8718( 3) - 07/11/00 09:44 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8718 Version: 3 Check-in Date: 11-Jul-2000 09:43:55 1.Proposal Title: Understanding the Anomalous Hot Stellar Population in Galactic Globular Clusters ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Giampaolo Piotto Universita di Padova CoI: Prof. Ivan R. King University of California, Berkeley N CoI: Prof. S. George Djorgovski California Institute of Technology N CoI: Dr. R. Michael Rich University of California at Los Angeles N CoI: Prof. Alvio Renzini European Southern Observatory N CoI: Dr. Allen V. Sweigart NASA/Goddard Space Flight Center N CoI: Dr. Marcio Catelan University of Virginia N CoI: Dr. Wayne B. Landsman NASA/Goddard Space Flight Center N CoI: Dr. Manuela Zoccali Universita di Padova N CoI: Lcda. Alejandra Recio Blanco Universidad de La Laguna N ------------------------------------------------------------------------------------ 5. Abstract We propose to image in mid- (F255W) and near-UV (F336W) the globular clusters NGC5986, NGC6273, NGC6388, and NGC6441. During our HST survey we found that these objects have a horizontal branch (HB) with an anomalously extended blue tail (EBT) and puzzling gaps along it. The survey F439W and F555W data indicate that the EBTs of the proposed targets extend up to T_eff>=40000K, implying that the hottest HB stars have lost almost all their envelope. The EBTs represent the most extreme of the mixed bag of anomalies known as ``second parameter problem'', and likely hold the key for understanding the factors affecting the HB morphology. The UV data, coupled with the existing optical images, will allow us: 1) To isolate the HB stars from other evolutionary sequences, enabling the measurement of the fraction of cluster stars in the EBTs. 2) Measure the main physical parameters (as mass and temperature) of the EBT stars. 3) Locate the gaps on the EBTs and measure their statistical significance 4) to provide tests for the origin of the EBT stars. Also the blue stragglers (BS) are better isolated in UV, allowing us to check if there is any relation between the presence of EBTs and the fraction of BS. Understanding the structure of the EBT stars, and why they are found in some, but not all systems will help to understand the stellar population of distant galaxies, and in particular the intringuing UV-upturn phonomenon in ellipticals. ------------------------------------------------------------------------------------ 8718( 3) - 07/11/00 09:44 - [ 2] Observations Description ------------------------ We propose to observe in the F255W and the F336W bands the cores of four galactic globular clusters (GCs) which we discovered to have anomalously extended horizontal branches (HB): NGC 5986, NGC 6273, NGC 6388, NGC 6441. We choose the F255W filter because it is more efficent than the F218W filter, the absorption is less effective than in the F218W band, and the corresponding exposure times needed to measure the faintest stars of interest are sensibly reduced. We will image the same fields covered in F439W and F555W bands during our previous surveys (GO6095 and GO7470). The purpose is to have mid- and near- UV flux in order to obtain the physical parameters of the hot stars in the extended blue tail (EBT) of the HB. The exposure times have been calculated using the WFPC2 exposure time calculator. We propose to reach the hottest point on the EBT of the target clusters with a S/N=10. In calculating the exposure times, we assumed a spectral type B0, appropriate for the hottest HB stars (which are NOT the brightest HB stars in the F255W band), and a S/N=10 at the magnitude of the hottest HB point. With the present choice of exposure times we expect to not saturate any cluster star. Note that our aim is to detect relatively faint UV-bright objects in a globular cluster field dominated by redder subgiants and giants. In principle, red leak should be a major concern. However, our UV-bright stars have such blue continua that there is little risk of red light from those stars affecting our colors. In any event, F255W virtually cuts off at 4500Angstrom. And while F336W has a red leak, it is nearly 3 orders of magnitude down from peak transmission and at 8000A. So we expect to easily separate our HB stars by color, and we expect that the observed colors will be unaffected by red leak (note that we will use the full filter transmission curve, including red leak, in convolution with the Kurucz atmosphere models). Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8718( 3) - 07/11/00 09:44 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC5986 STELLAR CLUSTER, RA=15H 46M 3.5S +/- 0.1S, DEC=- J2000 SURF(V) = 16.89 GLOBULAR CLUSTER 37D 47' 10.3" +/- 0.1" Coordinate Source: OTHER_SOURCE Comments: Same coordinates as exposure U49N1101R in GO7470 2 NGC6273 STELLAR CLUSTER, RA=17H 2M 37.7S +/- 0.1S, DEC=- J2000 SURF(V) = 15.65 GLOBULAR CLUSTER 26D 16' 5.0" +/- 0.1" Coordinate Source: OTHER_SOURCE Comments: Same coordinates as exposure U49N1801R in GO7470 3 NGC6388 STELLAR CLUSTER, RA=17H 36M 17.0S +/- 0.1S, DEC=- J2000 SURF(V) = 13.45 GLOBULAR CLUSTER 44D 44' 06.0" +/- 0.1" Coordinate Source: OTHER_SOURCE Comments: Same coordinates as exposure U2VO0701T in GO7470 4 NGC6441 STELLAR CLUSTER, RA=17H 50M 12.9S +/- 0.1S, DEC=- J2000 SURF(V) = 13.53 GLOBULAR CLUSTER 37D 03' 04.0" +/- 0.1" Coordinate Source: OTHER_SOURCE Comments: Same coordinates as exposure U2VO0801T in GO7470 8718( 3) - 07/11/00 09:44 - [ 4] Visit: 01 Visit Priority: Visit Requirements: PCS MODE F ORIENT 2.0D TO 3.0D On Hold Comments: Additional Comments: Same orientation as exposure U49N1101R in GO7470 (Roll Angle=182.5, i.e. orient=2.5) Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC5986 WFPC2 IMAGE PC1-FIX F336W CR-SPLIT=NO,ATD-GAIN=7 2 160 S ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC5986 WFPC2 IMAGE PC1-FIX F255W CR-TOLERANCE=1,ATD-GAIN=7 1 2000 S SAME POS AS 11 ------------------------------------------------------------------------------------------------------------------------------------ 8718( 3) - 07/11/00 09:44 - [ 5] Visit: 02 Visit Priority: Visit Requirements: PCS MODE F ORIENT 280.7D TO 281.7D On Hold Comments: Additional Comments: Same orientation as exposure U49N1801R in GO7470 (Roll Angle=101.2, i.e. orient=281.2) Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 21 NGC6273 WFPC2 IMAGE PC1-FIX F336W CR-SPLIT=NO,ATD-GAIN=7 1 300 S POS TARG 0.000, 0.000 ------------------------------------------------------------------------------------------------------------------------------------ 22 NGC6273 WFPC2 IMAGE PC1-FIX F336W CR-SPLIT=NO,ATD-GAIN=7 1 400 S POS TARG 0.000, 0.000 EXPAND MAX DUR 140% ------------------------------------------------------------------------------------------------------------------------------------ 23 NGC6273 WFPC2 IMAGE PC1-FIX F255W CR-TOLERANCE=1,ATD-GAIN=7 1 1400 S POS TARG 0.000, 0.000 ------------------------------------------------------------------------------------------------------------------------------------ 24 NGC6273 WFPC2 IMAGE PC1-FIX F255W CR-TOLERANCE=1,ATD-GAIN=7 1 2600 S POS TARG 0.567, 0.333 EXPAND MAX DUR 120% Comments: POS TARG for dithering ------------------------------------------------------------------------------------------------------------------------------------ 25 NGC6273 WFPC2 IMAGE PC1-FIX F255W CR-TOLERANCE=1,ATD-GAIN=7 1 2600 S POS TARG 0.333, 0.567 EXPAND MAX DUR 120% Comments: POS TARG for dithering ------------------------------------------------------------------------------------------------------------------------------------ 8718( 3) - 07/11/00 09:44 - [ 6] Visit: 03 Visit Priority: Visit Requirements: PCS MODE F ORIENT 264.9D TO 265.9D On Hold Comments: Additional Comments: Same orientation as exposure U2VO0701T in GO6095 (Roll Angle=85.4, i.e. orient=265.4) Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 31 NGC6388 WFPC2 IMAGE PC1-FIX F336W CR-TOLERANCE=.1,ATD-GAIN=7 1 800 S POS TARG 0.000, 0.000 ------------------------------------------------------------------------------------------------------------------------------------ 32 NGC6388 WFPC2 IMAGE PC1-FIX F336W CR-SPLIT=NO,ATD-GAIN=7 1 200 S POS TARG 0.000, 0.000 EXPAND MAX DUR 140% ------------------------------------------------------------------------------------------------------------------------------------ 33 NGC6388 WFPC2 IMAGE PC1-FIX F255W CR-SPLIT=NO,ATD-GAIN=7 1 1100 S POS TARG 0.000, 0.000 ------------------------------------------------------------------------------------------------------------------------------------ 34 NGC6388 WFPC2 IMAGE PC1-FIX F255W CR-TOLERANCE=1,ATD-GAIN=7 1 2600 S POS TARG 0.000, 0.250 EXPAND MAX DUR 120% Comments: POS TARG for dithering ------------------------------------------------------------------------------------------------------------------------------------ 35 NGC6388 WFPC2 IMAGE PC1-FIX F255W CR-TOLERANCE=1,ATD-GAIN=7 1 2600 S POS TARG 0.250, 0.000 EXPAND MAX DUR 120% Comments: POS TARG for dithering ------------------------------------------------------------------------------------------------------------------------------------ 36 NGC6388 WFPC2 IMAGE PC1-FIX F255W CR-TOLERANCE=1,ATD-GAIN=7 1 2600 S POS TARG 0.250, 0.250 EXPAND MAX DUR 120% Comments: POS TARG for dithering ------------------------------------------------------------------------------------------------------------------------------------ 8718( 3) - 07/11/00 09:44 - [ 7] Visit: 04 Visit Priority: Visit Requirements: PCS MODE F ORIENT 106.3D TO 116.3D On Hold Comments: Additional Comments: Same orientation as exposure U2VO0801T in GO6095 (Roll Angle=291.3, i.e. orient=111.3) Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 41 NGC6441 WFPC2 IMAGE PC1-FIX F336W CR-TOLERANCE=1,ATD-GAIN=7 1 800 S POS TARG 0.000, 0.000 ------------------------------------------------------------------------------------------------------------------------------------ 42 NGC6441 WFPC2 IMAGE PC1-FIX F336W CR-SPLIT=NO,ATD-GAIN=7 1 200 S POS TARG 0.000, 0.000 EXPAND MAX DUR 140% ------------------------------------------------------------------------------------------------------------------------------------ 43 NGC6441 WFPC2 IMAGE PC1-FIX F255W CR-SPLIT=NO,ATD-GAIN=7 1 1100 S POS TARG 0.000, 0.000 ------------------------------------------------------------------------------------------------------------------------------------ 44 NGC6441 WFPC2 IMAGE PC1-FIX F255W CR-TOLERANCE=1,ATD-GAIN=7 1 2600 S POS TARG 0.000, 0.250 EXPAND MAX DUR 120% Comments: POS TARG for dithering ------------------------------------------------------------------------------------------------------------------------------------ 45 NGC6441 WFPC2 IMAGE PC1-FIX F255W CR-TOLERANCE=1,ATD-GAIN=7 1 2600 S POS TARG 0.250, 0.000 EXPAND MAX DUR 120% Comments: POS TARG for dithering ------------------------------------------------------------------------------------------------------------------------------------ 46 NGC6441 WFPC2 IMAGE PC1-FIX F255W CR-TOLERANCE=1,ATD-GAIN=7 1 2600 S POS TARG 0.250, 0.250 EXPAND MAX DUR 120% Comments: POS TARG for dithering ------------------------------------------------------------------------------------------------------------------------------------ 8718( 3) - 07/11/00 09:44 - [ 8] Summary Form for Proposal 8718 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ATD-GAIN=7 CR-TOLERANCE=1 CR-TOLERANCE=.1 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE F ORIENT 2.0D TO 3.0D SAME POS AS 11 PCS MODE F ORIENT 280.7D TO 281.7D POS TARG 0.000, 0.000 POS TARG 0.000, 0.000 EXPAND MAX DUR 140% POS TARG 0.567, 0.333 EXPAND MAX DUR 120% POS TARG 0.333, 0.567 EXPAND MAX DUR 120% PCS MODE F ORIENT 264.9D TO 265.9D POS TARG 0.000, 0.250 EXPAND MAX DUR 120% POS TARG 0.250, 0.000 EXPAND MAX DUR 120% POS TARG 0.250, 0.250 EXPAND MAX DUR 120% PCS MODE F ORIENT 106.3D TO 116.3D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F336W F255W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC5986 NGC6273 NGC6388 NGC6441 ------------------------------------------------------------------------------------------------------------------------------------