Population effects on the red giant clump absolute magnitude : The K - band

We present a detailed analysis of the behaviour of the Red Clump K-band absolute magnitude (M K ) in simple and composite stellar populations, in light of its use as standard candle for distance determinations. The advantage of using M K , following recent empirical calibrations of its value for the solar neighbourhood, arises from its very low sensitivity to the extinction by interstellar dust. We show that, as in the case of the V and I-band results, M K is a complicated function of the stellar metallicity Z and age t. In general, M K is more sensitive to t and Z than M I , for high t and low Z. Morever, for ages above ∼1.5 Gyr, M K decreases with increasing Z, the opposite behaviour with respect to M V and M I . We provide data and equations which allow the determination of the K-band population correction ∆M K (difference between the Red Clump brightness in the solar neighbourhood and in the population under scrutiny) for any generic stellar population. These data complement the results presented in Girardi & Salaris (2001) for the V and I-band. We show how data from galactic open clusters consistently support our predicted ∆M V , ∆M I and ∆M K values. Multiband V IK population corrections for various galaxy systems are provided. They can be used in conjunction with the method devised by Alves et al. (2002), in order to derive simultaneously reddening and distance from the use of V IK observations of Red Clump stars. We have positively tested this technique on the Galactic globular cluster 47 Tuc, for which both an empirical parallax-based main sequence fitting distance and reddening estimates exist. We have also studied the case of using only V and I photometry, recovering consistent results for both reddening and distance. Application of this method to an OGLE-II field, and the results by Alves et al. (2002), confirm a LMC distance modulus of about 18.50, in agreement with the HST extragalactic distance scale zero-point.