The unusual pre-main-sequence star V718 Persei (HMW 15) - Photometry and spectroscopy across the eclipse

Context. The remarkable pre-main-sequence object V718 Per (HMW 15, H187) in the young cluster IC 348 periodically undergoes long-lasting eclipses caused by variable amounts of circumstellar dust in the line-of-sight to the star. It has been speculated that the star is a close binary and similar to another unusual eclipsing object, KH 15D. Aims. We have submitted V718 Per to a detailed photometric and spectroscopic study to investigate how regular the recurrent eclipses are, to find out more about the properties of the stellar object and the occulting circumstellar material, and to look for signatures of a possible binary component. Methods. V718 Per was monitored photometrically from the optical to the near-infrared (NIR). We also obtained high-resolution optical spectra with the Keck telescope at minimum as well as at maximum brightness. We derived the fundamental photospheric parameters of this star by comparing with synthetic spectra. Results. Our photometric data show that the eclipses are very symmetric and persistent, and that the extinction law of the foreground occulting dust deviates only little from what is expected for "normal" interstellar material. The stellar parameters of V718 Per indicate a primordial abundance of Li and a surface temperature of T(eff) approximate to 5200 K. Remarkably, the in-eclipse spectrum shows a significant broadening of the photospheric absorption lines, as well as a slightly lower stellar surface temperature. In addition, weak emission components appear in the absorption lines of Ha and the Ca II IR triplet lines. We did not detect any signs of atomic or molecular features related to the occulting body in the in-eclipse spectrum. We also found no evidence of any radial velocity changes in V718 Per to within about +/- 80 ms(-1), which for an edge-on system corresponds to a maximum companion mass of similar to 6 M(Jup). Conclusions. Our observations suggest that V718 Per is a single star, and thus very different from the related binary system KH 15D. We conclude that V718 Per is surrounded by an edge-on circumstellar disk with an irregular mass distribution orbiting at a distance of 3.3 AU from the star, presumably at the inner disk edge. To produce the prolonged eclipses, the occulting feature must extend along more than half of the inner disk edge. The change in stellar surface temperature and the emission line activity observed could be related to spot activity. We ascribe the broadening of photospheric absorption lines during the eclipse to forward scattering of stellar light in the circumstellar dust feature.