X-Shooting EF Eridani: further evidence for a massive white dwarf and a sub-stellar secondary

TL;DR

This study uses high-resolution spectral data of EF Eridani in a low state to provide evidence for a massive white dwarf and a sub-stellar secondary, refining the system's physical parameters.

Contribution

It presents new spectral observations revealing the secondary's emission lines and constrains the white dwarf's mass and the system's distance, supporting EF Eri as a post period-minimum binary.

Findings

White dwarf mass > 0.65 Msun at 3sigma confidence

Distance to EF Eri approximately 111 pc

Secondary mass less than 0.06 Msun

Abstract

High resolution spectral observations of the polar EF Eridani obtained in a low state with X-Shooter revealed narrow emission lines from the irradiated secondary. The lines were most prominent in the near-IR Ca2-triplet, the more intensive Halpha line had additional emission likely originating from an accretion stream. The lines with a radial velocity amplitude, K2' = 385 +- 4 kmps, serve as tracer of the otherwise unseen companion. The mass function implies a massive white dwarf with Mwd > 0.65 Msun at 3sigma confidence, and a short distance to the binary, d ~ 111 pc (<145pc at 3sigma confidence). The spectral energy distribution from the UV to the IR together with the high mass ratio gives further strong evidence of EF Eri being a post period-minimum object with M2 < 0.06 Msun