The distant star AU Microscopii may have mysterious cold spots. It seems to contain pockets of hydrogen that are more than 1500°C cooler than the surrounding areas, and astronomers aren’t sure why.
AU Microscopii, or AU Mic, is a relatively small star about 32 light years away. at Cornell University in New York was testing an algorithm using data on AU Mic collected by the Hubble Space Telescope in 1998 when she noticed something peculiar in the starlight’s spectrum: it contained evidence of hydrogen gas at temperatures between 725°C and 1725°C. The star’s photosphere – its outer shell, from which its light shines – has an average temperature of more than 3000°C.
Flagg and her colleagues performed a deeper analysis of the 1998 data, as well as new data taken by Hubble in 2020, and found that the cool hydrogen is probably in the star itself, rather than on a nearby planet or orbiting debris. This only deepened the mystery. “There is no obvious answer as to where in the star this cold hydrogen is coming from,” says Flagg.
Advertisement
The researchers came up with two hypotheses: AU Mic may contain an uncharacteristically cold layer at the top of its photosphere, or it may have “starspots” similar to those found on our sun, but with unexpectedly low temperatures. Both of these are plausible based on observations of the sun, but AU Mic is only half the sun’s mass, and neither effect has been spotted in such a small star before.
“Our understanding of stellar structure, especially for low mass stars, is really limited,” says Flagg. We need to improve this, she says, because in order to understand planets orbiting such stars, we have to be able to separate out what light comes from the star and what comes from the planet.
“We are going to look with the James Webb Space Telescope to try to find out about planets around stars like AU Mic, and if we don’t understand starspots, it’s going to be very difficult to understand the signatures of the planets around these stars,” says Flagg. Understanding those signatures is crucial for the search for life on other worlds.
Reference:
