A new look at the long-period eclipsing binary V383 Scorpii

Context. The system V383 Sco was discovered to be an eclipsing binary star at the beginning of the twentieth century. This system has one of the longest orbital periods known (13.5 yr) and was initially classified as zeta Aur-type eclipsing variable. It was then forgotten about for decades, with no progress made in understanding it. Aims. This study provides a detailed look at the system V383 Sco, using new data obtained before, during, and after the last eclipse, which occurred in 2007/8. There was a suspicion that this system could be similar to eclipsing systems with extensive dusty disks like EE Cep and epsilon Aur. This and other, alternative hypotheses are considered here. Methods. The All Sky Automated Survey (ASAS-3) V and I light curves have been used to examine apparent magnitude and colour changes. Low-and high-resolution spectra have been obtained and used for spectral classification, to analyse spectral line profiles, as well as to determine the reddening, radial velocities and the distance to the system. The spectral energy distribution (SED) was analysed using all available photometric and spectroscopic data. Using our own original numerical code, we performed a very simplified model of the eclipse, taking into account the pulsations of one of the components. Results. The low-resolution spectrum shows apparent traces of molecular bands, characteristic of an M-type supergiant. The presence of this star in the system is confirmed by the SED, by a strong dependence of the eclipse depth on the photometric bands, and by the nature of pulsational changes. The presence of a very low excitation nebula around the system has been inferred from [Oi] 6300 angstrom emission in the high-resolution spectrum. Analysis of the radial velocities, reddening, and period-luminosity relation for Mira-type stars imply a distance to the V383 Sco system of 8.4 +/- 0.6 kpc. The distance to the nearby V381 Sco is 6.4 +/- 0.8 kpc. The very different and oppositely directed radial velocities of these two systems (89.8 kms(-1) vs. -178.8 kms(-1)) seem to be in agreement with a bulge/bar kinematic model of the Galactic centre and inconsistent with purely circular motion. Conclusions. We have found strong evidence for the presence of a pulsating M-type supergiant in the V383 Sco system. This supergiant periodically obscures the much more luminous F0 I-type star, causing the deep (possibly total) eclipses which vary in duration and shape.