We have re-analyzed the case of the puzzling apsidal motion of DI Her using new observational and theoretical data. Some possible alternatives already explored to explain the discrepancy were reviewed briefly. We have also introduced new possible explanations for the problem. When compared with observational data some hypothesis present some inconsistencies or are directly in conflict with the data. There are some hypothesis that can not stand comparison even with theoretical data as for the case of high viscosity for the stellar interior, or the high density required if we assume that a circumstellar material is the responsible for the slow apsidal motion of DI Her.
Some more plausible explanations are debilitated by recent observations. As for example, that based on the possibility of no alignment of the orbital and spin momenta. In fact, the observation of the Rossiter effect in DI Her performed by Maloney & Guinan 1989 restrict seriously such a hypothesis although systematic observations are needed to enable us more conclusive analysis.
We can also quote the use of an alternative equation for the apsidal motion rate based on the work by Moffat (1984) which, when applied to others systems, gives an over-correction that leads to poorer results than when we use General Relativity. On the other hand the formulation by Moffat (1989) is also dependent on a pre-calibration, using DI Her itself among other systems. This theory does not predict the apsidal motion a priori and there is a strong dependence on the baryon to cosmions ratio which is not a well constrained physical parameter (Claret 1997).
The presence of a third body although is an acceptable explanation, depends obviously on observational evidence.
The magnitude of the period of apsidal motion U, the position of the periastron and the values of D may affect the apsidal motion measurements. However due to the small number of points in Fig. 3, 4 and 5 it is not possible to conclude definitively that some of these effects are responsible for the high deviations of . But the tendency shown (mainly in Fig. 3) deserves certainly a more careful investigation under the observational and theoretical point of view. Still concerning these questions, some natural questions can be made as for example: how to evaluate the errors in taking into account the relatively short observation time (compared with U)? What is the degree of confidence when the observations of apsidal motion are obtained near unfavorable positions of the periastron?
The main goal of this paper is not to solve the case of DI Her. The explanation for the case of DI Her is certainly more complex than commented here and it is possible that more than one perturbing process is acting simultaneously on the system. With this paper we mainly try to stimulate and renew the discussion on the observational and theoretical methods often used in the analysis of apsidal motion in close binary systems.
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998