The Lower Main Sequence of omega Centauri from Deep Hubble Space Telescope NICMOS Near-IR Observations

Astronomy and Astrophysics – Astronomy

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Stars: Luminosity Function, Mass Function, Galaxy: Globular Clusters: General, Galaxy: Globular Clusters: Individual Ngc Number: Ngc 5139

Scientific paper

A 20" x 20" field located ~7' from the center of the massive galactic globular cluster omega Centauri (NGC 5139) was observed by the NIC2 camera of the Near-Infrared Camera and Multiobject Spectrometer 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 3000 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 m160 magnitudes between 20 and 26, the deepest probe yet of a globular cluster in this wavelength region. These objects form a well-defined sequence in the m160 versus m110-m160 plane that is consistent with the theoretical near-IR color-magnitude diagram expected from recent low-mass stellar model calculations. The resulting stellar luminosity function increases steadily with increasing magnitude up to a peak at m160~=25 , where it turns over and drops slowly down to the detection limit set by the incompleteness limit of 60% at m160~=26 . With the theoretical mass-luminosity relationship that provides the best fit to the IR color-magnitude diagram, we obtain an excellent fit to the observed luminosity function down to a mass of ~0.2 M solar with a power-law mass function having a slope of alpha =-1 .

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