6823( 7) - 07/25/96 16:45 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6823 Version: 7 Check-in Date: 25-Jul-1996 16:42:09 1.Proposal Title: UV and HAlpha-bright Stars in the Core of NGC 6752 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle BINARIES AND STAR FOR GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: MICHAEL SHARA SPACE TELESCOPE SCIENCE INSTITUTE CoI: Rex Saffer Space Telescope Science Institute Y CoI: Sabine Moehler Landessternwarte Heidelberg N CoI: David Zurek Space Telescope Science Institute N ------------------------------------------------------------------------------------ 6. Abstract The cores of globular clusters (GCs) are predicted to harbor numerous close, interacting binaries. HST's FOC has revealed the presence of four UV-excess objects in the core of the GC \thecluster. Photometric variability, expected if the stars are cataclysmic variables (CVs), has not been detected. Further, for two of the stars for which archival HST/PC R and HAlpha images exist, no HAlpha excess has been detected. The lack of both variability and HAlpha excess, as well as the fact that none of the stars has been observed in outburst, suggests, but does not rule out, that they are not canonical cataclysmic variables. We also have extracted post- COSTAR observations from the archive which reveal some two dozen new HAlpha-bright objects in the core, which might well be CVs. We propose to obtain red FOS spectra of the two brightest UV stars and two HAlpha excess stars to determine their nature from an examination of their spectral features. The detection of flat continuua and/or emission lines, however weak, would lead to identifications as dwarf novae. If instead weak, broad hydrogen and/or helium absorption features are detected, possibly with emission reversals, the candidates would be identified as novalike CVs. Finally, if narrower photospheric absorption features are observed, model atmosphere analyses to determine the stellar effective temperatures, surface gravities, and helium abundances would be possible. In combination with the known cluster distance modulus, these would in turn allow estimates of the stellar masses. ------------------------------------------------------------------------------------ 6823( 7) - 07/25/96 16:45 - [ 2] Observations Description ------------------------ We propose to obtain FOS/RD G570H spectra through the 0.44"\\ ( 0.5) circular aperture of two UV-bright and two HAlpha- bright sources the core of \\thecluster. Our first two targets are the two brightest objects marked by boxes in the UV CMD shown in Fig. 1, obtained with the COSTAR-corrected FOC (Shara et al. 1995). These have U-band magnitudes of 18.0 and 18.6, and since CVs have flat optical energy distributions, the R-band magnitudes will be similar. Spectra with S/N \\twid 20 per diode in the continuum will be required to detect possibly weak emission lines, in the event the targets prove to be dwarf novae in quiescence or magnetic systems, or broad, shallow absorption features, should the targets prove to be dwarf novae in eruption or nova-like systems. Our last two targets are HAlpha-bright objects shown in Fig. 2. These targets have R-band magnitudes near 18.8 and 19.0, but are demonstrated to have HAlpha excesses, so that spectra having S/N = 15 in the continuum will suffice. The incident fluxes in the continuum near HAlpha thus range from 6--16 * 10^-17 erg s^-1 cm^-2 Angstrom ^-1. Using Tables 1-7 and 1-8 in the FOS Instrument Handbook, for FOS/RD G570H these translate to count rates per diode of 0.035-- 0.089. These are nearly of the order of the dark current of 0.01 counts s^-1 diode^-1, and we use the formula on the bottom of page to estimate the exposure times: begintableh begintabular|c|c|c|c|c|c| Target & R & F_Lambda & N_Lambda & S/N & Tau(min) , 1 & 18.0 & 16 * 10^-17 & 0.088 & 20 & \\pz 84 , 2 & 18.6 & \\pz 9 * 10^-17 & 0.051 & 20 & 156 , 3 & 18.8 & \\pz 7 * 10^-17 & 0.042 & 15 & 110 , 4 & 19.0 & \\pz 6 * 10^-17 & 0.033 & 15 & 148 , endtabular endtable \\thecluster\\ is in the Continuous Viewing Zone, thus these exposure time calculations mean that we require a total of 5.2 orbits to obtain the target spectra. We have consulted with ST ScI FOS Instrument Scientist Dr. Tony Keyes about acquisition procedures. Due to the extreme crowding in the cluster core and faintness of the targets, peakups in the ( 0.3) aperture have a significant failure probability. Thus we will use blind offsets from the bright star marked with a triangle in Fig. 1. A consideration of the astrometry error budget reveals the following: Peaking up on the offset star will result in a 1-Sigma positional uncertainty of 0arcs 04, comparable to the errors in measuring target offsets from the HST archival images. These will add in quadrature for a total 1-Sigma positional uncertainty of \\twid 0arcs 06. This makes for an unacceptable 2-Sigma error equal to the radius of the ( 0.3) aperture. We therefore will use the ( 0.5) aperture, whose 0arcs 22 radius exceeds the 3- Sigma blind offset uncertainty. We have confirmed that no other bright sources lie within 0arcs 25 of our targets, larger than the radius of the ( 0.5) aperture, and in this way we ensure that no significant contamination of the spectra will occur. The acquisition and observational sequences then will proceed as follows: We will acquire the bright offset star and peak up in the ( 0.3) aperture using a Z+D1+D3 sequence (overhead 43 min), achieving 0arcs 04 positional accuracy, then blind offset into the ( 0.5) aperture and obtain the observation. Between each subsequent observation we will re-peak on the offset star with a 3 X 3 grid search with a step size of 0arcs 052 (overhead 10 min). Thus the total acquisition overhead is 73 min. Guide star acquisition and instrument overheads account for another 33 min, for a total of just over one orbit. Therefore, our program requires a total of 7 spacecraft orbits to execute. There are no special calibration requirements; standard pipeline wavelength and flux calibrations will suffice for our analysis. Real Time Justification ----------------------- NGC 6752 is suitable for CVZ observing. We therefore require CVZ scheduling for this target, since this increases the efficiency of the proposed observations without altering the quality of the data we need. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6823( 7) - 07/25/96 16:45 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: Rex A. Saffer*3700 San Martin Drive*Baltimore*21218 Ship Via: UPS Email: saffer@stsci.edu , ------------------------------------------------------------------------------------ 6823( 7) - 07/25/96 16:45 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6752-RGB1 STAR, K III-I RA = 19H 10M 53.284S +/- 0.07S, 2000 V = 12.88+/-0.1 -OFFSET DEC = -59D 58' 57.93" +/- 0.5", B-V = +0.8+/-0.1 PLATE-ID=03D3 2 NGC6752-HAB1 STAR, EMISSION LINE RA-OFF = -0.607S +/- 0.004S, DEC 2000 V = 19.81+/-0.05 STAR -OFF = +5.08" +/- 0.03", FROM 1 B-V = +0.39+/-0.05 3 NGC6752-HAB2 STAR, EMISSION LINE RA-OFF = +0.263S +/- 0.004S, DEC 2000 V = 19.41+/-0.05 STAR -OFF = +0.06" +/- 0.03", FROM 1 B-V = +0.23+/-0.05 4 NGC6752-UVB1 STAR, INTERACTING RA-OFF = -0.809S +/- 0.004S, DEC 2000 V = 19.84+/-0.05 BINARY -OFF = -0.53" +/- 0.03", FROM 1 B-V = +0.26+/-0.05 5 NGC6752-UVB2 STAR, INTERACTING RA-OFF = -1.205S +/- 0.004S, DEC 2000 V = 18.81+/-0.05 BINARY -OFF = -8.34" +/- 0.03", FROM 1 B-V = +0.11+/-0.1 6823( 7) - 07/25/96 16:45 - [ 5] Visit: 01 Visit Requirements: CVZ 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6752-RGB FOS/RD ACQ/PEA 4.3 G570H SCAN-STEP-X = 3.3,SCAN-STEP-Y = 1 1.0 S ONBOARD ACQ FOR 2 1-OFFSET K 1.23,SEARCH-SIZE-X = 2, SEARCH-SIZE-Y = 6 Comments: Exposure time set at recommended minimum value, calculated value is 0.085S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC6752-RGB FOS/RD ACQ/BIN 4.3 MIRROR 1 0.9 S ONBOARD ACQ FOR 3 1-OFFSET ARY ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC6752-RGB FOS/RD ACQ/PEA 0.5 G570H SCAN-STEP-X = 0.29,SCAN-STEP-Y = 1 1.0 S ONBOARD ACQ FOR 4 1-OFFSET K 0.29,SEARCH-SIZE-X = 3, SEARCH-SIZE-Y = 3 Comments: Exposure time set at recommended minimum value, calculated value is 0.119S ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC6752-RGB FOS/RD ACQ/PEA 0.3 G570H SCAN-STEP-X = 0.17,SCAN-STEP-Y = 1 1.0 S ONBOARD ACQ FOR 5 1-OFFSET K 0.17,SEARCH-SIZE-X = 3, SEARCH-SIZE-Y = 3 Comments: Exposure time set at recommended minimum value, calculated value is 0.124S ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC6752-RGB FOS/RD ACQ/PEA 0.3 G570H SCAN-STEP-X = 0.052,SCAN-STEP-Y = 1 1.24 S ONBOARD ACQ FOR 6-8 1-OFFSET K 0.052,SEARCH-SIZE-X = 5, SEARCH-SIZE-Y = 5 Comments: CRITICAL PEAKUP, EXPOSURE TIME 10X FOR V=12.88 ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC6752-RGB FOS/RD ACCUM 0.5 G570H 1 375 S NO SPLIT 1-OFFSET ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC6752-HAB FOS/RD ACCUM 0.5 G570H 1 6820 S NO SPLIT 1 ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC6752-RGB FOS/RD ACQ/PEA 0.3 G570H SCAN-STEP-X = 0.052,SCAN-STEP-Y = 1 1.24 S ONBOARD ACQ FOR 9-10 1-OFFSET K 0.052,SEARCH-SIZE-X = 3, SEARCH-SIZE-Y = 3 Comments: CRITICAL PEAKUP, EXPOSURE TIME 10X FOR V=12.88 ------------------------------------------------------------------------------------------------------------------------------------ 9 NGC6752-HAB FOS/RD ACCUM 0.5 G570H 1 4455 S NO SPLIT 2 Comments: Aperture offset 0.05" SE to minimize contamination ------------------------------------------------------------------------------------------------------------------------------------ 6823( 7) - 07/25/96 16:45 - [ 6] 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-RGB FOS/RD ACQ/PEA 0.3 G570H SCAN-STEP-X = 0.052,SCAN-STEP-Y = 1 1.24 S ONBOARD ACQ FOR 11-12 1-OFFSET K 0.052,SEARCH-SIZE-X = 3, SEARCH-SIZE-Y = 3 Comments: CRITICAL PEAKUP, EXPOSURE TIME 10X FOR V=12.88 ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC6752-UVB FOS/RD ACCUM 0.3 G570H 1 13770 S NO SPLIT 1 ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC6752-RGB FOS/RD ACQ/PEA 0.3 G570H SCAN-STEP-X = 0.052,SCAN-STEP-Y = 1 1.24 S ONBOARD ACQ FOR 13 1-OFFSET K 0.052,SEARCH-SIZE-X = 3, SEARCH-SIZE-Y = 3 Comments: CRITICAL PEAKUP, EXPOSURE TIME 10X FOR V=12.88 ------------------------------------------------------------------------------------------------------------------------------------ 13 NGC6752-UVB FOS/RD ACCUM 0.5 G570H 1 4295 S NO SPLIT 2 ------------------------------------------------------------------------------------------------------------------------------------ 6823( 7) - 07/25/96 16:45 - [ 7] Summary Form for Proposal 6823 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/RD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/PEAK ACQ/BINARY ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SCAN-STEP-X = 3.3 SCAN-STEP-Y = 1.23 SEARCH-SIZE-X = 2 SEARCH-SIZE-Y = 6 SEARCH-SIZE-Y = 3 SEARCH-SIZE-X = 3 SEARCH-SIZE-Y = 5 SCAN-STEP-Y = 0.052 SCAN-STEP-X = 0.052 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category BINARIES AND STAR FORMATION ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ ONBOARD ACQ FOR 2 ONBOARD ACQ FOR 3 ONBOARD ACQ FOR 4 ONBOARD ACQ FOR 5 ONBOARD ACQ FOR 6-8 NO SPLIT ONBOARD ACQ FOR 9-10 ONBOARD ACQ FOR 11-12 ONBOARD ACQ FOR 13 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G570H MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6752-RGB1-OFFSET NGC6752-HAB1 NGC6752-HAB2 NGC6752-UVB1 NGC6752-UVB2 ------------------------------------------------------------------------------------------------------------------------------------