AU Microscopii (AU Mic) is a young red dwarf star located away – about 8 times as far as the closest star after the Sun. The apparent visual magnitude of AU Microscopii is 8.73, which is too dim to be seen with the naked eye. It was given this designation because it is in the southern constellation Microscopium and is a variable star. Like β Pictoris, AU Microscopii has a circumstellar disk of dust known as a debris disk and at least two exoplanets, with the presence of an additional two planets being likely.
Stellar properties
AU Mic is a young star at only 22 million years old; less than 1% of the age of the Sun. With a stellar classification of M1 Ve, it is a red dwarf star with a physical radius of 75% that of the Sun. Despite being half the Sun's mass, it is radiating only 9% as much luminosity as the Sun. This energy is being emitted from the star's outer atmosphere at an effective temperature of 3,700 K, giving it the cool orange-red hued glow of an M-type star. AU Microscopii is a member of the β Pictoris moving group. AU Microscopii may be gravitationally bound to the binary star system AT Microscopii.
AU Microscopii has been observed in every part of the electromagnetic spectrum from radio to X-ray and is known to undergo flaring activity at all these wavelengths. Its flaring behaviour was first identified in 1973. Underlying these random outbreaks is a nearly sinusoidal variation in its brightness with a period of 4.865 days. The amplitude of this variation changes slowly with time. The V band brightness variation was approximately 0.3 magnitudes in 1971; by 1980 it was merely 0.1 magnitudes.
Planetary system
AU Microscopii's debris disk has an asymmetric structure and an inner gap or hole cleared of debris, which has led a number of astronomers to search for planets orbiting AU Microscopii. By 2007, no searches had led to any detections of planets. However, in 2020 the discovery of a Neptune-sized planet was announced based on transit observations by TESS. Its rotation axis is well aligned with the rotation axis of the parent star, with the misalignment being equal to 5°.
Since 2018, a second planet, AU Microscopii c, was suspected to exist. It was confirmed in December 2020, after additional transit events were documented by the TESS observatory.
A third planet in the system was suspected since 2022 based on transit-timing variations, and "validated" in 2023, although several possible orbital periods of planet d cannot be ruled out yet. This planet has a mass comparable to that of Earth. Radial velocity observations have also found evidence for a fourth, outer planet as of 2023.
Debris disk
AU Microscopii harbors its own disk of dust, first resolved at optical wavelengths in 2003 by Paul Kalas and collaborators using the University of Hawaii 2.2-m telescope on Mauna Kea, Hawaii. This large debris disk faces the earth edge-on, and measures at least 200 AU in radius. At these large distances from the star, the lifetime of dust in the disk exceeds the age of AU Microscopii. The disk has a gas to dust mass ratio of no more than 6:1, much lower than the usually assumed primordial value of 100:1. The debris disk is therefore referred to as "gas-poor". The total amount of dust visible in the disk is estimated to be at least a lunar mass, while the larger planetesimals from which the dust is produced are inferred to have at least six lunar masses.
The spectral energy distribution of AU Microscopii's debris disk at submillimetre wavelengths indicate the presence of an inner hole in the disk extending to 17 AU, while scattered light images estimate the inner hole to be 12 AU in radius. Combining the spectral energy distribution with the surface brightness profile yields a smaller estimate of the radius of the inner hole, 1 - 10 AU.
The inner part of the disk is asymmetric and shows structure in the inner 40 AU. The inner structure has been compared with that expected to be seen if the disk is influenced by larger bodies or has undergone recent planet formation.
The surface brightness (brightness per area) of the disk in the near infrared as a function of projected distance from the star follows a characteristic shape. The inner of the disk appear approximately constant in density and the brightness is unchanging, more-or-less flat. Around the density and surface brightness begins to decrease: first it decreases slowly in proportion to distance as ; then outside , the density and brightness drops much more steeply, as . This "broken power-law" shape is similar to the shape of the profile of β Pic's disk.