8617( 2) - 02/28/00 06:23 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8617 Version: 2 Check-in Date: 28-Feb-2000 06:22:45 1.Proposal Title: Ultraviolet Spectroscopy of Hot Horizontal-Branch Stars in the Globular Cluster M13 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Bradford Behr California Institute of Technology CoI: Prof. Judith Cohen California Institute of Technology N ------------------------------------------------------------------------------------ 5. Abstract Blue horizontal-branch (BHB) stars in the metal-poor globular clusters M13, M15, NGC 288 and NGC 6752 have recently been found to exhibit remarkable metallicity enhancements and helium depletions relative to the canonical cluster composition. These abundance anomalies are most likely due to diffusion processes --- radiative levitation of the metals, and gravitational settling of helium --- in the stable radiative atmospheres of these hot stars. With available ground-based facilities, we have observed stars in M13 as hot as 19000 K, but beyond this point we are constrained by low V- and B-band flux and an insufficient number of visible- wavelength spectral lines. We therefore propose HST/STIS spectroscopic observations in the wavelength range 1700-3100 Angstrom to extend the abundance measurements in M13 to yet hotter stars. Such measurements will provide a new and important confrontation between theory and observations of the diffusion mechanisms, and will also offer insights into poorly-understood aspects of CMD morphology, including the BHB `gaps' and overluminous BHB stars observed in many clusters. ------------------------------------------------------------------------------------ 8617( 2) - 02/28/00 06:23 - [ 2] Observations Description ------------------------ Because the target stars are quite faint (U magnitude ~ 17), we are limited to the lower-resolution spectroscopic modes of STIS. The G230LB grating and the 52 * 0.1 arcsec slit will provide a spectral resolution DeltaLambda = 2.7 Angstrom per 2-pixel resolution element over a spectral range, 1680-3060 Angstrom, which includes several strong metal lines, arising from Fe III, C II, C III, O III, and Mg II transitions. As shown in the appended synthesis plot, the spectrum will be very crowded at this resolution. We will need to use spectral synthesis techniques to derive element abundances, instead of the simpler equivalent- width fitting approach that we employed in previous studies. The LINFOR line formation codes that we plan to use are equipped to handle such synthesis. To derive element abundances to an accuracy of 0.1 dex, we will need S/N of approximately 50 per resolution element in our spectra. The STIS Exposure Time Calculator suggests 9400 seconds of integration, or 160 minutes, to reach this level at 2500 Angstrom for a U = 17 B1 star. At a declination of +36 degrees, we will have 54 minutes of visibility per orbit, of which 13 will be spent on guide star acquisition (5-6 min), target acquisition (6 min), and instrument setup (1 min), leaving 41 minutes of exposure. We thus require 4 orbits per visit. Target stars have been selected from archival HST/WFPC photometry. Their T_eff run from 17000 K, similar to the bluest stars previously studied via ground-based observations, up to 30000 K. This range spans the photometric gap which appears in M13's BHB color distribution at about 22000 K. We will re- observe one of the hottest stars previously studied from the ground, in order to verify that the metallicity values derived from the two different spectral regimes are in agreement. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8617( 2) - 02/28/00 06:23 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6205- STAR, HORIZONTAL BRANCH RA=16H 41M 48.41S +/- 1", J2000 V = - U = 16.233+/-0.013 164148+36273 STAR DEC=36D 27' 33.4" +/- 1", PLATE- 246.6 TYPE=B3V 4 ID=00CK E(B-V) = 0.02+/-0.01 U-V = -0.838+/-0.027 2 NGC6205- STAR, HORIZONTAL BRANCH RA=16H 41M 47.85S +/- 1", J2000 V = - U = 16.449+/-0.015 164148+36281 STAR DEC=36D 28' 17.3" +/- 1", PLATE- 246.6 TYPE=B3V 8 ID=00CK E(B-V) = 0.02+/-0.01 U-V = -0.916+/-0.021 3 NGC6205- STAR, HORIZONTAL BRANCH RA=16H 41M 45.31S +/- 1", J2000 V = - U = 16.689+/-0.010 164145+36263 STAR DEC=36D 26' 33.8" +/- 1", PLATE- 246.6 TYPE=B3V 4 ID=00CK E(B-V) = 0.02+/-0.01 U-V = -1.110+/-0.024 4 NGC6205- STAR, HORIZONTAL BRANCH RA=16H 41M 44.32S +/- 1", J2000 V = - U = 16.695+/-0.014 164144+36264 STAR DEC=36D 26' 43.3" +/- 1", PLATE- 246.6 TYPE=B3V 4 ID=00CK E(B-V) = 0.02+/-0.01 U-V = -1.056+/-0.025 5 NGC6205- STAR, HORIZONTAL BRANCH RA=16H 41M 43.55S +/- 1", J2000 V = - U = 16.936+/-0.015 164144+36280 STAR DEC=36D 28' 2.6" +/- 1", PLATE- 246.6 TYPE=B3V 3 ID=00CK E(B-V) = 0.02+/-0.01 U-V = -1.093+/-0.024 8617( 2) - 02/28/00 06:23 - [ 4] Visit: 01 Visit Priority: Visit Requirements: 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 NGC6205-164 STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 15 S 148+362734 CD P ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC6205-164 STIS/C ACQ/PEA 52X0.1 MIRROR 1 20 S 148+362734 CD K ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=2 1 30 M 148+362734 CD ------------------------------------------------------------------------------------------------------------------------------------ 13 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 148+362734 CD ------------------------------------------------------------------------------------------------------------------------------------ 14 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 148+362734 CD ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 148+362734 CD ------------------------------------------------------------------------------------------------------------------------------------ 8617( 2) - 02/28/00 06:23 - [ 5] Visit: 02 Visit Priority: Visit Requirements: 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 ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC6205-164 STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 20 S 148+362818 CD P ------------------------------------------------------------------------------------------------------------------------------------ 21 NGC6205-164 STIS/C ACQ/PEA 52X0.1 MIRROR 1 25 S 148+362818 CD K ------------------------------------------------------------------------------------------------------------------------------------ 22 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=2 1 30 M 148+362818 CD ------------------------------------------------------------------------------------------------------------------------------------ 23 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 148+362818 CD ------------------------------------------------------------------------------------------------------------------------------------ 24 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 148+362818 CD ------------------------------------------------------------------------------------------------------------------------------------ 25 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 148+362818 CD ------------------------------------------------------------------------------------------------------------------------------------ 8617( 2) - 02/28/00 06:23 - [ 6] Visit: 03 Visit Priority: Visit Requirements: 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 ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC6205-164 STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 20 S 145+362634 CD P ------------------------------------------------------------------------------------------------------------------------------------ 31 NGC6205-164 STIS/C ACQ/PEA 52X0.1 MIRROR 1 25 S 145+362634 CD K ------------------------------------------------------------------------------------------------------------------------------------ 32 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=2 1 30 M 145+362634 CD ------------------------------------------------------------------------------------------------------------------------------------ 33 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 145+362634 CD ------------------------------------------------------------------------------------------------------------------------------------ 34 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 145+362634 CD ------------------------------------------------------------------------------------------------------------------------------------ 35 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 145+362634 CD ------------------------------------------------------------------------------------------------------------------------------------ 8617( 2) - 02/28/00 06:23 - [ 7] Visit: 04 Visit Priority: Visit Requirements: 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 ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC6205-164 STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 25 S 144+362644 CD P ------------------------------------------------------------------------------------------------------------------------------------ 41 NGC6205-164 STIS/C ACQ/PEA 52X0.1 MIRROR 1 30 S 144+362644 CD K ------------------------------------------------------------------------------------------------------------------------------------ 42 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=2 1 30 M 144+362644 CD ------------------------------------------------------------------------------------------------------------------------------------ 43 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 144+362644 CD ------------------------------------------------------------------------------------------------------------------------------------ 44 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 144+362644 CD ------------------------------------------------------------------------------------------------------------------------------------ 45 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 144+362644 CD ------------------------------------------------------------------------------------------------------------------------------------ 8617( 2) - 02/28/00 06:23 - [ 8] Visit: 05 Visit Priority: Visit Requirements: 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 ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC6205-164 STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 30 S 144+362803 CD P ------------------------------------------------------------------------------------------------------------------------------------ 51 NGC6205-164 STIS/C ACQ/PEA 52X0.1 MIRROR 1 30 S 144+362803 CD K ------------------------------------------------------------------------------------------------------------------------------------ 52 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=2 1 30 M 144+362803 CD ------------------------------------------------------------------------------------------------------------------------------------ 53 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 144+362803 CD ------------------------------------------------------------------------------------------------------------------------------------ 54 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 144+362803 CD ------------------------------------------------------------------------------------------------------------------------------------ 55 NGC6205-164 STIS/C ACCUM 52X0.1 G230LB 2375 CR-SPLIT=3 1 45 M 144+362803 CD ------------------------------------------------------------------------------------------------------------------------------------ 8617( 2) - 02/28/00 06:23 - [ 9] Summary Form for Proposal 8617 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT CR-SPLIT=2 CR-SPLIT=3 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G230LB ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6205-164148+362734 NGC6205-164148+362818 NGC6205-164145+362634 NGC6205-164144+362644 NGC6205-164144+362803 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 2375 ------------------------------------------------------------------------------------------------------------------------------------