6424( 5) - 02/26/96 10:32 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6424 Version: 5 Check-in Date: 26-Feb-1996 10:27:59 1.Proposal Title: Calibration of Stellar Evolutionary Ages and Variations in the IMF ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle STELLAR POPULATIONS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Ted von Hippel University of Wisconsin CoI: Gerry Gilmore Institute of Astronomy, University of N Cambridge CoI: Derek Jones Royal Greenwich Observatory N ------------------------------------------------------------------------------------ 6. Abstract There are two related and fundamental aspects of star formation and evolution which can be studied uniquely with HST/WFPC2 data for nearby open clusters: calibration of stellar isochrone ages independently of stellar structure models, and the dependence of the stellar luminosity - and hence mass - function on metallicity and environment. We have proven that HST photometry of nearby open clusters can determine the white dwarf luminosity function to its limit, and hence calibrate evolutionary and cooling age scales directly. The same observations determine the M dwarf luminosity function. Combination of our (metal-rich) open cluster data with HST data for (metal-poor) globular clusters is determining the stellar luminosity and mass function over 2 dex in abundance. Cycle 4 data were allocated to prove technical feasibility: we will complete the experiment, provide independent robust calibrations of stellar evolutionary models, and quantify the dependence of the stellar luminosity function on metallicity. HST data are necessary for this project since only HST has the resolution to distinguish cluster white dwarfs from the far more abundant background galaxies with similar colors. ------------------------------------------------------------------------------------ 6424( 5) - 02/26/96 10:32 - [ 2] Observations Description ------------------------ We propose to observe two open clusters (NGC 2420, NGC 2477) spanning the age range ~ 1 to 2 Gyrs with WFPC2 on HST. Both clusters will be observed to V ~ 26.0, I ~ 25.5 (S/N ~ 10) by exposing for a total of 2 hours (~ 126 minutes taking into account the details of efficiently using orbits, differing by a few minutes between clusters) through F555W and 1.3 hours (~ 82 minutes) through F814W. Two fields would be observed in both NGC 2420 and 2477. These exposure times will be accumulated from multiple exposures in I and V, which would make efficient use of the orbits, help remove cosmic rays, and decrease the impact of the brightest stars in each field to an acceptable level. Our choice of the F555W and F814W filters is motivated by the need for good calibration, flat fields, maximum depth in a given integration time, optimum sensitivity to lower main sequence and cool WD stars, and compatibility with our Cycle 4 data. Table 1 lists our candidate objects, ordered by increasing RA. These clusters were chosen after an exhaustive literature search followed by detailed simulations of each promising cluster, in order to optimize the age range of the clusters and the enhancement of the clusters against the Galactic stellar background. Column 1 lists the object name, columns 2 & 3 the object RA & Dec (J2000.) with the Galactic longitude and latitude underneath, column 4 lists the distance and distance modulus, column 5 the A_v, column 6 the log age and age in Gyrs, column 7 the M_v of the expected end of the WD cooling sequence and the equivalent terminus in V, column 8 the cluster diameter in arc minutes, column 9 the expected number of cluster stars near the cluster center and the expected number of Galactic field stars per 3 WF CCDs (a single field), columns 10 & 11 the expected number of cluster low mass main sequence stars (0.6 Msun to 0.18 Msun) and WDs for the same field, and column 14 lists the number of WFPC2 fields needed for a high quality age determination from the WD terminus. We request several (2, 2 and 4) fields in each cluster as they occupy large solid angles and have somewhat low stellar densities. Table 1 also shows that the expected number of WDs (16, 16, and 10) compares favorably with the total number of WDs used to determine the age of the Galactic disk (43 in Liebert et al., 1988). Our S/N calculations are based on the WFPC2 Instrument Handbook. We calculate that a 2 hour exposure in V and 1.3 hour exposure in I would yield S/N ~ 15 at V = 25.5, I = 25 for a G0 or redder star. We chose this S/N value as after flat-field uncertainties, etc., we desire a S/N >= 10 at least 0.5 mag beyond the end of the WD cooling sequence, the apparent magnitude of which is listed in Table 1, column 7. (See also Figures 2 and 3 from our Cycle 4 data, from which we have improved our calculations of exposure times.) Since only the brighter cluster stars are bluer than G0, these limiting magnitudes and S/N values are appropriate for the WD terminus and the lower mass main sequence stars. We request 1 new WFPC2 field in both NGC 2420 and 2477. Each of these new fields will need exposure times requiring 5 orbits. Since we already have exposures of 1 hour in each of V and I for 1 field in both NGC 2420 and 2477, we are requesting 2 orbits each to finish these two cluster fields. Our total request is thus 2 new fields * 5 orbits + 2 old fields * 2 orbits = 14 orbits. Real Time Justification ----------------------- No special requirements. We are pursuing a related project at the 2.5 meter telescope on La Palma. The fundamental difference between HST and ground-based capabilities is that the latter comes many magnitudes short of reaching the WD cooling terminus for old clusters. Furthermore, only HST has the ability to resolve the very faint galaxies which would otherwise dominate the very faint cool WDs. Therefore ground-based observations can only trace the WD cooling sequence in very nearby and young clusters. Similarly, only in very nearby clusters can one trace the main sequence to sufficiently low mass stars. Older clusters are favored for IMF studies due to the significant effects and uncertainties in corrections to low mass stellar luminosities. In practice this means that clusters with ages <= 1 Gyr can be studied from the ground in the manner we propose here, whereas older clusters cannot. While the young clusters we are studying from the ground (to date, Praesepe and Coma Bernices with T = 0.8 and 0.5 Gyrs, respectively) help in the comparison of WD cooling ages and isochrone ages at early stages of stellar evolution, they do not address the later time stages where fundamental differences exist between stellar evolution models, and where this proposal focuses. We are also obtaining ground-based photometry as an independent photometric calibration for the HST fields. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6424( 5) - 02/26/96 10:32 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6424( 5) - 02/26/96 10:32 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC2420-POS1 STELLAR CLUSTER,OPEN RA=114.5466080281D +/- J2000 V = 25.6+/-0.2 CLUSTER, 0.00001D,DEC=21.55268860582D +/- B-V = 0.7+/-0.2 0.00001D,PLATE-ID=00EL E(B-V) = 0.02+/-0.02 V = 25.5+/-0.2 V-I = 2.6+/-0.2 E(B-V) = 0.02+/-0.02 Comments: Re-acquisition of Cycle 4 field to add exposure time. Flux values are for typical cluster cool white dwarfs and low mass main sequence stars, respectively. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) B1983 0.0 0.0 0.0 2 NGC2420-POS2 STELLAR CLUSTER, OPEN RA=7H 38M 28.45S +/- 2000 V = 25.6+/-0.2 CLUSTER 0.1S,DEC=21D 35' 29.06" +/- B-V = 0.7+/-0.2 0.1",PLATE-ID=00EL E(B-V) = 0.02+/-0.02 V = 25.5+/-0.2 V-I = 2.6+/-0.2 E(B-V) = 0.02+/-0.02 Comments: New field. Flux values are for typical cluster cool white dwarfs and low mass main sequence stars, respectively. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1983 0.0 0.0 0.0 3 NGC2477-POS1 STELLAR CLUSTER,OPEN RA=118.0700295561D +/- J2000 V = 24.5+/-0.2 CLUSTER, 0.00001D,DEC=-38.59447576965D +/ B-V = 0.8+/-0.2 - 0.00001D,PLATE-ID=01Z2 E(B-V) = 0.3+/-0.08 V = 25.5+/-0.2 V-I = 3.4+/-0.2 E(B-V) = 0.3+/-0.08 Comments: Re-acquisition of Cycle 4 field to add exposure time. Flux values are for typical cluster cool white dwarfs and low mass main sequence stars, respectively. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) B1979 0.0 0.0 0.0 4 NGC2477-POS2 STELLAR CLUSTER, OPEN RA=7H 52M 26.20S +/- 0.1S,DEC=- 2000 V = 24.5+/-0.2 CLUSTER 38D 34' 40.3" +/- 0.1",PLATE- B-V = 0.8+/-0.2 ID=01Z2 E(B-V) = 0.3+/-0.08 V = 25.5+/-0.2 V-I = 3.4+/-0.2 E(B-V) = 0.3+/-0.08 Comments: New field. Flux values are for typical cluster cool white dwarfs and low mass main sequence stars, respectively. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1979 0.0 0.0 0.0 6424( 5) - 02/26/96 10:32 - [ 5] Visit: 01 Visit Requirements: ORIENT 99.4927D TO 99.4927D 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC2420- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 2 1200S LOW-SKY POS1 FIX ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC2420- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 2 1200S POS TARG 0.233,0.233 LOW-SKY POS1 FIX ------------------------------------------------------------------------------------------------------------------------------------ 6424( 5) - 02/26/96 10:32 - [ 6] 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 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC2420- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 1 700S LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC2420- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 2 800S SAME POS AS 3 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC2420- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 1 700S POS TARG 0.233,0.233 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC2420- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 2 800S SAME POS AS 5 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC2420- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 1 700S POS TARG -0.233,-0.233 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC2420- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 2 800S SAME POS AS 7 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 9 NGC2420- WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 700S SAME POS AS 7 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC2420- WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 2 800S SAME POS AS 7 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC2420- WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 700S POS TARG 0.0,0.0 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC2420- WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 2 800S SAME POS AS 11 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 6424( 5) - 02/26/96 10:32 - [ 7] Visit: 03 Visit Requirements: ORIENT 51.0716D TO 51.0716D 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 ------------------------------------------------------------------------------------------------------------------------------------ 13 NGC2477- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 4 500S LOW-SKY POS1 FIX ------------------------------------------------------------------------------------------------------------------------------------ 14 NGC2477- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 4 500S POS TARG 0.233,0.233 LOW-SKY POS1 FIX ------------------------------------------------------------------------------------------------------------------------------------ 6424( 5) - 02/26/96 10:32 - [ 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 ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC2477- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 4 500S LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 16 NGC2477- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 4 500S POS TARG 0.233,0.233 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 17 NGC2477- WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 4 500S POS TARG -0.233,-0.233 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 18 NGC2477- WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 4 500S SAME POS AS 17 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 19 NGC2477- WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 4 500S POS TARG 0.0,0.0 LOW-SKY POS2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 6424( 5) - 02/26/96 10:32 - [ 9] Summary Form for Proposal 6424 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 99.4927D TO 99.4927D LOW-SKY POS TARG 0.233,0.233 LOW-SKY SAME POS AS 3 LOW-SKY SAME POS AS 5 LOW-SKY POS TARG -0.233,-0.233 LOW-SKY SAME POS AS 7 LOW-SKY POS TARG 0.0,0.0 LOW-SKY SAME POS AS 11 LOW-SKY ORIENT 51.0716D TO 51.0716D SAME POS AS 17 LOW-SKY ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC2420-POS1 NGC2420-POS2 NGC2477-POS1 NGC2477-POS2 ------------------------------------------------------------------------------------------------------------------------------------