Millimetre Observations Reveal Variability in the Inner Circumstellar Envelope of the S-type AGB Star χ Cygni

Concepts de base

The observations reveal significant variability in the emission of the H12CN(3-2) line confined to the very close neighborhood of the S-type AGB star χ Cygni, as well as evidence for a recent mass ejection event enhanced over the red-shifted north-western octant of the circumstellar envelope.

Résumé

The paper presents new millimetre observations of the circumstellar envelope (CSE) of the S-type Asymptotic Giant Branch (AGB) star χ Cygni using the upgraded NOEMA array. The observations provide insights into the morpho-kinematics of the inner CSE.
Key highlights:

The H12CN(3-2) line emission is found to be significantly more extended in 2024 compared to 2023, suggesting variability in the close vicinity of the star. This raises questions about the nature of the enhanced HCN emission, whether it is due to increased molecular abundance or enhanced excitation.
Evidence is found for a recent mass ejection event, particularly enhanced over the red-shifted north-western octant of the CSE, leaving a depression of emission in its wake. This is interpreted as episodic outflows associated with the surface of convective cells.
Unexpected features are observed in the very close neighborhood 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, possibly confirming the important role played by shocks.
Observation of SiO maser emission in the (ν=1,J=6-5) transition is reported for the first time.
The results highlight the need for further high-resolution observations to better understand the mechanisms underlying the newly observed features in the inner CSE of this S-type AGB star.

Stats

The mass loss rate of χ Cygni is measured to be 2.4×10^-7 M_sun/yr.
The distance to χ Cygni is estimated to be 165 pc.
The stellar radius is estimated to be between 348 and 480 solar radii.

Citations

"Recent high-resolution imaging of nearby Asymptotic Giant Branch (AGB) stars at visible and infrared wavelengths has revealed inhomogeneous distributions of gas and dust in the inner circumstellar environment, with changes in morphology and grain sizes occurring over the course of weeks or months, giving support to 3-D hydro-dynamical models."
"Hinkle et al. (1982), using observations of a time series of infrared spectra (1.6-2.5 µm) spanning more than three pulsation cycles of χ Cygni, had given evidence for the photometric variability to be associated with an outwardly propagating wave, which traverses the photosphere during the pre-maximum and maximum phases of the light curve."
"Lacour et al. (2009), using optical interferometry IOTA observations, estimate the mass of χ Cygni between 1.4 and 3.6 M_sun, its radius between 348 and 480 solar radii (10-14 mas), its luminosity between 6000 and 9000 solar luminosities, and its temperature between 2440 and 2740 K."

Idées clés tirées de

Millimetre observations of the S-type AGB star $\chi$ Cygni: variability of the emission of the inner envelope

by D.T. Hoai, J... à arxiv.org 10-02-2024

https://arxiv.org/pdf/2408.17000.pdf

Questions plus approfondies

How do the observed variations in the H12CN(3-2) line emission over time relate to changes in the physical conditions and dynamics within the inner circumstellar envelope of χ Cygni?

The observed variations in the H12CN(3-2) line emission from χ Cygni over the years, particularly between the 2023 and 2024 observations, indicate significant changes in the physical conditions and dynamics within its inner circumstellar envelope (CSE). The H12CN(3-2) line emission exhibited a notable increase in spatial extent and variability, suggesting that the gas dynamics in the vicinity of the star are influenced by pulsation-related shock waves. These shock waves, which are a result of the star's pulsation and convection, can lead to localized heating and compression of the gas, facilitating the formation of HCN molecules.
In 2023, the H12CN(3-2) emission was characterized by a narrow profile and a high intensity, indicating a concentrated region of HCN formation close to the star, likely within a gravitationally bound static layer. However, by 2024, the emission showed a broader distribution, implying that the gas had expanded and possibly mixed with surrounding material. This change could be attributed to episodic mass ejection events or variations in the pulsation amplitude, which affect the dynamics of the CSE. The variability in the H12CN(3-2) line emission thus reflects the complex interplay of shock dynamics, gas expansion, and the thermal conditions within the inner CSE, highlighting the dynamic nature of the environment surrounding χ Cygni.

What are the potential implications of the evidence for episodic mass ejection events on our understanding of the mass loss processes in S-type AGB stars?

The evidence for episodic mass ejection events in χ Cygni has significant implications for our understanding of mass loss processes in S-type Asymptotic Giant Branch (AGB) stars. These mass ejection events, which appear to be localized and associated with shock waves generated during pulsation cycles, suggest that mass loss is not a continuous process but rather occurs in discrete bursts. This episodic behavior challenges the traditional view of mass loss in AGB stars, which often assumes a steady outflow driven by radiation pressure on dust.
The observed patterns of enhanced emission in the north-western quadrant of the CSE, along with the associated high Doppler velocities, indicate that these mass ejections are likely linked to the star's pulsation phase and the dynamics of convective cells on its surface. Such findings imply that the mass loss rate in S-type AGB stars may vary significantly over time, influenced by the star's pulsation cycle and the resulting shock dynamics. Understanding these episodic mass ejection events is crucial for accurately modeling the mass loss processes in AGB stars, as they contribute to the chemical enrichment of the interstellar medium and influence the evolution of the star itself.

Given the complex interplay between convection, pulsation, and shock dynamics in the inner envelope of χ Cygni, how can future observations at even higher angular resolution and sensitivity help to further elucidate the underlying mechanisms driving the observed morpho-kinematic features?

Future observations of χ Cygni at even higher angular resolution and sensitivity are essential for unraveling the complex interplay between convection, pulsation, and shock dynamics within its inner circumstellar envelope. Enhanced angular resolution would allow astronomers to resolve finer structures in the CSE, providing insights into the spatial distribution of various molecular species and their respective emission characteristics. This could lead to a better understanding of the localized regions of mass ejection and the dynamics of gas flows influenced by pulsation-induced shocks.
Increased sensitivity would enable the detection of fainter emissions and subtle variations in line profiles, which are critical for studying the temporal evolution of the CSE. By capturing high-resolution spectral data over time, researchers could track changes in molecular abundances and kinematics, revealing how the physical conditions within the envelope evolve in response to the star's pulsation cycle. Such observations could also help confirm or refute hypotheses regarding the role of shock waves in driving mass loss and the formation of complex molecular species like HCN.
Ultimately, these advanced observational capabilities would facilitate the development of more accurate hydrodynamic models that incorporate the effects of convection and pulsation, leading to a deeper understanding of the mechanisms governing mass loss in S-type AGB stars and their impact on stellar evolution and the surrounding interstellar medium.