Research News
NARIT Astronomer Helps Discover Highest-Energy SiO Emission Ever Detected in AGB Stars
Dr. Bannawit Pimpanuwat, a radio astrophysicist at the National Astronomical Research Institute of Thailand (Public Organization), or NARIT, has joined the international ATOMIUM collaboration in using the Atacama Large Millimeter/submillimeter Array (ALMA) to investigate the innermost gas and dust surrounding evolved AGB stars. The team detected silicon monoxide (SiO) emission from the highest-energy vibrational state ever observed in this type of star, providing new insights into the extreme environment where stellar winds are launched. The findings were published in Monthly Notices of the Royal Astronomical Society on May 9, 2026.
What Are AGB Stars?
Asymptotic Giant Branch (AGB) stars are low- to intermediate-mass stars, ranging from about 0.8 to 8 times the mass of the Sun, that have reached the final stage of their evolution. During this phase, the stars expand dramatically, their surface temperatures decrease, and they undergo strong pulsations that lift material away from the stellar surface. This material forms a dense circumstellar envelope (CSE) of gas and dust surrounding the star.
These envelopes play an essential role in the cosmic cycle of matter. Material expelled by dying stars is recycled into interstellar space, where it becomes the building blocks for future generations of stars, planetary systems, and the chemical elements that can eventually contribute to the emergence of life.
Why Observe SiO?
The region closest to the stellar surface, where stellar winds begin, is extremely dense and difficult to observe directly. SiO is one of the key molecules present in this region. It is involved in the formation of silicate dust and emits radiation at different energy levels depending on the surrounding environment. By observing these spectral lines, astronomers can measure the temperature, density, and motion of gas close to the wind-launching region with exceptional detail.
The Highest-Energy SiO Detection Ever Made in an AGB Star
ATOMIUM (ALMA Tracing the Origins of Molecules In dUst-forming oxygen-rich M-type stars) is a large ALMA program that investigates the composition and morphology of stellar winds from oxygen-rich evolved stars.
For this study, the team observed 14 oxygen-rich AGB stars and detected 16 SiO spectral lines between 214 and 270 GHz (or wavelengths of ~1 millimeter). These lines span vibrational states from v = 0 to v = 8, making this the highest SiO vibrational state ever detected in an AGB star. The corresponding upper-state energy is approximately 13,700 Kelvin, nearly ten times higher than the typical gas temperature in the innermost regions of AGB circumstellar envelopes.
Exciting SiO molecules to such extreme energy levels requires highly energetic and complex conditions near the stellar surface. Possible mechanisms include shock waves generated by stellar pulsations, strong infrared radiation fields through radiative pumping, collisions between molecules, and non-local thermodynamic equilibrium (non-LTE) conditions.
Masers: Probing Gas Close to the Stellar Surface
The researchers also found that several SiO lines exhibit maser emission, a natural amplification of radio and microwave radiation in space that operates on the same physical principle as a laser, but at much longer wavelengths.
Masers occur when molecules are driven into a population inversion, allowing stimulated emission to amplify the radiation dramatically. Because masers are exceptionally bright and originate from compact regions, they are much easier to detect than the fainter, more extended thermal emission. They also enable astronomers to pinpoint the locations of gas clumps with remarkable precision, revealing fine structures near the stellar surface where stellar winds are beginning to form.
High-resolution ALMA observations further show that this high-energy gas is not distributed in simple ring-like structures. Instead, it forms complex clumps and arcs with clear velocity gradients. These features are consistent with shock waves produced by stellar pulsations, which drive some gas outward while other material, lacking sufficient energy to escape the star's gravity, falls back toward the stellar surface.
Questions That Remain
One of the most intriguing findings is that stars with higher mass-loss rates do not necessarily exhibit more millimeter-wavelength SiO spectral lines. This suggests that stellar wind formation is governed by more than the amount of material a star loses.
Instead, the process is likely influenced by a combination of factors, including the three-dimensional structure of the gas, the timing of stellar pulsations, shock waves, and dust formation. Understanding how these processes interact will require more sophisticated computer models and long-term observations.
This study represents an important step toward connecting high-resolution observations with theoretical models of how Sun-like stars end their lives. It also provides new clues to one of astronomy's fundamental questions: how the final stages of stellar evolution help supply the raw material from which new stars and planetary systems are born.
Written by: Dr. Bannawit Pimpanuwat - Radio Astrophysics Researcher, NARIT
Reference: Pimpanuwat et al. (2026). ATOMIUM: inner circumstellar envelopes of oxygen-rich AGB stars as revealed by highly excited SiO lines. Monthly Notices of the Royal Astronomical Society, 549, stag888. https://doi.org/10.1093/mnras/stag888