HE 0017+0055 : A probable pulsating CEMP-rs star and long-period binary

TL;DR

This study investigates the binary nature and chemical composition of the CEMP-rs star HE 0017+0055, revealing complex velocity variations, elemental overabundances, and possible pulsations or binary interactions.

Contribution

It provides detailed elemental abundances and analyzes the origin of velocity variations, proposing new insights into the star's binary status and pulsation activity.

Findings

Detected 384-day radial velocity variability.

Identified overabundances of s- and r-process elements.

Suggested possible pulsations or binary companion effects.

Abstract

A radial-velocity monitoring of the Carbon-Enhanced Metal-Poor (CEMP) star HE 0017+0055 over 8 years with the Nordic Optical Telescope and Mercator telescopes reveals variability with a period of 384 d and amplitude of 540$\pm27$ m s$^{-1}$, superimposed on a nearly linear long-term decline of $\sim$1 m s$^{-1}$ day$^{-1}$. High-resolution HERMES/Mercator and Keck/HIRES spectra have been used to derive elemental abundances using 1-D LTE MARCS models. A metallicity of [Fe/H] $\sim -2.4$ is found, along with s-process overabundances on the order of 2 dex (with the exception of [Y/Fe] $\sim+0.5$), and most notably overabundances of r-process elements like Sm, Eu, Dy, and Er in the range 0.9 - 2.0 dex. With [Ba/Fe] $ > 1.9$ dex and [Eu/Fe] = 2.3 dex, HE 0017+0055 is a CEMP-rs star. It appears to be a giant star below the tip of the red giant branch (RGB). The s-process pollution must therefore originate from mass transfer from a companion formerly on the AGB, now a carbon-oxygen white dwarf (WD). If the 384 d velocity variations are attributed to the WD companion, its orbit must be seen almost face-on, with $i \sim 2.3^\circ$, because the mass function is very small: $f(M_1,M_2) = (6.1\pm1.1)\times10^{-6}$ Msun. Alternatively, the WD orbital motion could be responsible for the long-term velocity variations, with a period of several decades. The 384 d variations should then be attributed either to a low-mass inner companion (perhaps a brown dwarf, depending on the orbital inclination), or to stellar pulsations. The latter possibility is made likely by the fact that similar low-amplitude velocity variations, with periods close to 1 yr, have been reported for other CEMP stars in a companion paper (Jorissen et al., 2015). A definite conclusion about the origin of the 384 d velocity variations should however await the detection of synchronous low-amplitude photometric variations.