Astron. Astrophys. 324, 137-154 (1997)

4. Discussion

We have examined three eclipsing binaries possibly containing a CP star according to their spectral classification or low rotational velocity. Among them, only one, HD 185257, has a confirmed CP primary of the Am type. TV Nor, although classified EuCrSr, very probably has a normal secondary, while there is only very marginal evidence that the primary may be a mild Ap: Na seems slightly overabundant and the has a small positive value; furthermore, no out-of-eclipse variation is apparent, so that there is no intrinsic variation typical of magnetic Ap stars. One might wonder whether the primary of TV Nor is an HgMn-type Ap, since these stars do have a slightly larger than normal stars. Only the measurement of Hg and Mn lines will settle the question (Si, Cr and Sr lines should be examined too), and such a test would be worth the effort.

As for HD 184035, where km s^-1 (Levato 1975) or 53 km s^-1 (Hoffleit & Jaschek 1982) for the primary, no clear indication of any kind of chemical peculiarity can be found in the photometric data. Neither the shape of the lightcurve nor the peculiarity parameters betray the CP nature one could suspect from the relatively low equatorial velocity. Of course, more subtle chemical anomalies might be present which would have no effect on the photometry but could be seen on high-resolution spectra. This system is quite interesting in this respect, to test the idea of Abt & Morrell (1995) that is the only parameter that distinguishes normal and pronounced or mild CP stars. However, the tidal distorsion and corresponding variation of the temperature on the surface of the star would have to be taken into account in any detailed abundance analysis of the primary.

Another interest presented by these three systems is the synchronism, or lack thereof, between the spin and orbital periods. It seems that syncronism is nearly but not completely achieved in the case of TV Nor, and that it has not yet taken place in the case of HD 184035 - even for the evolved primary - if one takes the published at face value. Indeed, the expected of the primary of HD 184035 is only 40 km s^-1 in the case of synchronism. This may appear surprising at first sight, because of the shorter period of HD 184035: the effective temperatures of the components are very similar for HD 184035 and TV Nor; on the other hand, the primary of HD 184035 is significantly more massive than that of TV Nor and might have rotated faster in the past. No value has been published to date for HD 185257, but according to Nordström & Andersen (1985), it must lie between 50 and 100 km s^-1, so that synchronism has surely not been reached yet, at least for the primary. This is not unexpected, in view of the long period. It would be interesting to know the equatorial velocity of the secondary, which has a relatively thick outer convection zone (contrary to the primary) hence a different way to dissipate the tidal force, besides the magnetic braking through stellar wind. More spectroscopic data on both HD 184035 and HD 185257 would be extremely welcome in this respect, and also to obtain the radial-velocity curves of both companions. It would even be worthwhile to reobserve TV Nor at very high resolution () to obtain more accurate equatorial velocities.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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