8151( 4) - 10/06/99 11:05 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8151 Version: 4 Check-in Date: 06-Oct-1999 11:04:23 1.Proposal Title: NGC 1866: A Critical test of Stellar Evolution for Intermediate Mass Stars ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Alistair Walker National Optical Astronomy Observatories - CTIO CoI: Dr Enzo Brocato Osserv. Astronomico di Collurania, Teramo N CoI: Prof Vittorio Castellani Universit`a di Pisa N CoI: Dr Vanessa Hill European Southern Observatory N CoI: Gabriella Raimondo Osserv. Astronomico di Collurania, Teramo N ------------------------------------------------------------------------------------ 5. Abstract We propose to obtain V (F555W) and I (F814W) images centered on the LMC cluster NGC 1866, with a range of exposure times such that we can measure all cluster stars from the brightest at V ~ 15 down to V ~25. NGC 1866 has age ~ 100 Myr and is so populous that the evolving stars, with M ~ 5 M_sun, are caught even in short phases of evolution. As such, it provides a critical test of stellar evolution theory applicable to these intermediate mass stars. We will prepare a color- magnitude diagram that will, for the first time, include all the luminous evolved stars. This is impossible to achieve from ground-based observations due to crowding. We will also measure a luminosity function for the unevolved main sequence to M ~ 0.5 M_sun which allows both determination of the IMF for cluster stars with masses in the range 0.5 - 5 M_sun and an investigation of the cluster dynamical evolution by study of the spatial mass distribution. The deep and accurate MS photometry will also permit precise fitting of a fiducial ZAMS, and hence derivation of a distance relative to the Hyades, the basis of the ZAMS calibration. Since NGC 1866 contains at least 23 Cepheid variables, we will thus calibrate the Period-Luminosity relation zeropoint, and determine the distance to the LMC relative to the very accurate Hipparcos parallax distance to the Hyades with a minimum of steps and assumptions. ------------------------------------------------------------------------------------ 8151( 4) - 10/06/99 11:05 - [ 2] Observations Description ------------------------ Although most of the previous work in and around NGC 1866 has used B and V filters, we will here use V and I due to the relatively low WFPC2 CCD sensitivity in B compared to the longer wavelengths, which would severely limit S/N achieved on the lower MS. This choice has the important advantage that we can use the field star observations close to NGC 1866 (GO 5208, PI J. Trauger, see Geha et al., 1998) to remove LMC field and galactic foreground star contamination on and near the cluster MS. Brighter field star contamination is not a big issue, there are few field stars in a 40-80 arcsec annulus (Fig 5, Walker 1995) and they are almost all well away from the cluster sequences in the CMD. We will use our ground-based data to quantify this contamination. The brightest evolved stars have V = 15, V-I = 1.5; the top of the MS is V = 16.5, V-I = 0, the faintest stars we wish to measure have V = 25, V-I = 1.0. We want to avoid, except perhaps for the very faintest stars, any large CCD CTE corrections. Related, we wish to eliminate the difficulties that people, including ourselves, have had when trying to cover a dynamic range of ~ 10-12 magnitudes with only a short and long set of WFPC2 exposures. Given the steadily degrading CTE performance, we believe it is essential that we use three sets of exposure times in order to get uniform ~ 0.02 mag (S/N = 50) or better photometric accuracy from the brightest stars down to V ~ 23. We can relax this S/N requirement for fainter stars. This strategy is confirmed by the S/N calculations below, obtained using the WFPC2 Exposure Time Calculator. The exposure strategy is as follows. For the short exposures, V: 4 x 8s, saturates at V=14.5 (WF); S/N=50 at V=18.0 (PC) per exposure. I: 4 x 5s, saturates at I=13.1 (WF); S/N=50 at I=16.6 (PC) per exposure. The four exposures will be taken in a four-position grid, in order to be able to produce a combined frame with enhanced resolution (we will use the Drizzle software, found in STSDAS and the Drizzle II package). There will be few cosmic rays on such short exposures thus a four position grid will be adequate. For the medium exposures, we will take V: 8 x 60s, saturates at V=16.7 (WF); S/N=50 at V=20.3$ (PC) per exposure. I: 8 x 50s, saturates at I=15.6 (WF); S/N=50 at I=19.1 (PC) per exposure. These exposures will also be taken in a four-position grid, but this time with two exposures at each grid position, optimal both for efficient removal of cosmic rays and for resolution enhancement. Finally, the long exposures will be V: 4 x 500s, saturates at V=19.0 (WF); S/N=50 at V=22.6, S/N=12 at V=25.0 (PC) per exposure. I: 2 x 500s plus 2 x 600s, the latter saturates at I=18.3 (WF), S/N=50 at I=21.8, S/N=12 at I=24.1 (PC) per exposure. Although nominally I: 4 x 600s (as in Phase I) fits into an orbit, the amount of scheduleable time falls dramatically. If there is time available in the orbit when the HST schedule for this proposal is fixed, then it may be possible to lengthen the exposures back to the Phase I request. The slighter shorter total I band exposure time will not have significant impact on the science goals. The long exposures will not be dithered. These exposures will have many cosmic rays, and the center of the cluster (where increased resolution is most important) will be mostly saturated. The longer exposure times possible without the dither overhead are also a factor in the decision not to dither the long exposures. We will run the serial clocks (CLOCKS=YES) for the long exposures as we will have many saturated stars and wish to minimize the effects. We are aware of the caveats. For positioning HST, we have two requirements. Firstly, we wish to measure stars to the NGC 1866 center, which implies positioning the cluster on the PC. Secondly, we wish to measure all stars to a radius of 40 arcsec, outside of which we have ground-based data, otherwise we will not have sufficient evolved stars to achieve our first science goal. Our proposed strategy is to center NGC 1866 on the PC and take the short and intermediate exposure series. This occupies 2 orbits (1 for V, 1 for I), and allows us to measure all stars down to V=20, I=19 at S/N > 50 within a radius of approx 15 arcsec from the cluster center, with good pixel sampling. For the crowded center of the cluster, the finer pixel scale of the PC will allow us to achieve our desired photometric accuracy. We then move the cluster to near the center of WF3, repeat the short and medium exposures (2 orbits), which will measure stars with 10 < r < 40 arcsec, and also take the long exposures (2 orbits). This gives us near complete cover of the cluster, when combined with our ground-based data for r > 40 arcsec, losing only 2 percent of the "cluster area". The deep exposures, required for our second and third science goals, do not need to be complete to the cluster center for the faintest magnitudes and thus do not need to be repeated with the cluster centered on the PC. This strategy gives us good photometric overlap beween PC and WF, and between WF and ground (since stars measurable from the ground will be on WF1 and WF2). This will provide an independent check of the zeropoint calibrations from Holtzman et al. (1995), which we believe is essential to provide confidence in our distance measurement (science goal 3). A total of 6 orbits will thus be needed. If need be, the PC- centered (2 orbits) observations can be separated in time from the WF3-centered (4 orbits) observations. There are no roll-angle restrictions. Real Time Justification ----------------------- None Calibration Justification ------------------------- No special calibrations for the HST data are required to be provided via HST. We instead will use ground-based supporting observations, none of which affect HST scheduling. We have extensive photometry in and near NGC 1866 (Brocato et al. 1989, 1994) which has been used to determine the reddening, and to investigate the CMD and LF properties for the outer cluster stars. We also have available a set of 53 accurate local standards in V, B and I (Walker 1995). Using a CTIO ground-based telescope, we will carefully measure a few relatively uncrowded (chosen from the HST images) stars relative to this sequence, for comparison with the HST zeropoints (Holtzman et al. 1995). At the same time, we will measure the more distant cluster stars in the I band. In addition, the metallicity of the cluster has been already approximately estimated via Stromgren photometry for a few stars (Hilker et al. 1995) at Fe/H = -0.43 +/- 0.18, and from the integrated light spectrum at 1.6Mu (Oliva & Origlia 1998) whereby Fe/H =-0.55 +/- 0.4. In both cases the error is rather large and zeropoint uncertain. Moreover, the detailed chemical composition of the young matter in the Magellanic Clouds is known to differ somewhat from the solar mixture (the O/Fe ratio is lower, for example), so that knowledge of the overall metallicity is not sufficient. High resolution spectroscopic measurements are essential to provide a definitive composition and to allow a detailed comparison with theoretical predictions. These will be provided via observations with an ESO ground-based telescope, using well- proven techniques (Hill et al. 1997, Hill 1998). Additional Comments ------------------- ------------------------------------------------------------------------------------ 8151( 4) - 10/06/99 11:05 - [ 3] Data Distribution Paper Products: NO Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8151( 4) - 10/06/99 11:05 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1866 EXT-CLUSTER, Globular RA=05H 13M 39.14S +/- 0.1S, DEC= 2000 V = 15 Cluster -65D 27' 56.4" +/- 0.1", PLATE- B-V = 1.3 ID=06AF V-I = 1.5 Comments: Flux is for the brightest NGC 1866 stars. Total magnitude range of interest is V = 15-25, measured using three sets of exposure times. 8151( 4) - 10/06/99 11:05 - [ 5] Visit: 01 Visit Priority: 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1866 WFPC2 IMAGE PC1-FIX F555W CR-SPLIT=NO,DITHER-TYPE=BOX 1 8.00 S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1866 WFPC2 IMAGE PC1-FIX F555W CR-SPLIT=NO,DITHER-TYPE=BOX 2 60.00 S ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1866 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO,DITHER-TYPE=BOX 1 5.00 S ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC1866 WFPC2 IMAGE PC1-FIX F555W CR-SPLIT=NO,DITHER-TYPE=BOX 2 50.00 S ------------------------------------------------------------------------------------------------------------------------------------ 8151( 4) - 10/06/99 11:05 - [ 6] Visit: 02 Visit Priority: 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1866 WFPC2 IMAGE WF3-FIX F555W CR-SPLIT=NO,DITHER-TYPE=BOX 1 8.00 S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1866 WFPC2 IMAGE WF3-FIX F555W CR-SPLIT=NO,DITHER-TYPE=BOX 2 60.00 S ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1866 WFPC2 IMAGE WF3-FIX F814W CR-SPLIT=NO,DITHER-TYPE=BOX 1 5.00 S ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC1866 WFPC2 IMAGE WF3-FIX F814W CR-SPLIT=NO,DITHER-TYPE=BOX 2 50.00 S ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC1866 WFPC2 IMAGE WF3-FIX F814W CR-SPLIT=NO,CLOCKS=YES 2 600.00 ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC1866 WFPC2 IMAGE WF3-FIX F814W CR-SPLIT=NO,CLOCKS=YES 2 500.00 ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC1866 WFPC2 IMAGE WF3-FIX F555W CR-SPLIT=NO,CLOCKS=YES 4 500.00 ------------------------------------------------------------------------------------------------------------------------------------ 8151( 4) - 10/06/99 11:05 - [ 7] Visit: 51 Visit Priority: 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1866 WFPC2 IMAGE PC1-FIX F555W CR-SPLIT=NO,DITHER-TYPE=BOX 1 8.00 S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1866 WFPC2 IMAGE PC1-FIX F555W CR-SPLIT=NO,DITHER-TYPE=BOX 2 60.00 S ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1866 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO,DITHER-TYPE=BOX 1 5.00 S ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC1866 WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO,DITHER-TYPE=BOX 2 50.00 S ------------------------------------------------------------------------------------------------------------------------------------ 8151( 4) - 10/06/99 11:05 - [ 8] Summary Form for Proposal 8151 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1-FIX WF3-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO DITHER-TYPE=BOX CLOCKS=YES ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC1866 ------------------------------------------------------------------------------------------------------------------------------------