Modelling interstellar extinction in stellar populations
In stellar astrophysics, the determination of the magnitude of interstellar extinction is critical, due to its effect on the observed brightness and colour of the stars. Extinction is therefore an important factor in deriving scientific information from the colour-magnitude diagrams (CMDs) of stellar populations. The treatment of extinction in standard CMD analyses is to employ constant ratios of extinction in each photometric filter relative to the visual Johnson-V filter, denoted AX/AV in a generic filter X. This work presents a theoretical analysis of the behaviour of the extinction ratios AX/AV in multiple photometric systems as the values of three stellar parameters (effective temperature, surface gravity and metallicity) are varied. The results of this analysis show significant variations in the value of AX/AV with changes in the stellar parameters. For certain ultraviolet filters and an AV value of 1.0, the fractional flux lost to extinction is up to two orders of magnitude greater between different stellar atmospheres. Analytic functions of these stellar parameters are proposed to describe these variations. Also presented is an application of these functions to generic isochrones in multiple photometric filter systems. This was followed by an application of the extinction-ratio functions to the highly-reddened star cluster NGC 6793 whose members also have accurate Gaia parallax measurements. When a proper analysis of extinction, via the AX=AV functions, is used on the cluster data, it is shown that there is a non-negligible impact on the age determination for the cluster in multiple CMD axes and in different filter systems. For NGC 6793, the observational data predicts an age of 603 Myr, an AV value of 0.843 and a metallicity of [Fe/H] = 0.0 when the extinction in each filter is held constant. When the extinction is allowed to vary according to the AX/AV functions, the predicted values for these parameters become 500 Myr, 1.1 and +0.062, respectively. The uncertainties in the observational data, the models and all other factors considered were found to be insufficiently large to render the difference between these results insignificant. It was therefore concluded that changing the method of calculating extinction in isochrones results in a significant change in cluster parameter estimates, particularly for the age and AV values.
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