5 Standard Strömgren indices

The flat field correction surfaces in the Strömgren $vby-\beta$ filters found by using the observations of NGC 6134 have been applied to the observations of both clusters and the Strömgren standard stars.

The atmospheric extinction was determined by using observations of two bright standard stars which were observed as part of a programme to detect oscillations in two bright field stars (see Viskum et al. 1998). These stars were observed at the same time as the open clusters with the Danish 50 cm SAT telescope at La Silla. From these data we found the extinction coefficients of the standard Strömgren magnitudes, which were applied to the observations of the open clusters.

After the extinction correction we found the following transformation to the standard indices using the Strömgren standard stars:

$$+ \, 1.0444 \, (b-y)_I$$ b-y = +0.5026 (1)

$$+ \, 1.0067 \, y_I$$ V = -1.9027 (2)

$$+ \, 1.0118 \, (m_1)_I -0.1291 \, (b-y)$$ m₁ = -1.2662 (3)

$$\beta = + \, 1.4348 \, \beta_I$$ +0.7277 (4)

where I denotes the instrumental index.

Figure 3: Comparison of the b-y indices found by Nordström et al. (1997) and the present work. Stars with low standard deviation are plotted with diamonds while other stars are plotted with dots. The double stars "30'' and "121'' are marked with stars. The two datasets disagree appreciably for star "2095''. The lines labeled "Nordström'' and "Bruntt'' indicate the range in b-y of the stars used for calibration by Nordström et al. (1997) and in this paper

We have then compared the derived indices with previous Strömgren photometry of the open cluster NGC 3680. Nissen (1988) has published Strömgren indices from photoelectric measurements of the brightest stars in the field of NGC 3680, i.e. bright field stars and stars at the turnoff region in the cluster. More extended CCD photometric studies of this cluster have been done by Twarog et al. (1989) (uvby) and Nordström et al. (1997) (b-y). Their calibration both rely on the early work by Nissen (1988).

We find small offsets in the b-y and $\beta$ index and in V when comparing with Nissen (1988). The photometry of Nissen is of high quality and is well-calibrated as several ($\sim30$) standard stars were observed each night (compared to just five standard stars in our case). Hence we have corrected for the small offset errors, i.e. 0.01 and 0.02 mag in b-y and $\beta$ respectively calculated from 13 and 4 stars in common with Nissen (1988).

In Fig. 3 we show the difference in the b-y index found by Nordström et al. (1997) and the present work. As can be seen there is a significant dependency on colour. Also indicated in Fig. 3 is the range in b-y of the stars used for the standard transformation. Nordström et al. (1997) only have two stars in the dashed range in Fig. 3. As noted by Nordström et al. (1997) their calibration rely on the turnoff stars observed by Nissen (1988), i.e. with only a narrow range in b-y. Hence we consider our calibration of the fainter stars in the cluster as being more reliable as our standard stars cover a much more extended range in b-y.

   

Table 2: Photometric precision of the Strömgren indices in each cluster for stars around the turnoff (TO) and for the F type stars in NGC 6134

Stars	V	b-y	m1	$\beta$
NGC 6134, TO	0.004	0.007	0.009	0.010
NGC 6134, F	0.010	0.020	0.020	0.020
NGC 3680, TO	0.004	0.007	0.008	0.016

The photometric precision of the Strömgren indices are estimated using the standard deviation of the derived instrumental magnitudes. The result for the turnoff (hence brightest) stars in each cluster is shown in Table 2. These are A type stars for the younger cluster NGC 6134, while the turnoff stars in NGC 3680 are F type stars. Also quoted is the precision of the F type stars in NGC 6134. The F type stars are particularly interesting, as their m₁ index correlates with metallicity. The results found in Table 2 are comparable to what has been achieved earlier with photoelectric equipment, e.g. Nissen (1988).

We have also estimated the standard errors of the Strömgren indices by comparing our results to Nissen (1988) after the removal of offset errors and ignoring 3$\sigma$ outliers. The standard errors (sd. of the mean) for the four indices are 0.0018 (22 stars), 0.0022 (23 stars), 0.0032 (23 stars), 0.0032 (4 stars) in V, b-y, m₁, and $\beta$ respectively.