Evidences of extragalactic origin and planet engulfment in the metal-poor twin pair HD 134439/HD 134440

Abstract

Recent studies of chemical abundances in metal-poor Halo stars show the existence of different populations, which is important for studies of Galaxy formation and evolution. Here we revisit the twin pair of chemically anomalous stars HD 134439 and HD 134440, using high resolution (R $\sim 72 000$) and high S/N ratio (S/N $\sim 250$) HDS/Subaru spectra. We compare them to the well-studied Halo star HD 103095, using the line-by-line differential technique to estimate precise stellar parameters and LTE chemical abundances. We present the abundances of C, O, Na, Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Sr, Y, Ba, La, Ce, Nd, and Sm. We compare our results to the precise abundance patterns of Nissen & Schuster (2010) and data from dwarf Spheroidal Galaxies (dSphs). We show that the abundance pattern of these stars appears to be closely linked to that of dSphs with [$\alpha$/Fe] knee below [Fe/H] $<-1.5$. We also find a systematic difference of $0.06\pm0.01$ dex between the abundances of these twin binary stars, which could be explained by the engulfment of a planet, thus suggesting that planet formation is possible at low metallicities ([Fe/H]=$ -1.4$).