Millimetre observations of the S-type AGB star χ Cygni: Variability of the emission of the inner envelope

¹ Department of Astrophysics, Vietnam National Space Center (VNSC), Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Ha Noi, Vietnam
² Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, Domaine Universitaire, 38406 St. Martin d’Hères, France
³ LERMA, UMR 8112, CNRS and Observatoire de Paris, PSL Research University, 61 av. de l’Observatoire, 75014 Paris, France

^★ Corresponding author; dthoai@vnsc.org.vn, winters@iram.fr

Received: 4 June 2024
Accepted: 23 September 2024

Abstract

New observations are presented of millimetre line emissions of the circumstellar envelope (CSE) of the asymptotic giant branch (AGB) star χ Cygni, using the recently upgraded NOEMA array. χ Cygni is an S-type Mira variable, at the border between oxygen-rich and carbon-rich stars. It has been observed for over 40 years to display features that suggest evidence for the strong role played by pulsation- associated shock waves in the generation of its wind. These new observations provide evidence of a bright H¹²CN(3–2) line emission confined to the very close neighbourhood of the star; however, this emission appears significantly more extended in 2024 than in 2023. The interpretation of such variability in terms of maser emission has been considered and found to raise significant unanswered questions. Moreover, other unexpected features are observed in the very close neighbourhood of the star, including low Si¹⁶O(6–5)/ Si¹⁷O(6–5), ²⁸SiO(5–4)/²⁹SiO(5–4), and ¹²CO(2–1)/¹³CO(2–1) line emission ratios. We discuss several features, which possibly confirm the important role played by shocks: a measurement of the SiO(5–4)/SiO(6–5) emission ratio; the observation of a recent mass ejection, particularly enhanced in the north-western red-shifted octant, which has left a depression in its wake; patterns of enhanced CO(2–1) line emission, which suggest an interpretation in terms of episodic outflows, on a time scale of a few decades, enhanced over solid angles associated with the surface of convective cells. Unravelling the mechanisms underlying such newly observed features is very challenging. Thus, a confirmation of the reported observations with improved sensitivity and angular resolution would be highly welcome. The observation of SiO maser emission in the (ν=1, J=6–5) transition is reported for the first time.