On the Orbits of Low-mass Companions to White Dwarfs and the Fates of the Known Exoplanets

TL;DR

This paper investigates the orbital evolution and survival of low-mass companions and exoplanets around white dwarfs during stellar evolution, considering factors like mass loss, tides, and spin changes, with implications for detecting habitable planets.

Contribution

It introduces a comprehensive model of binary evolution that includes primary spin changes and maps conditions for companion survival through stellar phases.

Findings

Minimum initial semimajor axis for companion survival determined.

Regions identified where companions can survive the common envelope phase.

Earth-like planets cannot survive a common envelope, affecting habitable zone prospects.

Abstract

The ultimate fates of binary companions to stars (including whether the companion survives and the final orbit of the binary) are of interest in light of an increasing number of recently discovered, low-mass companions to white dwarfs (WDs). In this Letter, we study the evolution of a two-body system wherein the orbit adjusts due to structural changes in the primary, dissipation of orbital energy via tides, and mass loss during the giant phases; previous studies have not incorporated changes in the primary's spin. For companions ranging from Jupiter's mass to ~0.3 Msun and primaries ranging from 1-3 Msun, we determine the minimum initial semimajor axis required for the companion to avoid engulfment by the primary during post-main-sequence evolution, and highlight the implications for the ultimate survival of the known exoplanets. We present regions in secondary mass and orbital period space where an engulfed companion might be expected to survive the common envelope phase (CEP), and compare with known M dwarf+WD short-period binaries. Finally, we note that engulfed Earth-like planets cannot survive a CEP. Detection of a first-generation terrestrial planet in the white dwarf habitable zone requires scattering from a several-AU orbit to a high-eccentricity orbit (with a periastron of ~Rsun) from which it is damped into a circular orbit via tidal friction, possibly rendering it an uninhabitable, charred ember.