Kavli Affiliate: Huawei Zhang
| First 5 Authors: Huiling Chen, Yang Huang, Wei Zhu, Timothy C. Beers, Renjing Xie
| Summary:
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
$it{V}_mathrm{phi}=mathrm{244.96 km s^{-1}}$, and possesses a Solar
metallicity ([Fe/H] $= -0.05 pm 0.07$) and $alpha$-element abundance ratio
([$alpha$/Fe] $= +0.01 pm 0.03$). By adopting well-established relationships
between age and space velocities/elemental abundances, we estimate a kinematic
age of $3.50^{+3.00}_{-2.00}$ Gyr, and a chemical age of $3.25^{+2.50}_{-1.50}$
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
($1.90^{+1.15}_{-0.30}$ 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.
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