14174( 3) - 23-Sep-2015 16:15:29 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 14174 Version: 3 Check-in Time: 23-Sep-2015 16:15:29 STScI Edit Number: 0 Title Probing Extended Star Formation in the Young Massive Cluster NGC 1850 ------------------------------------------------------------------------------------ Type Cycle GO 23 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Paul Goudfrooij Space Telescope Science Institute Y CoI: Dr. Thomas H. Puzia Pontificia Universidad Catolica de Chile N CoI: Dr. Matteo Correnti Space Telescope Science Institute N CoI: Dr. Leo Girardi Osservatorio Astronomico di Padova N CoI: Dr. Jason S. Kalirai Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract Recently, deep HST images revealed that several massive intermediate-age star clusters (SCs) 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 SCs and their well-known light-element abundance variations. While many recent studies indicate that the eMSTOs are caused by a range in stellar age of about 100-500 Myr among cluster stars, the obvious and important question is: Why has this not been observed yet in SCs in that age range? Our recent studies suggest that prolonged star formation in SCs requires a cluster escape velocity in excess of about 15 km/s, and young SCs with such high escape velocities are extremely rare in the Local Group. We recently identified a SC in the Large Magellanic Cloud (LMC) that has the right mass and size to be able to retain mass loss in slow winds of the first stellar generation while being young enough to still host some of its second stellar generation (if present) on the pre-main sequence (PMS) and showing emission-line signatures: the ~90 Myr old cluster NGC 1850 in the LMC. Since this SC has no adequate HST imaging to detect an eMSTO or PMS stars associated with the cluster, we propose to obtain deep WFC3/UVIS broad-band imaging with filters F275W and F814W to analyze the MSTO region and use F656N imaging to search for Balmer-line emission signatures from PMS stars of the second stellar generation. The detection of an eMSTO and/or PMS stars in this SC (or a lack thereof) 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 F275W 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 14174( 3) - 23-Sep-2015 16:15:29 - [ 2] 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 F555W, F658N, and F814W. This will allow a robust determination of the field population contribution to the star cluster pointing. ------------------------------------------------------------------------------------ 14174( 3) - 23-Sep-2015 16:15:29 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC-1850 STELLAR CLUSTER, RA=05H08M45.7900S +/- 0.1", J2000 V = 8.96 GLOBULAR CLUSTER DEC=-68D45'39.79" +/- 0.1" Reference Frame: ICRS Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. ------------------------------------------------------------------------------------------------------------------------------------ 14174( 3) - 23-Sep-2015 16:15:29 - [ 4] Visit: 01 Visit Requirements: SCHED 30%; ORIENT 320D TO 359.9 D; ORIENT 50D TO 111 D; ORIENT 155D TO 202 D; ORIENT 233D TO 250 D On Hold Comments: Additional Comments: The visit orientation requirements are set to (a) avoid bright stars in or near the ACS parallels, (b) avoid catching another star cluster by the ACS parallels (and hence not sampling enough "background field" area), and (c) to still make sure NGC 1850B stays within the field of view of the prime WFC3 images. 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-1850 WFC3/UVI ACCUM UVIS2 F814W FLASH=12, BLADE=A 1 7 S (7 S) POS TARG 0.18, 0.18 S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC-1850 WFC3/UVI ACCUM UVIS2 F275W FLASH=12. 1 150 S (150 SAME POS AS 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC-1850 WFC3/UVI ACCUM UVIS2 F275W FLASH=12 1 780 S (780 PAR 4 WITH 3 S S) ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY ACS/WFC ACCUM WFC F555W 1 600 S see below (670 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC-1850 WFC3/UVI ACCUM UVIS2 F275W FLASH=12 1 790 S (790 POS TARG 0.18, -2.414; S S) PAR 6 WITH 5 ------------------------------------------------------------------------------------------------------------------------------------ 6 ANY ACS/WFC ACCUM WFC F555W 1 720 S see below (780 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC-1850 WFC3/UVI ACCUM UVIS2 F656N FLASH=12 1 900 S (900 SAME POS AS 5; PAR 8 S S) WITH 7 ------------------------------------------------------------------------------------------------------------------------------------ 8 ANY ACS/WFC ACCUM WFC F658N 1 730 S (730 S) ------------------------------------------------------------------------------------------------------------------------------------ 9 NGC-1850 WFC3/UVI ACCUM UVIS2 F814W FLASH=4 1 350 S (350 SAME POS AS 5; PAR 10 S S) WITH 9 ------------------------------------------------------------------------------------------------------------------------------------ 10 ANY ACS/WFC ACCUM WFC F814W 1 330 S (330 S) ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC-1850 WFC3/UVI ACCUM UVIS2 F656N FLASH=12 1 1050 S POS TARG 0, 0; PAR 12 S (1050 S) WITH 11 ------------------------------------------------------------------------------------------------------------------------------------ 12 ANY ACS/WFC ACCUM WFC F658N 1 900 S see below (1014 S) 14174( 3) - 23-Sep-2015 16:15:29 - [ 5] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------------------------------------------ 13 NGC-1850 WFC3/UVI ACCUM UVIS2 F656N FLASH=12 1 925 S (925 POS TARG 0.18, 0.18; PAR S S) 14 WITH 13 ------------------------------------------------------------------------------------------------------------------------------------ 14 ANY ACS/WFC ACCUM WFC F658N 1 850 S see below (902 S) ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC-1850 WFC3/UVI ACCUM UVIS2 F814W FLASH=4 1 440 S (440 SAME POS AS 3; PAR 16 S S) WITH 15 ------------------------------------------------------------------------------------------------------------------------------------ 16 ANY ACS/WFC ACCUM WFC F814W 1 340 S (340 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-1850 1 WFC3/UVI ACCUM UVIS2 F814W none none none none 1 N/A S NGC-1850 2 WFC3/UVI ACCUM UVIS2 F275W none none none none 1 N/A S NGC-1850 3 WFC3/UVI ACCUM UVIS2 F275W none none none none 1 N/A S ANY 4 ACS/WFC ACCUM WFC F555W none none none none 1 670.0 S NGC-1850 5 WFC3/UVI ACCUM UVIS2 F275W none none none none 1 N/A S ANY 6 ACS/WFC ACCUM WFC F555W none none none none 1 780.0 S NGC-1850 7 WFC3/UVI ACCUM UVIS2 F656N none none none none 1 N/A S ANY 8 ACS/WFC ACCUM WFC F658N none none none none 1 N/A NGC-1850 9 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S ANY 10 ACS/WFC ACCUM WFC F814W none none none none 2 N/A NGC-1850 11 WFC3/UVI ACCUM UVIS2 F656N none none none none 2 N/A S ANY 12 ACS/WFC ACCUM WFC F658N none none none none 2 1014.0 S NGC-1850 13 WFC3/UVI ACCUM UVIS2 F656N none none none none 2 N/A S ANY 14 ACS/WFC ACCUM WFC F658N none none none none 2 902.0 S NGC-1850 15 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S ANY 16 ACS/WFC ACCUM WFC F814W none none none none 2 N/A 14174( 3) - 23-Sep-2015 16:15:29 - [ 6] Summary Form for Proposal 14174 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2, WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BLADE=A, FLASH=12, FLASH=12., FLASH=4 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 155D TO 202 D, ORIENT 233D TO 250 D, ORIENT 320D TO 359.9 D, ORIENT 50D TO 111 D, PAR 10 WITH 9, PAR 12 WITH 11, PAR 14 WITH 13, PAR 16 WITH 15, 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, SAME POS AS 1, SAME POS AS 3, SAME POS AS 5, SCHED 30% ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F275W, F555W, F656N, F658N, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ANY, NGC-1850 ------------------------------------------------------------------------------------------------------------------------------------