! Hubble Space Telescope Cycle 6 (1996) Phase II Proposal Template ! ID: 6425 ! ! From gdc@merlin.anu.edu.au Tue Jan 30 18:13:58 1996 ! ! All keywords with multiple entries are comma delimited except the ! Visit_Requirements and Special_Requirements keywords which can be ! delimited with carriage returns or semi-colons, but not commas ! ! For help call your Program Coordinator: Mutchler ! Phone: 410-338-1321 , E-mail: mutchler@stsci.edu Proposal_Information Title: UV-Visible Observations of Hot Stars in Young Magellanic Cloud Star Clusters Proposal_Category: GO Scientific_Category: STELLAR POPULATIONS Cycle: 6 Investigators PI_name: Dr. Michael Bessell PI_Institution: Mt Stromlo and Siding Spring Observatories CoI_Name: Dr. Gary Da Costa CoI_Institution: Mt Stromlo and Siding Spring Observatories Contact: ! Y or N (designate at most one contact) Abstract: ! Free format text (please update) We propose to use WFPC2 to image 4 young populous star clusters in the Magellanic Clouds (NGC 330 in the SMC, NGC 1818, 2004, and 2100 in the LMC) using the far-UV F160BW filter and the visible F555W filter. The resulting F160BW- F555W colors will allow accurate temperature determinations for both main sequence and blue supergiant stars in these clusters. These observations will then be compared with the predictions of standard and non-standard evolutionary codes to improve constraints on the evolution of mbox ~10 to mbox20Msun stars. In particular, these HST observations will help resolve the following differences between optical ground-based color-magnitude diagrams and current evolutionary theory:, o the observed and predicted temperatures of the main sequence turnoff apparently differ by several thousand degrees, while the luminosities differ by up to 1 mag in V;, o the luminosity of the red giants and the main sequence turnoff are not consistent with the predictions of standard models;, o the observed ratios of red and blue supergiants are much greater than expected on the basis of standard models., The HST is essential for this project since only it provides both the far -UV sensitivity needed for excellent temperature discrimination and the high spatial resolution needed for photometry in these crowded cluster fields. Questions ! Free format text (please update) Observing_Description: The brightest blue supergiants in the LMC clusters NGC 1818, 2004 and 2100 have V magnitudes between 12 and 13 and temperatures between 17,000K and 10,000K, while the equivalent stars in the SMC cluster NGC 330 are approximately 0.5 magnitude fainter. Similarly, the bright blue stars that define the top of the evolved main sequence in the LMC clusters have V magnitudes between 13.5 and 14.5 while they are again approximately 0.5 mag fainter in NGC 330. Finally, the bulk of the unevolved main sequence stars are fainter than V = 14.5 in the LMC clusters and fainter than V = 15 in the SMC cluster. Exposure times have been calculated taking into account two criteria: first, the brightest blue supergiants must not be saturated on the shortest exposures, and second, the unevolved main sequence stars must have adequate signal on the long exposures but at the same time there must be a reasonable degree of overlap between the long and short exposures. The clusters will be centered on the PC in order to take advantage of the better spatial sampling with this camera and the observations will use the 7e-/DN gain setup so as to better sample the readnoise (there will be little background on any of the exposures). With these parameters then, and using the information contained in the June 1995 Version of the WFPC2 Handbook, the short F555W exposures have been set so that V = 12 (LMC) and V = 12.5 (SMC) stars centered on a PC pixel remain below the saturation limit. These exposures are then 1sec and 1.6sec, respectively. The short F160BW exposures have been set in an identical manner, but in addition assume a spectral type of B0, a reddening E(B--V) of 0.1 and the A_160/E(B-V) ratio given by Watson et al (1994). These exposures are then 60sec and 100sec, respectively. For the long exposures, to ensure sufficient overlap, exposure times have been set so that saturation (centered on a PC-pixel) occurs for stars two magnitudes fainter than the brightest blue supergiants. This corresponds to F555W exposures of 6sec for the LMC clusters and 10sec for the SMC cluster. Typical unevolved main sequence stars (V ~ 15.5 LMC, V ~ 16.0 SMC) then have S/N ~ 40 for such an exposure, averaging the cases for the star centered on the center of a PC-pixel and centered on the corner of a PC-pixel. For the F160WB filter, again assuming spectral type B0, a reddening E(B-V) of 0.1 and the A_160/E(B -V) ratio given by Watson et al (1994), the exposure times are 400sec (LMC) and 700sec (SMC). To facilitate removal of cosmic rays 3 400sec exposures will be taken for the LMC clusters, and 2 700sec exposures for the SMC cluster. Finally, to aid in the separation of Be stars from non-Be stars, an exposure of each cluster will be taken through the HAlpha filter F656N. Here the exposure has been set basically to fill the time remaining in one orbit visibility period, but the times (350sec LMC, 400sec SMC) are such that the Be stars among the stars near the top of the cluster main sequence, which can be up to 0.75 mag brighter than a non-Be star of the same V magnitude in an HAlpha filter, will be easily detected in a F656N-F555W versus F160WB-F555W diagram. For the LMC clusters then we request: F555W (1sec, 6sec), F160W (60sec, 3x400sec) and F656N (300sec). With the 8min guide star acquisition overhead and the 3min per exposure overhead, this corresponds to a total time of 55.12min which is within the 57min orbit visibility. Similarly, for the SMC cluster we request: F555W (1.6sec, 10sec), F160W (100sec, 2x700sec) and F656N (350sec). With the overheads this corresponds to 57.03min, again within the orbit visibility (58 min for NGC 330). Thus the total request is for 4 orbits. We note also that we have investigated the suitability of archive data for this project. Three of the clusters (NGC 330, NGC 2004, NGC 2100) have been observed in F555W and the F450W filter (wide-B) in the Cycle 4 GO program 5475, but unfortunately, the bright stars in which we are most interested are saturated on the frames taken for this program. For example, stars brighter than V ~ 15.9 are saturated on the PC frame in the available 40sec F555W image of the SMC cluster NGC 330. Two of these clusters are targets in Cycle 5 programs: NGC 1818 is a target in GTO program 6277 and NGC 2004 is a target in GO program 5904. Both of these programs are concerned primarily with the mass function of the clusters and do not intend to observe the clusters with the F160WB filter. We believe our program, concerned as it is with the temperatures of the brighter stars in these clusters, does not duplicate either of these proposals. Real_Time_Justification: There are no special requirements for this program. There are no plans or requirements for new supporting observations for this program. Calibration_Justification: ! Move appropriate text from Real_Time_Justification Additional_Comments: Fixed_Targets ! Section 5.1 Target_Number: 1 Target_Name: NGC330 Alternate_Names: Description: EXT-CLUSTER, OPEN CLUSTER Position: RA=0H 56M 18.0S +/- 2", DEC=-72D 27' 47.5" +/- 2" ! PLATE-ID=0000 Equinox: 2000. RV_or_Z: Epoch: Flux: V=14.0 +/- 0.3, B-V=0.0 +/- 0.2 ! Include at least V and B-V Comments: Flux given is for stars at the top of the evolved main sequence Target_Number: 2 Target_Name: NGC1818 Alternate_Names: Description: EXT-CLUSTER, OPEN CLUSTER Position: RA=5H 4M 13.8S +/- 2", DEC=-66D 25' 59.0" +/- 2" ! PLATE-ID=0000 Equinox: 2000.0 RV_or_Z: Epoch: Flux: V=13.5 +/- 0.3, B-V=0.0 +/- 0.2 ! Include at least V and B-V Comments: Flux given is for stars at the top of the evolved main sequence Target_Number: 3 Target_Name: NGC2004 Alternate_Names: Description: EXT-CLUSTER, OPEN CLUSTER Position: RA=5H 30M 40.7S +/- 2", DEC=-67D 17' 15.1" +/- 2" ! PLATE-ID=0000 Equinox: 2000.0 RV_or_Z: Epoch: Flux: V=13.5 +/- 0.3, B-V=0.0 +/- 0.2 ! Include at least V and B-V Comments: Flux given is for stars at the top of the evolved main sequence Target_Number: 4 Target_Name: NGC2100 Alternate_Names: Description: EXT-CLUSTER, OPEN CLUSTER Position: RA=5H 42M 08.4S +/- 2", DEC=-69D 12' 39.5" +/- 2" ! PLATE-ID=0000 Equinox: 2000.0 RV_or_Z: Epoch: Flux: V=13.5 +/- 0.3, B-V=0.0 +/- 0.2 ! Include at least V and B-V Comments: Flux given is for stars at the top of the evolved main sequence Visits Visit_Number: 1 Visit_Requirements: GROUP 1-4 WITHIN 1D Visit_Comments: Group Requirement is set so that the F160BW throughput loss due to contamination is approximately the same for all 4 clusters. Exposure_Number: 1 Target_Name: NGC330 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F555W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 1.60S Special_Requirements: Comments: Exposure_Number: 2 Target_Name: NGC330 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F555W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 10.0S Special_Requirements: SAME POS AS 1 NO SPLIT Comments: Exposure_Number: 3 Target_Name: NGC330 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F160BW Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 100S Special_Requirements: SAME POS AS 1 NO SPLIT Comments: Exposure_Number: 4 Target_Name: NGC330 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F160BW Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 2 Time_Per_Exposure: 800S Special_Requirements: ! Section 7.2 ! Uncomment or copy exposure level special requirements needed SAME POSition AS 1 NO SPLIT Comments: Exposure_Number: 5 ! Section 6.5 Target_Name: NGC330 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F656N Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 400S Special_Requirements: ! Section 7.2 ! Uncomment or copy exposure level special requirements needed SAME POSition AS 1 !END ORBIT NO SPLIT Comments: Visit_Number: 2 Visit_Requirements: ! Section 7.1 ! Uncomment or copy visit level special requirements needed ! Most of these requirements (including ORIENT) will limit scheduling ! PCS MODE [Fine | Gyro] ! GUIDing TOLerance ! DROP TO GYRO IF NECESSARY [NO REACQuisition] ! ORIENTation TO ! ORIENTation TO FROM ! ORIENTation TO FROM NOMINAL ! SAME ORIENTation AS ! CVZ ! PARallel ! SCHEDulability ! AFTER [BY [TO ]] ! AFTER ! BEFORE ! BETWEEN AND ! GROUP 1-4 WITHIN 1D ! PERIOD