6833( 4) - 07/13/97 20:57 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6833 Version: 4 Check-in Date: 13-Jul-1997 20:55:54 1.Proposal Title: A Cepheid Search in the Virgo Cluster Galaxy NGC 4571 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle COSMOLOGY GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: MICHAEL PIERCE INDIANA UNIVERSITY CoI: Douglas Welch McMaster University Y CoI: Robert McClure Dominion Astrophysical Observatory N CoI: Sidney van den Bergh Dominion Astrophysical Observatory N CoI: Peter Stetson Dominion Astrophysical Observatory N CoI: Rene Racine University of Montreal N ------------------------------------------------------------------------------------ 6. Abstract We propose to undertake a survey for Cepheid Variables in the Virgo Cluster galaxy NGC 4571 using WFPC2. The determination of the distance to Virgo is key to establishing the value of the H_zero given that the relative distances between more distant clusters and Virgo are now established. Cepheids are arguably the most precise and well-tested method for estimating extragalactic distances. Unfortunately, the existence of a significant line-of-sight depth in the spiral population of Virgo means that a distance to a randomly chosen spiral does not necessarily establish the distance to the core of ellipticals which define the cluster. However, there is strong circumstantial evidence that places some of the Virgo spirals at or very near the core. NGC 4571 is located less than 2.5 degrees from M 87 and is severely stripped of its H I envelope, providing strong evidence for it being in the core, particularly given that the spirals with significant stripping are all within 3 degrees of M 87. In addition, its velocity is 1.5 Sigma below the mean of the cluster. Such galaxies are predicted to be moving very nearly along the line-of-sight and very near the core of the cluster. We plan to obtain 12 epochs of F814W imaging and 4 epochs with F555W. This should result in ~ 20 Cepheids with minimal crowding and allow for a determination of both the distance and internal extinction. The resulting distance should be known to +- 10%. Such a measurement will significantly reduce the uncertainty in the Virgo distance and will be far more effective than observing several randomly chosen spirals spread over the Virgo region. ------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 2] Observations Description ------------------------ We plan to obtain 12 epochs of I-band imaging (F814W) and 4 epochs of V-band imaging (F555W) for NGC 4571. These data will require single orbit visits for 8 of 12 epochs with the remaining 4 visits requiring 2 orbits each. The latter are necessary to estimate the internal reddening. The somewhat unconventional approach of concentrating on the I-band, rather than V, for period determination is based upon a detailed examination of the sources of error in the resulting Cepheid based distance (particularly the uncertainty in reddening). With CR-splitting and the overhead for guide star acquisition and WFPC2 readout we should obtain 2 exposures of 21 min over each orbit (and constituting a single epoch). This will enable stellar photometry with a precision of ~ 0.15 mag at V = 26.5. By obtaining time-critical observations (see below) we should be able to minimize aliasing for periods of between 10 and 55 days with only 12 epochs of observations and discover ~ 20 Cepheids. Real Time Justification ----------------------- We require time-critical observations to provide observations with a broad range of spacings in time in order to minimize the aliasing in our period determination. We have applied an optimization scheme which takes beginning and ending times as well as period interval and minimizes the power over this observational window. Such an approach has significantly power (i.e.aliasing) than a simple power-law distribution. By far our strongest observational constraint is to acquire observations at a fixed roll-angle over as large a range in time as possible and at the same time avoid a V=8 star located approx. 3 arcmin NE of field center. We propose to begin the observational sequence on 2 May 1996 +/- 1 day with the first V and I exposures, followed with observations spaced at 6.0 +/- 0.2 days (V+I), 11.0 +/- 0.2 days, 15.3 +/- 0.2 days, 19.9 +/- 0.3 days, 21.9 +/- 0.3 days (V+I), 25.8 +/- 0.3 days, 30.2 +/- 0.3 days, 34.6 +/- 0.3 days (V+I), 40.1 +/- 0.3 days, 47.8 +/- 0.3 days, and 55.0 +/- 0.3 days. Special orientation: because of a nearby bright star, and the undersampling of WF CCDs, the position and orientation of the observations must be fixed for the entire exposure sequence. The length of the cycle (set by the need to sample the longest period Cepheids), and the roll constraints, allow for only a few days tolerance in the start of a sequence. The phasing requirements of the observations mean that the entire observational sequence must be completed once they have begun. We have already established a ground- based calibration of the field to be observed using HRCam on the CFHT in seeing with FWHM <= 0.50 arcsec. Additional checks of our calibration are planned via observations with the WIYN telescope at KPNO and with the Adaptive Optics Bonnette (AOB) on the CFHT when it becomes available. We are also proposing for time on AOB to obtain H-band photometry of some Cepheids we find in NGC 4571. AOB requires a bright star within about 30 arcsec of its field center in order to produce an estimated FWHM = 0.10 arcsec. The bright star NE of NGC 4571, and a working AOB, should allow a high-precision distance to NGC 4571 to be obtained without any concerns regarding extinction. NOTE: The expected start date has been updated from the above. It is now expected that the observations may take place around the period between 98.123 and 98.178 or so. The ORIENT angle has been updated also to the initial range of U3 ORIENT = 105.00D TO 118.00D, with all subsequent data being taken at the SAME ORIENT as whatever the first set turns out to be. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: Indiana University*Department of Astronomy*Swain West Rm 319*Bloomington, IN 47405 Ship Via: UPS Email: ------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 GALAXY, Spiral RA = 12H 36M 56.44S +/- 0.3", 2000 V = 348 SURF(V) = 24.5 DEC = +14D 13' 02.5" +/- 0.3", B-V = 0.51 PLATE-ID=00G0 Comments: Single target for proposal 6833( 4) - 07/13/97 20:57 - [ 5] Visit: 01 Visit Requirements: ORIENT 105.00D TO 118.00D On Hold Comments: Additional Comments: Assumes that observations are done over period of 55 days between 98.123 - 98.178. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 WFPC2 IMAGE WFALL-F F814W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2500 S END ORBIT IX ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4571 WFPC2 IMAGE WFALL-F F555W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2600 S SAME POS AS 1 END ORBIT IX ------------------------------------------------------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 6] Visit: 02 Visit Requirements: AFTER 1 BY 5.80D TO 6.20D SAME ORIENT AS 1 On Hold Comments: Additional Comments: Return as close as possible to position of visit 1. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 WFPC2 IMAGE WFALL-F F814W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2500 S POS TARG +0.3 , +0.0 END IX ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4571 WFPC2 IMAGE WFALL-F F555W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2600 S SAME POS AS 1 END ORBIT IX ------------------------------------------------------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 7] Visit: 03 Visit Requirements: AFTER 1 BY 10.80D TO 11.20D SAME ORIENT AS 1 On Hold Comments: Additional Comments: Return as close as possible to position of visit 1 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 WFPC2 IMAGE WFALL-F F814W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2500 S POS TARG +0.3 , +0.3 END IX ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 8] Visit: 04 Visit Requirements: AFTER 1 BY 15.10D TO 15.50D SAME ORIENT AS 1 On Hold Comments: Additional Comments: Return as close as possible to position of visit 1 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 WFPC2 IMAGE WFALL-F F814W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2500 S POS TARG +0.0 , +0.3 END IX ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 9] Visit: 05 Visit Requirements: AFTER 1 BY 19.60D TO 19.90D SAME ORIENT AS 1 On Hold Comments: Additional Comments: Return as close as possible to position of visit 1 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 WFPC2 IMAGE WFALL-F F814W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2500 S POS TARG -0.3 , +0.3 END IX ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 10] Visit: 06 Visit Requirements: AFTER 1 BY 21.60D TO 22.20D SAME ORIENT AS 1 On Hold Comments: Additional Comments: Return as close as possible to position of visit 1 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 WFPC2 IMAGE WFALL-F F814W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2500 S POS TARG -0.3, +0.0 END IX ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4571 WFPC2 IMAGE WFALL-F F555W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2600 S SAME POS AS 1 END ORBIT IX ------------------------------------------------------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 11] Visit: 07 Visit Requirements: AFTER 1 BY 25.50D TO 26.10D SAME ORIENT AS 1 On Hold Comments: Additional Comments: Return as close as possible to position of visit 1 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 WFPC2 IMAGE WFALL-F F814W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2500 S POS TARG -0.3 , -0.3 END IX ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 12] Visit: 08 Visit Requirements: AFTER 1 BY 29.90D TO 30.50D SAME ORIENT AS 1 On Hold Comments: Additional Comments: Return as close as possible to position of visit 1 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 WFPC2 IMAGE WFALL-F F814W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2500 S POS TARG +0.0 , -0.3 END IX ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 13] Visit: 09 Visit Requirements: AFTER 1 BY 34.30D TO 34.90D SAME ORIENT AS 1 On Hold Comments: Additional Comments: Return as close as possible to position of visit 1 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 WFPC2 IMAGE WFALL-F F814W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2500 S POS TARG +0.3 , -0.3 END IX ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4571 WFPC2 IMAGE WFALL-F F555W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2600 S SAME POS AS 1 END ORBIT IX ------------------------------------------------------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 14] Visit: 10 Visit Requirements: AFTER 1 BY 39.80D TO 40.40D SAME ORIENT AS 1 On Hold Comments: Additional Comments: Return as close as possible to position of visit 1 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 WFPC2 IMAGE WFALL-F F814W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2500 S POS TARG +0.4 , +0.0 END IX ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 15] Visit: 11 Visit Requirements: AFTER 1 BY 47.50D TO 48.10D SAME ORIENT AS 1 On Hold Comments: Additional Comments: Return as close as possible to position of visit 1 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 WFPC2 IMAGE WFALL-F F814W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2500 S POS TARG +0.0 , +0.4 END IX ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 16] Visit: 12 Visit Requirements: AFTER 1 BY 54.70D TO 55.30D SAME ORIENT AS 1 On Hold Comments: Additional Comments: Return as close as possible to position of visit 1 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4571 WFPC2 IMAGE WFALL-F F814W CR-SPLIT = 0.5,CR-TOLERANCE = 0.1 1 2500 S POS TARG -0.4 , +0.0 END IX ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 6833( 4) - 07/13/97 20:57 - [ 17] Summary Form for Proposal 6833 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT = 0.5 CR-TOLERANCE = 0.1 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category COSMOLOGY ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 105.00D TO 118.00D END ORBIT SAME POS AS 1 END ORBIT AFTER 1 BY 5.80D TO 6.20D SAME ORIENT AS 1 POS TARG +0.3 , +0.0 END ORBIT AFTER 1 BY 10.80D TO 11.20D SAME ORIENT AS 1 POS TARG +0.3 , +0.3 END ORBIT AFTER 1 BY 15.10D TO 15.50D SAME ORIENT AS 1 POS TARG +0.0 , +0.3 END ORBIT AFTER 1 BY 19.60D TO 19.90D SAME ORIENT AS 1 POS TARG -0.3 , +0.3 END ORBIT AFTER 1 BY 21.60D TO 22.20D SAME ORIENT AS 1 POS TARG -0.3, +0.0 END ORBIT AFTER 1 BY 25.50D TO 26.10D SAME ORIENT AS 1 POS TARG -0.3 , -0.3 END ORBIT AFTER 1 BY 29.90D TO 30.50D SAME ORIENT AS 1 POS TARG +0.0 , -0.3 END ORBIT AFTER 1 BY 34.30D TO 34.90D SAME ORIENT AS 1 POS TARG +0.3 , -0.3 END ORBIT AFTER 1 BY 39.80D TO 40.40D SAME ORIENT AS 1 POS TARG +0.4 , +0.0 END ORBIT AFTER 1 BY 47.50D TO 48.10D SAME ORIENT AS 1 POS TARG +0.0 , +0.4 END ORBIT AFTER 1 BY 54.70D TO 55.30D SAME ORIENT AS 1 POS TARG -0.4 , +0.0 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W F555W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC4571 ------------------------------------------------------------------------------------------------------------------------------------