R145

R145 (HD 269928) is a spectroscopic binary star system in the Tarantula Nebula of the Large Magellanic Cloud, within the constellation Dorado. Both stars in the system are among the most luminous known.

The star first appears in the Henry Draper Catalogue with a photographic magnitude of 11.8 and a spectral type of O, a classification then used for all hot stars with emission lines. It was included in the first Henry Draper Extension volume, published in 1925. In 1960, it was added to the Radcliffe Magellanic Catalogue as RMC R145, with a somewhat uncertain spectral type of WN6-7. In the first catalogue of LMC Wolf-Rayet stars, it appears as number 90 (Brey 90) with a WN7 type; the fourth catalogue, published in 1999, lists it as BAT99-119.

The 2011 VLT FLAMES survey designated it VFTS 695 and reclassified it as WN6h, indicating it retains significant hydrogen in its atmosphere. That survey also identified a second luminous star in the system, though its spectral type could not be determined. Orbital and physical parameters for both stars were calculated from FLAMES data in 2016.

System

The system is a double-lined spectroscopic binary with an orbital period of 159 days. The two stars follow an eccentric orbit, with separation varying from less than one AU to nearly eight AU.

Their orbital velocities are nearly identical, implying very similar masses. Exact masses depend on the orbital inclination, which polarimetry places at 39°; the formal error of 6° introduces considerable uncertainty. Mass estimates from other methods yield larger values, suggesting the inclination may be smaller than 39°.

Components

The primary component dominates the spectrum with strong, broad emission lines. It is a WN6h Wolf-Rayet star with a temperature near 50,000 K. Despite its WR spectral type, it is a young star with about 40% hydrogen in its atmosphere. The atmospheric enhancement of helium and nitrogen results from strong convection and heavy mass loss driven by high luminosity and likely rapid rotation.

The secondary component is marginally more massive and visually brighter than the primary. It has a lower temperature of about 43,000 K, a larger size, and a spectral type of O3.5If/WN7. Each star has a bolometric luminosity over two million times that of the Sun. Masses derived from the orbit are uncertain due to inclination, but both are likely between 100 and 200 solar masses.