Warnings related to the decrease of WFPC2 exposure times to the next legal value are acknowledged ----- Attempting Submission 1 (Thu May 10 07:07:16 GMT 2007) ----- Warnings related to the decrease of WFPC2 exposure times to the next legal value are acknowledged. This proposal replaces ACS Phase I ID #337 Assigned ID: 1149 ----- Attempting Submission 1 (Thu Feb 08 12:42:57 CET 2007) ----- Phase I Proposal 1149 (ngc3603) successfully submitted. A comprehensive study of the low-mass stellar population in the Galactic starburst region NGC 3603 NGC 3603, located in the Carina spiral arm, is one of the most luminous giant HII regions in the Milky Way, and as such it is often referred to as a prime template for extragalactic starbursts. While previous studies were focussing on the high and intermediate mass stellar content of the central starburst cluster, which powers the HII region, the effects of the starburst environment with its large number of ionizing O stars on the emerging low-mass population are unknown. As the most nearby, most easily accessible starburst, NGC 3603 provides the best testbed to study the long-lived, low-mass stars originating from a starburst environment. Taking advantage of the large field of view and high sensitivity of WFPC2, we want to survey the stellar population in an area of 10pc x 10pc (6' x 6') down to a mass limit of 0.2 to 0.5 Mo. This will enable us to derive the total cluster mass, look for spatial variations in the initial mass function, determine the age of the dispersed low-mass population in the HII region and search for evidence of sequential star formation. Ultimately, we aim at reconstructing the low-mass stellar initial mass function of the starburst epoch in NGC 3603, which in turn will advance our understanding of extragalactic starburst phenomena and the emerging low-mass stars as observed in ancient populations. The observations of NGC 3603 are part of our larger effort to study intense star-forming regions in the Milky Way, LMC and SMC.
The primary observations will be carried out with WFPC2. We aim to cover roughly half of the giant HII region NGC 3603, with the central starburst cluster located in the south-western corner of a 2 times 2 sparse WFPC2 mosaic. The region has been selected to have the lowest contamination by nebular emission and foreground clouds. Each visit will include observations in the F555W, F675W, and F814W filters, with short and longer integration times in order to improve the dynamic range. A 4-point dither pattern will be implemented as proper PSF sampling is most critical to overcome crowding for deriving the mass function down to 0.5 M_Sun. For visit 1, the range of HST orientations is limited in order to avoid the brightest patches of diffuse emission of the HII region, and to provide optimal overlap with archival WFPC2 data of the cluster. Observations in F675W are required in order to estimate the number of low-mass stars with masses <0.5 M_Sun missed in F555W due to patchy extinction, and to distinguish very low-mass pre-main sequence stars from bluer foreground and reddened background stars. Including overheads for acquisition, instrumental set-up, dithering and filter changes, the described sequence can be fitted into 2 orbits. For each of the 4 fields, a separate visit is planned, resulting in a total of 8 orbits in order to cover a ~ 6 arcmin times 6 arcmin region. The coordinated parallel observations with NIC2 will provide additional samples of the dispersed field, fore- and background stellar populations in the direction of NGC 3603. For NIC2, we will use the F110W, F160W, and F205W filters. Observing times will be scaled according to the WFPC2 observations. 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