RHO AQR (Rho Aquarii), with a nod toward THETA AQR (Theta Aquarii). Within Aquarius (the Water Bearer) what comes most to mind is his "Water Jar," from which he pours the streaming liquid into the mouth of the Southern Fish, Piscis Austrinus. We might also admire the two great supergiants off to the west, Sadalmelik (Alpha Aquarii) and Sadalsuud (Beta). The numerous stars to the south are less appreciated. Among them, some eight degrees below the Water Jar, is a pair a bit over three-fourths of a degree apart. The more westerly and brighter of the two, fourth magnitude Theta Aquarii, is an ordinary class G (G8 giant-subgiant) that serves as a guide star to more interesting, though fainter, fifth magnitude (5.37) Rho Aqr to the east, a class B (B8) "giant" (but see below). Though proximate on the sky, they have nothing to do with each other. At a distance of 882 light years (give or take 91), Rho is almost five times farther than Theta (187 l-y, plus/minus under 3). The distance plus a well-defined temperature of 4970 Kelvin (needed to allow for infrared radiation) yields a modest luminosity for Theta Aqr of just 16.6 Suns, from which we find radius of 11.8 solar. Theory suggests that it's a helium-fusing giant of perhaps two solar masses. Even though Theta is just over two degrees north of the ecliptic, there seems to be no observation of radius via lunar occultation, which tells how ignored the star really is. But then there is Rho. It's an underobserved "mercury-manganese" (HgMn) star, one with huge overabundances of these chemical elements as well as enrichments of rare earths such as europium, alongside depletions of some others, exemplified by calcium. For relatively slow rotators, the outer stellar layers are fairly peaceful and unstirred. As a result, some elements are lofted upward by radiation pressure, while others sink under the force of gravity. With a projected equatorial spin-speed of 69 kilometers per second rotation (which translates to a rotation period of at most 4.4 days), Rho apparently qualifies. The HgMn stars are higher temperature versions of class A metallic line stars like Sirius. The odd abundances make classification problematic. With a temperature of 12,450 Kelvin (needed for evaluation of ultraviolet light) and a 0.1 magnitude correction for dimming by interstellar dust, Rho radiates at a rate of 773 Suns, the radius 6.6 solar. Theory reveals that the star is either a subgiant with a dead helium core or a dwarf that is close to becoming one with a mass of 4.3 to 4.5 Suns, its age some 112 million years. Slight Doppler shifts in the spectrum suggest a companion with a 220-day period, which yields a separation for low companion-mass of 1.2 Astronomical Units. As different as they are now, the two stars have similar fates. Both are destined to lose their outer layers and die as white dwarfs with respective masses of 0.63 and 0.82 times that of the Sun. Because of its higher mass, Rho goes first, not that Theta gives a hoot.

Written by Jim Kaler 4/10/15. Return to STARS.