A surprisingly large asymmetric ejection from Mira A

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 MiraA 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 MiraA, 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 200years 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.

💡 Research Summary

The authors present a multi‑epoch, high‑resolution study of the inner circumstellar environment of the oxygen‑rich AGB star Mira A (o Ceti). Using VLT/SPHERE‑ZIMPOL they obtained six polarized‑light images between 2015 and 2023 (angular resolution 19–28 mas), and with ALMA they observed several molecular lines (13CO J=3‑2, CO J=3‑2, multiple SO and SO₂ transitions, plus AlO, AlF and PO) at two epochs (2017 Nov and 2023 Jul) with ∼15 mas resolution. The polarized images reveal two prominent lobes extending from the star; dust scattering is confined almost exclusively to the edges of these lobes, while the molecular emission fills the interior. The northern‑eastern (NE) lobe can be tracked across all epochs and shows clear outward motion.

By measuring the position of a filamentary reference point in the NE lobe, the authors fit two kinematic models. A constant‑velocity model yields a plane‑of‑sky expansion speed of 10.5 ± 1.6 km s⁻¹ and implies an ejection date around late 2010–early 2011. A ballistic (decelerating) model gives an initial speed of 31.6 ± 0.6 km s⁻¹ and an ejection around late 2012–early 2013. Both fits reproduce the data, so the exact launch time and initial velocity remain ambiguous, but the observed deceleration suggests an episodic, rather than a steady, wind.

Radiative‑transfer analysis of the SO and SO₂ lines provides column densities and excitation temperatures, leading to abundances X(SO) ≈ 1.5 × 10⁻⁶ and X(SO₂) ≈ 2.5 × 10⁻⁶ relative to H₂. AlO and AlF are detected at very low levels (∼2 × 10⁻¹⁰ and 6.5 × 10⁻¹⁰, respectively), while PO is more abundant (∼4 × 10⁻⁷). These values are consistent with a chemistry where sulfur quickly forms SO/SO₂ in an oxygen‑rich environment, aluminum is largely locked in refractory Al₂O₃ dust, and phosphorus appears mainly as PO.

Integrating the gas density over the observed volume gives a total gas mass in the two lobes of ≈2 × 10⁻⁵ M⊙, substantially higher than expected from the large‑scale mass‑loss rate of Mira A at the same radii. This excess mass, together with the inferred ejection dates, leads the authors to propose a quasi‑periodic ejection mechanism with a recurrence time of 50–200 yr. Such events could dominate the overall mass‑loss budget if they repeat with similar magnitude, although the global mass‑loss rate remains uncertain.

A striking feature of the polarized‑light data is the strong, epoch‑dependent variation in brightness of individual lobe structures. The authors argue that an asymmetric stellar radiation field—perhaps caused by large convective cells or hot spots on the stellar surface—illuminates different portions of the circumstellar envelope at different times, producing a “lighthouse” effect. This interpretation naturally explains the rapid changes in scattered‑light intensity without invoking changes in the dust distribution itself.

In summary, the paper provides compelling evidence that Mira A experiences episodic, highly asymmetric mass‑ejection events that are not captured by simple, spherically symmetric wind models. The combination of dust confined to lobe edges, dense gas filling the lobes, unusual molecular abundances, and time‑variable illumination points to a complex interplay of stellar pulsation, convection, dust nucleation, and radiative transfer in shaping the mass‑loss process of oxygen‑rich AGB stars.