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.