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X-Ray Evidence of the Common Envelope Phase of V471 Tauri

Chandra Low Energy Transmission Grating Spectrograph observations of the precataclysmic binary V471 Tauri have been used to estimate the C/N abundance ratio of the K dwarf component for the first time. While the white dwarf component dominates the spectrum longward of 50 Å, at shorter wavelengths the observed X-ray emission is entirely due to coronal emission from the K dwarf. The H-like 2p 2P3/2, 1/2 → 1s 2S1/2 resonance lines of C and N yield an estimate of their logarithmic abundance ratio relative to the Sun of [C/N] = -0.38 ± 0.15—half of the currently accepted solar value. We interpret this result as the first clear observational evidence for the presumed common envelope phase of this system, during which the surface of the K dwarf was contaminated by CN cycle-processed material dredged up into the red giant envelope. We use the measured C/N ratio to deduce that 0.015-0.04 M☉ was accreted by the K dwarf while engulfed and to show that this is consistent with a recent tentative detection of 13C in the K dwarf photosphere and with the measured Li abundance in the scenario in which the red giant companion was Li-rich during the common envelope phase.