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

• Published in: Astronomy and astrophysics (Berlin) 2002-07, Vol.389 (3), p.931-944
• Main Authors: Tubbesing, S., Kaufer, A., Stahl, O., Wolf, B., Schmid, H. M., Korn, A. J., Maintz, M., Rivinius, T., Szeifert, T., Arentoft, T., Sterken, C.
• Format: Article
• Language: English
• Subjects: outflows; stars: binaries: eclipsing; stars: early-type; stars: emission-line; stars: individual: R 81; stars: supergiants; stars: winds
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20020682

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 Strömgren system. The stellar parameters derived for the primary are $T_{\rm eff} = 19\,500$ K and $R = 96~ R_\odot$. 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 first time, the orbital motion of the primary has been detected. The system is close and eccentric ($e=0.569$) and both components nearly fill 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 profile variations with a period of about 11 days, probably caused by non-radial pulsation, were observed. The line profiles 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.