6797( 5) - 04/22/97 15:28 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6797 Version: 5 Check-in Date: 22-Apr-1997 15:14:20 1.Proposal Title: The Very-Low-Mass Star Content of the Globular Cluster NGC 6397 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle STELLAR POPULATIONS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Ivan R. King University of California CoI: Adrienne M. Cool San Francisco State University N CoI: Jay Anderson University of California N CoI: William van Altena YaleUniversity N CoI: Giampaolo Piotto Universita di Padova N ------------------------------------------------------------------------------------ 6. Abstract Very few constraints exist on the population of very-low-mass stars in globular clusters. Observations with HST have come close to reaching the hydrogen-burning limit at the lower end of the main sequence in the globular cluster NGC 6397, the cluster with lowest distance modulus. Determination of the faintest part of the luminosity function is limited, however, by serious field-star contamination. This proposal is to take second-epoch images for determination of proper motions of individual faint stars. Tests of the positional accuracy that we now achieve show that over the time baseline that will be available by Cycle 6, separation of cluster members from field stars will be close to perfect. This will allow us to extend the measurable main-sequence luminosity function two magnitudes beyond its present limit, to I ~eq 25, or V ~eq 28, which corresponds to an absolute-magnitude of M_V ~eq 15.7. This will allow us to come closer than anyone ever has to the hydrogen- burning limit at the bottom of the main sequence. ------------------------------------------------------------------------------------ 6797( 5) - 04/22/97 15:28 - [ 2] Observations Description ------------------------ The second-epoch data would be taken with the F814W filter only, which is the filter for which our first-epoch material reaches the faintest stars. Additional observations in a second filter are not needed, as the existing V data already allow us to distinguish white dwarfs from red dwarfs for all stars detected in I. Since the majority of the stars in the first epoch were covered by 3 orbits' worth of exposure time, we propose 3 orbits again for the second epoch, in order to achieve the same astrometric accuracy. The area covered by our 1994 images is irregular, due to a series of shifts and two orientations, and covers an area equivalent to ~ 1.8 WFPC2 fields. Two pointings will be required to insure coverage of at least 85\\% of the area that our first-epoch observations encompass, irrespective of the spacecraft orientation. Given that the cluster LF may be falling, it is important to cover a maximum of the area of the existing observations, so that membership can be assessed for as many stars as possible. The exposures in each orbit will be divided into one short (80s) and three long exposures. Considering overhead times and the 57-minute visibility time for this cluster, the long exposures will each be >= 12 min long. The short exposure will be used to correct saturated centers of the many bright stars in the images, a step that is absolutely essential in deriving the accurate PSFs that are needed for the astrometry. Each orbit will have a CR- split, but the 3 orbits at each pointing will be dithered with respect to each other, to combat the undersampling of the WFC. We know from our previous observations that we can do accurate astrometry down to I=25 (as demonstrated in Fig.\\ 3). To maximize the time baseline, the observations would best be made late in Cycle 6. However, the project is feasible if carried out at any time in Cycle 6. Real Time Justification ----------------------- None. Calibration Justification ------------------------- ustification Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: ------------------------------------------------------------------------------------ 6797( 5) - 04/22/97 15:28 - [ 3] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6397-1 STELLAR CLUSTER, RA=17H 41M 00.83S +/- 0.1S, DEC= J2000 V = 5.25 Globular Cluster -53D 43' 55" +/- 1" B-V = 0.73 2 NGC6397-2 STELLAR CLUSTER, RA=17H 41M 00.41S +/- 0.1S, DEC= J2000 V = 5.25 Globular Cluster -53D 42' 29" +/- 1" B-V = 0.73 6797( 5) - 04/22/97 15:28 - [ 4] Visit: 01 Visit Requirements: ORIENT 345.7D TO 345.9D AFTER 1-JAN-97 On Hold Comments: Additional Comments: The orientation was chosen because it is the one at which we can get ideal coverage of our previous field. The target positions are tied to this orientation. Other periods in July and September could do nearly as well, but we would then have to change the orientation and the target positions. In addition, this proper-motion program needs a long time baseline; hence we need to be scheduled near the end of the Cycle. Please note that the duration of visibility peaks sharply near the center of the scheduling window, and the science would benefit from longer visibility. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6397-1 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 80 S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC6397-1 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 1 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC6397-1 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 1 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC6397-1 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 700 S SAME POS AS 1 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC6397-1 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 80 S POS TARG +0.400,+0.400 ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC6397-1 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 5 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC6397-1 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 5 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC6397-1 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 5 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 9 NGC6397-1 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 80 S POS TARG -0.400,-0.400 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC6397-1 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 9 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC6397-1 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 9 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC6397-1 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 9 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 13 NGC6397-2 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 80 S ------------------------------------------------------------------------------------------------------------------------------------ 14 NGC6397-2 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 13 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC6397-2 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 13 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 16 NGC6397-2 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 13 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 17 NGC6397-2 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 80 S POS TARG +0.400,+0.400 ------------------------------------------------------------------------------------------------------------------------------------ 18 NGC6397-2 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 17 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 19 NGC6397-2 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 17 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC6397-2 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 17 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 6797( 5) - 04/22/97 15:28 - [ 5] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 21 NGC6397-2 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 80 S POS TARG -0.400,-0.400 ------------------------------------------------------------------------------------------------------------------------------------ 22 NGC6397-2 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 21 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 23 NGC6397-2 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 21 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 24 NGC6397-2 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 800 S SAME POS AS 21 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 6797( 5) - 04/22/97 15:28 - [ 6] Summary Form for Proposal 6797 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 345.7D TO 345.9D AFTER 1-JAN-97 SAME POS AS 1 EXPAND POS TARG +0.400,+0.400 SAME POS AS 5 EXPAND POS TARG -0.400,-0.400 SAME POS AS 9 EXPAND SAME POS AS 13 EXPAND SAME POS AS 17 EXPAND SAME POS AS 21 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6397-1 NGC6397-2 ------------------------------------------------------------------------------------------------------------------------------------