A surprisingly large asymmetric ejection from Mira A

TL;DR

This study analyzes the inner circumstellar environment of Mira A, an AGB star, by combining high-resolution SPHERE/ZIMPOL polarized-light imaging and ALMA molecular-line data to characterize two expanding lobes. Through radiative-transfer modeling with LIME, it derives a two-region structure (inner region and lobes) with $M_{ m gas} \\sim 8\times10^{-5}$ and $2.1\times10^{-5} M_\\odot$ in the inner region and lobes, respectively, and constrains excitation temperatures, gas densities, and molecular abundances (e.g., $SO \\sim 1.5\times10^{-6}$, $SO_2 \\sim 2.5\times10^{-6}$; AlO, AlF, PO at lower levels). The ejection is dated to roughly 2010–2012, and a periodicity of 50–200 years is discussed as a potential driver for episodic mass loss, though the large-scale mass-loss-rate uncertainties temper this conclusion. A notable redshifted SO$_2$ component and strong polarization-brightness variations lead to the proposal of a lighthouse-like asymmetric radiation field shaping the observed emission. Overall, the work suggests that substantial, possibly periodic, mass ejection events can contribute significantly to Mira A’s mass loss and highlights the need for continued high-resolution, multi-wavelength monitoring to link inner-envelope dynamics to the star’s long-term evolution.

Abstract

Stars with masses between roughly 1 and 8~$M_\odot$ end their lives on the asymptotic giant branch (AGB), when intense mass loss takes place. The outflows are generally accepted to be driven by radiation pressure acting on dust grains that form in the dense extended atmospheres created by the action of convection and stellar pulsations. The complex physics underlying convection, stellar pulsations, and dust nucleation precludes predicting AGB mass loss from first principles. We investigated the evolution of two lobes observed to be expanding away from the AGB star Mira~A using images of polarized light obtained at six epochs using SPHERE on the VLT and of molecular emission at two epochs obtained with ALMA. While dust seems confined to the edges of the lobes, gas fills the lobes and displays higher densities than expected at the observed radii based on the large-scale mass-loss rate of Mira~A, with a total gas mass in the lobes of $\sim 2 \times 10^{-5}~M_\odot$. We find the expansion of the lobes to be consistent with both a constant velocity (ejection time in 2010 or 2011) or a decelerating expansion (ejection time in 2012). If ejection events with a similar magnitude happen periodically, we derive periods between 50 and 200~years to account for the mass-loss rate of Mira~A. This periodicity is uncertain because the average mass-loss rate of Mira A on larger scales is uncertain. We find abundances in the lobes of $\sim 1.5 \times 10^{-6}$ and $\sim 2.5 \times 10^{-6}$ for SO and SO$_2$, respectively, and of $2\times10^{-10}$, $6.5\times10^{-10}$, and $4\times10^{-7}$ for AlO, AlF, and PO. The strong variation in brightness of the different features identified in the polarized-light images is puzzling. We suggest that an asymmetric stellar radiation field preferentially illuminates specific regions of the circumstellar envelope at a given time, producing a lighthouse-like effect.

Figures (22)

• Figure 1: Images of the polarization degree (color map) toward Mira at different epochs in filters CntHa for all epochs except for 2017, which was observed with filter NR. The approximate pulsation phases ($\phi$) of Mira A at the times of the observations are given above each panel. The white contours show the total intensity emission from Mira A at 20%, 40%, and 80% of the peak value. The red dots mark the position of the companion in the total intensity images, while the gray dots in the bottom right panel indicate the approximate position of the companion in previous epochs. The straight line used to determine the reference point on the NE lobe is shown in the upper middle panel. The 3$\sigma$ contour of the SO $J_K = {8_8-7_7}$ emission in 2017 November (shown in Fig. \ref{['fig:SO']}) is marked by the magenta line in comparison to the polarized-light map at the same epoch. The gray ellipse in the bottom right corner of each panel indicates the full width at half maximum of the point spread function in the obtained images.
• Figure 2: Moment-zero maps of the emission in the SO $J_K = 8_8-7_7$ line observed in 2017 November 9 and 2023 July 6 and 14. The approximate pulsation phases ($\phi$) of Mira A at the times of the observations are given at the top of each panel. The white contours mark where emission reaches three times the RMS noise level in each image. The straight white line in the right panel indicates the direction used to measure the size of the NW lobe in the two epochs.
• Figure 3: Observed distance on the plane of the sky of the reference point of the NE (maroon circles) and NW (magenta squares) lobes. The magenta solid line and blue dashed line show the fits to the observed extents of the NW and NE lobes, respectively, assuming constant expansion. The magenta dot-dashed and green dotted line shows a fit to the extent of the NW and NE lobes, respectively, assuming a ballistic trajectory for a star with 1.2 $M_\odot$. The black intervals mark the uncertainty interval on the ejection times of the lobe to the NE based on the two models.
• Figure 4: Results from the SO$_2$ populations diagram. From left to right, the panels show the obtained maps of $T_{\rm SO_2}^{\rm exc}$ in Kelvin, the uncertainty on $T_{\rm SO_2}^{\rm exc}$ in percentage, log$_{10}(N_{\rm SO_2}~{\rm cm^{-2}}$), and the uncertainty on $N_{\rm SO_2}$ in percentage. The white contours indicate the 30% uncertainty level on $N_{\rm SO_2}$.
• Figure 5: Observed emission (left) and inferred optical depth (middle) of the $^{13}$CO $J=3-2$ line and corresponding calculated column density of $^{13}$CO (right) assuming optically thin emission and the same excitation temperature distribution derived for SO$_2$. The dashed white circle in the middle panel marks the 60 mas radius inside of which the derived column densities are not reliable because of high optical depths. The white and magenta contours in the left and right panels, respectively, mark emission equal to three times the RMS noise level.