Some stars produce dust. But unlike the stuff underneath your TV cabinet, this dust plays a major part in the total luminosity of a galaxy. In a new study, scientists have used long-term observational data to see how interstellar dust correlates with a star’s brightness.
Asymptotic giant branch (AGB) stars are generally slightly older and bigger than our Sun. They are also the main producers of space dust, particularly those aptly named ‘dusty AGB stars.’ Because AGB stars are bloated red giant stars, their outer atmospheres are cool, allowing condensation and radiation pressure to push the dust outward.
Interstellar dust is a key component in the formation of solid objects, including planets. Despite studies investigating how this dust originates, the process is not well understood. Some AGB stars are thought to control dust formation through the expansion and contraction of their outer layers, a phenomenon called stellar pulsation that causes changes in a star’s luminosity.
Investigating the relationship between stellar pulsation and dust formation requires mid-infrared (IR) monitoring. IR radiation is emitted by any object that radiates heat, so almost all celestial objects emit some IR. The wavelength at which an object radiates most intensely depends on its temperature, meaning that some IR wavelengths are better for studying certain objects than others. Mid-IR operates at a temperature range of 10 to 140 Kelvin (-263 °C/-442 °F to -133 °C/-208 °F) and can be used to see planets, comets, asteroids, and, importantly, dust warmed by starlight.
Researchers from the University of Tokyo have used the data from two IR space telescopes, used on the AKARI and WISE satellites, to gain insights into the relationship between the dust of AGB stars and their brightness.
“We study stars, and IR light from them is a key source of information that helps us unlock their secrets,” said Kengo Tachibana, lead author of the study. “Until recently, most IR data was from very short-period surveys due to the lack of advanced dedicated platforms. But missions like AKARI and WISE have allowed us to take longer-period surveys.”
Out of commission since 2011, AKARI was the first Japanese satellite dedicated to making IR observations, surveying the sky using near-, mid- and far-IR. The Wide-field Infrared Survey Explorer (WISE) was a NASA IR space telescope originally launched in December 2009, placed in hibernation mode in February 2011, and brought back to life in 2013 as the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE).
Combining the mid-IR data collected by AKARI and WISE, researchers were able to generate long-term observational data. The study is the largest on mid-IR variability, a measure of the amount of dust a star produces.
The researchers found a correlation between the light intensity of dusty AGB stars and variations in their dust production.
“Thanks to long-period IR observations, we have found that the light from dusty AGBs varies,” Tachibana said. “We also found that the spherical shells of dust produced by and then ejected by these stars have concentrations of dust that vary in step with the stars’ changes in luminosity. Of the 169 dusty AGBs surveyed, no matter their variability period, the concentrations of dust around them would coincide. So, we’re certain these are connected.”
This study was just the beginning for the researchers who intend to explore the mechanisms of interstellar dust production using IR observations obtained by the University of Tokyo Atacama Observatory in Chile, which may help uncover more information about how our own Solar System formed.
The study was published in the journal Publications of the Astronomical Society of Japan.
Source: University of Tokyo
