Hubble Space Telescope NICMOS Variability Study of Massive Stars in the Young Dense Galactic Starburst NGC 3603
We have used the relatively long data string of the 1997-1999 NICMOS focus tests on NGC 3603 to extract ~J-band light curves for several hundred stars in the cluster core. Given the relatively modest photometric precision [σ(J)>=0.05 mag], we were able to isolate only a half-dozen variable candidates with peak-to-valley amplitudes above ~0.2 mag. One of the variables is one of the two outstandingly brightest cluster members, A1, located in the very dense cluster center. A1 shows double eclipses on each orbital cycle, with the same period (P=3.7724 days) as found previously and independently in unresolved ground-based radial velocity variations of the Wolf-Rayet (WR) emission component in the central core of NGC 3603. Very rough best estimates for the masses of the components of A1 are in the range 30-90 Msolar for the brighter and more massive H-rich WR component (WN6ha) and 25-50 Msolar for its assumed O star companion. A more detailed study is urgently needed, given the potential for this extremely luminous system to harbor the most massive main-sequence star ever ``weighed.'' Another variable, HST 12, escaped the original search, which was based on larger than average standard deviation. It is a probable field-star eclipsing variable with a moderately long period.