14688( 2) - 14-Jul-2016 17:40:41 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 14688 Version: 2 Check-in Time: 14-Jul-2016 17:40:41 STScI Edit Number: 0 Title Extended Star Formation or a Range of Stellar Rotation Velocities? The Nature of Extended Main Sequence Turnoffs in Intermediate-Age Star Clusters ------------------------------------------------------------------------------------ Type Cycle GO 24 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Paul Goudfrooij Space Telescope Science Institute Y CoI: Dr. Matteo Correnti Space Telescope Science Institute N CoI: Dr. Leo Girardi Osservatorio Astronomico di Padova N CoI: Dr. Thomas H. Puzia Pontificia Universidad Catolica de Chile N ------------------------------------------------------------------------------------ Abstract Recently, deep color-magnitude diagrams (CMDs) from HST data revealed that several massive intermediate-age star clusters in the Magellanic Clouds exhibit extended main-sequence turn-offs (eMSTOs), and in some cases also dual red clumps. This poses serious questions regarding the mechanisms responsible for the formation of massive star clusters and their well-known light-element abundance variations. The nature of eMSTOs is currently a hotly debated topic of study. Several recent studies indicate that the eMSTOs are caused by an age spread of about 100-500 Myr among cluster stars, while other studies indicate that eMSTOs can be caused by a coeval population in which the relevant stars span a range of rotation velocities. Formal evidence to (dis-)prove either scenario still remains at large, mainly because the available stellar tracks that incorporate the effects of rotation are only available for masses > 1.7 Msun whereas the stars in the known eMSTOs of intermediate-age clusters are less massive. To circumvent this issue, we identified a massive star cluster in the Large Magellanic Cloud (LMC) that has the right dynamical properties to host an eMSTO along with an age at which the effects of age spreads to CMD morphology are substantially different from those of spreads of rotation rates: the ~600 Myr old cluster NGC 1831. We propose to obtain deep WFC3/UVIS imaging with filters F336W and F814W to analyze the morphologies of the MSTO and upper MS regions of NGC 1831 at high precision and compare with model predictions. This will have a lasting impact on our understanding of the eMSTO phenomenon and of star cluster formation in general. ------------------------------------------------------------------------------------ Observations Description ------------------------ The target star cluster is observed with WFC3 using the F336W and F814W broad-band filters. We make sure we include one F814W 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. Hence we also include at least two longer exposures per filter to achieve deep photometry. At least two dither positions are used for each filter to allow a robust elimination of hot pixels and cosmic ray hits during the analysis. ACS is used in parallel to the primary WFC3 exposures, using the filters F435W and F814W. This will allow a robust determination of 14688( 2) - 14-Jul-2016 17:40:41 - [ 2] the field population contribution to the star cluster pointing, both in terms of the star distribution in the color-magnitude diagram and the overall radial distribution of cluster stars. Used the ORIENT special requirement to avoid bright stars just outside the WFC3 and ACS fields of view, which would cause scattered light issues. ------------------------------------------------------------------------------------ 14688( 2) - 14-Jul-2016 17:40:41 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC-1831 STELLAR CLUSTER, RA=05H06M16.2934S, J2000 V = 11.18 GLOBULAR CLUSTER DEC=-64D55'5.92" Reference Frame: Other: SIMBAD Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. ------------------------------------------------------------------------------------------------------------------------------------ 14688( 2) - 14-Jul-2016 17:40:41 - [ 4] Visit: 01 (NGC 1831 visit) Visit Requirements: SCHED 30%; ORIENT 332D TO 346 D; ORIENT 319D TO 328 D; ORIENT 303D TO 313 D; ORIENT 285D TO 291 D; ORIENT 269D TO 279 D; ORIENT 227D TO 245 D; ORIENT 214D TO 218 D; ORIENT 193D TO 201 D; ORIENT 168D TO 180 D; ORIENT 110D TO 130 D; ORIENT 81D TO 93 D; ORIENT 72D TO 75 D; ORIENT 32D TO 43 D; ORIENT 9D TO 11 D; ORIENT 0D TO 3 D; ORIENT 357D TO 359.9 D On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC-1831 WFC3/UVI ACCUM UVIS2 F814W FLASH=12 1 100 S (100 POS TARG 0.18, 0.18 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC-1831 WFC3/UVI ACCUM UVIS2 F814W 1 660 S (660 POS TARG 0., 0. S S) ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC-1831 WFC3/UVI ACCUM UVIS2 F336W FLASH=10 1 975 S (975 PAR 4 WITH 3 S S) ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY ACS/WFC ACCUM WFC F435W 1 830 S see below (890 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC-1831 WFC3/UVI ACCUM UVIS2 F336W FLASH=10 1 975 S (975 POS TARG 0.18, -2.414; S S) PAR 6 WITH 5 ------------------------------------------------------------------------------------------------------------------------------------ 6 ANY ACS/WFC ACCUM WFC F435W 1 850 S (850 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC-1831 WFC3/UVI ACCUM UVIS2 F336W FLASH=9 1 1115 S POS TARG 0.18, 0.18; PAR S (1115 S) 8 WITH 7 ------------------------------------------------------------------------------------------------------------------------------------ 8 ANY ACS/WFC ACCUM WFC F435W 1 980 S (980 S) ------------------------------------------------------------------------------------------------------------------------------------ 9 NGC-1831 WFC3/UVI ACCUM UVIS2 F336W FLASH=9 1 1115 S POS TARG 0.36, -2.234; S (1115 S) PAR 10 WITH 9 ------------------------------------------------------------------------------------------------------------------------------------ 10 ANY ACS/WFC ACCUM WFC F814W 1 950 S (950 S) ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC-1831 WFC3/UVI ACCUM UVIS2 F814W 1 720 S (720 POS TARG 0.18, -2.414; S S) PAR 12 WITH 11 ------------------------------------------------------------------------------------------------------------------------------------ 12 ANY ACS/WFC ACCUM WFC F814W 1 600 S (600 S) ------------------------------------------------------------------------------------------------------------------------------------ 14688( 2) - 14-Jul-2016 17:40:41 - [ 5] 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 | ------------------------------------------------------------------------------------------------------------------------------------ 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-1831 1 WFC3/UVI ACCUM UVIS2 F814W none none none none 1 N/A S NGC-1831 2 WFC3/UVI ACCUM UVIS2 F814W none none none none 1 N/A S NGC-1831 3 WFC3/UVI ACCUM UVIS2 F336W none none none none 1 N/A S ANY 4 ACS/WFC ACCUM WFC F435W none none none none 1 890.0 S NGC-1831 5 WFC3/UVI ACCUM UVIS2 F336W none none none none 1 N/A S ANY 6 ACS/WFC ACCUM WFC F435W none none none none 1 N/A NGC-1831 7 WFC3/UVI ACCUM UVIS2 F336W none none none none 2 N/A S ANY 8 ACS/WFC ACCUM WFC F435W none none none none 2 N/A NGC-1831 9 WFC3/UVI ACCUM UVIS2 F336W none none none none 2 N/A S ANY 10 ACS/WFC ACCUM WFC F814W none none none none 2 N/A NGC-1831 11 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S ANY 12 ACS/WFC ACCUM WFC F814W none none none none 2 N/A 14688( 2) - 14-Jul-2016 17:40:41 - [ 6] Summary Form for Proposal 14688 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2, WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=10, FLASH=12, FLASH=9 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 0D TO 3 D, ORIENT 110D TO 130 D, ORIENT 168D TO 180 D, ORIENT 193D TO 201 D, ORIENT 214D TO 218 D, ORIENT 227D TO 245 D, ORIENT 269D TO 279 D, ORIENT 285D TO 291 D, ORIENT 303D TO 313 D, ORIENT 319D TO 328 D, ORIENT 32D TO 43 D, ORIENT 332D TO 346 D, ORIENT 357D TO 359.9 D, ORIENT 72D TO 75 D, ORIENT 81D TO 93 D, ORIENT 9D TO 11 D, PAR 10 WITH 9, PAR 12 WITH 11, PAR 4 WITH 3, PAR 6 WITH 5, PAR 8 WITH 7, POS TARG 0., 0., POS TARG 0.18, -2.414, POS TARG 0.18, 0.18, POS TARG 0.36, -2.234, SCHED 30% ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F336W, F435W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ANY, NGC-1831 ------------------------------------------------------------------------------------------------------------------------------------