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Photometric behaviour of Carinae, a celestial Chinese lantern: 1974-1998 *
A.M. vanGenderen 1,
C. Sterken ** 2,
M. deGroot 3 and
G. Burki 4
Received 19 October 1998 / Accepted 16 December 1998
We discuss 24 y of optical photometry of Carinae, among which new Geneva photometry made between 1994 and 1998. Various conclusions from our previous photometric studies are confirmed. The core hides a normal S Dor variable (or LBV): it shows light variations on a time scale of 1-4 y, with superimposed micro oscillations whose quasi-period indicates a temperature in the order of 22 000 K. Therefore, a more complicated model for Car is necessary to explain its extraordinary appearance and phenomena exhibited in the past and at present.
An analysis of the brightness of Car in the ultraviolet (UV) passbands of three photometric systems (Walraven, Strömgren and Geneva) reveals the presence of an important variable UV source, which appears to be modulated with the 5.52 y period of the spectroscopic events, related to the possible revolution of an excentric binary of the type proposed by Damineli et al. (1997).
Our new data support the luminous disk model suggested by van Genderen et al. (1994, 1995). A very hot companion of the LBV would be responsible for the excitation of the disk.
We suspect that the flare-like event in the X-ray flux and in the optical and near-infrared light around 1998.0 was the result of the encounter of the interface of the colliding winds of the binary with an arm-shaped density enhancement in a disk around the LBV (not necessarily "the" luminous disk). We suppose that this encounter created an intense X-ray/hot spot region. The subsequent steep decline of the flare is ascribed to an eclipse of the X-ray/hot spot by the wind interface.
The radio flux variation of the gas torus in the equatorial plane at a distance of from the core, could be the result of the luminous disk becoming optically thin. This would, obviously, start abruptly near the time of periastron passage and would last for a few years thereafter, so that a hot star, normally enshrouded by the disk, is able to excite the outer gas torus. The creation of the X-ray/hot spot, with a life-time of at most a few months, could also be the cause of the instantanious physical change of the luminous disk mentioned above (and its 5.52 y modulation) visible in the UV, since both happen at the same time.
Apart from the 5.52 y period in the UV, we found a striking 200 d-oscillation, also in the UV, during the last orbital cycle beween 1992.5 and 1998.0. Its possible explanation depends on whether it is cyclic or truly periodic (in the latter case Car could hide a triple star).
Key words: stars: individual: j Carinae stars: oscillations stars: supergiants stars: variables: general techniques: photometric
Send offprint requests to: A.M. van Genderen
Online publication: March 1, 1999