International observational campaigns of the last two eclipses in EE Cephei: 2003 and 2008/9^⋆,⋆⋆

¹ Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100 Toruń, Poland
e-mail: cgalan@astri.uni.torun.pl; mamiko@astri.uni.torun.pl; tomtom@astri.uni.torun.pl
² Olsztyn Planetarium and Astronomical Observatory, Al. Marszałka J. Piłsudskiego 38, 10-450 Olsztyn, Poland
³ Universidad de Concepción, Departamento de Astronomia, Casilla 160-C, Concepción, Chile
⁴ Institute of Physics, Faculty of Science, Ss. Cyril and Methodius University, PO Box 162, 1000 Skopje, FYROM, Macedonia
⁵ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko Shose Blvd., 1784 Sofia, Bulgaria
⁶ Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, NOA PO Box 20048, 11810 Athens, Greece
⁷ Dept. of Physics and Earth Science, University of North Alabama, Florence, 35632 AL, USA
⁸ David Dunlap Observatory, Department of Astronomy and Astrophysics, University of Toronto, 50 St. George St., Toronto, ON M5S 3H4, Canada
⁹ International Centre for Astronomical and Medico-Ecological Research, Terskol, Russia
¹⁰ Variable Star and Exoplanet Section ofCzech Astronomical Society, Czech Republic
¹¹ Altan Observatory, Velka Upa 193, Pec pod Snezkou, Czech Republic
¹² Tadeusz Banachiewicz Astronomical Observatory, Wȩglówka, 32-412 Wiśniowa, Poland
¹³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁴ Department of Earth and Space Sciences, University of California at Los Angeles, 595, Charles E. Young Dr. East CA 90095, USA
¹⁵ Mt. Suhora Observatory, Pedagogical Univ., ul. Podchora¸żych 2, 30-084 Kraków, Poland
¹⁶ Rolling Hills Observatory Clermont, FL, USA
¹⁷ University of Hawaii Maui College, Kahului, Hawaii
¹⁸ Instituto de Astronomia, Universidad Catolica del Norte, Av. Angamos 0610, Antofagasta, Chile
¹⁹ Department of Astrophysics, Astronomy and Mechanics, National and Kapodistrian University of Athens, 157 84 Zografos, Athens, Greece
²⁰ Instituto de Astronomá, Universidad Nacional Autónoma de México Apdo. postal 70264, Ciudad Universitaria, México D.F. 04510, México
²¹ Department of Experimental Physics and Astronomical Observatory, University of Szeged, Dom ter 9, 6720 Szeged, Hungary
²² Pulkovo Astronomical Observatory, Russian Academy of Sciences, Pulkovskoe sh. 65, 196140 St. Petersburg, Russia
²³ Space Research Centre, Polish Academy of Sciences, Bartycka 18A, 00-716 Warsaw, Poland
²⁴ Nicolaus Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
²⁵ Observatorio Astronómico “Las Pegueras”, NAVAS DE ORO (Segovia), Spain
²⁶ Hopkins Phoenix Observatory, 7812 West Clayton Drive, Phoenix, 85033-2439 Arizona, USA
²⁷ Observatory and Planetarium of Johann Palisa, VŠB – Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava-Poruba, Czech Republic
²⁸ Instytut Astronomiczny, Uniwersytet Wrocławski, Kopernika 11, 51-622 Wrocław, Poland
²⁹ Astronomical Observatory, Jagiellonian Univ., ul. Orla 171, 30-244 Kraków, Poland
³⁰ Las Cumbres Observatory, 6740 Cortona Drive Suite 102, Goleta, CA 93117, USA
³¹ National Centre for Nuclear Research, 00-681 Warsaw, Poland
³² Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
³³ Furzehill House, Ilston, Swansea, SA2 7LE, UK
³⁴ Variable Star Section of the British Astronomical Association, Furzehill House, Ilston, Swansea, SA2 7LE, UK
³⁵ INAF, Osservatorio Astronomico di Padova, via dell Osservatorio 8, 36012 Asiago ( VI), Italy
³⁶ Special Astrophysical Observatory of the Russian AS, 369167 Nizhnij Arkhyz, Russia
³⁷ GRAS Observatory, Mayhill, New Mexico, USA
³⁸ Department of Physics and Space Sciences, 150 W. University Blvd, Florida Institute of Technology, Melbourne, FL 32901, USA
³⁹ Green Island Observatory (B34), North Cyprus
⁴⁰ Hankasalmi Observatory, Jyvaskylan Sirius ry, Vertaalantie 419, 40270 Palokka, Finland
⁴¹ Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland
⁴² Sonoita Research Observatory/AAVSO, USA
⁴³ Department of Physics and Astronomy, Box 516, 751 20 Uppsala, Sweden
⁴⁴ Centrum Hewelianum, PKFM “Twierdza Gdańsk”, ul. 3 Maja 9a, 80-802 Gdańsk, Poland
⁴⁵ University of Ljubljana, Faculty of Mathematics and Physics, Jadranska 19, 1000 Ljubljana, Slovenia

Received: 30 November 2010
Accepted: 17 April 2012

Abstract

Context. EE Cep is an unusual long-period (5.6 yr) eclipsing binary discovered during the mid-twentieth century. It undergoes almost-grey eclipses that vary in terms of both depth and duration at different epochs. The system consists of a Be type star and a dark dusty disk around an invisible companion. EE Cep together with the widely studied ε Aur are the only two known cases of long-period eclipsing binaries with a dark, dusty disk component responsible for periodic obscurations.

Aims. Two observational campaigns were carried out during the eclipses of EE Cep in 2003 and 2008/9 to verify whether the eclipsing body in the system is indeed a dark disk and to understand the observed changes in the depths and durations of the eclipses.

Methods. Multicolour photometric data and spectroscopic observations performed at both low and high resolutions were collected with several dozen instruments located in Europe and North America. We numerically modelled the variations in brightness and colour during the eclipses. We tested models with different disk structure, taking into consideration the inhomogeneous surface brightness of the Be star. We considered the possibility of disk precession.

Results. The complete set of observational data collected during the last three eclipses are made available to the astronomical community. The 2003 and 2008/9 eclipses of EE Cep were very shallow. The latter is the shallowest among all observed. The very high quality photometric data illustrate in detail the colour evolution during the eclipses for the first time. Two blue maxima in the colour indices were detected during these two eclipses, one before and one after the photometric minimum. The first (stronger) blue maximum is simultaneous with a “bump” that is very clear in all the UBV(RI)_C light curves. A temporary increase in the I-band brightness at the orbital phase  ~0.2 was observed after each of the last three eclipses. Variations in the spectral line profiles seem to be recurrent during each cycle. The Na i lines always show at least three absorption components during the eclipse minimum and strong absorption is superimposed on the Hα emission.

Conclusions. These observations confirm that the eclipsing object in EE Cep system is indeed a dark, dusty disk around a low luminosity object. The primary appears to be a rapidly rotating Be star that is strongly darkened at the equator and brightened at the poles. Some of the conclusions of this work require verification in future studies: (i) a complex, possibly multi-ring structure of the disk in EE Cep; (ii) our explanation of the “bump” observed during the last two eclipses in terms of the different times of obscuration of the hot polar regions of the Be star by the disk; and (iii) our suggested period of the disk precession (~11–12 P_orb) and predicted depth of about 2 for the forthcoming eclipse in 2014.