Astron. Astrophys. 363, 901-916 (2000)

Appendix

In this Appendix a comparison is presented between the DENIS I and OGLE I and DENIS JK and 2MASS JK for the respectively 173 and 141 LMC and SMC Cepheids in common. This has its limitations since the comparison is done using variable stars. Yet it may still be of interest to other workers, as such a comparison has not been done yet. The variability would increase the spread in any correlation but should not wash out any colour terms, if they exist.

The observed IJK magnitudes are first corrected for reddening. The of the respective OGLE field is used (U99b,c), a selective reddening of 3.1 is used, and the extinction curve of Cardelli et al. (1989). In particular, , and are used. Since the effective wavelengths of the DENIS and OGLE I and the DENIS and 2MASS JK are very similar, no distinction between the respective filtersets is made. At the present level of accuracy this effect of differential reddening between the slightly different filters should be entirely negligible.

Fig. A.1 shows the differences between 2MASS and DENIS and plotted versus 2MASS , and OGLE and DENIS and plotted versus DENIS . In the left panel, all stars are plotted, in the right panel a sub-sample is plotted using a criterium on the total error. Linear least-square fits were made and the results are listed in Table A.1.

Fig. A.1. Comparison of DENIS IJK with OGLE I and 2MASS JK. The left hand panel contains all sources, for the right hand a selection on the combined error in the x- and y-axis is made.

Table A.1. Comparing photometric systems. Fits of the form

From this data, there is no evidence for a colour-term in I and J and the difference between OGLE and DENIS , and 2MASS and DENIS is less than 0.10 magnitude. In K there seems to be a color term at the 3 level when all stars are used, which still is present at the 2 level when a subsample of stars with the smaller photometric errors is being used. The average difference (2Mass) - (Denis) is +0.20 magnitudes, which is comparable to the observed scatter, which is an upper limit to the intrinsic scatter as two single-epoch data are being compared.

The conclusion is drawn from the present dataset that there is no evidence for colour terms when comparing OGLE and DENIS , and 2MASS and DENIS , and that any difference between the systems is less than 0.10 magnitude. Regarding 2MASS and DENIS , there is evidence for a colour term, which amounts to a difference (2Mass) - (Denis) = +0.20 magnitudes for a typical colour of .

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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