AT Microscopii

Lore & Background

The system was first noted in 1926 by Dutch-American astronomer Willem Jacob Luyten, who reported spectral line variations in the star. A photographic plate from June 23, 1895, showed bright hydrogen lines that were much weaker on a plate taken June 29, 1895, and absent on a plate from July 1, 1903. The net brightness variation did not exceed 0.5 magnitude. Luyten also noted a large proper motion, with the star shifting 0.43 arcseconds between 1899 and 1923. By 1927, the object was resolved as a pair of stars with an angular separation of 2.95 arcseconds, both of dwarf Me type—the first such matching pair discovered. Parallax measurements gave an annual shift of about 0.1 arcseconds, and the radial velocity was measured at +5 km/s away from the Sun. Nearby star HD 197981 (AU Microscopii) showed a similar radial velocity of +10 km/s, suggesting a physical association. In 1954, both components were listed as suspected flare stars by Czech solar physicist Zdeněk Švestka. Photometric monitoring in 1969 revealed 54 flares over 16.31 hours, with the combined magnitude increasing by more than 0.05 for over half that period. By 1972, the pair received the variable star designation AT Microscopii. Hipparcos measurements gave a parallax of 0.0935 arcseconds, confirming the distance of about 35 ly. The angular separation is now 4.0 arcseconds. Both stars are pre-main sequence red dwarfs; component A has about 27% of the Sun's mass and 3.6% of its luminosity, while component B has 25% mass and 3.3% luminosity. They have active coronae, show BY Draconis-type variability, and are X-ray emitters. The average flare rate is 2.8 per hour. Their X-ray spectrum indicates a plasma density and a magnetic field strength of at least 100 G in flare regions. Neither star shows lithium in its spectrum, having depleted it through nuclear fusion. The system lies near AU Microscopii, with a projected separation suggesting they may form a wide hierarchical triple system, with the AT Microscopii pair orbiting AU Microscopii over about 10 million years. All three are candidate members of the Beta Pictoris moving group, an association of stars sharing common motion, with an estimated age between 10 and 21 million years.

Reader's Guide

AT Microscopii holds significance as the first discovered binary system of matching dwarf Me stars, establishing a class of active red dwarf pairs. Its extreme flare activity—54 flares in 16.31 hours—set a benchmark for stellar flare studies in the mid-20th century, demonstrating the high energy output of young, low-mass stars. The system's proximity to AU Microscopii and their similar radial velocities support the hypothesis of a wide hierarchical triple system, contributing to understanding of multiple star formation and dynamics. As candidate members of the Beta Pictoris moving group, these stars provide a laboratory for studying the early evolution of red dwarfs and their magnetic activity. The depletion of lithium in both components indicates they have undergone core nuclear fusion, offering constraints on stellar evolution models for very low-mass stars. The system's X-ray emission and magnetic field measurements (at least 100 G in flare regions) inform models of coronal heating and flare physics. Its legacy includes serving as a prototype for flare star research and a key object in the study of young stellar associations near the Sun.

Did You Know?

• In 1969, 54 flares were observed from the system over 16.31 hours.
• Both components have depleted lithium through nuclear fusion at their cores.
• The system is a candidate member of the Beta Pictoris moving group, aged 10–21 million years.