Professor of Mathematics
Paata Ivanisvili
Department of Mathematics, University of California, Irvine
Candidate 20: a bright known binary whose companion is probably a hot star
Why this estimate. The binary has long been known, and the star gives off more far-ultraviolet light than a star of its type should, which points to a hot companion that shines on its own rather than a dark one. The companion’s mass is also poorly constrained.
How to read the pictures. The orbit figure has three panels. On the left is the small ellipse that Gaia’s orbit traces on the sky, measured in thousandths of an arcsecond, with grey orbits that fit the data about equally well. In the middle is the mass the unseen companion would need for each possible mass of the visible star; the shaded band is the likely range, the red line marks the star’s estimated mass, and the dotted and dashed lines mark 3 and 1.4 solar masses, roughly where black holes begin and where a typical neutron star lies. On the right, the star (red) sits among other stars in Gaia’s diagram of colour against brightness. The finder chart shows a patch of sky two arcminutes across, about a fifteenth of the Moon’s width, with north up and east to the left; the star sits between the two yellow ticks, and the white bar is 30 arcseconds long.
What Gaia found. This is a bright white star of type A or F, about 700 parsecs away in Hydra, catalogued as a spectroscopic binary, with five references in the SIMBAD database. Gaia combined its motion on the sky with its speeds along the line of sight into a nearly circular 958-day orbit. If the star has about 2.1 solar masses, the companion has about 2.6 solar masses (1.7 to 5.9) and gives off no visible light.
Why the estimate is low. The binary is long recorded. GALEX, NASA’s ultraviolet space telescope, found the star brighter in far-ultraviolet light (magnitude 16.6) than a star of its type should be, which points to a hot companion that shines on its own. A hot white dwarf would fit the ultraviolet light, but it cannot weigh 1.7 to 5.9 solar masses. The companion’s mass is poorly constrained in any case.
The binary is known, but no record of a companion this heavy could be found. The star is far too bright for Pan-STARRS1 and ZTF, so there is no light curve.
- Star
- Gaia DR3 5664612407982550528 (HD 84286), an A- or F-type star in a known binary
- Location
- RA 145.93223°, Dec −21.75301° (J2000), in Hydra
- Distance
- about 700 parsecs (2,300 light-years)
- Brightness
- Gaia G = 9.8, visible in binoculars
- Probability
- This candidate is unconfirmed. The percentage under the title is an estimate of the chance that it is a real, previously unrecorded case of the kind described. The estimate has not been checked by outside experts. See the introduction to all fifty-eight candidates.
- Caveat
- The binary is already known, the ultraviolet light points to a hot companion that shines, and the companion’s mass is poorly constrained.
