AU Microscopii (AU Mic) is a young

red dwarf A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

located away – about 8 times as far as the closest star after the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

. The

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light ca ...

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 Microscopium ("the Microscope") is a minor constellation in the southern celestial hemisphere, one of twelve created in the 18th century by French astronomer Nicolas-Louis de Lacaille and one of several depicting scientific instruments. The name ...

and is a

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

. Like β Pictoris, AU Microscopii has a circumstellar disk of dust known as a

debris disk A debris disk (American English), or debris disc ( Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris ...

and at least three

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

s, with the presence of an additional planet 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 In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...

of M1 Ve, it is a red dwarf star with a physical radius of 75% that of the

. 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 The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of 3,700 K, giving it the cool orange-red hued glow of an

M-type star In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

. AU Microscopii is a member of the β Pictoris moving group. AU Microscopii may be gravitationally bound to the binary star system

AT Microscopii AT Microscopii is a binary star system located at a distance of from the Sun in the constellation of Microscopium. Both members are flare stars, meaning they are red dwarf stars that undergo random eruptions that increase their brightn ...

AU Microscopii has been observed in every part of the

electromagnetic spectrum The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. From low to high ...

from

radio Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

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 A sine wave, sinusoidal wave, or sinusoid (symbol: ∿) is a periodic wave whose waveform (shape) is the trigonometric sine function. In mechanics, as a linear motion over time, this is '' simple harmonic motion''; as rotation, it correspond ...

variation in its brightness with a period of 4.865 days. The amplitude of this variation changes slowly with time. The

V band The V band ("vee-band") is a standard designation by the Institute of Electrical and Electronics Engineers (IEEE) for a band of frequencies in the microwave portion of the electromagnetic spectrum ranging from 40 to 75 gigahertz (GHz). The ...

brightness variation was approximately 0.3

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

s 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 Transit may refer to: Arts and entertainment Film * ''Transit'' (1980 film), a 1980 Israeli film * ''Transit'' (1986 film), a Canadian short film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countrie ...

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 2024 study which performed measurements of

Rossiter–McLaughlin effect The Rossiter–McLaughlin effect is a spectroscopic phenomenon observed when an object moves across the face of a rotating star. The star is seen to undergo a redshift anomaly caused by the obscuration of different parts of its disk. Descriptio ...

for the planet c revealed that the planet is possibly misaligned with the star's rotation axis, returning a poorly constrained value of projected

obliquity In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbital ...

''λ''_c = . A third planet in the system was suspected since 2022 based on

transit-timing variation Transit-timing variation is a method for detecting exoplanets by observing variations in the timing of a transit. This provides an extremely sensitive method capable of detecting additional planets in the system with masses potentially as small a ...

s, 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 The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...

observations have also found evidence for a fourth, outer planet as of 2023. Observations of the AU Microscopii system with the

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, Lis ...

were unable to confirm the presence of previously unknown companions. Observations with

CHEOPS Khufu or Cheops (died 2566 BC) was an ancient Egyptian monarch who was the second pharaoh of the Fourth Dynasty, in the first half of the Old Kingdom period (26th century BC). Khufu succeeded his father Sneferu as king. He is generally accep ...

also detected strong TTVs of AU Mic c, which can be explained with planet d on a 12.6 day orbit. The mass of planet d is found to be only about 20% of the mass of earth (or two

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

masses) according to this study.

Debris disk

All-sky observations with the Infrared Astronomy Satellite revealed faint infrared emission from AU Microscopii. This emission is due to a circumstellar disk of dust which first resolved at optical wavelengths in 2003 by Paul Kalas and collaborators using the University of Hawaii 2.2-m telescope on

Mauna Kea Mauna Kea (, ; abbreviation for ''Mauna a Wākea''); is a dormant Shield volcano, shield volcano on the Hawaii (island), island of Hawaii. Its peak is above sea level, making it the List of U.S. states by elevation, highest point in Hawaii a ...

, Hawaii. This large debris disk faces the earth edge-on at nearly 90 degrees, 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", as the primordial gas within the circumstellar system has been mostly depleted. The total amount of dust visible in the disk is estimated to be at least a lunar mass, while the larger

planetesimal Planetesimals () are solid objects thought to exist in protoplanetary disks and debris disks. Believed to have formed in the Solar System about 4.6 billion years ago, they aid study of its formation. Formation A widely accepted theory of pla ...

s from which the dust is produced are inferred to have at least six lunar masses. The

spectral energy distribution A spectral energy distribution (SED) is a plot of energy versus frequency or wavelength of light (not to be confused with a 'spectrum' of flux density vs frequency or wavelength). It is used in many branches of astronomy to characterize astron ...

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 In astronomy, surface brightness (SB) quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background. An object's surface brightness depends on ...

(brightness per area) of the disk in the near infrared

\scriptstyle I

as a function of projected distance

\scriptstyle r

from the star follows a characteristic shape. The inner

\scriptstyle r\,<\,15 AU

of the disk appear approximately constant in density and the brightness is unchanging, more-or-less flat. Around

\scriptstyle r\, \approx\, 15 AU

the density and surface brightness begins to decrease: first it decreases slowly in proportion to distance as

\scriptstyle I\, \propto \,  r^

; then outside

\scriptstyle r\, \approx\, 43 AU

, the density and brightness drops much more steeply, as

\scriptstyle I\, \propto \, r^

. This "broken power-law" shape is similar to the shape of the profile of β Pic's disk. In October 2015 it was reported that astronomers using the

Very Large Telescope The Very Large Telescope (VLT) is an astronomical facility operated since 1998 by the European Southern Observatory, located on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each equipped with ...

(VLT) had detected very unusual outward-moving features in the disk. By comparing the VLT images with those taken by the

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

in 2010 and 2011 it was found that the wave-like structures are moving away from the star at speeds of up to 10 kilometers per second (22,000 miles per hour). The waves farther away from the star seem to be moving faster than those close to it, and at least three of the features are moving fast enough to escape the gravitational pull of the star. Follow-up observations with the

SPHERE A sphere (from Ancient Greek, Greek , ) is a surface (mathematics), surface analogous to the circle, a curve. In solid geometry, a sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three ...

instrument on the

were able to confirm the presence of the fast-moving features, and James Webb Space Telescope observations found similar features within the disk in two NIRCam filters; however, these features have not been detected in the radio with

Atacama Large Millimeter Array The Atacama Large Millimeter/submillimeter Array (ALMA) is an astronomical interferometer of 66 radio telescopes in the Atacama Desert of northern Chile, which observe electromagnetic radiation at millimeter and submillimeter wavelengths. The ar ...

observations. These fast-moving features have been described as "dust avalanches", where dust particles catastrophically collide into planetesimals within the disk.

Methods of observation

AU Mic's disk has been observed at a variety of different

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

s, giving humans different types of information about the system. The light from the disk observed at optical wavelengths is stellar light that has reflected (scattered) off dust particles into Earth's line of sight. Observations at these wavelengths utilize a coronagraphic spot to block the bright light coming directly from the star. Such observations provide high-resolution images of the disk. Because light having a wavelength longer than the size of a dust grain is scattered only poorly, comparing images at different wavelengths (visible and near-infrared, for example) gives humans information about the sizes of the dust grains in the disk. Optical observations have been made with the

and

Keck Telescope The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have aperture primary mirrors, and, when c ...

s. The system has also been observed at

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

and sub-millimeter wavelengths with the

James Clerk Maxwell Telescope The James Clerk Maxwell Telescope (JCMT) is a submillimetre-wavelength radio telescope at Mauna Kea Observatory in Hawaii, US. The telescope is near the summit of Mauna Kea at . Its primary mirror is 15 metres (16.4 yards) across: it is the la ...

Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003, that was deactivated when operations ended on 30 January 2020. Spitzer was the third space telescope dedicate ...

, and the

. This light is emitted directly by dust grains as a result of their internal heat (modified

blackbody A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The radiation emitted by a black body in thermal equilibrium with its environment is ...

radiation). The disk cannot be resolved at these wavelengths, so such observations are measurements of the amount of light coming from the entire system. Observations at increasingly longer wavelengths give information about dust particles of larger sizes and at larger distances from the star.

Gallery

Image:The View from Within AU Microscopii's Disk.jpg, Artist's impression of AU Microscopii Credit: NASA/ESA/G. Bacon (STScI) File:Hubble sees evaporating planet getting the hiccups (opo2315a).jpg, Artist's impression of AU Microscopii and planet AU Mic b, which is losing mass.

References

External links

* {{DEFAULTSORT:AU Microscopii Microscopium M-type main-sequence stars Flare stars Circumstellar disks Microscopii, AU 197481 Articles containing video clips 0803 102409 Durchmusterung objects Planetary systems with two confirmed planets Beta Pictoris moving group