The low-mass initial mass function of the field population in the Large Magellanic Cloud with Hubble Space Telescope WFPC2 Observations

We present V- and I-equivalent HST/WFPC2 stellar photometry of an area in the Large Magellanic Cloud (LMC), located on the western edge of the bar of the galaxy, which accounts for the general background field of its inner disk. The WFPC2 observations reach magnitudes as faint as V=25 mag, and the large sample of more than 80,000 stars allows us to determine in detail the Present-Day Mass Function (PDMF) of the detected main-sequence stars, which is identical to the Initial Mass Function (IMF) for masses M <~ 1 M_solar. The low-mass main-sequence mass function of the LMC field is found not to have a uniform slope throughout the observed mass range, i.e. the slope does not follow a single power law. This slope changes at about 1 M_solar to become more shallow for stars with smaller masses down to the lowest observed mass of ~ 0.7 M_solar, giving clear indications of flattening for even smaller masses. We verified statistically that for stars with M <~ 1 M_solar the IMF has a slope Gamma around -2, with an indicative slope Gamma =~ -1.4 for 0.7 <~ M/M_solar <~ 0.9, while for more massive stars the main-sequence mass function becomes much steeper with Gamma =~ -5. The main-sequence luminosity function (LF) of the observed field is in very good agreement with the Galactic LF as it was previously found. Taking into account several assumptions concerning evolutionary effects, which should have changed through time the stellar content of the observed field, we reconstruct qualitatively its IMF for the whole observed mass range (0.7 <~ M/M_solar <~ 2.3) and we find that the number of observed evolved stars is not large enough to have affected significantly the form of the IMF, which thus is found almost identical to the observed PDMF.