The Large Magellanic Cloud globular cluster NGC 1866: New data, new models, new analysis

We present a new deep (down to V similar to 24) photometry of a wide region (similar to 6' x 6') around the Large Magellanic Cloud globular cluster NG C 1866. Our sample is much larger (by more than a factor of 3) than any pre vious photometry and with a main sequence (MS) that may be considered compl ete, down to at least 3 mag below the brightest MS star; such an occurrence allows a meaningful and robust comparison with various theoretical scenari os produced by means of models computed with the evolutionary code FRANEC. Both age and present-day mass function slope, a, are derived by a fit to th e available MS and by the use of the parameter Delta sigma, which is simply the difference, in a, between the observed and the predicted integrated MS luminosity functions. Our main conclusions are as follows: (1) The adoptio n of standard models (i.e., computed by adopting the Schwarzschild criterio n to fix the border of the convective core) allows a fair fit to the MS for an age of the order of 100-140 Myr and a present-day mass function having a slope alpha between 2.3 and 1.9, the exact values depending on the adopte d distance modulus, It is moreover possible to reproduce the average He clu mp luminosity while the total number of stars predicted in the He clump is twice the observed value; this means that we reobtain and confirm the first finding of Becker & Mathews, according to whom the simple adoption of a "c lassical" scenario leads to a neat discrepancy concerning the prediction of the number of stars in the He clump. (2) The adoption of models computed b y increasing the size of the convective core by a certain amount-i.e., 0.25 H(p)-leads to a fair fit to the MS only for a visual distance modulus (m - M)(v) similar or equal to 18.6, an age t similar or equal to 200 Myr, and a mass function slope alpha similar or equal to 2.2. In this case, the total number of He clump stars is well reproduced, although the luminosity funct ion of the He clump itself is predicted to be systematically less luminous than observed. The previous conclusions are based on the assumption that th ere is no appreciable population of binaries in NGC 1866. Though there are not yet sufficient data on the frequency of binary systems in these cluster s, we analyze how the previous scenarios would change if a consistent (simi lar or equal to 30%) population of binary systems were present in the clust er. This choice is based on the fact that a fraction of binaries of the ord er of 30% has already been found in NGC 1818, a cluster similar to NGC 1866 . The inclusion of a 30% binary population leads to the following additiona l conclusions: (3) The adoption of the standard models now leads to a good fit to the entire luminosity function-i.e., MS, turnoff, and He clump stars -for a visual distance modulus (m - M)(v) = 18.8, an age t similar or equal to 100 Myr, and a mass function slope alpha similar or equal to 2.4, thus largely removing the "classical" discrepancy between observed and predicted number of stars in the He-burning clump. The quoted visual distance modulu s constrains the unreddened distance modulus (m - M)(0), within 18.50 and 1 8.62, depending on the reddening (whose most common values available in the literature range from 0.05 to 0.10). (4) At variance with the last point, the fit obtained by using models computed with an enlarged convective core gets worse when a binary component is taken into account. This is because o f the fact that the presence of binary systems increases the existing discr epancy between the observed and predicted clump luminosity, since the He cl ump is predicted to be even less luminous than in absence of binaries. As a consequence of this analysis, we think that the next step toward a pro per understanding of NGC 1866 and similar clusters, must include the accura te determination of the frequency of binary systems, which we hope will be performed with the incoming Cycle 8 Hubble Space Telescope observations of NGC 1866.