Session 80 - Star Clusters in the Milky Way.
Display session, Friday, January 09
Exhibit Hall,

[80.02] Deep HST-NICMOS Observations of Low Mass Stars in ømega Cen

G. De Marchi, F. Paresce, L. Pulone (ESO), F. Allard (CRAL-ENS Lyon)

A 20\arcsec \times 20\arcsec field located \sim 7\arcmin\, away from the center of the massive galactic globular cluster ømega\,Cen (NGC5139) was observed by the NIC\,2 camera of the NICMOS instrument on board HST through the F110W and F160W broad band filters centered at 1.1 \mu and 1.6 \mu for a total of 3,000 and 4,000 seconds for the two filters, respectively. Standard photometric analysis of the resulting images yields 340 stars with a signal above a 8 \sigma threshold in both filters covering the range of HST H-band magnitudes between 20 and 26, the deepest probe yet in these bands of a globular cluster. These objects form a well defined sequence in the H vs J-H 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 H\simeq 24.5, where it turns over and drops slowly down to the detection limit set by the incompleteness limit of 60% at H\simeq 25.5. 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 \sim 0.2\,M\sun with a power law mass function having a \alpha = -1 slope.