Rho1 Arae

Rho1 Arae is the Bayer designation for a star in the southern constellation of Ara. Unusually for a star with a Bayer designation, it was not catalogued by Bayer in his Uranometria. It was instead first catalogued by Nicolas Lacaille, in his Coelum Australe Stelliferum published in 1763. This star gained the Bayer designation of Rho1 Arae in Bode's Uranographia, published in 1801. Rho1 Arae is one of the dimmest stars with a Bayer designation, having an apparent visual magnitude of just +6.275 According to the Bortle Dark-Sky Scale, this means the star is just barely visible to the naked eye in dark rural skies. Based upon parallax measurements, it is about 640 ly distant from the Sun, give or take a 50-light-year margin of error.

This is a spectroscopic binary system, which means that the presence of an orbiting companion is indicated by shifts in the spectrum. However, because the primary component is spinning rapidly with a projected rotational velocity of 370 ± 10, it is difficult to obtain reliable orbital elements. The orbital period has been estimated at 0.439 day. The spectrum of this system matches a stellar classification of B3 Vnpe, which may indicate the primary is a B-type main-sequence star. The 'e' suffix indicates the presence of emission lines, indicating this is a Be star. For Rho1 Arae, the emission lines are prominent and variable.

Rho1 Arae has a peculiar velocity of 27.4 ± 4.9 km/s relative to its neighbors, making it a runaway star system. A scenario that it was ejected from the Scorpius–Centaurus OB association as a result of a past supernova explosion seems unlikely because of its binarity.