Guanajuato, Mexico) and 78 archive TNG/HARPS-N spectra from 2021 to 2024. Surprisingly, HD 110067 has a high and persistent activity level of S_MWO = 0.32. From the estimated empirical and Rossby number of 0.4, and with the parameterised spin-down timescale, we derive an activity age of ≈2.5(±0.8) Gyr. Similarly, a possible rotation period of 20 days, consistent with TESS photometric variations and our vsin(i), suggests Ro = 0.32 and an age of just 1.7 Gyr. Such a relatively young activity age is indeed consistent with a very small lithium signature (the equivalent width of the 6707.8 Å doublet is 1.1 ± 0.2 mÅ) and implies that HD110067 can be directly compared to its virtual twin σ Draconis, which has an even weaker lithium presence and an activity cycle around ⟨S⟩_MWO ≈ 0.22.