AU and AT Microscopii AB

Arnold O. Benz, Institute of Astronomy, ETH Zurich

High resolution and jumbo images (Benz et al, 1998).

AU and AT Mic AB are flare stars, like UV Ceti (Luyten
726-8 B) shown flaring at left. UV Ceti is an extreme
example of a flare star that can boost its brightness by
five times in less than a minute, then fall somewhat
slower back down to normal luminosity within two or three
minutes before flaring suddenly again after several hours.

AU Microscopii

This cool and dim, main sequence red dwarf (M0-1 Ve) has only around 50 percent of Sol's mass (Paul Kalas), around 66 to 67 percent of its diameter (Johnson and Wright, 1983, page 697), and about 2.5 to 2.9 percent of its visual luminosity but an absolute luminosity (including infrared wavelengths) around 10 percent of Sol's (Paul Kalas). However, AU Microscopii is a very young, active flare star, only around 12 million years old -- but may be as young as eight and as old as 20 million years old. In addition, it is also classified as a BY Draconis variable due to prominent starspots and a fast rotation. Some alternative names and useful star catalogue numbers are: AU Mic, Gl 803, Hip 102409, HD 197481, CD-31 17815, CP(D)-31 6335, SAO 212402, LTT 8214, LDS 720 A, and V 824.

Paul ,
U.H.'s 2.2-m Telescope,
IfA, U.C. Berkeley
(Used with permission)




Larger infrared image.




A planet may be creating a hole in the
dust disk of AU Microscopii, within 17
AUs of the star (more).

Dust Disk and Planets?

In the 1980s, the Infrared Astronomical Satellite (IRAS) revealed an "excess" of far-infrared radiation in the spectrum of AU Mic. Infrared excesses around stars are common (around 15 percent of all stars), but direct images are quite rare -- obtained in barely a handful of cases thus far. Some strong detections of an infrared excess have been found later to be associated with substantial dust disks around some very young star systems such as Beta Pictoris; indeed, AU Mic is likely to have been born from the same nebula as the more massive Beta Pic, an A-type star, which has three to four times more dust than AU Mic in its disk. In the case of AU Mic, a dust disk was later imaged as a nebulosity around the star using a charged-coupled device (CCD) at the University of Hawaii 2.2 m telescope on Mauna Kea by a team astronomers, including Paul Kalas as the lead investigator (Kalas et al, March 2004; in pdf).

J.E. Krist, D.R. Ardila,
D.A. Golimowski, M. Clampin,
H.C. Ford, G.D. Illingworth,
G.F. Hartig, ACS Team,
STScI, ESA, NASA


Larger and annotated images


The disk extends from around
35 to more than 100 AUs from
the star (more).

In 2004, a similar team of astronomers (including Michael Liu, as lead investigator, Kalas, Jonathan Williams, and Barbara Matthews) made a new measurement of the infrared excess around AU Mic using the James Clerk Maxwell Telescope (JCMT) on Mauna Kea. According to Kalas, the data indicated that the star is surrounded by dust of a given temperature, and that temperature suggests how far away from the star the dust is located (i.e., closer is hotter). In the case of AU Mic, the team found that the dust is quite cold (40 Kelvin), and the lack of hot grains in infrared emission indicates that dust is missing within 17 AUs of AU Mic -- which was confirmed optically within 12 AUs (Krist et al, 2004). The disk extends around 35 to more than 100 AUs from the star (Krist et al, 2004; and Michael Liu, 2004).

Greg Bacon, John Krist,
STScI, ESA, NASA



Larger illustration



The disk has a "hole" centered
around the star that may have
cleared out by a planet(s) (more).

Infrared images of AU Mic's disk were obtained by blocking out the glare of the star using a device known as a coronagraph. Although the earlier CCD image indicated that the dust around AU Mic is contained in a circumstellar disk, the occulting spot for the star that made those images possible has a radius of 50 AUs and so the 17-AU hole in AU Mic's disk could not be seen directly; subsequently, however, the Hubble Space Telescope was used to resolve the disk to within 8 AUs of the star. Astronomers have interpreted the data as indicating that there is dust depletion within 12 AUs of the star, and that one way to keep grains away from the star is if a planet is sweeping up the dust (Krist et al, 2004; Kalas et al, 2004; and Liu et al, Astrophysical Journal, forthcoming June 2004).

Michael Liu,
IfA/University of Hawaii,
Keck Observatory





Clumps and other asymmetries in
AU Mic's inner dust disk suggest
that planets as large as Pluto
but smaller than Jupiter have
formed (more information and
images from IfA).

On August 12, 2004, the same astronomer team announced that they had found multiple clumps in the dust disk about 25 to 40 AUs from AU Mic (between the orbital distances of Neptune and Pluto in the Solar System) which are probably produced by the gravitational interactions of unseen, newly formed planets. According to the team's calculations of asymmetries in the inner disk that exhibit a sharp change at around 35 AUs (32-38 AUs) of the star, these planetary bodies could be as small as Pluto. However, they are likely to be smaller than Jupiter as very massive planets should still retain sufficient heat from planetary formation to have been directly visible with the Keck telescope(s) used by the astronomers (more from IfA's press release, Michael Liu, 2004, space.com, and Science -- requires subscription).

Greg Bacon, STScI,
ESA, NASA



Larger illustration



A rocky inner planet with moon
within the "hole" around AU Mic's
dust disk, as imagined by Greg
Bacon (more).

On January 7, 2007, astronomers using the Hubble Space Telescope announced that the observation of "a blizzard of particles" that they believe to have been shed from unseen snowball-sized objects in AU Mic's disk, which reflects the process by which planets grow from tiny dust grains (new release). The icy as well as dusty snowballs are believed to reside in a region called AU Mic's "birth ring," which was first hypothesized in 2005 by Linda Strubbe and Eugene Chiang. Located between 3.7 and 4.6 billion miles from the star in an icy cold region of AU Mic, these objects bump into each other and release fluffy particles that are propelled outward by the intense pressure from starlight. Many astronomers believe that such objects are the seeds of planets that have or will eventually develop.

James R. Graham,
Paul G. Kalas,
Brenda Matthews,
STScI, ESA, NASA




Larger image.




With the dust disk,
particles shed from
snow-balled sized
bodies have been
detected (more).

On May 7, 2007, astronomers modelling the vertical thickness and dust production in AU Mic's disk based on Hubble observations submitted a paper arguing that its disk likely contains planetary embryos as large as Pluto that are undergoing runaway growth into larger bodies (Quillen et al, 2007). Their model predicts how large the planetary bodies in a dust or debris disk must be to puff it up to a certain thickness. Although a developing star system's dust disk should thin as the system ages, more dust grains may be "knocked" into eccentric orbits that "puff out" the disk if dust agglomeration has proceeded enough for planetesimals to form increasingly large embryonic planets. In the case of AU Mic, the model indicates that a Pluto-sized object may have already formed at an orbital distance of around 30 AUs from the red dwarf star.

NASA



Larger image.



A Pluto-sized planetary
embryo may have already
formed at an orbital
distance of around 30
AUs from AU Mic (which
may look like Neptune's
icy moon, Triton).

The distance from AU Microscopii where an Earth-type planet would be comfortable with liquid water may be centered around 0.3 AU. Even if Earth-type life managed to develop on an inner planet within this very young system (which has an estimated age of only eight to 20 million years), AU Mic's frequent flares would "fry" any surface microbes that are not already affected by frequent asteroidal or cometary impactors.

AT Microscopii A

This cool and dim, main sequence red dwarf (M4.5 Vpe) probably has less than half of Sol's mass, 41 percent of its smaller diameter (Johnson and Wright, 1983, page 697), and only around 0.35 percent of its brightness. However, AT Microscopii is a flare star, probably around 12 million years old like AU Mic. Some alternative names and useful star catalogue numbers are: AT Mic A, Gl 799 A, Hip 102141, HD 196982, CD-31 16135 A, CP(D)-31 6181, SAO 212355, LTT 8181, LDS 720 B, and U or UGP 502.

Star A has a binary companion. In 1962, it was separated from its companion star by almost 40 AUs (SepAB= 3.9"). According to the Sixth Catalog of Orbits of Visual Binaries, the latest data indicate that the orbit of the two stars Stars A and B has an average separation (semi-major axis of 3.18") of 32.5 AUs. However, the eccentricity of the orbit (e= 0.256) moves the stars as close as 24.2 AUs but as far apart as 40.8 AUs. The two stars take around 209 years to complete an orbit around each other, which is inclined by about 137 degrees from the perspective of an observer on Earth (Hartkopf and Mason, 2003).

AT Microscopii B

This cool and dim, main sequence red dwarf (around M4.5 Ve) probably has less than half of Sol's mass, 37 percent of its diameter (Johnson and Wright, 1983, page 697), and less than 0.35 percent of its brightness of its brightness. However, it is probably around 12 million years old like AU Mic. Some alternative names and useful star catalogue numbers are: AT Mic B, Gl 799 B, CD-31 16135 B, CP(D)-31 6181 B, and LDS 720 C.