The eclipsing hypergiant R 81 (B2.5Ia-O) in the Large Magellanic Cloud

Abstract

The eclipsing P Cygni-type star R 81 (HDE 269128, HIP 24080) of spectral type B2.5 Ia-0 in the Large Magellanic Cloud was studied on the basis of a long continuous time series with simultaneous high-resolution Feros spectra and photometric measurements in the Stromgren system. The stellar parameters derived for the primary are Te = 19 500 K and R =9 6R. The orbital period of the binary is 74.566 days. The mean light curve shows two eclipses, a brightness maximum just after the eclipse of the hypergiant and a slow decline of brightness between the two minima. For the rst time, the orbital motion of the primary has been detected. The system is close and eccentric (e =0 :569) and both components nearly ll their Roche volumes. A spectral signature of the companion of the hypergiant has not been found. We suspect that the secondary is embedded in a shell or disk of material accreted from the primary. In addition, line prole variations with a period of about 11 days, probably caused by non-radial pulsation, were observed. The line proles indicate a strong wind from the primary with an outflow velocity of about 150 km s 1 . Near primary eclipse, strong absorptions in low excitation lines emerge abruptly that point to an outflow of enhanced density and higher velocity in the direction towards and beyond the secondary.