13011( 2) - 10-Jul-2012 18:20:48 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 13011 Version: 2 Check-in Time: 10-Jul-2012 18:20:48 STScI Edit Number: 0 Title Searching for the H-alpha 'Smoking Gun' of Prolonged Star Cluster Formation ------------------------------------------------------------------------------------ Type Cycle GO 20 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Thomas H. Puzia Pontificia Universidad Catolica de Chile Y CoI: Dr. Paul Goudfrooij Space Telescope Science Institute N CoI: Dr. Selma E. de Mink Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract Recently, deep HST images provided conclusive evidence that several massive intermediate-age star clusters (SC) in the LMC and SMC present extended main-sequence turn-offs (eMSTOs), and in some cases also dual red clumps. This poses serious questions regarding the mechanisms responsible for SC formation. While most recent studies indicate that the eMSTOs are caused by a range in stellar age of about 150-500 Myr among cluster stars, the obvious and important question is: Why has this not been observed yet in SCs in that age range? Our calculations show that prolonged star formation in SCs requires a cluster escape velocity in excess of about 10 km/s, and there really is only one SC in the Local Group that has the right mass and size to be able to retain mass loss in slow winds of the first stellar generation and be young enough to have some of its second stellar generation being still on the pre-main sequence (PMS) and showing emission-line signatures: the 300 Myr old cluster NGC 1856 in the LMC. Since this SC has no adequate HST imaging in the archive to detect an eMSTO, we propose to obtain deep broad-band imaging in F438W, F555W, F814W with WFC3/UVIS to analyze the MSTO and use F656N to search for Balmer-line emission signatures from PMS stars of the secondary stellar generation. The detection of the PMS phase in this cluster (or lack thereof) will have a lasting impact on our understanding of the eMSTO phenomenon. This SC represents a unique opportunity to render insights on the relative importance of various types of polluter stars of the first generation that have been proposed to be responsible for the chemical enrichment of second-generation stars in star clusters. ------------------------------------------------------------------------------------ Observations Description ------------------------ The target star cluster is observed with WFC3 using the F438W, F555W, and F814W broad-band filters and the F656N narrow-band Halpha filter. For the broad-band filters, we make sure we include one exposure that does not saturate the brightest stars expected from its age mentioned in the literature. On the other hand, we need color uncertainties less than 0.05 mag down to well beyond the estimated magnitude of the main sequence turnoff to achieve an accurate age determination and to enable a robust identification of multiple or broad turnoff structures if the target cluster in question is indeed found to host multiple stellar populations. Hence we also include at least two longer exposures per filter to achieve deep photometry. At least two dither positions are used for each target star cluster to allow a robust elimination of hot pixels and cosmic ray hits during the analysis. ACS is used in parallel to the 13011( 2) - 10-Jul-2012 18:20:48 - [ 2] primary WFC3 exposures, using the filters F555W, F658N, and F814W. This will allow a robust determination of the field population contribution to the star cluster pointing. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 13011( 2) - 10-Jul-2012 18:20:48 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC-1856 STELLAR CLUSTER, RA=05H09M30.3900S +/- 0.1", J2000 V = 10.06 GLOBULAR CLUSTER DEC=-69D07'45.40" +/- 0.1" Reference Frame: ICRS Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. ------------------------------------------------------------------------------------------------------------------------------------ 13011( 2) - 10-Jul-2012 18:20:48 - [ 4] Visit: 01 Visit Requirements: SCHED 30%; ORIENT 0D TO 87 D; ORIENT 205D TO 359.9 D On Hold Comments: Additional Comments: The visit orientation requirements are set to avoid a bright star in the WFC3 field of view. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC-1856 WFC3/UVI ACCUM UVIS1 F814W 1 51 S POS TARG -0.3, -0.3 S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC-1856 WFC3/UVI ACCUM UVIS1 F438W 1 430 S SAME POS AS 1 S ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC-1856 WFC3/UVI ACCUM UVIS1 F438W 1 430 S POS TARG 0.18, 2.414 S ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC-1856 WFC3/UVI ACCUM UVIS1 F814W 1 360 S POS TARG .18, 2.414; PAR S 5 WITH 4 ------------------------------------------------------------------------------------------------------------------------------------ 5 ANY ACS/WFC ACCUM WFC F555W 1 280 S ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC-1856 WFC3/UVI ACCUM UVIS1 F656N 1 735 S POS TARG 0.0, null; PAR S 7 WITH 6 ------------------------------------------------------------------------------------------------------------------------------------ 7 ANY ACS/WFC ACCUM WFC F555W 1 630 S see below ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC-1856 WFC3/UVI ACCUM UVIS1 F814W 1 450 S POS TARG 0.0, null; PAR S 9 WITH 8 ------------------------------------------------------------------------------------------------------------------------------------ 9 ANY ACS/WFC ACCUM WFC F814W 1 350 S ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC-1856 WFC3/UVI ACCUM UVIS1 F438W 1 185 S POS TARG 0.0, null S ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC-1856 WFC3/UVI ACCUM UVIS1 F555W 1 350 S S ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC-1856 WFC3/UVI ACCUM UVIS1 F555W 1 350 S POS TARG 0.18, 2.414; S PAR 13 WITH 12 ------------------------------------------------------------------------------------------------------------------------------------ 13 ANY ACS/WFC ACCUM WFC F814W 1 280 S see below ------------------------------------------------------------------------------------------------------------------------------------ 14 NGC-1856 WFC3/UVI ACCUM UVIS1 F656N 1 940 S POS TARG 0.18, 2.414; S PAR 15 WITH 14 ------------------------------------------------------------------------------------------------------------------------------------ 15 ANY ACS/WFC ACCUM WFC F658N 1 850 S 13011( 2) - 10-Jul-2012 18:20:48 - [ 5] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------------------------------------------ 16 NGC-1856 WFC3/UVI ACCUM UVIS1 F656N 1 940 S POS TARG -0.3, -0.3; PAR S 17 WITH 16 ------------------------------------------------------------------------------------------------------------------------------------ 17 ANY ACS/WFC ACCUM WFC F658N 1 850 S ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ NGC-1856 1 WFC3/UVI ACCUM UVIS1 F814W none none none none 1 N/A S NGC-1856 2 WFC3/UVI ACCUM UVIS1 F438W none none none none 1 N/A S NGC-1856 3 WFC3/UVI ACCUM UVIS1 F438W none none none none 1 N/A S NGC-1856 4 WFC3/UVI ACCUM UVIS1 F814W none none none none 1 N/A S ANY 5 ACS/WFC ACCUM WFC F555W none none none none 1 N/A NGC-1856 6 WFC3/UVI ACCUM UVIS1 F656N none none none none 1 N/A S ANY 7 ACS/WFC ACCUM WFC F555W none none none none 1 742.0 S NGC-1856 8 WFC3/UVI ACCUM UVIS1 F814W none none none none 1 N/A S ANY 9 ACS/WFC ACCUM WFC F814W none none none none 1 N/A NGC-1856 10 WFC3/UVI ACCUM UVIS1 F438W none none none none 2 N/A S NGC-1856 11 WFC3/UVI ACCUM UVIS1 F555W none none none none 2 N/A S NGC-1856 12 WFC3/UVI ACCUM UVIS1 F555W none none none none 2 N/A S ANY 13 ACS/WFC ACCUM WFC F814W none none none none 2 339.0 S NGC-1856 14 WFC3/UVI ACCUM UVIS1 F656N none none none none 2 N/A S ANY 15 ACS/WFC ACCUM WFC F658N none none none none 2 N/A NGC-1856 16 WFC3/UVI ACCUM UVIS1 F656N none none none none 2 N/A S ANY 17 ACS/WFC ACCUM WFC F658N none none none none 2 N/A 13011( 2) - 10-Jul-2012 18:20:48 - [ 6] Summary Form for Proposal 13011 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS1, WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 0D TO 87 D, ORIENT 205D TO 359.9 D, PAR 13 WITH 12, PAR 15 WITH 14, PAR 17 WITH 16, PAR 5 WITH 4, PAR 7 WITH 6, PAR 9 WITH 8, POS TARG -0.3, -0.3, POS TARG .18, 2.414, POS TARG 0.0, null, POS TARG 0.18, 2.414, SAME POS AS 1, SCHED 30% ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F438W, F555W, F656N, F658N, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ANY, NGC-1856 ------------------------------------------------------------------------------------------------------------------------------------