6697( 6) - 07/16/96 12:44 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6697 Version: 6 Check-in Date: 16-Jul-1996 12:38:40 1.Proposal Title: The Blue Stragglers in the Core of NGC 6397 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle HOT STARS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Guido De Marchi ESO CoI: Mario Livio Space Telescope Science Institute N CoI: Dante Minniti European Southern Observatory N CoI: Nino Panagia Space Telescope Science Institute N CoI: Francesco Paresce European Southern Observatory N CoI: Rex Saffer Space Telescope Science Institute Y ------------------------------------------------------------------------------------ 6. Abstract Six bright blue stragglers dominate the core of the globular cluster NGC 6397. They are the most massive objects detected in this cluster. No other blue stragglers are found within one core radius of the cluster center, and although a total of 26 blue stragglers are known in NGC 6397, none of them is as bright as the six objects in the core. Photometric data and low resolution spectroscopy that we obtained with the HST in the UV classify them in the late-B to mid-A spectral type range. Although a merger of two stars is likely to have given birth to these stars (in view of their mass, estimated photometrically), no direct indication exists on their formation mechanism: both hypotheses of a binary merger (in which stars in full contact evolve into coalescence after mass transfer) as well as coalescence following a collision are compatible with the available data. We, therefore, propose to obtain medium resolution FOS spectra to investigate the nature of these objects. From the spectral analysis of the Balmer jump and Balmer line profiles and from the known UV colors, we will be able to determine T(eff)\ and \logg. Knowing the distance to the cluster, masses will accurately be determined for the first time, and will allow us to assess more reliably the formation mechanism. To better understand their evolutionary scenario, we will also compare their photospheric He abundances (higher after a collision) and rotational velocities (higher in a merged binary) with those of a blue horizontal branch star near the core. ------------------------------------------------------------------------------------ 6697( 6) - 07/16/96 12:44 - [ 2] Observations Description ------------------------ NOTE: The original observing description below mentions the 0.3 aperture as the aperture of choice, but this has been modified after further discussions between the Contact Scientist and the observers. Original text: We propose to obtain optical (G400H, G570H) spectra through the 0.26"\\ ( 0.3) circular aperture of six very blue and bright blue stragglers (BSS) in the core of NGC 6397. We have performed simulations, analysing synthetic spectra to which noise has been added, showing that with a signal-to-noise ratio (S/N) of 30, precise estimates of the atmospheric parameters T(eff)\\ and \\logg\\ can be made (DeltaT(eff)\\ ~ 500 K, Delta\\logg\\ ~ 0.1 dex). Saffer (1991) has shown convincingly that despite the correlation of effective temperature and surface gravity errors, the internal error estimates (precision) agree well with external estimates based on repeated observations of standard stars. Further, for a subsample of bright, well-studied objects analyzed by Saffer et al. (1994), the zero points (accuracy) also agree well with temperature and gravity determinations based on a variety of independent analysis techniques in the literature. The random and systematic errors associated with the proposed analysis thus are shown to be well understood, and the error estimates cited above are reliable. Combined with the distance to NGC 6397, the temperature and gravity determinations will allow mass estimates to within ~ 25\\%. To discriminate efficiently between the two competing hypotheses of BSS formation in this cluster (binary mergers and collisions), we also intend to compare the photospheric He abundance of these stars as well as their rotational velocities with those of a bright, blue horizontal branch (HB) star (V~eq 13), located ~ 10" away from the cluster center (outside the field shown in Figure 1). We intend to use this star as representative of the properties of the cluster population of hot stars, and therefore propose to obtain a spectrum with S/N ~eq 100 (with an exposure time of ~ 5 min). Should the BSS have anomalously high He abundance, this would be evident from a comparison of the HeI Lambda4026 and Lambda4471 lines in their spectra with the corresponding features in the spectrum of the HB star. We expect these lines to be easier to detect in the spectra of the two bluer BSS objects (Figure 1), for which we intend to obtain longer exposures in order to reach S/N of ~ 50 (with an exposure time of ~ 10--15 min). Rotational velocities, if larger than ~ 250 kms, would result in a noticeable broadening, detectable by analyzing the profiles of the HeI and Balmer \\balpha\\ lines. We have used the calcspec task in the STSDAS synphot package to calculate the FOS count rates for our selected targets, which predict exposure times ranging from 3 to 6 minutes to achieve spectra having S/N = 30. These count rates and exposure time estimates agree to within 5\\% with those made using Tables 1--7 and 1--8. Using the FOS Worksheet and following the guidelines in the Cycle 6 Phase I proposal instructions, we have calculated we require 6 spacecraft orbits to properly acquire and observe all our targets. The steps that we intend to adopt are the following: acquire the bright HB star and place it securely inside the 0.3 aperture; obtain two spectra of it (with the G570H and G400H gratings); offset to the first of the six bright BSS and acquire it, with a 3 * 3 step peakup sequence within the aperture; obtain two spectra (one per grating); move consecutively to all the other BSS and repeat these steps for all of them. We have already collected FOC F/48 images of a field larger than that shown in Figure 1, which includes the HB star as well as all the six BSS. These images provide us with extremely precise positions for our targets with respect to the aquisition stars (better than ~ 0.05" ). This will allow us to accurately locate the BSS inside the FOS 0.3 aperture. We notice here that we can accomodate the short exposures needed to obtain a spectrum of the HB star right into the target acquisition orbit, so that no additional spacecraft time will be required to obtain this ``free'' observation. The brightness of the HB star and of the six BSS makes the acquisition procedure very reliable and accurate, and ensures that contamination due to possible nearby objects will be negligible. As reported in the Observing Summary on page 2 of this proposal, a total of 6 orbits are requested for the observations proposed here. Real Time Justification ----------------------- There are no special calibration requirements; standard pipeline wavelength and flux calibrations will suffice for our analysis. Calibration Justification ------------------------- Additional Comments ------------------- The following Phase II proposal utilizes a non-standard 4-stage peakup acquisition procedure designed by the proposer. The offset star is too bright to acquire with ACQ/BIN and the MIRROR element. The sequence will use existing Patterns A and B1, followed by new Pattern C2 (3X3 at 0.23 arcsec in the (0.5) aperture), and finally existing Pattern E4 to achieve a critical peakup pointing accuracy of 0.04 arcsec. I (Rex A. Saffer, Contact CoI) have had extensive discussions with Instrument Scientists Tony Keyes and Michael Dahlem about the pointing accuracies resulting from the procedure, and I am confident that the procedure has an extremely high probability of success. I am grateful for their expert assistance and advice. ------------------------------------------------------------------------------------ 6697( 6) - 07/16/96 12:44 - [ 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 , ------------------------------------------------------------------------------------ 6697( 6) - 07/16/96 12:44 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6397-HB1 STAR, HORIZONTAL BRANCH RA = 17H 40M 41.14S +/- 0.06S, 2000 V = 13.65+/-0.1 STAR DEC = -53D 40' 25.27" +/- 0.5", B-V = +0.16+/-0.1 PLATE-ID=0688 Comments: Plate ID is for Region XV181. The target is located at x,y coords 6777.7,9938.3 out of 14000x14000 pixels, so there is no overlap possible with other adjacent GS plates, even for the FGSs. It was also checked with Conrad Sturch of GSSS to verify that there were no other special catalogs in place in the GSC for this target so that the GSs have to have come from XV181, Plate-ID=0688. ( Reference to pixels above is referring to the digitized GSSS Plate. Also, the epoch is just the epoch of that plate which is automatically accounted for since the GSs used to make the 1994 FOC observations were already in the Equinox J2000.0/Epoch 1987.70 reference frame of XV181 = Plate-ID 0688. ) - Ray Lucas - 9 July, 1996 2 NGC6397-BSS1 STAR, BLUE STRAGGLER RA-OFF = +0.765S +/- 0.011S, DEC 2000 V = 14.57+/-0.1 -OFF = -0.63" +/- 0.10", FROM 1 B-V = +0.29+/-0.1 3 NGC6397-BSS2 STAR, BLUE STRAGGLER RA-OFF = +0.810S +/- 0.011S, DEC 2000 V = 14.27+/-0.1 -OFF = -1.94" +/- 0.10", FROM 1 B-V = +0.33+/-0.1 4 NGC6397-BSS3 STAR, BLUE STRAGGLER RA-OFF = +0.950S +/- 0.011S, DEC 2000 V = 14.42+/-0.1 -OFF = -3.93" +/- 0.10", FROM 1 B-V = +0.16+/-0.1 5 NGC6397-BSS4 STAR, BLUE STRAGGLER RA-OFF = +1.185S +/- 0.011S, DEC 2000 V = 14.91+/-0.1 -OFF = -5.23" +/- 0.10", FROM 1 B-V = +0.22+/-0.1 6 NGC6397-BSS5 STAR, BLUE STRAGGLER RA-OFF = +1.095S +/- 0.011S, DEC 2000 V = 14.42+/-0.1 -OFF = -6.85" +/- 0.10", FROM 1 B-V = +0.23+/-0.1 7 NGC6397-BSS6 STAR, BLUE STRAGGLER RA-OFF = +1.315S +/- 0.011S, DEC 2000 V = 14.43+/-0.1 -OFF = -7.39" +/- 0.10", FROM 1 B-V = +0.24+/-0.1 6697( 6) - 07/16/96 12:44 - [ 5] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: USE SAME GUIDE STARS AS FOR GO 5644, RELEVANT DATA FOLLOW: Visit Nr: 1 SU ID: 01 Target: NGC6397 Config: FOC/96 Start: Aug 19, 1994 05:10:23 UT (1994.231) End: Aug 19, 1994 10:59:41 UT (1994.231) GS Star IDs from GO 5644 dataset x2g10101t.shh are: Dominant GS: 0872901553F1 ( Star in FGS1 ) Subdominant GS: 0872902125F3 ( Star in FGS3 ) ( GS info from Ray Lucas - 9 July, 1996 ) 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-HB1 FOS/RD ACQ/PEA 4.3 G190H SCAN-STEP-Y = 1.23,SEARCH-SIZE-X 1 2.0 S ONBOARD ACQ FOR 4 K = 1,SEARCH-SIZE-Y = 3 Comments: Target Acquisition Pattern A Texp revised upward to 2.0s as per Michael Dahlem. ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC6397-HB1 FOS/RD ACQ/PEA 1.0 G190H SCAN-STEP-X = 0.61,SCAN-STEP-Y = 1 2.0 S ONBOARD ACQ FOR 4 K 0.61,SEARCH-SIZE-X = 6, SEARCH-SIZE-Y = 2 Comments: Target Acquisition Pattern B1 Texp revised upward to 2.0s as per Michael Dahlem. ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC6397-HB1 FOS/RD ACQ/PEA 0.5 G190H SCAN-STEP-X = 0.23,SCAN-STEP-Y = 1 2.0 S ONBOARD ACQ FOR 4 K 0.23,SEARCH-SIZE-X = 3, SEARCH-SIZE-Y = 3 Comments: Target Acquisition Pattern C2 Texp revised upward to 2.0s as per Michael Dahlem. ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC6397-HB1 FOS/RD ACCUM 1.0 G570H 1 720 S ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC6397-HB1 FOS/RD ACCUM 1.0 G400H 1 600 S SAME POS AS 4 ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC6397-HB1 FOS/RD ACQ/PEA 0.3 G400H SCAN-STEP-X = 0.052,SCAN-STEP-Y = 1 1.8 S ONBOARD ACQ FOR 8 K 0.052,SEARCH-SIZE-X = 4, SEARCH-SIZE-Y = 4 Comments: Target Acquisition Pattern E3, CRITICAL PEAKUP, EXPOSURE TIME 10X FOR V=13.65 ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC6397-BSS FOS/RD ACQ/PEA 0.3 G400H SCAN-STEP-X = 0.052,SCAN-STEP-Y = 1 3.2 S ONBOARD ACQ FOR 8 2 K 0.052,SEARCH-SIZE-X = 3, SEARCH-SIZE-Y = 3 Comments: CRITICAL PEAKUP, EXPOSURE TIME 10X FOR V=14.27 ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC6397-BSS FOS/RD ACCUM 0.5 G400H 1 300 S 2 ------------------------------------------------------------------------------------------------------------------------------------ 9 NGC6397-BSS FOS/RD ACCUM 0.5 G570H 1 360 S SAME POS AS 8 2 ------------------------------------------------------------------------------------------------------------------------------------ 6697( 6) - 07/16/96 12:44 - [ 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 NGC6397-BSS FOS/RD ACQ/PEA 0.3 G570H SCAN-STEP-X = 0.052,SCAN-STEP-Y = 1 5.8 S ONBOARD ACQ FOR 11 4 K 0.052,SEARCH-SIZE-X = 3, SEARCH-SIZE-Y = 3 Comments: CRITICAL PEAKUP, EXPOSURE TIME 10X FOR V=14.91 ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC6397-BSS FOS/RD ACCUM 0.5 G570H 1 640 S 4 ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC6397-BSS FOS/RD ACCUM 0.5 G400H 1 530 S SAME POS AS 11 4 ------------------------------------------------------------------------------------------------------------------------------------ 13 NGC6397-BSS FOS/RD ACQ/PEA 0.3 G400H SCAN-STEP-X = 0.052,SCAN-STEP-Y = 1 4.2 S ONBOARD ACQ FOR 14 1 K 0.052,SEARCH-SIZE-X = 3, SEARCH-SIZE-Y = 3 Comments: CRITICAL PEAKUP, EXPOSURE TIME 10X FOR V=14.57 ------------------------------------------------------------------------------------------------------------------------------------ 14 NGC6397-BSS FOS/RD ACCUM 0.5 G400H 1 390 S 1 ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC6397-BSS FOS/RD ACCUM 0.5 G570H 1 470 S SAME POS AS 14 1 ------------------------------------------------------------------------------------------------------------------------------------ 16 NGC6397-BSS FOS/RD ACQ/PEA 0.3 G570H SCAN-STEP-X = 0.052,SCAN-STEP-Y = 1 3.7 S ONBOARD ACQ FOR 17 3 K 0.052,SEARCH-SIZE-X = 3, SEARCH-SIZE-Y = 3 Comments: CRITICAL PEAKUP, EXPOSURE TIME 10X FOR V=14.42 ------------------------------------------------------------------------------------------------------------------------------------ 17 NGC6397-BSS FOS/RD ACCUM 0.5 G570H 1 470 S 3 ------------------------------------------------------------------------------------------------------------------------------------ 18 NGC6397-BSS FOS/RD ACCUM 0.5 G400H 1 400 S SAME POS AS 17 3 ------------------------------------------------------------------------------------------------------------------------------------ 19 NGC6397-BSS FOS/RD ACQ/PEA 0.3 G400H SCAN-STEP-X = 0.052,SCAN-STEP-Y = 1 3.7 S ONBOARD ACQ FOR 20 5 K 0.052,SEARCH-SIZE-X = 3, SEARCH-SIZE-Y = 3 Comments: CRITICAL PEAKUP, EXPOSURE TIME 10X FOR V=14.42 ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC6397-BSS FOS/RD ACCUM 0.5 G400H 1 460 S 5 ------------------------------------------------------------------------------------------------------------------------------------ 21 NGC6397-BSS FOS/RD ACCUM 0.5 G570H 1 470 S SAME POS AS 20 5 ------------------------------------------------------------------------------------------------------------------------------------ 6697( 6) - 07/16/96 12:44 - [ 7] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 22 NGC6397-BSS FOS/RD ACQ/PEA 0.3 G570H SCAN-STEP-X = 0.052,SCAN-STEP-Y = 1 3.7 S ONBOARD ACQ FOR 23 6 K 0.052,SEARCH-SIZE-X = 3, SEARCH-SIZE-Y = 3 Comments: CRITICAL PEAKUP, EXPOSURE TIME 10X FOR V=14.43 ------------------------------------------------------------------------------------------------------------------------------------ 23 NGC6397-BSS FOS/RD ACCUM 0.5 G570H 1 470 S 6 ------------------------------------------------------------------------------------------------------------------------------------ 24 NGC6397-BSS FOS/RD ACCUM 0.5 G400H 1 400 S SAME POS AS 23 6 ------------------------------------------------------------------------------------------------------------------------------------ 6697( 6) - 07/16/96 12:44 - [ 8] Summary Form for Proposal 6697 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/RD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SCAN-STEP-Y = 1.23 SEARCH-SIZE-X = 1 SEARCH-SIZE-Y = 3 SEARCH-SIZE-Y = 2 SEARCH-SIZE-X = 3 SEARCH-SIZE-Y = 4 SCAN-STEP-Y = 0.052 SCAN-STEP-X = 0.052 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category HOT STARS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 4 SAME POS AS 4 ONBOARD ACQ FOR 8 SAME POS AS 8 ONBOARD ACQ FOR 11 SAME POS AS 11 ONBOARD ACQ FOR 14 SAME POS AS 14 ONBOARD ACQ FOR 17 SAME POS AS 17 ONBOARD ACQ FOR 20 SAME POS AS 20 ONBOARD ACQ FOR 23 SAME POS AS 23 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G190H G570H G400H ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6397-HB1 NGC6397-BSS2 NGC6397-BSS4 NGC6397-BSS1 NGC6397-BSS3 NGC6397-BSS5 NGC6397-BSS6 ------------------------------------------------------------------------------------------------------------------------------------