SIMBAD references

We have used the WFPC2 on board HST to investigate the stellar population in a field located 4.6' NW of the center of the globular cluster M15 by means of deep imaging in wide bands centered at 606 and 812nm. A total of 7770 stars are accurately classified by two-color photometry to form a color-magnitude diagram extending down to a limiting magnitude m₈₁₄≃m_I≃25.5, determined reliably and solely by counting statistics. At m₈₁₄≃22.5 we detect a steepening of the main sequence slope, in accordance with theoretical models that predict such a feature as a consequence of the opacity change due to the effect of H₂ in the atmosphere. The main sequence luminosity function increases with decreasing luminosity following a power-law trend with index α≃1.3 in the range 3.5<M₈₁₄<8.5, but then drops sharply down to the measurement limit. Average photometric incompleteness is only ∼15% in the faintest magnitude bin, and is due exclusively to crowding. On the basis of the available mass-luminosity relation for this metallicity, we transform the luminosity function into a mass function that shows a power-law increase in numbers for decreasing masses in the range 0.8-0.3M_☉, consistent with ground based results, but flattens below 0.25M_☉. The observed trend in the luminosity and mass functions is in good agreement, in the region of overlap, with that shown by the metal poor cluster NGC 6397 that we have previously investigated with the same instrumentation (PaperI). This implies that the cluster population at these locations is largely insensitive to the interaction with the Galaxy, as well as to mass segregation. The ensuing result is that very low mass stars are not produced in any dynamically significant amount by globular clusters, a conclusion consistent with recent determinations of the mass function in the disk and in the halo, implying the existence of some limitation in the mechanism of stellar formation.