V686 Coronae Australis

Lore & Background

V686 Coronae Australis has been recognized as variable since 1974, initially classified as a helium variable with a period of 3.71 days. Earlier spectral classifications included B7.5 V, B8 IV, and B9 III:, but the accepted type is B3V, a B-type main-sequence star. In 1976, a large magnetic field ranging from −5,000 to 7,000 gauss was detected; later measurements gave a value of gauss. The star is chemically peculiar, catalogued as B6 He wk. Si (helium-weak) and also as a mercury-manganese star.

The variability period is 3.674 days, but it has been proposed that the true rotation period is 7.34 days, with two near-identical minima in the light curve. The star spins slowly, with a projected rotational velocity of . It is a member of the Scorpius–Centaurus association and the Local Association. Its heliocentric radial velocity is , and interstellar dust dims its average brightness by 0.35 magnitudes.

V686 CrA has 4.73 times the Sun's mass, 3.52 times its radius, and radiates 504 solar luminosities. Its effective temperature is . It has an unusually high metallicity of [Fe/H] = +1.13, over ten times solar, though this reflects only the spectrum, not the entire composition. Age estimates vary: one analysis of surface magnetic fields gives 63 million years, while another source suggests 7.9 million years, having completed only 0.9% of its main-sequence lifetime.

Reader's Guide

V686 Coronae Australis is significant as a chemically peculiar magnetic variable star that exemplifies the α² Canum Venaticorum class. Its large, varying magnetic field and helium-weak spectrum make it a key object for studying stellar magnetism and chemical stratification in B-type stars. The star's membership in the Scorpius–Centaurus association and the Local Association places it within a well-studied moving group, aiding in age and evolutionary context. The discrepancy in its rotation period—whether 3.674 days or the proposed 7.34 days with twin minima—highlights the complexity of interpreting light curves from magnetic rotators. Its extreme metallicity ([Fe/H] = +1.13) is unusual for a B star and points to surface chemical anomalies rather than bulk composition. The conflicting age estimates (63 million vs. 7.9 million years) underscore ongoing debates about the star's evolutionary stage. As a solitary, bright variable visible to the naked eye, V686 CrA serves as a practical target for amateur and professional monitoring of magnetic variability and rotational modulation.

Did You Know?

• It is classified as both a helium-weak star and a mercury-manganese star.
• The star's metallicity is [Fe/H] = +1.13, over ten times that of the Sun.
• Its rotation period may be 7.34 days, with two near-identical minima per cycle.