6113( 7) - 11/22/95 15:26 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6113 Version: 7 Check-in Date: ********** 1.Proposal Title: Searching for Low Mass Stars: the Mass Function at the Hydrogen Burning Limit ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Stellar Populations GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Francesco Paresce Space Telescope Science Institute CoI: Guido De Marchi Space Telescope Science Institute N CoI: Georges Meylan European Southern Observatory N CoI: Martino Romaniello Universita di Pisa N CoI: Ralph Bohlin Space Telescope Science Institute N CoI: Roberto Buonanno Osservatorio di Roma N CoI: Francesca D'Antona Osservatorio di Roma N CoI: Vittorio Castellani Universita di Pisa N CoI: Enzo Brocato Osservatorio di Teramo N ------------------------------------------------------------------------------------ 6. Abstract We propose to use the unprecedented imaging capabilities of the WFPC2 to investigate the low-mass Main Sequence end in five globular clusters, in order to derive the luminosity and mass functions of very low mass stars (VLM). Deep photometric observations are required through the F606W and F814W wide band filters. Among the closest clusters, five objects have been selected with the same metallicity but very different dynamical histories, different destruction times, and different scale heights. This survey will obtain a detailed map of the internal properties of the clusters and, through the study of mass segregation, will gain a closer insight into the relaxation mechanisms that govern their evolution and into the effects of dynamics on stellar evolution. A thorough intercomparison of the luminosity and mass functions of several different clusters measured at various locations inside each cluster will provide useful tests on VLM stars evolution, will deliver long awaited hints on the origin of the dark matter, and will improve the understanding of the dynamical processes at work in globular clusters. As a byproduct, several cluster white dwarfs will be detected and their cooling sequence will be investigated. ------------------------------------------------------------------------------------ 6113( 7) - 11/22/95 15:26 - [ 2] Observations Description ------------------------ We propose the use of the WFPC2 to obtain V and I band images of five globular clusters to study the low mass end of the luminosity and mass functions. We have selected our sample of clusters (listed in Table 1) in such a way that the effects of the dynamical processes (both internal to the clusters and external) on the stellar population and on the present-day mass function can be investigated for VLM stars. Therefore, we have selected five clusters with different destruction times (column 3, in Gyr) and dynamical histories ("c" in column 1 means collapsed), but always with almost identical values of the metallicity (column 3). Clusters' distances have been kept as close as possible in order to minimize the overall exposure time. To see how exactly the dynamical evolution reflects on the local stellar population, we propose to observe from one to three different regions inside each cluster, depending on the concentration and angular size of the cluster itself. The number of regions and their distance from the center are indicated in Table 1 (column 9). Our strategy is to observe near the core (not exactly at the core to avoid overcrowding), at the half-mass (~ half-light) radius, and, for NGC 6809, also at ~ 1.5 r_hl. This will provide us with a detailed map of the effects of mass segregation inside each cluster. The exposure times have been estimated on the basis of our past experience with the same WFPC2 filters: we have used our previous observations of NGC 6397 taken with the same instrumental set-up and same scientific purpose (Paresce et al. 1995) and have scaled the corresponding exposure times proportionally to the distance of each cluster, in order to reach the low end of the MS at M_I ~ 12 still with a SNR about 5 (see Figure 1). Our experience shows, in fact, that a SNR of ~ 5 in the PSF peak is mandatory to efficiently identify faint stars and to guarantee accurate photometric measurements. Below this threshold, dark current fluctuations, as well as hot pixels and cosmic rays not properly removed make detection very problematic and photometry increasingly less reliable (i.e. incomplete). Isolated pixels with a peak SNR of 3 or more surrounded by pixels at (or near) the local background level are often encountered (especially when the detector is operated above - 80 ^oC). The severe PSF undersampling of the WF2--4 detectors makes it difficult to decide unambiguously whether these structures are real stars or dark current fluctuations or even cosmic ray residuals. To partly overcome the PSF undersampling we propose to apply a few (5-10) pixel offset (dithering) between consecutive images taken of each region in each filter. Based on our own experience and on that of other colleagues, this is the most efficient way of recovering faint stars with reliability and eliminate high frequency noise, thus providing complete and accurate photometry of low mass MS stars. Improving the sampling will also make it easier to discriminate, through profile fitting, between stars and faint galaxies, which are expected to be a statistically significant source of contamination at V> 26 (Bahcall et al. 1990; Paresce et al. 1995). We note in passing that one additional cluster within the same range of metallicity and distance, NGC 6752, is now being observed in Cycle 4 by Bailyn (GO 5318, Aug. 1994) and Renzini (GO 5439, Oct. 1994). These observations have completely different scientific aims from each other and from the purpose of our survey, but nevertheless include several exposures through the V and I WFPC2 filters. These two series of observations will image respectively a small area at the cluster center (where Bailyn is looking for binary stars to assess their contribution to the overall energy balance of the cluster), and a larger area located at approximately the half light radius (where Renzini has proposed to investigate the WD cooling sequence which will be used as a standard candle for the GC system). Although these observations are not intended at studying the LF at the VLM end of the MS, they will nevertheless be useful to this aim. The LFs derived from these two independent data sets (either published by the GOs or measured by ourselves when the data become public) will allow us to increase the statistical significance of the survey that we propose. We do not require any special calibration, since standard PIPELINE procedures for dark current, bias, and flat field calibration have proven most adequate to our purposes. The standard IRAF digiphot.apphot aperture photometry package (as well as the digiphot.daophot PSF-fitting photometry package for very crowded fields) will be used for data reduction. Real Time Justification ----------------------- None. As our HST WFPC2 observations of NGC 6397 (Paresce et al. 1994) have shown, the LFs of VLM MS stars as derived from ground-based data are plagued by a number of observational uncertainties. The observational difficulties inherent in the project that we propose here (Hesser 1993) make the HST the only instrument, at present, capable of reaching the scientific objectives of our investigation. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6113( 7) - 11/22/95 15:26 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6113( 7) - 11/22/95 15:26 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6254 STELLAR CLUSTER, RA=16H 57M 8.9S +/- 1S, DEC=-4D 2000 V=28 GLOBULAR CLUSTER 5' 58" +/- 1", PLATE-ID=00F5 B-V=1.8 E(B-V)=0.28 Comments: V MAGNITUDE AND B-V COLOR REFER TO THE FAINTEST STARS WE EXPECT TO DETECT 3 NGC6656 STELLAR CLUSTER, RA=18H 36M 24.2S +/- 1S, DEC=- 2000 V=28 GLOBULAR CLUSTER 23D 54' 12" +/- 1", PLATE- B-V=1.8 ID=067T E(B-V)=0.28 Comments: V MAGNITUDE AND B-V COLOR REFER TO THE FAINTEST STARS WE EXPECT TO DETECT 5 NGC6809 STELLAR CLUSTER, RA=19H 39M 59.4S +/- 1S, DEC=- 2000 V=28 GLOBULAR CLUSTER 30D 57' 44" +/- 1", PLATE- B-V=1.8 ID=02EH E(B-V)=0.28 Comments: V MAGNITUDE AND B-V COLOR REFER TO THE FAINTEST STARS WE EXPECT TO DETECT 6113( 7) - 11/22/95 15:26 - [ 5] Visit: 01 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 NGC6254 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2200S POS TARG +54,-203 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC6254 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 960S PAR WITH 10 D ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC6254 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 960S PAR WITH 10 ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC6254 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2400S SAME POS AS 10 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 21 NGC6254 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 20 D ------------------------------------------------------------------------------------------------------------------------------------ 22 NGC6254 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 20 ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC6254 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2400S SAME POS AS 10 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 31 NGC6254 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 30 D ------------------------------------------------------------------------------------------------------------------------------------ 32 NGC6254 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 30 ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC6254 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2400S SAME POS AS 10 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 41 NGC6254 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 40 D ------------------------------------------------------------------------------------------------------------------------------------ 42 NGC6254 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 40 ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC6254 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2400S SAME POS AS 10 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 51 NGC6254 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 50 D ------------------------------------------------------------------------------------------------------------------------------------ 52 NGC6254 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 50 ------------------------------------------------------------------------------------------------------------------------------------ 6113( 7) - 11/22/95 15:26 - [ 6] Visit: 02 Visit Requirements: SAME ORIENT AS 1 On Hold Comments: Additional Comments: This visit should follow Visit 1 as close as possible, and with the exact same pointing. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 60 NGC6254 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2200S POS TARG +54,-203 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 61 NGC6254 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 960S PAR WITH 60 D ------------------------------------------------------------------------------------------------------------------------------------ 62 NGC6254 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 960S PAR WITH 60 ------------------------------------------------------------------------------------------------------------------------------------ 70 NGC6254 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2400S SAME POS AS 60 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 71 NGC6254 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 70 D ------------------------------------------------------------------------------------------------------------------------------------ 72 NGC6254 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 70 ------------------------------------------------------------------------------------------------------------------------------------ 80 NGC6254 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2400S SAME POS AS 60 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 81 NGC6254 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 80 D ------------------------------------------------------------------------------------------------------------------------------------ 82 NGC6254 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 80 ------------------------------------------------------------------------------------------------------------------------------------ 90 NGC6254 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2400S SAME POS AS 60 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 91 NGC6254 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 90 D ------------------------------------------------------------------------------------------------------------------------------------ 92 NGC6254 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 90 ------------------------------------------------------------------------------------------------------------------------------------ 95 NGC6254 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2400S SAME POS AS 60 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 96 NGC6254 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 95 D ------------------------------------------------------------------------------------------------------------------------------------ 97 NGC6254 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 95 ------------------------------------------------------------------------------------------------------------------------------------ 6113( 7) - 11/22/95 15:26 - [ 7] Visit: 03 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 ------------------------------------------------------------------------------------------------------------------------------------ 300 NGC6656 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2200S POS TARG +54,-203 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 301 NGC6656 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 960S PAR WITH 300 D ------------------------------------------------------------------------------------------------------------------------------------ 302 NGC6656 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 960S PAR WITH 300 ------------------------------------------------------------------------------------------------------------------------------------ 310 NGC6656 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2400S SAME POS AS 300 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 311 NGC6656 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 310 D ------------------------------------------------------------------------------------------------------------------------------------ 312 NGC6656 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 310 ------------------------------------------------------------------------------------------------------------------------------------ 320 NGC6656 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2400S SAME POS AS 300 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 321 NGC6656 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 320 D ------------------------------------------------------------------------------------------------------------------------------------ 322 NGC6656 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 320 ------------------------------------------------------------------------------------------------------------------------------------ 330 NGC6656 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2400S SAME POS AS 300 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 331 NGC6656 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 330 D ------------------------------------------------------------------------------------------------------------------------------------ 332 NGC6656 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 330 ------------------------------------------------------------------------------------------------------------------------------------ 6113( 7) - 11/22/95 15:26 - [ 8] Visit: 04 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 ------------------------------------------------------------------------------------------------------------------------------------ 400 NGC6809 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2200S POS TARG +54,-203 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 401 NGC6809 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 960S PAR WITH 400 D ------------------------------------------------------------------------------------------------------------------------------------ 402 NGC6809 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 960S PAR WITH 400 ------------------------------------------------------------------------------------------------------------------------------------ 410 NGC6809 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2400S SAME POS AS 400 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 411 NGC6809 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 410 D ------------------------------------------------------------------------------------------------------------------------------------ 412 NGC6809 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 410 ------------------------------------------------------------------------------------------------------------------------------------ 420 NGC6809 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2400S SAME POS AS 400 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 421 NGC6809 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 420 D ------------------------------------------------------------------------------------------------------------------------------------ 422 NGC6809 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 420 ------------------------------------------------------------------------------------------------------------------------------------ 430 NGC6809 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2400S SAME POS AS 400 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 431 NGC6809 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 430 D ------------------------------------------------------------------------------------------------------------------------------------ 432 NGC6809 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 430 ------------------------------------------------------------------------------------------------------------------------------------ 440 NGC6809 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2400S SAME POS AS 400 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 441 NGC6809 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 440 D ------------------------------------------------------------------------------------------------------------------------------------ 442 NGC6809 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 440 ------------------------------------------------------------------------------------------------------------------------------------ 450 NGC6809 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2400S SAME POS AS 400 END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 451 NGC6809 FOC/96 IMAGE 512X512 F430W,F1N PIXEL=50X25 1 1100S PAR WITH 450 D ------------------------------------------------------------------------------------------------------------------------------------ 452 NGC6809 FOC/96 IMAGE 512X512 F342W PIXEL=50X25 1 1100S PAR WITH 450 ------------------------------------------------------------------------------------------------------------------------------------ 6113( 7) - 11/22/95 15:26 - [ 9] Summary Form for Proposal 6113 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 FOC/96 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=DEF PIXEL=50X25 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Stellar Populations ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements POS TARG +54,-203 END ORBIT PAR WITH 10 SAME POS AS 10 END ORBIT PAR WITH 20 PAR WITH 30 PAR WITH 40 PAR WITH 50 SAME ORIENT AS 1 PAR WITH 60 SAME POS AS 60 END ORBIT PAR WITH 70 PAR WITH 80 PAR WITH 90 PAR WITH 95 PAR WITH 300 SAME POS AS 300 END ORBIT PAR WITH 310 PAR WITH 320 PAR WITH 330 PAR WITH 400 SAME POS AS 400 END ORBIT PAR WITH 410 PAR WITH 420 PAR WITH 430 PAR WITH 440 PAR WITH 450 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W F430W,F1ND F342W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6254 NGC6656 NGC6809 ------------------------------------------------------------------------------------------------------------------------------------