6. The nature of the secondaries
6.1. Photometric evidence
The photometric analysis of the Am stars in the Hyades and Praesepe shows several interesting facts. All the photometric data were taken in the WEBDA (Mermilliod 1999 1) compilation of literature. The Am stars are off the main relation for normal stars by about 0.03 mag, in a (, ) diagram (Fig. 16). The () excess results from the line-blanketing in the Am stars. In a coulour magnitude diagram, part of the vertical distance to the ZAMS, usually interpreted as a sign of duplicity, is due to the metallicity effect. The parameter is preferred to () because is probably less sensitive to metallicity.This effect offers an interesting possibility of identifying Am stars in the main sequence of more distant clusters, provided that the photometry has a precision of 0.01 mag and that the clusters have no differential reddening.
In a (, ) diagram (Fig. 17), where is estimated from the apparent magnitude and the distance of each open cluster (Mermilliod 1999), the distribution of the Am stars is clearly bimodal. The double-lined systems (open squares) are accordingly located on the binary ridge, about 0.75 mag above the ZAMS, while most other systems are close to or on the ZAMS. This implies that the secondaries are fainter than the primaries by at least 3 to 4 magnitudes, which fixes an upper limit on the secondary masses. The values for the masses derived from the present data are given in Table 6.
Table 6. Minimum and maximum masses of the secondaries
6.2. Secondary masses
The three synchronised stars (KW 279, vB 38 and vB 45) present quite similar masses of the secondary () of about 0.5 . For KW 40 the difference between , which is derived form the photometry, and , calculated from Vsync, results from the composition of two errors. Firstly, the luminosity of KW 40 is not corrected from the third component luminosity, and secondly, a small error on the V value would result in a significant change of the derived mass. A secondary mass of 1.81 would imply that the secondary is an A-type star, which probably has a large rotational-velocity and might not be detected by CORAVEL. For vB 169, the value of the secondary mass can be affected by the presence of the third and fourth components of the system.
The six systems not detected in X-rays have secondary masses compatible with that of a white dwarf.
© European Southern Observatory (ESO) 2000
Online publication: February 25, 2000