A detailed view on the circumstellar environment of the M-type AGB star EP Aquarii

Abstract

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 M⊙, 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″. Decreased signal along a PA of 138° suggests that the dust is confined to an inclined ring-like structure, consistent with the previously determined wind morphology.

Morpho-kinematics of the wind of AGB star L$_2$ Pup

“…The formation of such expanding discs, or equatorial enhancements, is not understood. In the case of EP Aqr, Homan et al (2020) invoked the possible presence of a companion. In the present case of L 2 Pup, the presence of another disc within some 20 au from the centre of the star makes it even more difficult to propose a sensible interpretation.…”

Atomium: The astounding complexity of the near circumstellar environment of the M-type AGB star R Hydrae. I. Morpho-kinematical interpretation of CO and SiO emission

“…Alternatively, the void in R Hya is reminiscent of a similar void in the inner wind of the AGB star EP Aquarii (Homan et al 2018b), which was later interpreted as a potential photodissociation region generated by the radiation field of a white dwarf (WD; Homan et al 2020a). Considering that the void in R Hya consistently appears in approximately the same channels and at the same place in different molecules observed in the atomium survey, the presence of a photodissociating companion (i.e.…”

ATOMIUM: The astounding complexity of the near circumstellar environment of the M-type AGB star R Hydrae

“…Alternatively, the void in R Hya is reminiscent of a similar void in the inner wind of the AGB star EP Aquarii (Homan et al 2018b), which was later interpreted as a potential photodissociation region generated by the radiation field of a white dwarf (WD; Homan et al 2020a). Considering that the void in R Hya consistently appears in approximately the same channels and at the same place in different molecules observed in the atomium survey, the presence of a photodissociating companion (i.e., either a cool WD or hot main-sequence star) cannot be immediately excluded.…”

“…Considering that the void in R Hya consistently appears in approximately the same channels and at the same place in different molecules observed in the atomium survey, the presence of a photodissociating companion (i.e., either a cool WD or hot main-sequence star) cannot be immediately excluded. Depending on its properties, such a companion could easily remain hidden in spectra or SEDs, as was the case for the WD companion of EP Aqr (Homan et al 2020a), or the companion of π 1 Gruis (Homan et al 2020b). It must be noted, however, that it is unlikely that R Hya possesses two companions of significant mass within a radius of 50 au, since this would make the system highly unstable, implying that it would probably not have survived up to the AGB phase.…”