The Hamburg/ESO R-process Enhanced Star survey (HERES) X. HE 2252-4225, one more r-process enhanced and actinide-boost halo star

TL;DR

This study provides a detailed chemical abundance analysis of the r-process enhanced star HE 2252-4225, revealing its neutron-capture element pattern, a possible common r-process origin, and an anomalously high Th/Eu ratio affecting age estimates.

Contribution

First detailed abundance analysis of HE 2252-4225, confirming its r-process enhancement and comparing its pattern with other r-process stars and solar r-process.

Findings

Abundance pattern matches classical r-process in multiple element regions.

Star's Th/Eu ratio suggests an unexpectedly young age.

Consistent neutron-capture element pattern with other r-process enhanced stars.

Abstract

We report on a detailed abundance analysis of the r-process enhanced giant star, HE 2252-4225 ([Fe/H] = -2.63, [r/Fe] = 0.80). Determination of stellar parameters and element abundances was based on analysis of high-quality VLT/UVES spectra. The surface gravity was calculated from the NLTE ionisation balance between Fe I and Fe II. Accurate abundances were determined for a total of 38 elements, including 22 neutron-capture elements beyond Sr and up to Th. This object is deficient in carbon, as expected for a giant star with Teff < 4800 K. The stellar Na-Zn abundances are well fitted by the yields of a single supernova of 14.4 Msun. For the neutron-capture elements in the Sr-Ru, Ba-Yb, and Os-Ir regions, the abundance pattern of HE 2252-4225 is in excellent agreement with the average abundance pattern of the strongly r-process enhanced stars CS 22892-052, CS 31082-001, HE 1219-0312, and HE 1523-091. This suggests a common origin of the first, second, and third r-process peak elements in HE 2252-4225 in the classical r-process. We tested the solar r-process pattern based on the most recent s-process calculations of Bisterzo et al. (2014) and found that elements in the range from Ba to Ir match it very well. No firm conclusion can be drawn about the relationship between the fisrt neutron-capture peak elements, Sr to Ru, in HE 2252-4225 and the solar r-process, due to the uncertainty in the solar r-process. The investigated star has an anomalously high Th/Eu abundance ratio, so that radioactive age dating results in a stellar age of tau = 1.5+-1.5 Gyr that is not expected for a very metal-poor halo star.