The distant star AU Microscopii may have mysterious cold spots. It seems to contain pockets of hydrogen that are more than 1500掳C cooler than the surrounding areas, and astronomers aren鈥檛 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鈥檚 spectrum: it contained evidence of hydrogen gas at temperatures between 725掳C and 1725掳C. The star鈥檚 photosphere 鈥� its outer shell, from which its light shines 鈥� has an average temperature of more than 3000掳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. 鈥淭here is no obvious answer as to where in the star this cold hydrogen is coming from,鈥� says Flagg.
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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 鈥渟tarspots鈥� 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鈥檚 mass, and neither effect has been spotted in such a small star before.
鈥淥ur understanding of stellar structure, especially for low mass stars, is really limited,鈥� says Flagg. We need to improve this, she says,聽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.
鈥淲e 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鈥檛 understand starspots, it鈥檚 going to be very difficult to understand the signatures of the planets around these stars,鈥� says Flagg. Understanding those signatures is crucial for the search for life on other worlds.
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