Abundance Analysis of HE 2148-1247, A Star with Extremely Enhanced Neutron Capture Elements

Abundances for 27 elements in the very metal-poor dwarf star HE 2148-1247 are presented, including many of the neutron capture elements. We establish that HE 2148-1247 is a very highly s-process-enhanced star with anomalously high Eu as well, Eu/H ~ half-solar, demonstrating the large addition of heavy nuclei at [Fe/H] = -2.3 dex. Ba and La are enhanced by a somewhat larger factor and reach the solar abundance, while Pb significantly exceeds it, thus demonstrating the addition of substantial s-process material. Ba/Eu is 10 times the solar r-process ratio but much less than that of the s-process, indicating a substantial r-process addition as well. C and N are also very highly enhanced. We have found that HE 2148-1247 is a radial velocity variable; it is probably a small-amplitude long-period binary. The C, N, and the s-process element enhancements were thus presumably produced through mass transfer from a former asymptotic giant branch (AGB) binary companion. The large enhancement of heavy r-nuclides also requires an additional source as this is far above any inventory in the interstellar medium at such low [Fe/H]. We consider that the s-process material was added by mass transfer of a more massive companion during its thermally pulsating AGB phase and ending up as a white dwarf. We further hypothesize that accretion onto the white dwarf from the envelope of the star caused accretion-induced collapse of the white dwarf, forming a neutron star, which then produced heavy r-nuclides and again contaminated its companion. This mechanism in a binary system can thus enhance the envelope of the lower mass star in s- and r-process material sequentially. Through analysis of the neutron capture element abundances taken from the literature for a large sample of very metal-poor stars, we demonstrate, as exemplified by HE 2148-1247, that mass transfer in a suitable binary can be very efficient in enhancing the heavy elements in a star; it appears to be capable of enhancing the s-process elements in very metal-poor stars to near the solar abundance but not substantially above it. The yield of Pb relative to Ba appears to vary among very metal-poor stars.

Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration.