Fig. 6.2 Geometry of the secondary eclipse of RT Lacertae. This figure [taken from Fig. 3 in Milone (1977, p. 1003)], shows the shape and depth relations of the secondary eclipse, i.e., the interception of the x functions and the depth line for the fully rectified minimum infrared excess. RT Lac seems to be both an RS CVn-type variable and also what Plavec (1980) called a "W Serpentis" type3 variable, where a hot spot on the accretion disk of one of the stars excites far-ultraviolet emission. Huenemoerder (1988) discovered just such variable ultraviolet emission in RT Lac. If sufficiently thick, an accretion disk can resemble a shroud enveloping the hotter component, thereby dimming its light but not necessarily altering its color significantly. In the Russell-Merrill analysis by Milone (1976, 1977), both eclipses are shown to be occultation eclipses, because the envelope around the hotter star occults light but does not radiate it. This kind of model cannot be explored with most of the modern light curve programs - at present. Yet, because the Russell model permits the primary and secondary eclipses to be treated independently, the apparent paradoxes of the system can be resolved, at least to a degree.
The fitted optical and infrared light curves are shown in Fig. 6.3 . The Milone (1976, 1977) solution is not unique, however. Eaton & Hall (1979) analyzed the system under the assumption that the light curve and color index anomalies can be explained by fortuitous combinations of large numbers of small spot regions. Such a model agrees with the results of a study by Crawford (1992), who found that a
3 According to Wilson (1989), these stars can be described as a group of long-period Algollike mass-transferring binaries characterized by very substantial disks around the more massive components, strange and poorly repeating light curves, prominent optical emission lines, and large secular period changes.
particular spectroscopic feature is better matched with a spot model, at a particular epoch of observations. However, the analysis still fails to account for all the light curve features, despite the increase in the number of parameter-fitting elements.
In summary, it is quite likely that the components of RT Lac are spotted, but the presence of a transient stream and a thick disk in the system is also likely. It is probable that these various features may alternate in importance with epoch.
For those readers who are interested in the nature of such a curious system, a description of the investigation history and the properties of RT Lac can be found in Milone (2002). Lanza et al. (2002) carried out spot modeling on a long series of synoptic observations by Turkish observers. Popper (1991, 1992) identified it as a cool Algol (an Algol system in which both components are relatively cool and thus are evolving on similar timescales; physically, the M — Teff relation for these objects is opposite that of normal Algols). Finally, its evolutionary state has been discussed and compared to those of other such systems by Eggleton & Kiseleva-Eggleton (2002).
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