SIMBAD references

We have investigated the properties of the stellar mass function in the globular cluster NGC 6397 through the use of a large set of Hubble Space Telescope (HST) observations. The latter include existing WFPC 2 images in the V and I bands, obtained at ∼4'.5 and 10' radial distances, as well as a series of deep images in the J and H bands obtained with the NIC 2 and NIC 3 cameras of the NICMOS instrument pointed, respectively, to regions located ∼4'.5 and ∼3'.2 from the center. These observations span the region from ∼1 to ∼3 times the cluster's half-light radius (r_hl≃3') and have been subjected to the same, homogeneous data processing so as to guarantee that the ensuing results could be directly compared to one another. We have built color-magnitude diagrams that we use to measure the luminosity function of main-sequence stars extending from just below the turnoff all the way down to the hydrogen-burning limit. All luminosity functions derived in this way show the same, consistent behavior in that they all increase with decreasing luminosity up to a peak at M_I≃8.5 or M_H≃7 and then drop precipitously well before photometric incompleteness becomes significant. Within the observational uncertainties, at M_I≃12 or M_H≃10.5 (∼0.09 M_☉) the luminosity functions are compatible with zero. The direct comparison of our NIC 2 field with previous WFPC 2 observations of the same area shows that down to M_H≃11 there are no more faint, red stars than those already detected by the WFPC 2, thus excluding a significant population of faint, low-mass stars at the bottom of the main sequence. By applying the best available mass-luminosity relation appropriate to the metallicity of NGC 6397 and consistent with our color-magnitude diagrams to both the optical and the IR data, we obtain a mass function that shows a break in slope at ∼0.3 M_☉. No single-exponent power-law distribution is compatible with these data, regardless of the value of the exponent. We find that a dynamical model of the cluster can simultaneously reproduce the luminosity functions observed in the core, at ∼3'.2, 4'.5, and 10' away from the center, as well as the surface brightness and velocity dispersion profiles of red giant stars, only if the model initial mass function (IMF) rises as m^–1.6±0.2 in the range 0.8-0.3 M_☉ and then drops as m^0.2±0.1 below ∼0.3 M_☉. Adopting a more physical lognormal distribution for the IMF, we find that all these data taken together imply a best-fit distribution with m_c≃0.3 and σ≃1.8.