8124( 4) - 10/28/99 12:42 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8124 Version: 4 Check-in Date: 19-Feb-1999 16:53:45 1.Proposal Title: The Stellar Initial Mass Function of Globular Clusters ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Giampaolo Piotto University of Padova CoI: Dr. Enrico Vesperini University of Massachusetts-Amherst N CoI: Dr. Gordon Drukier Tel Aviv University N CoI: Dr. M. Zoccali Universita di Padova N CoI: Prof. Adrienne Cool San Francisco State University N CoI: Prof. Ivan R. King University of California, Berkeley N CoI: Prof. Douglas Heggie University of Edinburgh N CoI: Dr. Francesca D'Antona Osservatorio Astronomico di Roma N ------------------------------------------------------------------------------------ 5. Abstract Knowledge of star formation in old populations is a vital ingredient in understanding galactic evolution. Globular star clusters offer the possibility of observing the mass functions of homogeneous old stellar populations. Those for which reliable mass functions have been determined already, however, have generally been subject to dynamical evolution (relaxation, escape of stars, and tidal shocks), so that the observed mass function is not primordial. We have therefore examined tables of properties of globular clusters in order to choose those that are least likely to have suffered from relaxation, escape, and tidal shocks. We propose two new targets (NGC 2808 and M53), and dynamical rediscussion of existing data on one other (M3). To minimize mass-segregation corrections, in each cluster we will select a field at the radius where the local mass function most closely resembles the global mass function. The range of metallicities of the targets will also allow us to estimate the influence of metallicity on the IMF. The team has leading-edge expertise in all the techniques required: making the observations and reducing them to luminosity functions, converting to mass functions, and all types of dynamical modeling, which are required to make the small corrections to global and then initial mass functions. ------------------------------------------------------------------------------------ 8124( 4) - 10/28/99 12:42 - [ 2] Observations Description ------------------------ In each cluster we will choose a field where our dynamical models suggest that the local MF does not differ much from the global one. We propose to use the F606W and F814W filters of WFPC2 for V and I. Although F606W is less close to standard V, it has a better throughput, allowing us to halve the exposure time in V. (We can compare the theoretical models as easily with F606W observations as with F555W.) In this way, the exposure time will be the same in the V and I bands. Note that the choice of the F606W filter does not in any way affect the LF comparison. Within each orbit we will obtain 3 (NGC2808) or 2 (NGC5024) long exposures (for cosmic-ray removal) and one short exposure (100s). Small shifts of a few pixels between orbits will remove ambiguities associated with hot pixels, and the shifts will include fractional-pixel dithers to improve our photometry. In each orbit, the short exposures are necessary to correct saturated pixels in the longer exposures, which is essential for obtaining the high- quality point spread functions needed to carry out accurate photometry and to extend the photometry upward to stars at the base of the red giant branch. Our experience with WFPC2 images of moderately crowded cluster fields shows that in V and I the detection limit is invariably set by a diffuse background of light that is, at least in part, the result of large-angle scattering from the many bright stars in the field (Krist & Burrows 1994). In fields with surface brightnesses comparable to the ones we propose to observe, the typical background level is ~ 15 DN (~eq 100 e^-) per 1000s (similar in both filters), which combines with the read noise (5 e^-) to yield a noise of ~eq 11 e^- per pixel. The faint (V ~eq 27.5), red (V-I ~eq 1.5- 2.0) stars we wish to detect will, according to the WFPC2 Handbook and synphot, yield in F606W approximately 90 e^- in a detection aperture of radius 1.5 WF pixels (7- pixel area), resulting in S/N = 3.0 in a single image, and 7.5 in the 6 planned exposures, combined. In the same size aperture, these stars (which have I ~eq 25.5--26.0) should generate ~eq 160 e^- in 1000s in F814W, for S/N = 5.4 in one exposure, and 13 in the 6 exposures combined. Combining these results suggests that we will measure colors for these faint stars to an accuracy of 0.15 magnitude. These estimates are in good agreement with the results we have obtained in our previous work on five globular clusters with WFPC2. This means that we will be able to count stars down to an absolute magnitude M_V>~11.1, corresponding to m<~0.2m_\odot, in the farthest cluster (NGC5024), faint enough to give a significant comparison among the IMFs in the 2 clusters of our sample. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8124( 4) - 10/28/99 12:42 - [ 3] Data Distribution Paper Products: NO Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8124( 4) - 10/28/99 12:42 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC2808 STELLAR CLUSTER, RA=9H 12M 02.6S +/- 0.1S, DEC=- J2000 SURF(V) = 14.430 GLOBULAR CLUSTER 64D 51' 47" +/- 0.1" 2 NGC5024 STELLAR CLUSTER, RA=13H 12M 55.3S +/- 0.1S, J2000 SURF(V) = 17.370 GLOBULAR CLUSTER DEC=+18D 10' 09" +/- 0.1" 8124( 4) - 10/28/99 12:42 - [ 5] Visit: 01 Visit Priority: Visit Requirements: PCS MODE F ORIENT 295D TO 345D ORIENT 50D TO 155D On Hold Comments: Additional Comments: The orientation was chosen in order to avoid a few bright stars. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC2808 WFPC2 IMAGE WF2-FIX F606W CR-SPLIT=NO,ATD-GAIN=7 1 100 S POS TARG -75.0,+75.0 Comments: POS TARG was chosen in order to orient the WF2-WF4 diagonal in radial direction and the inner corner of WF2 at 50 arcsec from the cluster center ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC2808 WFPC2 IMAGE WF2-FIX F606W CR-SPLIT=NO,ATD-GAIN=7 1 800 S SAME POS AS 11 ------------------------------------------------------------------------------------------------------------------------------------ 13 NGC2808 WFPC2 IMAGE WF2-FIX F606W CR-TOLERANCE=0.01,ATD-GAIN=7 1 1400 S SAME POS AS 11 ------------------------------------------------------------------------------------------------------------------------------------ 14 NGC2808 WFPC2 IMAGE WF2-FIX F606W CR-SPLIT=NO,ATD-GAIN=7 1 100 S POS TARG -75.25,+75.25 Comments: Dithering ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC2808 WFPC2 IMAGE WF2-FIX F606W CR-TOLERANCE=0.01,ATD-GAIN=7 1 1600 S SAME POS AS 14 ------------------------------------------------------------------------------------------------------------------------------------ 16 NGC2808 WFPC2 IMAGE WF2-FIX F606W CR-SPLIT=NO,ATD-GAIN=7 1 700 S SAME POS AS 14 ------------------------------------------------------------------------------------------------------------------------------------ 17 NGC2808 WFPC2 IMAGE WF2-FIX F606W CR-SPLIT=NO,ATD-GAIN=7 1 100 S POS TARG -74.875,+74.875 Comments: Dithering ------------------------------------------------------------------------------------------------------------------------------------ 18 NGC2808 WFPC2 IMAGE WF2-FIX F606W CR-TOLERANCE=0.01,ATD-GAIN=7 1 1600 S SAME POS AS 17 ------------------------------------------------------------------------------------------------------------------------------------ 19 NGC2808 WFPC2 IMAGE WF2-FIX F606W CR-SPLIT=NO,ATD-GAIN=7 1 700 S SAME POS AS 17 ------------------------------------------------------------------------------------------------------------------------------------ 8124( 4) - 10/28/99 12:42 - [ 6] Visit: 02 Visit Priority: Visit Requirements: PCS MODE F SAME ORIENT AS 1 On Hold Comments: Additional Comments: The orientation was chosen in order to avoid a few bright stars. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 21 NGC2808 WFPC2 IMAGE WF2-FIX F814W CR-SPLIT=NO,ATD-GAIN=7 1 100 S POS TARG -75.0,+75.0 Comments: POS TARG was chosen in order to orient the WF2-WF4 diagonal in radial direction and the inner corner of WF2 at 50 arcsec from the cluster center ------------------------------------------------------------------------------------------------------------------------------------ 22 NGC2808 WFPC2 IMAGE WF2-FIX F814W CR-SPLIT=NO,ATD-GAIN=7 1 800 S SAME POS AS 21 ------------------------------------------------------------------------------------------------------------------------------------ 23 NGC2808 WFPC2 IMAGE WF2-FIX F814W CR-TOLERANCE=0.01,ATD-GAIN=7 1 1400 S SAME POS AS 21 ------------------------------------------------------------------------------------------------------------------------------------ 24 NGC2808 WFPC2 IMAGE WF2-FIX F814W CR-SPLIT=NO,ATD-GAIN=7 1 100 S POS TARG -75.25,+75.25 Comments: Dithering ------------------------------------------------------------------------------------------------------------------------------------ 25 NGC2808 WFPC2 IMAGE WF2-FIX F814W CR-TOLERANCE=0.01,ATD-GAIN=7 1 1600 S SAME POS AS 24 ------------------------------------------------------------------------------------------------------------------------------------ 26 NGC2808 WFPC2 IMAGE WF2-FIX F814W CR-SPLIT=NO,ATD-GAIN=7 1 700 S SAME POS AS 24 ------------------------------------------------------------------------------------------------------------------------------------ 27 NGC2808 WFPC2 IMAGE WF2-FIX F814W CR-SPLIT=NO,ATD-GAIN=7 1 100 S POS TARG -74.875,+74.875 Comments: Dithering ------------------------------------------------------------------------------------------------------------------------------------ 28 NGC2808 WFPC2 IMAGE WF2-FIX F814W CR-TOLERANCE=0.01,ATD-GAIN=7 1 1600 S SAME POS AS 27 ------------------------------------------------------------------------------------------------------------------------------------ 29 NGC2808 WFPC2 IMAGE WF2-FIX F814W CR-SPLIT=NO,ATD-GAIN=7 1 700 S SAME POS AS 27 ------------------------------------------------------------------------------------------------------------------------------------ 8124( 4) - 10/28/99 12:42 - [ 7] Visit: 03 Visit Priority: Visit Requirements: PCS MODE F 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 ------------------------------------------------------------------------------------------------------------------------------------ 31 NGC5024 WFPC2 IMAGE WF2-FIX F606W CR-SPLIT=NO,ATD-GAIN=7 1 100 S POS TARG -61.0,+61.0 Comments: POS TARG was chosen in order to orient the WF2-WF4 diagonal in radial direction and the inner corner of WF2 at 30 arcsec from the cluster center ------------------------------------------------------------------------------------------------------------------------------------ 32 NGC5024 WFPC2 IMAGE WF2-FIX F606W CR-SPLIT=NO,ATD-GAIN=7 1 1000 S SAME POS AS 31 ------------------------------------------------------------------------------------------------------------------------------------ 33 NGC5024 WFPC2 IMAGE WF2-FIX F606W CR-SPLIT=NO,ATD-GAIN=7 1 1100 S SAME POS AS 31 ------------------------------------------------------------------------------------------------------------------------------------ 34 NGC5024 WFPC2 IMAGE WF2-FIX F606W CR-SPLIT=NO,ATD-GAIN=7 1 100 S POS TARG -61.25,+61.25 Comments: Dithering ------------------------------------------------------------------------------------------------------------------------------------ 35 NGC5024 WFPC2 IMAGE WF2-FIX F606W CR-TOLERANCE=0.01,ATD-GAIN=7 1 2200 S SAME POS AS 34 ------------------------------------------------------------------------------------------------------------------------------------ 36 NGC5024 WFPC2 IMAGE WF2-FIX F606W CR-SPLIT=NO,ATD-GAIN=7 1 100 S POS TARG -60.875,+60.875 Comments: Dithering ------------------------------------------------------------------------------------------------------------------------------------ 37 NGC5024 WFPC2 IMAGE WF2-FIX F606W CR-TOLERANCE=0.01,ATD-GAIN=7 1 2200 S SAME POS AS 36 ------------------------------------------------------------------------------------------------------------------------------------ 8124( 4) - 10/28/99 12:42 - [ 8] Visit: 04 Visit Priority: Visit Requirements: PCS MODE F SAME ORIENT AS 3 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 ------------------------------------------------------------------------------------------------------------------------------------ 41 NGC5024 WFPC2 IMAGE WF2-FIX F814W CR-SPLIT=NO,ATD-GAIN=7 1 100 S POS TARG -61.0,+61.0 Comments: POS TARG was chosen in order to orient the WF2-WF4 diagonal in radial direction and the inner corner of WF2 at 30 arcsec from the cluster center ------------------------------------------------------------------------------------------------------------------------------------ 42 NGC5024 WFPC2 IMAGE WF2-FIX F814W CR-SPLIT=NO,ATD-GAIN=7 1 1000 S SAME POS AS 41 ------------------------------------------------------------------------------------------------------------------------------------ 43 NGC5024 WFPC2 IMAGE WF2-FIX F814W CR-SPLIT=NO,ATD-GAIN=7 1 1100 S SAME POS AS 41 ------------------------------------------------------------------------------------------------------------------------------------ 44 NGC5024 WFPC2 IMAGE WF2-FIX F814W CR-SPLIT=NO,ATD-GAIN=7 1 100 S POS TARG -61.25,+61.25 Comments: Dithering ------------------------------------------------------------------------------------------------------------------------------------ 45 NGC5024 WFPC2 IMAGE WF2-FIX F814W CR-TOLERANCE=0.01,ATD-GAIN=7 1 2200 S SAME POS AS 44 ------------------------------------------------------------------------------------------------------------------------------------ 46 NGC5024 WFPC2 IMAGE WF2-FIX F814W CR-SPLIT=NO,ATD-GAIN=7 1 100 S POS TARG -60.875,+60.875 Comments: Dithering ------------------------------------------------------------------------------------------------------------------------------------ 47 NGC5024 WFPC2 IMAGE WF2-FIX F814W CR-TOLERANCE=0.01,ATD-GAIN=7 1 2200 S SAME POS AS 46 ------------------------------------------------------------------------------------------------------------------------------------ 8124( 4) - 10/28/99 12:42 - [ 9] Summary Form for Proposal 8124 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WF2-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ATD-GAIN=7 CR-TOLERANCE=0.01 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE F ORIENT 295D TO 345D ORIENT 50D TO 155D POS TARG -75.0,+75.0 SAME POS AS 11 POS TARG -75.25,+75.25 SAME POS AS 14 POS TARG -74.875,+74.875 SAME POS AS 17 PCS MODE F SAME ORIENT AS 1 SAME POS AS 21 SAME POS AS 24 SAME POS AS 27 PCS MODE F POS TARG -61.0,+61.0 SAME POS AS 31 POS TARG -61.25,+61.25 SAME POS AS 34 POS TARG -60.875,+60.875 SAME POS AS 36 PCS MODE F SAME ORIENT AS 3 SAME POS AS 41 SAME POS AS 44 SAME POS AS 46 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC2808 NGC5024 ------------------------------------------------------------------------------------------------------------------------------------