The Kinematic and Chemical Properties of the Close-in Planet Host Star 8 UMi

Abstract

A recent study by Hon et al. reported that a close-in planet around the red clump star, 8 UMi, should have been engulfed during the expansion phase of its parent star's evolution. They explained the survival of this exoplanet through a binary-merger channel for 8 UMi. The key to testing this formation scenario is to derive the true age of this star: is it an old "imposter" resulting from a binary merger, or a genuinely young red clump giant? To accomplish this, we derive kinematic and chemical properties for 8 UMi using astrometric data from {\it Gaia} DR3 and the element-abundance pattern measured from a high-resolution ($R\sim 75,000$) spectrum taken by SOPHIE. Our analysis shows that 8 UMi is a normal thin-disk star with orbital rotation speed of $V_{\ddot{\mathrm{I}}}•=244.96km{\mathrm{s}}^{-1}$, and possesses a Solar metallicity ([Fe/H] $=-0.05\pm 0.07$) and $\hat{I}\pm$-element abundance ratio ([$\hat{I}\pm$/Fe] $=+0.01\pm 0.03$). By adopting well-established relationships between age and space velocities/elemental abundances, we estimate a kinematic age of $3.50_{-2.00}^{+3.00}$ Gyr, and a chemical age of $3.25_{-1.50}^{+2.50}$ Gyr from [C/N] and $3.47\pm 1.96$ Gyr from [Y/Mg] for 8 UMi, respectively. These estimates are consistent with the isochrone-fitting age ($1.90_{-0.30}^{+1.15}$ Gyr) of 8 UMi, but are all much younger than the timescale required in a binary-merger scenario. This result challenges the binary-merger model; the existence of such a closely orbiting exoplanet around a giant star remains a mystery yet to be resolved.

16 pages, 5 figures, 2 table; accepted by ApJL