A detailed view on the circumstellar environment of the M-type AGB star
  EP Aquarii I. High-resolution ALMA and SPHERE observations
  Cool evolved stars are known to be significant contributors to the enrichment
of the interstellar medium through their dense and dusty stellar winds. High
resolution observations of these outflows have shown them to possess high
degrees of morphological complexity. We observed the asymptotic giant branch
(AGB) star EP Aquarii with ALMA in band 6 and VLT/SPHERE/ZIMPOL in four filters
the visible. Both instruments had an angular resolution of 0.025''. These are
follow-up observations to the lower-resolution 2016 ALMA analysis of EP
Aquarii, which revealed that its wind possesses a nearly face-on,
spiral-harbouring equatorial density enhancement, with a nearly pole-on
bi-conical outflow. At the base of the spiral, the SiO emission revealed a
distinct emission void approximately 0.4'' to the west of the continuum
brightness peak, which was proposed to be linked to the presence of a
companion. The new ALMA data better resolve the inner wind and reveal that its
morphology as observed in CO is consistent with hydrodynamical
companion-induced perturbations. Assuming that photodissociation by the
UV-field of the companion is responsible for the emission void in SiO, we
deduced the spectral properties of the tentative companion from the size of the
hole. We conclude that the most probable companion candidate is a white dwarf
with a mass between 0.65 and 0.8 Msol , though a solar-like companion could not
be definitively excluded. The radial SiO emission shows periodic, low-amplitude
perturbations. We tentatively propose that they could be the consequence of the
interaction of the AGB wind with another much closer low-mass companion. The
polarised SPHERE/ZIMPOL data show a circular signal surrounding the AGB star
with a radius of 0.1''...
