TW Piscis Austrini

The proximity of TW Piscis to Fomalhaut was first reported in 1897 by Thomas Jefferson Jackson See (1866-1962). Subsequently, the two stars have been determined to be distant physical companions (D. Barrado y Navascues, 1998). However, another K5 dwarf (LTT 8273) later observed in proper motion studies is now believed to be an optical companion, although it also may be a remaining member of a low-density star cluster including Fomalhaut, Vega, and Castor that has gradually dispersed over hundreds of millions of years.

As a relatively bright star in Earth's night sky, TW PsA is catalogued as Harvard Revised (HR) 8721, a numbering system derived from the 1908 Revised Harvard Photometry catalogue of stars visible to many Humans with the naked eye. The HR system has been preserved through its successor, the Yale Bright Star Catalogue -- updated and expanded through the hard work of E. Dorrit Hoffleit and others. HR 8721 is also listed as HD 216803 in the Henry Draper (1837-82) Catalogue with extension (HDE), a massive photographic stellar spectrum survey carried out by Annie Jump Cannon (1863-1941) and Edward Charles Pickering (1846-1919) from 1911 to 1915 under the sponsorship of a memorial fund created by Henry's wife, Anna Mary Palmer. (More discussion on star names and catalogue numbers is available from Alan MacRobert at Sky and Telescope and from Professor James B. Kaler's Star Names.)

The Star

TW Piscis Austrini is a orange-red main sequence dwarf star of spectral and luminosity type K4-5 Vpe. This star may have around 81 percent of Sol's mass, 76 to 85 percent of its diameter (Pasinetti-Fracassini et al, 2001), and 12 to 13 percent of its visual luminosity. It may be only about 200 million years old (D. Barrado y Navascues, 1998).

The Yale Bright Star Catalogue's notes entry for HR 8721 indicate that TW PsA is a BY Draconi-type variable (Vogt et al, 1983) that varies in apparent visual magnitude from 6.44 to 6.49 over 10 days and shares common proper motion with Fomalhaut (HR 8728). As a flare star, TW Piscis Austrini is its variable star designation. Other useful star catalogue designations for the star include: TW PsA, HR 8721*, Gl 879, Hip 113283, HD 216803, CD-32 17321, CP(D)-32 6550, SAO 214197, and LTT 9283.

Hunt for Substellar Companions

Since TW Piscis Austrini is sort of like a distant cousin to Sol, some speculate whether it might just be bright enough to support Earth-type life on a planet lucky enough to orbit in its water zone. The distance from TW PsA where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.36 AU -- within Mercury's orbital distance in the Solar System. At that distance from the star, such a planet would have an orbital period of about 88 days -- about a fourth of an Earth year. Astronomers would find it very difficult to detect using present methods.

Life Around a Flare Star

Many dim, red (M) and some orange-red (K) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs make up more than half of all stars in our galaxy. Although flares do occur on the Sun every so often, the amount of energy released in a Solar flare is small compared to the total amount of energy that Sol produces. However, a flare the size of a solar flare occurring on a orange-red dwarf star (such as TW Piscis Austrini) that normally has less than 14 percent of than Sol's luminosity would be more noticeable.

Arnold O. Benz, Institute of Astronomy, ETH Zurich

High resolution and jumbo images (Benz et al, 1998).

TW PsA is a flare star, like UV Ceti (Luyten
726-8 B) shown flaring at left. UV Ceti is an extreme
example of a flare star that can boost its brightness by
five times in less than a minute, then fall somewhat slower
back down to normal luminosity within two or three
minutes before flaring suddenly again after several hours.

Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized Solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be less likely because their planets must be located very close to dim orange-red dwarfs to be warmed sufficiently by star light to have liquid water (about 0.36 AU for TW Piscis Austrini), which makes flares even more dangerous around such stars. In any case, the light emitted by late orange-red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.