8630( 6) - 09/17/02 14:19 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8630 Version: 6 Check-in Date: 17-Sep-2002 14:18:31 1.Proposal Title: The Deepest Far-UV Imaging Survey of Globular Clusters: NGC 6752 and NGC 6397 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Michael Shara American Museum of Natural History CoI: Mr. David Zurek Space Telescope Science Institute N CoI: Dr. Christian Knigge Columbia University N ------------------------------------------------------------------------------------ 5. Abstract A few very close binaries can drive the dynamical evolution of an entire globular cluster. We propose the deepest UV imagery ever obtained on globular clusters to search for cataclysmic variables (CVs). CVs should be relatively easy to find in globular cores with HST, but there is a remarkable dearth of detected CVs in globular clusters relative to the large numbers predicted by tidal capture theory. This calls into serious doubt all of our understanding of globular cluster dynamical evolution. If most CVs in globular clusters are much fainter than canonical classical and dwarf novae, then tidal capture theory (and our claim to understand cluster evolution) can be salvaged. We propose to image the globular clusters NGC 6752 and NGC 6397 in the passband where CVs emit most of their radiation: the far UV, and at Lyalpha. Using the FUV-MAMA detector on STIS, we will reach an equivalent optical limiting magnitude of M_V = 13 at S/N=10 which is sensitive enough to detect even the faintest known CVs. If few or no faint CVs are found, then theorists will have run out of phase space and simple tidal capture theory will be shown to have made an incorrect prediction. This would force a major revision in our theory of tidal capture, and our understanding of globular cluster dynamical evolution. ------------------------------------------------------------------------------------ 8630( 6) - 09/17/02 14:19 - [ 2] Observations Description ------------------------ We propose to carry out a far-UV imaging survey of the globular clusters NGC 6752 and NGC 6397 for a total of 10 HST/CVZ orbits. We will use the F25MAMA (Clear) filter during dark time to reduce geocoronal Lyman alpha and OI, and the F25SrF2 during bright time. This will not only provide the deepest possible FUV image, it will allow us to construct a ``pseudo filter'' Lyman alpha. A short, ~2 minute, STIS/CCD image will be taken to determine the positions of the FUV objects in the optical by matching the STIS/CCD image with the deep (V_lim=26) WFPC2 archival images of NGC 6752 and NGC 6397. NGC 6752 and NGC 6397 are ideally suited for this proposal for several reasons (basic data on GCs taken from Harris 1996): (1) They lie near the heart of the CVZ, allowing the most efficient use of HST time. (2) They provide the best combination of reddening and distance for a FUV imaging study; E(B-V)=0.04, A_UV=0.28 and (m- M)_0 = 13.08 for 6752, E(B-V)=0.18, A_UV=1.26 and (m-M)_0 = 12.31 for 6397. (Note: A_UV~7E(B-V) Mihalas & Binney 1981, fig 3-28.) (3) NGC 6752 has a core radius of R_c=10", so the entire core will fit into the STIS/MAMA field of view (25"X25"). (See Fig. 2, which also emphasizes the power of STIS in the UV.) NGC 6397 has R_c =2.5" so it, too, comfortably fits in the STIS FOV. (4) Deep WFPC2 F555W images already exist in the archive for both clusters. We have determined that we will reach a limiting magnitude of M_V = 13.0 at a S/N = 10 in 26600 seconds for several prototypical CVs (U Gem, SS Syg, AM Her and DQ Her) at the distance and reddening of both clusters. This requires 5 CVZ orbits per cluster including guide star acquisition. (The short CCD observations and overheads are taken into account.) CVs with M_FUV = 11.0 will have a S/N = 10 in a single 10 minute exposure. Thus we will be able to search for variability for any CVs brighter than this. The nature of the MAMA detectors allows us to combine all the 10 minute exposures into one very deep image with essentially no read noise penalty to S/N. We have used existing WFPC2 and pre-COSTAR WFPC/F336W and FOC/F220W images of both clusters to verify that our observations will not violate either global or local STIS/MAMA brightness limits. The brightest pixel of a BHB (the brightest stars in the FUV) will have 33 counts/sec which is well below the bright limit of 100 counts/sec. The global bright limit of 200000 counts/sec would only be violated if there were in excess of 100 BHBs in the field of view of the STIS/MAMA. There are 20 or fewer BHBs in the STIS/MAMA fields of view of NGC 6752 and NGC 6397. Real Time Justification ----------------------- Both clusters lie in the heart of the continuous viewing zone (CVZ). We have therefore performed our exposure time calculations on the basis of a CVZ visibility of 96 min/orbit. There are a sufficient total number of CVZ orbits and scheduling opportunities available at the positions of the clusters for us to easily carry out our observing program. We have not scheduled separate visits for MAMA and CCD exposures, since only 2 minutes will be needed to obtain the CCD images we require. This strategy has been cleared with the STIS instrument team, and the necessary overheads have been accounted for. To take full advantage of the FUV filter set we request to schedule 25MAMA observations during dark time (when geocoronal lines are at a minimum) and F25SRF2 during bright time blocking geocoronal LyAlpha. The sum of all the data will provide the deepest FUV image ever and the difference will result in a pseudo filter centered on LyAlpha. There exists in the HST archive WFPC2 images, F555W (V band) and F814W (I band), of the cores of both clusters which when summed, reach a V magnitude of 26 with a S/N=10. These images will allow us to identify and/or place limiting magnitudes in the optical for the FUV objects detected. The UV-V and UV-I CMDs will allow us to differentiate the WD cooling sequence 8630( 6) - 09/17/02 14:19 - [ 3] from most CVs in the cluster core (see figure 4); the pseudo Ly-alpha image, as well as variability will further differentiate WD dominated CVs from WDs. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8630( 6) - 09/17/02 14:19 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6752 STELLAR CLUSTER, RA=19H 10M 51.78S +/- 0.1", J2000 V = - V = 17.0+/-0.5 GLOBULAR CLUSTER DEC=-59D 58' 54.7" +/- 0.1" 27.4 E(B-V) = 0.04 B-V = -0.2 U-B = -0.7 Coordinate Source: Other_Source: Harris Catalog 2 NGC6397 STELLAR CLUSTER, RA=17H 40M 41.36S +/- 0.1", J2000 V = 18.9 E(B-V) = 0.18 GLOBULAR CLUSTER DEC=-53D 40' 25.3" +/- 0.1" V = 14+/-0.5 B-V = 0.1 Coordinate Source: Other_Source: Harris Catalog 8630( 6) - 09/17/02 14:19 - [ 5] Visit: 01 Visit Priority: Visit Requirements: PCS MODE FINE CVZ On Hold Comments: Additional Comments: I have set up this visit such that it will execute with in the calendar given for day 089. There is about 60 seconds that things can slip by. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC6752 STIS/C ACCUM 50CCD MIRROR GAIN=1,CR-SPLIT=3 1 180 S CD Comments: This set of exposures is needed at the beginning of the 5 orbit CVZ visit to match the observations with WFPC2 observations already in the archive. ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC6752 STIS/C ACCUM F28X50L MIRROR GAIN=1,CR-SPLIT=3 1 180 S CD P Comments: This set of exposures is needed at the beginning of the 5 orbit CVZ visit to match the observations with WFPC2 observations already in the archive. ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC6752 STIS/F ACCUM F25QTZ MIRROR 3 900 S UV-MAM A Comments: These exposures should occur immediately following the CCD images of exposures 10 and 15. The start time should be 02:55:14 and should finish at 03:43:53. ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC6752 STIS/F ACCUM 25MAMA MIRROR 2 1000 S UV-MAM A Comments: This observation should be done in shadow. Shadow is at 03:44:14. This observation is set to start at 03:44:13 and end at 04:19:39. ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC6752 STIS/F ACCUM F25QTZ MIRROR 5 650 S UV-MAM A Comments: This is a out of shadow exposure. It's start time is 04:19:59 and it's end time is 05:19:24. ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC6752 STIS/F ACCUM 25MAMA MIRROR 2 1000 S UV-MAM A Comments: This exposure should be taken in shadow (near shadow). It has a start time of 05:19:44 and an end time of 05:55:10. (shadow goes from 05:20:36 to 05:55:57) ------------------------------------------------------------------------------------------------------------------------------------ 8630( 6) - 09/17/02 14:19 - [ 6] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 60 NGC6752 STIS/F ACCUM F25QTZ MIRROR 5 650 S UV-MAM A Comments: This exposure is to occur out of shadow. It has a start time of 05:55:30 and an end time of 06:54:55. ------------------------------------------------------------------------------------------------------------------------------------ 70 NGC6752 STIS/F ACCUM 25MAMA MIRROR 2 1000 S UV-MAM A Comments: This exposure should occur in shadow or near shadow. It has a start time of 06:55:15 and an end time of 07:30:41. (Shadow is from 06:56:58 to 07:32:18) ------------------------------------------------------------------------------------------------------------------------------------ 80 NGC6752 STIS/F ACCUM F25SRF2 MIRROR 5 670 S UV-MAM A Comments: This exposure is to be done during non-shadow time. The exposure is set to begin at 07:31:01 and end at 08:32:06. ------------------------------------------------------------------------------------------------------------------------------------ 90 NGC6752 STIS/F ACCUM 25MAMA MIRROR 2 1000 S UV-MAM A Comments: This exposure should occur in shadow (or near shadow). It is set to start at 08:32:26 and end at 09:07:52. (Shadow goes from 08:33:20 to 09:08:40) ------------------------------------------------------------------------------------------------------------------------------------ 100 NGC6752 STIS/F ACCUM F25SRF2 MIRROR 5 670 S UV-MAM A Comments: This exposure should occur out of shadow. It has a start time of 09:08:12 and an end time of 10:09:17. ------------------------------------------------------------------------------------------------------------------------------------ 110 NGC6752 STIS/F ACCUM 25MAMA MIRROR 1 1000 S UV-MAM A Comments: This is to occur during shadow (or near shadow time). It has a start time of 10:09:37 and an end time of 10:30:40. (Shadow goes from 10:09:42 to 10:45:02) ------------------------------------------------------------------------------------------------------------------------------------ 8630( 6) - 09/17/02 14:19 - [ 7] Visit: 02 Visit Priority: Visit Requirements: PCS MODE FINE On Hold Comments: Additional Comments: This has been filled out with the normal visibility. If longer visibilities are possible the last exposure has been set to expand to the end of the orbit in each of the first 4 orbits. This is highly desirable as we wish to get as deep as possible thus the CVZ requirement before. If needed more exposures can be added to break up those visibilities. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC6397 STIS/C ACCUM 50CCD MIRROR CR-SPLIT=3,GAIN=1 1 180 S CD Comments: These exposures should go at the beginning of the 5 orbit visit. ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC6397 STIS/C ACCUM F28X50L MIRROR CR-SPLIT=3,GAIN=1 1 180 S CD P Comments: These exposures should go at the beginning of the 5 orbit visit. ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC6397 STIS/F ACCUM F25QTZ MIRROR 2 600 S UV-MAM A Comments: NOTE: The number of iterations and/or the length of the exposures may need to be changed to fill the orbit. ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC6397 STIS/F ACCUM F25QTZ MIRROR 1 600 S END ORBIT EXPAND UV-MAM A Comments: NOTE: The number of iterations and/or the length of the exposures may need to be changed to fill the orbit. ------------------------------------------------------------------------------------------------------------------------------------ 55 NGC6397 STIS/F ACCUM F25QTZ MIRROR 4 600 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 70 NGC6397 STIS/F ACCUM F25QTZ MIRROR 1 600 S END ORBIT EXPAND UV-MAM A Comments: NOTE: The number of iterations and/or the length of the exposure time may need to be changed to fill the orbit. ------------------------------------------------------------------------------------------------------------------------------------ 8630( 6) - 09/17/02 14:19 - [ 8] Visit: 03 Visit Priority: Visit Requirements: PCS MODE FINE AFTER 02 BY 42 D TO 60 D On Hold Comments: Additional Comments: This has been filled out with the normal visibility. If longer visibilities are possible the last exposure has been set to expand to the end of the orbit in each of the first 4 orbits. This is highly desirable as we wish to get as deep as possible thus the CVZ requirement before. If needed more exposures can be added to break up those visibilities. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC6397 STIS/C ACCUM 50CCD MIRROR CR-SPLIT=3,GAIN=1 1 180 S CD Comments: These exposures should go at the beginning of the 5 orbit visit. ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC6397 STIS/C ACCUM F28X50L MIRROR CR-SPLIT=3,GAIN=1 1 180 S CD P Comments: These exposures should go at the beginning of the 5 orbit visit. ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC6397 STIS/F ACCUM 25MAMA MIRROR 1 1000 S SHADOW UV-MAM A Comments: These exposures should occur during dark (or shadow) time. ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC6397 STIS/F ACCUM F25SRF2 MIRROR 1 600 S END ORBIT EXPAND UV-MAM A Comments: NOTE: The number of iterations and/or the length of the exposures may need to be changed to fill the orbit. ------------------------------------------------------------------------------------------------------------------------------------ 35 NGC6397 STIS/F ACCUM F25SRF2 MIRROR 1 600 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC6397 STIS/F ACCUM 25MAMA MIRROR 1 1000 S SHADOW UV-MAM A Comments: These exposures should occur during dark (or shadow) time. ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC6397 STIS/F ACCUM F25SRF2 MIRROR 1 600 S END ORBIT EXPAND UV-MAM A Comments: NOTE: The number of iterations and/or the length of the exposures may need to be changed to fill the orbit. ------------------------------------------------------------------------------------------------------------------------------------ 8630( 6) - 09/17/02 14:19 - [ 9] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 55 NGC6397 STIS/F ACCUM F25SRF2 MIRROR 1 600 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 60 NGC6397 STIS/F ACCUM 25MAMA MIRROR 1 1000 S SHADOW UV-MAM A Comments: These exposures should occur during dark (or shadow) time. ------------------------------------------------------------------------------------------------------------------------------------ 70 NGC6397 STIS/F ACCUM F25SRF2 MIRROR 1 600 S END ORBIT EXPAND UV-MAM A Comments: NOTE: The number of iterations and/or the length of the exposure time may need to be changed to fill the orbit. ------------------------------------------------------------------------------------------------------------------------------------ 8630( 6) - 09/17/02 14:19 - [ 10] Summary Form for Proposal 8630 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 50CCD F28X50LP F25QTZ 25MAMA F25SRF2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters GAIN=1 CR-SPLIT=3 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE FINE CVZ PCS MODE FINE END ORBIT EXPAND PCS MODE FINE AFTER 02 BY 42 D TO 60 D SHADOW ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6752 NGC6397 ------------------------------------------------------------------------------------------------------------------------------------