Dusting Off The Mysteries Surrounding Dying Stars

The imbalance between aluminum oxide and silicate in the dust surrounding dying stars has been explained by scientists in Japan.

AsianScientist (Nov. 14, 2017) – Scientists in Japan have obtained a better understanding of the composition of dust that surrounds dying stars. Their findings are published in the journal Science Advances.

Stars like our Sun eject large amounts of gas and dust into space, containing various elements and compounds. Asymptotic giant branch (AGB) phase stars, near their end of life, are particularly significant sources of such substances in our galaxy.

In space, silicon is ten times more abundant than aluminum. However, many oxygen-rich AGB stars are rich in aluminum oxide dust but poor in silicate dust, which has puzzled researchers. The question remains: Why is aluminum oxide dust so abundant around oxygen-rich AGB stars?

In this study, a research team led by Assistant Professor Aki Takigawa of Kyoto University has used the Atacama Large Millimeter/submillimeter Array (ALMA) to obtain detailed images of gas molecules forming dust surrounding an AGB star.

“Previously, there was a limit to how well we could observe dust forming regions close to stars,” Takigawa explained. “Now, thanks to the high spatial resolution of ALMA, we can obtain images of gases in these regions in finer detail. So, we pointed ALMA toward an aluminum oxide-rich AGB star, W Hydrae.”

Gas molecules of aluminum monoxide and silicon monoxide—AlO and SiO, respectively—eventually form aluminum oxide and silicate dust. The team observed that AlO was distributed within three stellar radii of W Hydrae, while SiO was detected beyond five stellar radii. Moreover, 70 percent of SiO remained gaseous, without forming into dust.

“These results indicate that as aluminum oxide grows and accumulates near a star, the addition of a small amount of silicate dust may trigger wind acceleration,” said Takigawa. “This decreases gas density, suppressing further silicate dust formation.”

“This may explain the presence of aluminum-oxide-rich but silicate-poor AGB stars,” he added.

These new results shed light not only on the dynamics of gas and dust surrounding stars, but also on the importance of studying both gas and dust in tandem. The team plans to continue using ALMA to elucidate gas and dust dynamics in the universe.

The article can be found at: Takigawa et al. (2017) Dust Formation and Wind Acceleration Around the Aluminum Oxide–rich AGB Star W Hydrae.


Source: Kyoto University; Photo: ALMA (ESO/NAOJ/NRAO), Takigawa et al.
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