ETA CRV (Eta Corvi). Within the set of large constellations -- Centaurus (the Centaur), Ursa Major (the Larger Bear), even Orion the Hunter -- the small ones act like enhancing jewels, among them Lyra (the Harp or Lyre), Canis Minor (the Smaller Dog), Crux (the Southern Cross), and our focus here, Corvus the Crow. Four of its stars (in order of brightness, Gienah, Kraz, Algorab, and Minkar, respectively Gamma, Beta, Delta, and Epsilon, all third magnitude) make an exquisite distorted box whose top stars are "pointers" that look eastward to Spica. Two more stars hang off the box. To the southwest is the Alpha star, oddly- fifth-ranked fourth magnitude Alchiba (one wonders what Bayer actually had in mind), and opposite, to the northeast, sixth-ranked also-fourth magnitude (4.31) Eta Corvi (the only one of the six with no proper name). While seeming rather ordinary, it's a star with a fascinating difference. While this class F (F2) star, 59 light years away, was originally known as a giant-subgiant (one that has just given hydrogen fusion in its core), it is now firmly known to be a hydrogen-fusing dwarf rather like the Sun, just somewhat more massive, as well as younger. With a well-determined temperature of 6815 Kelvin (a bit over a thousand degrees warmer than our 5780 Kelvin Sun), it radiates at a modest rate (for naked-eye stars) of 4.9 times solar, from which we find a radius of 1.6 solar. A projected equatorial rotation velocity of 60 kilometers per second leads to a short rotation period of under 1.33 days, while from the theory of stellar structure and evolution we find a mass of 1.4 times that of the Sun and an age of roughly 1 to 1.3 billion years that takes the star over a third of the way to when it WILL become a subgiant (then giant) with a dead helium core. The star was once thought to have a spectroscopic companion, but the tiny spectral wavelength shifts are much more likely to be caused by "Delta Scuti" type pulsational variability, which has not been researched (but which, given the spectral class, makes sense). The star also exhibits some X-ray emission from magnetic activity. Eta Corvi's outstanding characteristic, though, is that it is a "Vega" type star with a surrounding disk that may well hold planets (though none has as yet actually been detected).
Three far infrared images of Eta Corvi spread from left to right with increasing resolved detail. The star itself is not visible. Instead we see heat radiation from a dusty disk (at center) with a diameter more than 200 times the distance between Earth and Sun. (The radiating source toward the top is in the background and is not relevant.) At far right the disk is seen to break into a pair of blobs that lie opposite the unseen star and that may indicate the presence of a Neptune-like planet. (From a paper by M. C. Wyatt, J. S. Greaves, W. R. F. Dent, and I. M. Coulson in the Astrophysical Journal, vol. 620, p. 492, 2005.)
Long-wave infrared observations reveal a disk, probably from heated radiating dust grains, intriguingly similar to our Sun's extended "Kuiper Belt," a zone of small bodies left over from the formation of our Solar System and the reservoir of short-period comets. Similar disks are seen around Beta Pictoris, Vega, Fomalhaut, and Epsilon Eridani. Eta Corvi's disk is cleared out in the middle, and contains a pair of clumps. The data are interpreted as either a 300-AU-wide ring tilted through 45 degrees, or a less-tilted ring with a real clumpy structure. The clumps could have arisen from a planet similar to Neptune that, shortly after it formed, moved outward from 80 to 130 AU, shovelling the star's debris disk -- Kuiper Belt -- in front of it. The same thing seems to have happened in our own Solar System when our early Neptune moved outward (according to dynamical theory) by some 10 AU. Such a planet would be 3.5 times farther out from Eta Corvi than is our Neptune from the Sun, and would take 900 years to orbit. Where there is one possible planet, there may be others, so the little, almost anonymous, star in Corvus will be high on the list to watch.
Written by Jim Kaler. Return to STARS.