Deep luminosity functions of globular clusters – IV. NGC 6171 ⋆

This paper is aimed at determining the main-sequence properties, and the mass function of the metal-rich globular cluster NGC 6171. We present new deep BV CCD photometry for four fields ($$\text{size}\,1.9\times\,3.0\,\text{arcmin}^{2}$$) located between 2 and 5 core radii from the cluster centre. Two procedures have been used to estimate the apparent distance modulus, giving consistent results: $${(m-M)}_{V} = 14.63 \pm 0.20$$. A luminosity function down to $$V \thicksim 23.5({M}_{V} \thicksim 8.6)$$ has been constructed and corrected for incompleteness. A comparison with theoretical luminosity functions shows that the mass function (modelled as a power law) has a slope $$x\,\simeq\,-1$$ down to $$m\,\sim\,0.6{M}_{\odot}$$. The data corrected for mass segregation, using multimass King–Michie models, indicate that the global value for the power-law index is $${x}_{0} \thicksim -1.0$$. A comparison of the MS locus with the theoretical isochrones suggests an age of ~18 Gyr if an α-elements enhancement $$([\alpha/Fe] \thicksim 0.2)$$ is taken into account.