8 Metallicity

The reddening corrected m₀ index is correlated with metallicity for F type stars. In Fig. 7 we show the relation between the $\beta$ and m₀ index for stars in each of the observed clusters. Also plotted is the Hyades relation taken from Crawford (1975a, 1979). As may be inferred, the metallicity of NGC 3680 is somewhat lower than the Hyades cluster, while the metallicity of NGC 6134 is somewhat higher. Note that [Fe/H] $_{\rm Hyades}\simeq0.12$ according to recent observations by Cayrel (1985).

In the field of NGC 6134 a few of the F type stars (i.e. $\beta < 2.72$) have high m₀ indices, which indicate that they are not members of the cluster. On the other hand it can also be seen that quite a few of the A stars in NGC 6134 also have extremely high m₀ indices, which not necessarily exclude them from being cluster members, but may indicate that they are Am type stars. Note that they are all found on the upper part of the cluster main sequence. We have used the calibration of [Fe/H] for Am stars by Smalley et al. (1993) on the possible Am stars (i.e. $\delta m_0 < -0.05$ and $\beta > 2.72$) in the cluster. We find a mean metallicity of ${\rm [Fe/H]}=0.83\pm0.04$ (standard deviation of mean) from 12 possible Am stars.

Figure 7: The relation between m0 and $\beta$ for stars in NGC 3680 (top) and NGC 6134 (bottom). Also plotted is the standard Hyades relation taken from Crawford (1975a, 1979). The symbols are explained in the top plot

A calibration of the m₀ index with metallicity has been obtained by Nissen (1981). In Fig. 8 we have plotted the histogram of stars in NGC 6134 which are probable cluster members, i.e. stars with interstellar reddening within $3\sigma$ of the mean value. For NGC 3680 we have plotted the result for 12 stars which are cluster members according to the spectroscopy by Nordström et al. (1997). The mean value of each cluster is found by removing $3\sigma$ outliers. Our results are: NGC 6134: ${\rm [Fe/H]}=0.28\pm0.02$ and NGC 3680: ${\rm [Fe/H]}=0.09\pm0.02$, cf. Table 3.

The uncertainty of the metallicity may be as large as 0.1-0.2 dex for NGC 6134, as only few Strömgren standard stars with high b-ywere included in the calibration. This is a problem as the F stars in NGC 6134 are highly reddened. The result for NGC 6134 agrees with what is given in the catalogue by Lyngå (1987) ${\rm [Fe/H]}=0.25$. Twarog et al. (1997) have found ${\rm [Fe/H]}=0.18\pm0.09$ from revised DDO photometry. On the other hand Claria et al. (1992) found ${\rm [Fe/H]}=-0.05\pm0.12$ from photometry of red giant stars in the cluster. They used three different photometric methods, UBV, DDO and Washington which yielded ${\rm [Fe/H]}= 0.08$, -0.16, and -0.11 respectively.

Several estimates of the metallicity of NGC 3680 have been published. Nordström et al. (1997) found ${\rm [Fe/H]}=0.11\pm0.05$ when using their cluster membership assignment and the Strömgren photometry of Nissen (1988). Twarog et al. (1997) cite ${\rm [Fe/H]}=-0.12\pm0.05$ from DDO photometry of 8 giant stars.

It is apparent from the published results that it is difficult to determine the metallicities within 0.15 dex by means of photometry. A determination of the metallicities by using spectroscopy would certainly be of great value.

Figure 8: Histograms of the metallicity for stars in the field of NGC 6134 and NGC 3680. In the top plot the dashed line includes all stars, while the solid line is for the confirmed cluster members