THE LOWER MAIN-SEQUENCE OF OMEGA-CENTAURI FROM DEEP HUBBLE-SPACE-TELESCOPE NICMOS NEAR-INFRARED OBSERVATIONS

A 20 '' x 20 '' field located similar to 7' from the center of the mas sive galactic globular cluster omega Centauri (NGC 5139) was observed by the NIC2 camera of the Near-infrared Camera and Multiobject Spectro meter on board the Hubble Space Telescope (HST) through the F110W and F160W broadband filters centered at 1.1 and 1.6 mu m for a total of 30 00 and 4000 s for the two filters, respectively. Standard photometric analysis of the resulting images yields 340 stars with a signal above a 10 sigma threshold in both filters, covering the range of HST m(160) magnitudes between 20 and 26, the deepest probe yet of a globular clu ster in this wavelength region. These objects form a well-defined sequ ence in the m(160) versus m(110)-m(160) plane that is consistent with the theoretical near-IR color-magnitude diagram expected from recent l ow-mass stellar model calculations. The resulting stellar luminosity f unction increases steadily with increasing magnitude up to a peak at m (160) similar or equal to 25, where it turns over and drops slowly dow n to the detection limit set by the incompleteness limit of 60% at m(1 60) similar or equal to 26. With the theoretical mass-luminosity relat ionship that provides the best fit to the LR color-magnitude diagram, we obtain an excellent fit to the observed luminosity function down to a mass of similar to 0.2 M. with a power-law mass function having a s lope of alpha = -1.