6763( 4) - 04/22/97 17:55 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6763 Version: 4 Check-in Date: 22-Apr-1997 17:53:12 1.Proposal Title: Deep Imagery of NGC 3603: Low-Mass Star Formation in the Closest Starburst Region ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle STELLAR POPULATIONS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: LAURENT DRISSEN UNIVERSITE LAVAL CoI: Anthony Moffat Universite de Montreal N CoI: Gerhardt Meurer Johns Hopkins University N CoI: Eva Grebel Universitaet Bonn N ------------------------------------------------------------------------------------ 6. Abstract This proposal aims at studying the intermediate and low-mass stellar population in NGC 3603, the most luminous visible starburst region in the Galaxy, which more than rivals the dense core R136 of 30 Dor in the 7 times more distant LMC. Although the massive star content of this very dense cluster is now fairly well known, our knowledge of the medium- and low -mass part (1.5 - 10 M_\odot) of the IMF in NGC 3603, not to mention more distant starbursts, is marginal at best. We therefore propose to obtain deep WFPC2 images of NGC 3603 that will allow us to probe its stellar content and IMF down to V=22.5 (M_v = +3.5; M ~ 1.5 M_\odot). Questions such as a possible low-mass cutoff, variation of the IMF slope as a function of radius, and differential star formation will be addressed. In particular, we want to know if violent star formation of the most massive stars (and the subsequent strong ionization field and wind kinetic energy they generate) has prevented, or delayed, the formation of less massive stars in the very core of the cluster. Because of its proximity, NGC 3603 is the only truly dense starburst where we can directly observe whether or not violent, massive star formation efficiently quenches the formation of lower mass stars. ------------------------------------------------------------------------------------ 6763( 4) - 04/22/97 17:55 - [ 2] Observations Description ------------------------ The brightest stars in NGC 3603 (based on Cycle 1 HST data) have V ~ 11.2, and our objective is to detect stars down to V=23.5 (M_V = +4.5, corresponding to ~ 1.5 M_\\\\\\\\odot) with a S/N ratio of ~ 15 in order to be as complete as possible. We will take a series of short and long exposures in each of the filters F547M (V), F675W (R) and F814W (I) centered on the core of NGC 3603 in order to determine the stellar luminosity function. The use of 3 filters is required to determine the individual reddening for the stars. Although the global reddening of the cluster is known from ground-based observations, there are indications of variable, internal reddening. The F547M filter is prefered to the F555W filter in order to avoid contamination by nebular lines. In choosing the exposure times, we need to take into account the fact that bright stars will saturate on long exposures and blooming may mask a significant fraction of the CCD. We therefore chose to take multiple intermediate exposures instead of a long one. This strategy significantly increases the overheads, but insures that photometry of faint stars will be possible everywhere in the cluster. Example of exposure time calculation - We have used the WFPC2 Exposure Time Calculator on WWW to estimate the exposure times, and we have checked them using the specifications of the WFPC2 manual (N_e = 2.5 * 10^11 t QT * 10^-0.4 * mag, with QT = 0.01224 for the F547M filter.) noindent (a) F547M: We will first take a short (1 second) exposure; the 3 brightest stars (of Wolf-Rayet spectral type) will be slightly below saturation levels, and stars with V = 18.0 (M_V = -0.9; sp. B6; mass ~ 6 M_\\\\\\\\odot) will be detected with a S/N ratio ~ 10. Exposures longer than ~ 50 seconds will cause serious bleeding (more than 10 pixels on each side) for 20 stars; on the other hand, exposures shorter than 20 seconds will be dominated by the read-out noise at V=23. We therefore chose a series of 20 30-sec exposures. Blooming will occur for ~ 15 stars, but will only affect a small number of pixels, except for the three brightest stars (30 pixels each). This combined 480-sec image will give a S/N ratio of ~ 20 for V=22.5, and stars of V=24.0 will be detected with S/N=3. Because of the limited space on the on-board tape recorder, only the PC1 chip will be read (option READ=1) during 12 of the 20 30-s exposures. One long (200 s) exposure will be obtained; although the central region of the cluster will be saturated, a significant fraction of the image will be free of defects. Finally, to bridge the gap between the 1s and 30s exposures, we will obtain a 10s exposure. This will insure that intermediate- mass stars have a good S/N ratio without saturating. (b) F675W and F814W The same considerations for the other two filters lead to similar time requirements, since the total spacecraft time is dominated by the overheads. A grand total of 3 orbits are thus requested for this proposal. Background and foreground contamination of the CMD will be removed by using the data from the outer regions of the 3 WFC, and using the values of extinction determined from multi-color photometry. The effective radius of the cluster being ~ 30''^(10), a substantial fraction of the WF CCDs will include mostly field stars. Based on ground-based and HST data, this type of contamination is expected to be relatively small. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6763( 4) - 04/22/97 17:55 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 6763( 4) - 04/22/97 17:55 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC3603 STELLAR CLUSTER, OB RA=11H 15M 7.4S +/- 1',DEC=-61D 2000.0 V = 9.0 ASSOCIATION 15' 38" +/- 1',REGION B-V = +1.5 Comments: Value above is the total magnitude of the cluster; brightest individual stars have V=11.2, B-V=+1.5; faintest will have V=24.0 6763( 4) - 04/22/97 17:55 - [ 5] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC3603 WFPC2 IMAGE PC1-FIX F547M 3 1 S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC3603 WFPC2 IMAGE PC1-FIX F547M 12 10 S ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC3603 WFPC2 IMAGE PC1-FIX F547M 8 30 S ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC3603 WFPC2 IMAGE PC1-FIX F675W 3 0.4 S ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC3603 WFPC2 IMAGE PC1-FIX F675W 12 4 S ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC3603 WFPC2 IMAGE PC1-FIX F675W 8 20 S ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC3603 WFPC2 IMAGE PC1-FIX F814W 3 0.4 S ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC3603 WFPC2 IMAGE PC1-FIX F814W 12 5 S ------------------------------------------------------------------------------------------------------------------------------------ 9 NGC3603 WFPC2 IMAGE PC1-FIX F814W 8 20 S ------------------------------------------------------------------------------------------------------------------------------------ 6763( 4) - 04/22/97 17:55 - [ 6] Summary Form for Proposal 6763 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F547M F675W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC3603 ------------------------------------------------------------------------------------------------------------------------------------