pi1 Gru SPHERE images

The inner companion, pi1 Gru C was located at 37.4+/-2.0mas from the primary in June-July 2019 (projected separation of 6.05+/-0.55au at 161.7+/-11.7pc). The best orbital solution gives a companion mass of 0.86^+0.22^_-0.20_M_{sun}_ (using the derived mass of the primary), and a semi-major axis of 7.05^+0.54^_-0.57_au. This leads to an orbital period of 11.0^+1.7^_-1.5_yr.

The best solution is an elliptical orbit with eccentricity e=0.35^+0.18^_-0.17_, but a circular orbit cannot be totally excluded. The close companion can either be a K1V(F9.5V/K7V) star or a white dwarf. The ultraviolet and millimeter continuum photometry are consistent with the presence of an accretion disk around the close companion.

The ultraviolet emission could then either originate in hot spots in an overall cooler disk, or also from a hot disk in case the companion is a white dwarf. Though the close companion and the AGB star are interacting, and an accretion disk is observed around the companion, the mass-accretion rate is too low to cause a Ia supernova but could produce nove every ~900yr. Short wavelength spatially resolved observations are needed to further constrain the nature of the C companion.

Searches for close-in companions similar to this system will help to better understand the physics of mass- and angular-momentum transfer, and orbital evolution in the late evolutionary stages.