The s-process enriched star HD 55496: origin from a globular cluster or from the tidal disruption of a dwarf galaxy?

Abstract

We present a new abundance analysis of HD 55496, previously known as a metal-poor barium star. We found that HD 55496 has a metallicity [Fe/H] $=-1.55$ and is s-process enriched. We find that HD 55496 presents four chemical peculiarities: (i) a Na-O abundance anti-correlation; (ii) it is aluminum rich; (iii) it is carbon poor for a s-process enriched star and (iv) the heavy 2$^{nd}$ s-process peak elements, such as Ba, La, Ce, and Nd, present smaller abundances than the lighter s-process elements, such as Sr, Y and Zr, which is not usually observed among the chemically-peculiar binary stars at this metallicity. The heavy-element abundance pattern suggests that the main source of the neutrons is the $^{22}$Ne($\alpha$,n)$^{25}$Mg reaction. Taken all these abundance evidence together into consideration, this strongly suggests that HD 55496 is a "second generation of globular cluster star" formed from gas already strongly enriched in s-process elements and now is a field halo object. Our dynamical analysis, however, indicates that the past encounter probabilities with the known globular clusters is very small ($\leq 6\%$). This evidence, together with the fact of having a retrograde motion, points to a halo intruder possibly originated from the tidal disruption of a dwarf galaxy.