Evolutionary constraints on the planet-hosting subgiant ε Reticulum from its white dwarf companion

ABSTRACT

The planet-hosting and Sirius-type binary system ε Reticulum is examined from the perspective of its more evolved white dwarf secondary. The stellar parameters are determined from a combination of Balmer line spectroscopy, gravitational redshift and solid angle. These three methods conspire to yield the most accurate physical description of the companion to date: T_eff= 15 310 ± 350 K and M= 0.60 ± 0.02 M_⊙. Post-main-sequence mass-loss indicates that the current binary separation has increased by a factor of 1.6 from its primordial state when the current primary was forming its planet(s), implying a₀≥ 150 au and constraining stable planets to within 15–20 au for a binary eccentricity of e= 0.5. Almost 80 years have passed since the first detection of the stellar companion, and marginal orbital motion may be apparent in the binary, suggesting a near edge-on configuration with i≳ 70°, albeit with substantial uncertainty. If correct, and all known bodies are coplanar, the mass of the planet HD 27442b is bound between 1.6 and 1.7 M_J.

A search for photospheric metals in the DA white dwarf yields no detections, and hence there is no clear signature of an extant planetary system orbiting the previously more massive secondary. However, if the white dwarf mass derived via spectral fitting is correct, its evolution could have been influenced by interactions with inner planets during the asymptotic giant branch. Based on the frequency of giant planets and circumstellar debris as a function of stellar mass, it is unlikely that the primordial primary would be void of planets, given at least one orbiting its less massive sibling.