Professor of Mathematics

Paata Ivanisvili

Department of Mathematics, University of California, Irvine

Candidate 15: a giant whose wobble hints at a heavy dark companion

about 5%

Why this estimate. The orbit takes about as long as Gaia’s published observations lasted, so it is poorly pinned down. The companion outweighs the giant by less than the uncertainty in the giant’s own mass, there are no measurements of the star’s speed along the line of sight to confirm the orbit, and a close pair of ordinary stars orbiting the giant would look the same.

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.

Three panels: a thin blue ellipse among grey ones; a rising orange band crossed by a vertical red line; and a grey cloud of points with a red star marked on it. — Left: the orbit Gaia fitted, a 1,009-day ellipse traced on the sky, with grey orbits that fit about equally well. Middle: the companion’s mass for each assumed mass of the visible star, if the companion gives off no light. Right: the star (red) in Gaia’s diagram of colour against brightness, among stars with Gaia orbits and precise distances.

What Gaia found. This is a yellow giant, about 5,400 K, some 3,000 parsecs away in a crowded Milky Way field in Carina. Gaia found that it moves in a small ellipse on the sky every 1,009 days, seen almost edge-on. If the giant has its model mass of 2.5 solar masses, the companion has about 3.3 solar masses (2.9 to 3.8). An ordinary star that heavy would outshine the giant, so the companion would have to be dark, and at that mass it would be a black hole near the low end of the range black holes are known to have. The giant fills only a small part of the space available to it, so no gas is flowing between the two, and GALEX saw no ultraviolet light from a hot companion.

Why the estimate is low. The fitted period is about as long as the 1,038 days of Gaia’s third data release, so the orbit is poorly constrained. The companion is only 0.6 to 0.8 solar masses heavier than the giant, which is within the uncertainty of the giant’s model mass. Gaia has no measurements of the star’s speed along the line of sight to confirm the orbit, and two ordinary stars in a close orbit around each other, together orbiting the giant, would produce the same wobble without being dark.

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, and the claim rests on its motion, not its brightness.

Star: Gaia DR3 5238699738399933952, a yellow giant
Location: RA 165.33933°, Dec −65.92882° (J2000), in Carina, in the southern Milky Way
Distance: about 3,000 parsecs (10,000 light-years)
Brightness: Gaia G = 13.4
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 orbit is as long as Gaia’s observations, the masses overlap within their uncertainties, there are no measurements along the line of sight, and a triple system would look the same.