6 The Balmer discontinuity index

As the CCD was insensitive to light in the u filter we could not obtain the Balmer discontinuity index, c₀ = (u-v) - (v-b). The calibrations of the interstellar reddening include small terms involving the Balmer discontinuity c₀ index (Crawford 1975a, 1979), hence it is essential to get a somewhat reliable measure of this index. For NGC 3680 the c₀ index was obtained for a few stars in the turnoff region by Nissen (1988), and we have used his values.

Figure 4: Conversion of $\delta M_V$ to the c0 index for stars in NGC 6134. The solid lines are the standard relations found in Crawford (1975a, 1979)

The c₀ index measures to what extent a star has evolved above the ZAMS. We can measure the extent of evolution through $\delta M_V$ by fitting a ZAMS to the lower end of the observed colour-magnitude diagram. For the stars in NGC 6134 we calculated approximate c₀ indices indirectly by using the calibrations of the f factor by Nissen (1988) (F stars):

$$% \delta c_0 = \delta M_V / f = \delta M_V / (9.0 + 50 (2.72-\beta))$$ (5)

and Crawford (1979) (A stars):

$$% \delta c_0 = \delta M_V / f = \delta M_V / 9.0,$$ (6)

where $\delta c_0$ is defined as $\delta c_0 = c_0({\rm ZAMS}) - c_0$.

In Fig. 4 we show how $\delta M_V$ is converted to the c₀ index by using the f factor. Note that this was only done for stars with E(b-y) within 3$\, \sigma$ of the mean value, i.e. stars that are probable cluster members.