Professor of Mathematics
Paata Ivanisvili
Department of Mathematics, University of California, Irvine
Candidate 16: an F star with the cleanest of these orbits
Why this estimate. Of the nine Gaia orbits among candidates 15 to 23, this is the cleanest: it is well inside Gaia’s observing period and strongly significant. But the star sits well above the main sequence, so it is either evolved or a blend of two stars, which would upset the mass estimate; if the star is 30 per cent lighter than modelled, the companion drops below 1.4 solar masses, and a close pair of ordinary stars could mimic it.
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 slightly evolved yellow-white star of type F, about 6,500 K, some 1,800 parsecs away in the plane of the Milky Way. Gaia’s main orbit-fitting method found a 678-day orbit, well inside its observing period and with good significance. If the star has about 1.6 solar masses, the companion has about 1.6 solar masses too (1.5 to 1.8) and yet gives off no visible light: it could be a neutron star or a heavy white dwarf.
Why the estimate is low. If the star is 30 per cent lighter than its model mass, the companion drops below 1.4 solar masses, where ordinary white dwarfs are common. The star sits 2.2 magnitudes above the main sequence in Gaia’s colour–brightness diagram, so it is either evolved or a blend of two stars, and a blend would upset the mass estimate. A close pair of ordinary stars can mimic a dark companion of this mass, and there are no measurements of the star’s speed along the line of sight.
No record of a dark companion for this star could be found. The star is too far south for Pan-STARRS1 and ZTF, so there is no light curve.
- Star
- Gaia DR3 5351774995980250880, a slightly evolved F-type star
- Location
- RA 155.80550°, Dec −57.44321° (J2000), in Carina, in the plane of the Milky Way
- Distance
- about 1,800 parsecs (5,800 light-years)
- Brightness
- Gaia G = 13.9
- 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 star’s own mass is uncertain, it may be a blend of two stars, a close pair of ordinary stars could mimic the companion, and no measurements along the line of sight confirm the orbit.
