ATRIA (Alpha Trianguli Australis). Among the easiest constellations to invent are simple triangles. There are two of them, one north (Triangulum) and one far south (Triangulum Australe). Of the two, the latter is the larger and brighter, its Alpha star (from which we get the modern proper name, "Atria") a nice bright second magnitude (1.92), ranking 41st. Far to the south, Atria is one of the closest bright stars to the southern rotation pole (the South Celestial Pole. Offset from the pole by just 21 degrees, it is beaten out only by Miaplacidus (Beta Carinae), which is both brighter and a hair closer. The orange light from this class K (K2) bright giant contrasts nicely with the swarm of background stars from the southern Milky Way, the class showing that the star is quietly fusing helium into carbon and oxygen in its deep core. Atria's temperature is not well known, the estimates spanning 3970 to 4400 Kelvin. From the average of 4200 Kelvin (from which we get an accounting of its infrared radiation) and its distance of 415 light years, we derive a substantial luminosity of 4900 times that of the Sun and a radius of 130 solar, 60 percent the size of the Earth's orbit (and approaching that of Venus). Luminosity, temperature, and theory yield a mass 7 times that of the Sun, and an age of 45 million years. Atria has been called "metal rich," its iron content (relative to hydrogen) twice solar. Another study, however, showed it to be slightly deficient in iron. Most likely the metallicity is close to solar. Though Atria seems single, there are two indications that it might have a small companion. First, it has been classed as "barium star." Such stars are believed to have been contaminated in heavy elements by an initially more massive companion that evolved first and passed matter enriched by nuclear processes onto the star we now see, the companion turning into a dense white dwarf (which Atria itself will also someday become). Atria's youth, however, implies that the white dwarf should be hot enough to detect in ultraviolet light, and it is nowhere to be seen. More significant, Atria is the classic "hybrid star," a giant that shows evidence for blowing a cool wind from its surface, yet having a hot surrounding magnetic corona at the same time. In support, the star is a notable source of X-rays. Anomalous X-ray flares do not fit the picture, however, and they (and a very high X-ray temperature) are better explained as coming from a young solar type class G companion. So is the star single, double, or triple? We do not yet know. Thanks to Jerome Diekmann, who suggested this star, to Tom Ayres for discussion, and to Andre Bordeleau for correction.
Written by Jim Kaler. Return to STARS.