8643( 6) - 10/09/01 10:37 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8643 Version: 6 Check-in Date: 09-Oct-2001 10:36:24 1.Proposal Title: Ultraviolet Properties of the Metal Rich M87 Globular Cluster System ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Robert O'Connell The University of Virginia CoI: Dr. Ralph Bohlin Space Telescope Science Institute N CoI: Professor David Burstein Arizona State University N CoI: Professor Jay Frogel Ohio State University N CoI: Dr. Arunav Kundu University of Virginia N CoI: Dr. Wayne Landsman Goddard Space Flight Center N CoI: Professor Robert Rood University of Virginia N CoI: Professor James Rose University of North Carolina N ------------------------------------------------------------------------------------ 5. Abstract We propose to use STIS imaging to obtain far-ultraviolet photometry of the metal rich globular cluster system of the elliptical galaxy M87. This system represents a key link between the well understood populations of the clusters and the hot stars in elliptical galaxies, where our physical insight is presently limited. Our goal is to establish the relationship between cluster metal abundance and the production of UV-bright populations of stars on the ``extreme horizontal branch'' at T(eff) > 16000K. These stars are the source of the surprising ``ultraviolet-upturn'' phenomenon in elliptical galaxies. Our observations will fill a major gap in the present coverage of cluster metal abundances. This would be an important step in understanding the dependence of the upturn on its parent stellar population. A basic motivation is the expectation that the UV-upturn could be the most sensitive probe of the ages and abundances of elliptical galaxy populations. We plan to observe 3 fields in M87, which will provid e a sample of ~ 30--50 UV-detected objects in the brightest 3 magnitudes of its cluster luminosity function. The program is technically challenging but appears feasible. Relatively long integrations are needed, under conditions of minimum dayglow emission from Earth's atmosphere. ------------------------------------------------------------------------------------ 8643( 6) - 10/09/01 10:37 - [ 2] Observations Description ------------------------ We are planning STIS far-UV MAMA imaging observations of globular clusters in 3 fields in the gE galaxy M87. The brightest clusters have V ~ 20, implying (see above) estimated m_Lambda(1500) ~ 21--25 for the top 3 magnitudes of the V-band luminosity function. We will choose fields from existing HST/WFPC2 optiucal- band photometry from which we can obtain B--I or V--I colors for the clusters. These are well correlated with abundances as estimated from ground-based spectroscopy of absorption line features. The exact placement of the STIS imaging field within the pre- existing WFPC2 fields will be adjusted to optimize the number of clusters included. We estimate that near the galaxy center there will be 25-50 brighter clusters per MAMA field (25"\ square), based on ground-based and HST counts. This implies that 3 fields will suffice to yield our goal of at least 40 detected clusters. The clusters are essentially point sources at STIS resolution. We estimate that the total e xposure time with the far-UV MAMA in each field should be approximately 6000 secs. We have based our estimates on the STIS Imaging Exposure Time Calculator. Other than the faintness of the sources, the most important technical consideration is the dayglow sky background. Because the sources of interest are hot, with T_e >~ 22000 K, it is important to extend the bandwidth of our images as far to the blue as possible without including geocoronal Lyman-Alpha. This implies use of the F25SRF2 longpass filter, which has a blue cutoff at 1270Angstrom. However, this filter admits the dayglow emission lines of OI (1302, 1356 Angstrom) produced by the residual atmosphere when it is illuminated by sunlight. The dayglow can create a relatively bright sky background which may degrade our signal-to-noise. However, the only alternative FUV filter blocked against the dayglow is F25QTZ, which has a blue cutoff at 1460Angstrom. Because its bandwidth is reduced by ~ 200 Angstrom, this filter would require exposures over t wice as long as F25SRF2 to re ach a given S/N under shadow conditions, according to the ETC. We have examined the dayglow background on 21 FUV-MAMA F25SRF2 imaging frames taken as part of S. Heap's program P7432. The observed sky count rate depends strongly on the angle of the sun above the Earth limb. The maximum count rate observed (9.2e-4 cts/s/pixel) is still 40\ ``typical'' by the ETC. The background rate is minimized for sun angles less than about 30^degrees. Therefore, it is not necessary to observe only in Earth's shadow to obtain low background rates. (This sun angle dependence is also seen in the FOC study of OI emission by Caulet et al. 1994). Since dayglow line strengths are variable and hard to predict, our best strategy is to use the wider F25SRF2 longpass filter to facilitate source detection but observe in the MAMA TIMETAG mode. In this mode, the data consists of a stream of time-tagged photon events and coordinates. This will allow us to maximize the low-background observing time, however long it may be on a given orbit, while rejecting bright-background periods. To specify only SHADOW TIME for the exposures would unnecessarily restrict available observing periods (to about 25 min per orbit), although we do wish the visits to be scheduled at times during the year such that shadow time is maximized for each target. We estimate that this approach will increase the usable integration time per orbit by up to 50\ checked that the estimated total count rate (<2500 cts/s) is well within the capacity of the data system for TIMETAG mode. TIMETAG data are slightly more awkward to reduce than normal image frames, but this mode is widely used by X-ray astronomers, for instance. With this strategy, we estimate that a standard integration time of 6000 seconds with F25SRF2 will yield far-UV fluxes at S/N ~ 25 for m_Lambda(1500) = 22 and S/N ~ 8 for m_Lambda(1500) = 24 (comparable to or better than the photometry in Fig. 1). If we conservatively assume that we obtain only 25 minutes of suitable l ow-background time per orbit, a 6000 second exposure implies 4 orbits of far-UV exposure time per field. In Phase 2, we will use the timing requirement BETWEEN to set the scheduling window for our visits during the two-month period of best shadow- time visibility for our targets. It is also advisable to obtain STIS CCD imaging of the same fields during the same visits in order to verify pointing and provide astrometry and additional information on the optical fluxes of our targets. It should be possible to obtain a 500 second CCD exposure with the longpass filter (F28X50LP, blue cutoff at 5500 Angstrom) on either the first or last of the four orbits of the MAMA series. S/N will be 15 at V = 25. This combined MAMA-CCD pattern does not violate the single-visit rule, as we understand it. Additional photometry in the broad NUV-MAMA band would be useful in studying cluster metal abundances but would be much less sensitive to the EHB components of interest here. Since the NUV observations would nearly doubl e the orbital request, we defer th em to a later cycle. This program is challenging, though feasible, for STIS. Upcoming HST instrumentation will not be better suited for it. The far-UV channel on ACS will not perform significantly better, and WFC3 will not be able to observe in the far-UV. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- We require low dayglow observing conditions. We will not request SHADOW scheduling, however. Instead we will use a combination of MAMA/TIMETAG operation and BETWEEN visit scheduling to assure low background conditions. We have selected BETWEEN dates to maximize darkside visibility. We give preference to dates after opposition, when the source will rise in daylight but set in shadow, meaning that acquisition and setup occur in the less desirable daylight period. ------------------------------------------------------------------------------------ 8643( 6) - 10/09/01 10:37 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4486- EXT-CLUSTER, GLOBULAR RA=12H 30M 49.83S +/- 0.07S, J2000 V = 1282 V = 22 GCFIELD-1 CLUSTER DEC=12D 22' 51.2" +/- 1", PLATE- F(1500)=6.0e-18 ID=00IY SURF-BKG(1500)=2.0e-18 V-I = 1.0+/-0.3 Coordinate Source: HST_IMAGE 2 NGC4486- EXT-CLUSTER, GLOBULAR RA=12H 30M 53.09S +/- 0.07S, J2000 V = 1282 V = 22 GCFIELD-2 CLUSTER DEC=12D 24' 15.3" +/- 1", PLATE- F(1500)=6.0e-18 ID=00IY SURF-BKG(1500)=3.0e-19 V-I = 1.0+/-0.3 Coordinate Source: HST_IMAGE 3 NGC4486- EXT-CLUSTER, GLOBULAR RA=12H 30M 50.63S +/- 0.07S, J2000 V = 1282 V = 22 GCFIELD-3 CLUSTER DEC=12D 23' 42.2" +/- 1", PLATE- F(1500)=6.0e-18 ID=00IY SURF-BKG(1500)=8.0e-18 V-I = 1.0+/-0.3 Coordinate Source: HST_IMAGE Comments: Pattern_Data Pattern_Number: Primary_Pattern: Pattern_Type: Pattern_Purpose: Number_Of_Points: Point_Spacing: Coordinate_Frame: Pattern_Orient: Center_Pattern: Secondary_Pattern: Pattern_Type: Pattern_Purpose: Number_Of_Points: Point_Spacing: Coordinate_Frame: Pattern_Orient: Center_Pattern: 8643( 6) - 10/09/01 10:37 - [ 4] Visit: 01 Visit Priority: Visit Requirements: BETWEEN 10-Feb-2001 AND 10-Jun-2001 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2680 S IELD-1 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2900 S IELD-1 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 8643( 6) - 10/09/01 10:37 - [ 5] Visit: 02 Visit Priority: Visit Requirements: BETWEEN 10-Feb-2001 AND 10-Jun-2001 SAME ORIENT AS 01 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC4486-GCF STIS/C ACCUM F28X50L MIRROR CR-SPLIT=2 1 500 S IELD-1 CD P ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2000 S IELD-1 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2900 S IELD-1 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 8643( 6) - 10/09/01 10:37 - [ 6] Visit: 03 Visit Priority: Visit Requirements: BETWEEN 10-Feb-2001 AND 10-Jun-2001 BETWEEN 10-Feb-2002 AND 15-Jun-2002 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2680 S IELD-2 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2900 S IELD-2 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 8643( 6) - 10/09/01 10:37 - [ 7] Visit: 04 Visit Priority: Visit Requirements: BETWEEN 10-Feb-2001 AND 10-Jun-2001 SAME ORIENT AS 03 BETWEEN 10-Feb-2002 AND 15-Jun-2002 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC4486-GCF STIS/C ACCUM F28X50L MIRROR CR-SPLIT=2 1 500 S IELD-2 CD P ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2000 S IELD-2 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2900 S IELD-2 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 8643( 6) - 10/09/01 10:37 - [ 8] Visit: 05 Visit Priority: Visit Requirements: BETWEEN 10-Feb-2001 AND 10-Jun-2001 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2680 S IELD-3 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2900 S IELD-3 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 8643( 6) - 10/09/01 10:37 - [ 9] Visit: 06 Visit Priority: Visit Requirements: BETWEEN 10-Feb-2001 AND 10-Jun-2001 SAME ORIENT AS 05 BETWEEN 10-Feb-2002 AND 10-Jun-2002 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC4486-GCF STIS/C ACCUM F28X50L MIRROR CR-SPLIT=2 1 500 S IELD-3 CD P ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2000 S IELD-3 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2900 S IELD-3 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 8643( 6) - 10/09/01 10:37 - [ 10] Visit: 15 Visit Priority: Visit Requirements: BETWEEN 10-Feb-2002 AND 10-Jun-2002 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2680 S IELD-3 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC4486-GCF STIS/F TIME-TA F25SRF2 MIRROR BUFFER-TIME=800 1 2900 S IELD-3 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 8643( 6) - 10/09/01 10:37 - [ 11] Summary Form for Proposal 8643 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F25SRF2 F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/FUV-MAMA STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes TIME-TAG ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=800 CR-SPLIT=2 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 10-Feb-2001 AND 10-Jun-2001 BETWEEN 10-Feb-2001 AND 10-Jun-2001 SAME ORIENT AS 01 BETWEEN 10-Feb-2001 AND 10-Jun-2001 BETWEEN 10-Feb-2002 AND 15-Jun-2002 BETWEEN 10-Feb-2001 AND 10-Jun-2001 SAME ORIENT AS 03 BETWEEN 10-Feb-2002 AND 15-Jun-2002 BETWEEN 10-Feb-2001 AND 10-Jun-2001 SAME ORIENT AS 05 BETWEEN 10-Feb-2002 AND 10-Jun-2002 BETWEEN 10-Feb-2002 AND 10-Jun-2002 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC4486-GCFIELD-1 NGC4486-GCFIELD-2 NGC4486-GCFIELD-3 ------------------------------------------------------------------------------------------------------------------------------------