Title:Beryllium: The smoking gun of a rejuvenated star

Abstract:Context. The chemistry and Galactic velocity components of the star HD 65907 suggest that despite its young isochronal age of $\sim$5 Gyr, it is in fact a merger of two old Population II stars. Its low Li abundance is also consistent with a mass accretion episode. Aims. We determine Li and Be abundances for this star and evaluate its radial velocity time series, activity cycle, and spectral energy distribution in search of clues regarding the origin of this enigmatic star. Methods. Li and Be abundances were determined via spectral synthesis of their resonance lines using HARPS and UVES spectra, respectively. HARPS data were also used to study variations in the star's radial velocity and activity levels. Photometric data were adopted to evaluate the stellar spectral energy distribution. Results. HD 65908 is severely Li- and Be-depleted. Its radial velocity is nearly constant ($\sigma =$ 2 m/s), with a small modulation likely associated with stellar activity, and the star shows no further signs of an undetected close companion. The excess infrared emission is consistent with a 30 K blackbody, which is interpreted as a debris disk surrounding the star. The post-merger mass, rotation rate, and evolution of this star are discussed. Conclusions. The low Li and Be abundances, in addition to the lack of evidence for a companion, are strong pieces of evidence in favor of the stellar merger scenario. In this context, Be can be used to confirm other blue stragglers among field solar-type stars, as proposed in the literature.