IP Eri: A surprising long-period binary system hosting a He white dwarf

TL;DR

IP Eri is a long-period binary system with a He white dwarf and a K0 IV star, challenging common-envelope evolution models and suggesting alternative binary evolution pathways.

Contribution

This study provides detailed orbital and spectroscopic analysis of IP Eri, revealing its unique long period and eccentricity, and discusses implications for binary evolution theories.

Findings

Orbital period of 1071 days and eccentricity of 0.25.

The He white dwarf has a mass of 0.43 M$_\odot$, indicating a helium-core WD.

Abundance patterns match those of normal field stars of similar metallicity.

Abstract

We determine the orbital elements for the K0 IV + white dwarf (WD) system IP Eri, which appears to have a surprisingly long period of 1071 d and a significant eccentricity of 0.25. Previous spectroscopic analyses of the WD, based on a distance of 101 pc inferred from its Hipparcos parallax, yielded a mass of only 0.43 M$_\odot$, implying it to be a helium-core WD. The orbital properties of IP Eri are similar to those of the newly discovered long-period subdwarf B star (sdB) binaries, which involve stars with He-burning cores surrounded by extremely thin H envelopes, and are therefore close relatives to He WDs. We performed a spectroscopic analysis of high-resolution spectra from the HERMES/Mercator spectrograph and concluded that the atmospheric parameters of the K0 component are $T_{\rm eff} = 4960$ K, $\log{g} = 3.3$, [Fe/H] = 0.09 and $\xi = 1.5$ km/s. The detailed abundance analysis focuses on C, N, O abundances, carbon isotopic ratio, light (Na, Mg, Al, Si, Ca, Ti) and s-process (Sr, Y, Zr, Ba, La, Ce, Nd) elements. We conclude that IP Eri abundances agree with those of normal field stars of the same metallicity. The long period and non-null eccentricity indicate that this system cannot be the end product of a common-envelope phase; it calls instead for another less catastrophic binary-evolution channel presented in detail in a companion paper (Siess et al. 2014).