Reconciling the emission mechanism discrepancy in Mira's tail, and its evolution in an interface with shear

TL;DR

This study explains the complex shape and emission features of Mira's tail by modeling its interaction with a shear flow, reconciling previous discrepancies in emission mechanisms and tail morphology.

Contribution

It introduces a novel model of Mira's tail evolution incorporating shear flow effects, successfully reproducing observed tail curvature and emission characteristics.

Findings

Reproduces the kinked tail morphology of Mira.

Predicts recompression and reheating consistent with FUV emission.

Suggests Mira's tail evolution is unique due to its specific environment.

Abstract

GALEX observations of the Mira AB binary system revealed a surrounding structure that has been successfully hydrodynamically interpreted as a bow shock and tail of ram-pressure-stripped material. Even the narrow tail, initially difficult to model, has been understood as the effect of the passage of Mira from a warm neutral medium into a hot, low-density medium, postulated to be the Local Bubble. However, no model to date has explained the observed kink and associated general curvature of the tail. We test the hypothesis that before entering the Local Bubble, Mira was travelling through a shear flow with approximately 1/3 Mira's own velocity at an angle of ~30degrees to Mira's proper motion. The hypothesis reproduces the kinked nature of Mira's tail and predicts recompression and reheating of the tail material to the same or greater levels of density and temperature predicted in the shock. This provides a heat source for the FUV emission, allowing for an extended lifetime of the FUV emission in line with other estimates of the age of the tail. The uniqueness of Mira's situation implies that the chances of observing other FUV tails behind AGB stars is highly unlikely.