4. The Mercury-Manganese stars
The Mercury-Manganese (HgMn) stars are a group of non-magnetic Chemically Peculiar stars corresponding to normal middle through late B main sequence band stars. Their abundance anomalies range from near solar to very unsolar. Those of the trace elements minimally affect their energy distributions. But those of iron-peak elements may have more important consequences especially when their abundances approach that of iron. One of their most notable anomalies is the tendency of the Mn abundances to increase from near solar in the cooler stars with near 10000 K and to being substantially greater than solar among the hotter stars with near 13,000 K (Adelman 1992a). Further many HgMn stars are members of binary systems. For this study we studied single stars and those whose companions have not been detected or contribute very little to the optical region fluxes.
We determined the effective temperatures and surface gravities of HgMn stars as for the mCP stars using for the most part solar composition model atmospheres. Corrections were applied for reddening when indicated by the photometry using the values in Schild (1977). As high quality photometry is available for many class members, calibrations of effective temperatures and surface gravities are possible in many photometric systems. In some cases the relationships used are those for normal main sequence band stars without considering whether this is really appropriate.
Table 2 shows the same kind of systematic offset between the photometric and the spectrophotometric temperatures seen in the mCP stars which further suggests that the similar processes are at work. Fig. 5 shows the difference in effective temperature between the photometric and spectrophotometric temperatures for the HgMn stars as a function of T(uvby). The least squares fit is
In addition there is on the average a 0.04 dex decrease in log g from the photometric values for the spectrophotometric-H determination which is about half as great as that for the mCP stars ( CrV was not included in this mean value).
Table 2. Effective temperature and surface gravities of Mercury-Manganese Stars.
which indicates that the normal star relations based on uvby agree better than those based on [c1]. This probably reflects that the line blanketing of the HgMn stars is more similar to the normal stars than the mCP stars.
© European Southern Observatory (ESO) 2000
Online publication: March 17, 2000