10 Summary and conclusions

CI Cam is an sgB[e] star, and many of its characteristics, particularly the emission line spectrum, are representative of this class. Comparison with other sgB[e] stars suggests that CI Cam is among the hottest members of the class and viewed nearly pole-on. It differs from the other sgB[e] stars in interacting with a compact star which introduces an additional variable element. It seems most likely that the compact object is physically associated with CI Cam, i.e. that it is an HMXB. In this case, the compact object is probably in a long period, eccentric orbit. However we cannot rule out the possibility that this was a chance encounter and that the two stars are not physically associated. Resolution of this issue will likely involve waiting for a true periodicity, repeated over several cycles, or another outburst. In considering the outburst mechanism, there is actually little difference between a long period eccentric orbit and a chance encounter, so our discussion of the outburst applies to both cases.

We suggest that the majority of the optical emission lines originate from an equatorially concentrated outflow or circumstellar disc. During outburst, hydrogen and helium lines appear to have two components, a narrow rest component and a moderately blueshifted broad component. Metallic lines are mainly dominated by the narrow component at all times, although some asymmetry is seen in outburst suggesting that a broad component is present. The square profiles of the Fe II lines can be explained by the equatorial outflow model, if viewed pole-on, so the narrow component is likely associated with this. The broad component becomes weaker and narrower on the decline, and almost disappears in quiescence. This may come from material ejected in the outburst rather than from the equatorial outflow. Forbidden lines fall into two categories; [O I] and [Fe II] show similar line profiles to Fe II, but with a lack of low velocity material. These profiles could originate from the low density, upper layers of the equatorial outflow. Other forbidden lines, [N II] and [O III] are narrower and likely come from a much more extended region.

The outburst mechanism remains undetermined, although the outburst was probably precipitated by the passage of the compact object through the equatorial material. It is unlikely that X-ray heating of any component is responsible for the optical outburst. Instead the optical outburst is likely associated with the expanding remnant produced by the X-ray outburst, either through direct emission from the remnant or as a result of its interaction with the circumstellar material. The spectral shape of the outburst optical continuum, and the presence of broad, blue-shifted emission components, are both consistent with predictions for supercritical accretion resulting in ejection of much of the material (Shakura & Sunyaev 1973), and the peak mass transfer rate for an equatorial passage of the compact object is indeed predicted to be well above the Eddington limit.

After the outburst changes in the emission lines persist for at least three years, with Fe II lines stronger than before and He I, He II, and N II lines weaker. The timescale for the extended Fe II decay, at least, is similar to the expected viscous timescale of the disc of hundreds to thousands of days, so this may indicate the gradual recovery of the disc to its equilibrium state. As the system does not yet appear to have stabilised continued monitoring is important to determine if the system eventually recovers to the pre-outburst state or if it settles to a different level.

Acknowledgements

We would like to thank Simon Jeffrey, Amy Mioduszewski, Guy Pooley, John Porter, Rob Robinson, and Lev Titarchuk for information and many helpful thoughts and discussions which have helped us converge on the picture, albeit incomplete, that we now have of CI Cam. RIH would particularly like to thank Rob Robinson for access to an annotated high resolution spectrum of CI Cam which proved invaluable in identifying lines, and for permission to reproduce the data shown in Fig. 12b.

RIH, PAC, and CAH acknowledge support from grant F/00-180/A from the Leverhulme Trust. EAB and SNF acknowledge support from Russian RFBR grant N 00-02-16588. MRG acknowledges the support of NASA/LTSA grant NAG5-10889. PR acknowledges support via the European Union Training and Mobility of Researchers Network Grant ERBFMRX/CT98/0195. WFW was supported in part by the NSF through grant AST-9731416.

The William Herschel Telescope is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. The G. D. Cassini telescope is operated at the Loiano Observatory by the Osservatorio Astronomico di Bologna. Skinakas Observatory is a collaborative project of the University of Crete, the Foundation for Research and Technology-Hellas and the Max-Planck-Institut für Extraterrestrische Physik. This work also uses archival observations made at Observatoire de Haute Provence (CNRS), France. We would like to thank K. Belle, P. Berlind, N. V. Borisov, M. Calkins, A. Marco, D. N. Monin, S. A. Pustilnik, H. Quaintrell, T. A. Sheikina, J. M. Torrejón, A. V. Ugryumov, G. G. Valyavin, and R. M. Wagner for assistance with some of the observations.

This work has made use of the NASA Astrophysics Data System Abstract Service, the Vienna Atomic Line Database (Kupka et al. 1999) and Peter van Hoof's Atomic Line List v2.04 (http://www.pa.uky.edu/~peter/atomic/).