The Kinematic and Chemical Properties of the Close-in Planet Host Star 8 UMi
The Astrophysical Journal Letters(2024)
摘要
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
Gaia DR3 and the element-abundance pattern measured from a
high-resolution (R ∼ 75,000) spectrum taken by SOPHIE. Our analysis shows
that 8 UMi is a normal thin-disk star with orbital rotation speed of
V_ϕ=244.96 km s^-1, and possesses a Solar
metallicity ([Fe/H] = -0.05 ± 0.07) and α-element abundance ratio
([α/Fe] = +0.01 ± 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 ± 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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关键词
Planet hosting stars,Chemical abundances,Stellar evolution,Galaxy kinematics
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