6572( 5) - 12/03/96 13:26 - [ 0] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6572 Version: 5 Check-in Date: 03-Dec-1996 13:21:38 1.Proposal Title: Searching for Low Mass Stars: the Mass Function at the H Burning Limit ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle STELLAR POPULATIONS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Francesco Paresce EUROPEAN SOUTHER OBSERVATORY CoI: Martino Romaniello Scuola Normale Superiore 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 CoI: Guido De Marchi Space Telescope Science Institute N CoI: Georges Meylan European Southern Observatory N ------------------------------------------------------------------------------------ 6. Abstract This is the natural continuation of our Cycle-5 program aimed at deriving the luminosity and mass function of very low mass stars (VLM) in globular clusters (GCs). Cycle-5 observations, scheduled for 1995 November, will study three clusters all with the same metallicity, yet with completely different dynamical histories (both internal and due to the interaction with the Galaxy) to investigate the effects of dynamics on the present day mass function at the VLM end. We propose now to use the same technique (deep WFPC2 observations in the F606W and F814W bands) to analyze four clusters with very similar dynamical evolution, but spanning a large range of metallicity. Recent studies show that metallicity may affect the star formation process in GCs, resulting in a steeper IMF at low masses for metal-poor objects, although a flattening below ~ 0.3 Msun is expected in all cases. A thorough intercomparison of the luminosity and mass functions of several different clusters 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 sequences will be investigated. ------------------------------------------------------------------------------------ Observations Description ------------------------ We propose the use of the WFPC2 to obtain V and I band images of four 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 metallicity on the IMF of the stellar population can be investigated for VLM stars through the analysis of their present-day mass function. Therefore, we have selected four clusters with similar dynamical state (all of them are relaxed) and dynamical histories (destruction times, column 3 in units of T_ Hubb.), but with different values of the metallicity (column 4). Since we already have observed the VLM luminosity and mass function of two low- metallicity GCs (Fe/H~eq -1.9), and since our Cycle 5 program (to be executed in 1995 October) will provide observations of three more such clusters, we have selected here objects with higher metal content, by choosing two GCs with Fe/H~eq -0.5 and two with Fe/H~eq -1.0. Although, in principle, one object per metallicity class would suffice, the second pair is added to allow for a consistency check and to improve the statistics. Nonetheless, to minimize the overall exposure time clusters' distances have been kept as close as possible (distance moduli (m-M)_V in column 2), compatibly with the selection criteria. begintablet begintabularlcccccccc multicolumn9cTable1: Physical parameters of the selected clusters and required exposure times, multicolumn1cCluster &(*) &(^+) &(*) &(*) &(*) & t_ f606w & t_ f814w & Total, multicolumn1lNGC & multicolumn1c(m--M)_V & multicolumn1cT_d & multicolumn1cFe/H & multicolumn1cr_hl & multicolumn1cr_c& multicolumn1c(min)& multicolumn1c(min)& multicolumn1cOrb., 6838 & 13.8 & 2.9 & -0.58 & 127" & 38" & 90 & 125 & 5 , 6723 & 14.8 & 2.6 & -1.09 & 86" & 57" & 300 & 340 & 15 , 6352 & 14.5 & 3.0 & -0.51 & 117" & 50" & 170 & 210 & 9 , 6171 & 15.0 & 3.7 & -0.99 & 104" & 32" & 460 & 500 & 23 , endtabular, (*) Djorgovski (1993), (^+) Aguilar, Hut & Ostriker (1989) endtable Our strategy is to observe near the half-mass (~ half-light) radius, where the present day MF is expected be as close as possible to the IMF (Richer et al. 1991). In order to derive the MF we need to convert the LF (the observed data) by the use of the M-L relation. This is a very delicate issue, which has plagued previous determinations of the MF in GCs, because of the uncertainties associated with the M-L itself at these low masses. Models developed by one of us (FD), however, appear today to proceede secure and find confirmation in the observational data (D'Antona 1995; D'Antona & Mazzitelli 1995). We should, nevertheless, notice that the cross-comparison of the LFs themselves is already very significant, as we have accurately selected objects for which the dynamical history (both internal and resulting from the Galactic tidal field) is expected to be the same. As dynamics is the only process that can modify the IMF in time, having selected clusters with a similar dynamical evolution would ensure that any differences found in the LFs originate in the IMFs, in that the latter were different at birth due to metallicity. Columns 7 and 8 show the exposure times needed to reach, for each region in each cluster, the low end of the MS with the F606W and F814W broad band filters, while column 9 gives the total required number of orbits for each cluster. These 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, M 15, and 47 Tuc taken with the same instrumental set-up and same scientific purpose (Paresce et al. 1995; De Marchi & Paresce 1995a, 1995b) 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 ~eq 12 still with SNR ~eq 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). The exposure times calculated in this way are in fair agreement with those obtained by running the ``on-line'' WFPC2 simulator available on the STScI WWW server. 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 this inconvenience, 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, J.N., Guhathakurta, P., and Schneider, D.P. 1990, Science, 248, 178; Paresce et al. 1995a). 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 and ROMAFOT PSF-fitting photometry packages for more crowded fields, will be used for data reduction. Real Time Justification ----------------------- As our HST WFPC2 observations of NGC 6397 (Paresce et al. 1995a), M 15 (De Marchi & Paresce 1995a), and 47 Tuc (De Marchi & Paresce 1995b) have shown, the LFs of VLM MS stars as derived from ground-based data are plagued by a number of observational uncertainties (Hesser 1993). The observational difficulties inherent in the project that we propose here make the HST the only instrument, at present, capable of reaching the scientific objectives of our investigation. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6572( 5) - 12/03/96 13:26 - [ 1] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6352 STELLAR CLUSTER, RA=17H 25M 29.2S +/- 0.1S,DEC=- 2000 V = 28 GLOBULAR CLUSTER 48D 25' 22" +/- 1",PLATE-ID=0689 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 2 NGC6496 STELLAR CLUSTER, RA=17H 59M 2.0S +/- 0.1S,DEC=- 2000 V = 28 GLOBULAR CLUSTER 44D 15' 54" +/- 1",PLATE-ID=061A 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 6572( 5) - 12/03/96 13:26 - [ 2] 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 ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC6352 WFPC2 IMAGE WFALL F814W CR-SPLIT=NO 1 40 S POS TARG 83,83 ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC6352 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2400 S SAME POS AS 11 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 13 NGC6352 WFPC2 IMAGE WFALL F606W CR-SPLIT=NO 1 40 S SAME POS AS 11 ------------------------------------------------------------------------------------------------------------------------------------ 14 NGC6352 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2360 S SAME POS AS 11 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC6352 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2760 S SAME POS AS 11 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 16 NGC6352 WFPC2 IMAGE WFALL F814W CR-SPLIT=NO 1 30 S SAME POS AS 11 ------------------------------------------------------------------------------------------------------------------------------------ 17 NGC6352 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2760 S SAME POS AS 11 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 18 NGC6352 WFPC2 IMAGE WFALL F606W CR-SPLIT=NO 1 30 S SAME POS AS 11 ------------------------------------------------------------------------------------------------------------------------------------ 19 NGC6352 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2760 S SAME POS AS 11 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC6352 WFPC2 IMAGE WFALL F814W CR-SPLIT=NO 1 30 S SAME POS AS 11 ------------------------------------------------------------------------------------------------------------------------------------ 21 NGC6352 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2760 S SAME POS AS 11 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 22 NGC6352 WFPC2 IMAGE WFALL F606W CR-SPLIT=NO 1 30 S SAME POS AS 11 ------------------------------------------------------------------------------------------------------------------------------------ 23 NGC6352 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2760 S SAME POS AS 11 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 24 NGC6352 WFPC2 IMAGE WFALL F814W CR-SPLIT=NO 1 30 S SAME POS AS 11 ------------------------------------------------------------------------------------------------------------------------------------ 25 NGC6352 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2760 S SAME POS AS 11 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 26 NGC6352 WFPC2 IMAGE WFALL F606W CR-SPLIT=NO 1 30 S SAME POS AS 11 ------------------------------------------------------------------------------------------------------------------------------------ 6572( 5) - 12/03/96 13:26 - [ 3] Visit: 02 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 ------------------------------------------------------------------------------------------------------------------------------------ 31 NGC6496 WFPC2 IMAGE WFALL F814W CR-SPLIT=NO 1 40 S POS TARG 83,83 ------------------------------------------------------------------------------------------------------------------------------------ 32 NGC6496 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2300 S SAME POS AS 31 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 33 NGC6496 WFPC2 IMAGE WFALL F814W CR-SPLIT=NO 1 30 S SAME POS AS 31 ------------------------------------------------------------------------------------------------------------------------------------ 34 NGC6496 WFPC2 IMAGE WFALL F606W CR-SPLIT=NO 1 40 S SAME POS AS 31 ------------------------------------------------------------------------------------------------------------------------------------ 35 NGC6496 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2360 S SAME POS AS 31 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 36 NGC6496 WFPC2 IMAGE WFALL F606W CR-SPLIT=NO 1 30 S SAME POS AS 31 ------------------------------------------------------------------------------------------------------------------------------------ 37 NGC6496 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2760 S SAME POS AS 31 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 38 NGC6496 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2760 S SAME POS AS 31 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 39 NGC6496 WFPC2 IMAGE WFALL F814W CR-SPLIT=DEF 1 2760 S SAME POS AS 31 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC6496 WFPC2 IMAGE WFALL F606W CR-SPLIT=DEF 1 2760 S SAME POS AS 31 EXPAND MAX DUR 150% ------------------------------------------------------------------------------------------------------------------------------------ 6572( 5) - 12/03/96 13:26 - [ 4] Summary Form for Proposal 6572 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO CR-SPLIT=DEF ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements POS TARG 83,83 SAME POS AS 11 EXPAND MAX DUR 150% SAME POS AS 11 SAME POS AS 31 EXPAND MAX DUR 150% SAME POS AS 31 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W F606W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6352 NGC6496 ------------------------------------------------------------------------------------------------------------------------------------