9096( 8) - 03/06/03 13:22 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9096 Version: 8 Check-in Date: 06-Mar-2003 12:59:31 1.Proposal Title: Objective-Prism Spectroscopy of Massive Young Clusters ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 10 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Jesus Maiz-Apellaniz Space Telescope Science Institute CoI: Dr. Nolan R. Walborn Space Telescope Science Institute N CoI: Dr. John W. MacKenty Space Telescope Science Institute N CoI: Prof. Colin A. Norman The Johns Hopkins University N CoI: Dr. Enrique Pérez Instituto de Astrofísica de Andalucía N CoI: Dr. J. Miguel Mas-Hesse LAEFF/INTA N ------------------------------------------------------------------------------------ 5. Abstract Most of the present knowledge of the UV spectral properties of massive young clusters (MYCs) is based on IUE data with marginal spatial resolution. We propose to greatly improve the spatial information by obtaining STIS NUV-MAMA objective-prism spectroscopy in the 1300-3600 Angstrom range of three nearby extragalactic regions with a total of ~ 10 MYCs. Slitless techniques are seldom attempted on crowded clusters due to the overlap among different sources. We plan to overcome that problem by observing with two different roll angles, using comparison UV and optical images from the HST archive, and developing automatic extraction techniques based on codes already written by some of the authors. Some of the issues that will be tackled include: (i) Is there an age difference between the core and the outer parts of the clusters? (ii) What is the importance of differential extinction in the estimation of the cluster age? (iii) Is the absence of the 2175 Angstrom bump in starbursts due to geometrical effects or is it an intrinsic property of the dust? (iv) Are the properties of the dust homogeneous or do they vary with position? The STIS NUV prism is ideal for this type of study since it provides relatively higher spectral resolution for the features around 1500 Angstrom and lower resolution at longer wavelengths, where the sources are generally less intense and the background is higher. ------------------------------------------------------------------------------------ 9096( 8) - 03/06/03 13:22 - [ 2] Observations Description ------------------------ We propose to use the prism with the STIS NUV MAMA, the F25SRF2 filter, and no slit. The choice of a slitless mode is motivated by the large increase in the number of observed objects over a single-slit exposure. The prism is preferred over the G140L and G230L gratings for two reasons. First, because only one exposure per target is needed to cover the FUV+NUV ranges. Second, because in a slitless mode with the G140L or G230L grating a star has its full spectrum recorded only if it is centered along the spectral axis of the MAMA. On the other hand, the length of a full spectrum (1150-3600 Angstrom) using the prism is only ~ 550 pixels long, which allows the spectrum of half the stars in a uniform field to be fully recorded (see Fig. 2). This, of course, comes with a loss of spectral resolution but it is compensated by the dispersion relationship of the prism, which provides larger resolutions around the interesting region for spectral classification (~ 2 Angstrom/pixel at 1400 Angstrom and ~ 4 Angstrom/pixel at 1600 Angstrom) and small resolutions at longer wavelengths (lambda > 2000 Angstrom), where our main interest is only to measure the extinction law. The reason for using the F25SRF2 filter is to block the most intense of the geocoronal lines, Ly Alpha. The F25SRF2 does not block the more interesting area for UV spectral classification starting at lambda ~ 1300 Angstrom, and for that reason is preferred over the F25QTZ filter. Nevertheless, the use of F25SRF2 requires shadow time (see special requirements). We consider the worst case scenario to calculate our values of the S/N ratios; that in which only one of the two orientations provides a clearly separated spectrogram of the source. In that case, the total exposure time is only 3000 s (if the two orientations are good this should be multiplied by two). The S/N ratios at 1500 Angstrom for an unreddened early main sequence B star at the distance of NGC 604 or an early main sequence O star at the distance of NGC 4214 are in the range 10-20. The values around 2500 Angstrom for those same stars are in the range 30-40 due to the higher throughput of the prism at those wavelengths. We thus will be able to classify spectroscopically unreddened OB supergiants and early O V stars in NGC 4214 and to extend the classification to early B V stars for NGC 604. We also plan to be able to use the brightest stars (up to extinctions of E(B-V) ~ 1.0) to construct our extinction curve map, since the spectral resolution needed for that task is lower than that provided by the prism and several pixels could be grouped to increase the S/N ratio. We have also checked that our observations do not violate any of the MAMA limits. We have used UV spectral data and imaging from the literature to check that the fields in NGC 4214 are quite far from those limits. For example, a field including NGC 4214-I-As and its surrounding halo and clusters would have a peak local count rate of ~ 2 counts/s/pixel at 2637 Angstrom and a global count rate of ~ 4000-5000 counts/s (the total value depending on the exact positioning of the field). For NGC 604 we are closer to the limits but still in the safe region. We have used the combination of IUE spectroscopy and F170W WFPC2 imaging provided by González-Delgado & Pérez (2000) to calculate that a field of the size of the NUV MAMA will have at most a global count rate of 17 000 counts/s, with no individual source coming close to the peak local count rate. A target acquisition will not be required as a slitless mode is requested. Each of the three fields will be imaged twice with a different orientation in each case. This will require two visits for each field for a total of six visits. Each of the visits will be two orbits long for a total of twelve orbits. The observations will be dithered using the STIS- ALONG-SLIT pattern (i.e. along the direction perpendicular to the dispersion, see page 231 in the Cycle 10 STIS Handbook), which will require a small angle maneuver between the first and second orbits of each visit. The observations have to be carried out using shadow time (25 min/orbit), which yields a total of 50 minutes for the observations in each visit. The needed overheads for the 9096( 8) - 03/06/03 13:22 - [ 3] first (6 minutes for the guide star acquisition, 8 minutes for the first exposure using MAMA spectroscopy) and second (5 minutes for the guide star reacquisition, 1 minute for the second exposure using MAMA spectroscopy, 0.5 minutes for the small angle maneuver) orbits do not need to be subtracted from that figure, since those operations can be done outside the shadow window (see page 30 of the HST Cycle 10 Primer). The reduction of objective prism exposures of crowded field is inherently difficult. We will tackle the problem according to the following plan: 1. The two exposures obtained in each visit will be dithered along the spatial direction in order to improve the sampling of the line-spread function. This will simplify the separation of sources with small projected distances along axis 2 (the ``slit'' or non- spectral axis). It will also allow a simple elimination of the few hot pixels existent in the NUV MAMA. 2. WFPC2 F170W+F336W images (see Figs. 1 and 2) will be used to locate the relative positions of the individual sources. 3. The exposures on the two different visits for each field will be rotated by a large angle (~ 60 degrees or larger) in order to find at least one close-to-maximum projected distance along axis 2 for closely spaced sources. Also, this will permit the separation of those spectrograms which are superimposed by chance in one of the visits due to their alignment. 4. We are well advanced in developing and testing a code to solve iteratively for the cases with crowded fields. The code will work by finding easily separated cases in one of the two visits and using the result to subtract those stars from the data of the other visit. The process will then be repeated using each visit alternatively. The part of the code which separates closely spaced spectrograms has already been written and successfully applied (see the section on previous programs). The authors plan to make the software public after it has been tested with the data in this proposal so that similar observations can be undertaken with the prism by other groups. Another problem associated with prism observations is that of wavelength calibration, which is uncertain at the +- 50 Angstrom level for longer wavelengths in the NUV. Nevertheless, since our interest in that part of the spectrum is the analysis of the extinction law, such an uncertainty is acceptable. Furthermore, it is possible that we may actually be able to improve the present wavelength calibration by locating known features in our spectra. Real Time Justification ----------------------- The observations have to be carried out under shadow circumstances due to the presence of the O I 1304 Angstrom + O I 1356 Angstrom + O I 2471 Angstrom geocoronal emission lines, which are not blocked by the F25SRF2 filter. The F25SRF2 filter cannot be substituted by F25QTZ, which blocks the first two geocoronal lines, because it also blocks some interesting absorption lines around 1400 Angstrom (most notably, Si IV 1394+1403 Angstrom). The difference between shadow and non-shadow circumstances amounts to more than two orders of magnitude in the intensity of the geocoronal lines, which is enough to change the regime of our observations from being detector-limited to background- limited, sensibly reducing the S/N ratio. If the observations were carried under non-shadow, a large fractions of the targeted stars could not be properly studied. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9096( 8) - 03/06/03 13:22 - [ 4] 9096( 8) - 03/06/03 13:22 - [ 5] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4214-I- EXT-CLUSTER, STAR RA=12H 15M 39.8S +/- 1", DEC=36D J2000 V = 291 V = 15.0+/-0.5 ANGLE1 FORMING REGION 19' 34.0" +/- 1", PLATE-ID=00Y2 F-CONT(6563)=7.4+/-1.0e-16 F-CONT(2200)=2.5+/-0.5e-14 TYPE=O-B F-CONT-BKG(6563)=2.0+/-1.0e-14 F-CONT-BKG(2200)=5.0+/-1.0e-14 A(V) = 0.3+/-0.3 Coordinate Source: HST_IMAGE Comments: The flux values refer to the brightest compact source in the field, NGC 4214-I-As, which is resolved for the the wavelength and pixel size of interest. The background values refer to the whole field. Extinction is highly variable across the region. This is the same region as that of target 2 but with a different orientation. 2 NGC4214-I- EXT-CLUSTER, STAR RA=12H 15M 39.8S +/- 1", DEC=36D J2000 V = 291 V = 15.0+/-0.5 ANGLE2 FORMING REGION 19' 34.0" +/- 1", PLATE-ID=00Y2 F-CONT(6563)=7.4+/-1.0e-16 F-CONT(2200)=2.5+/-0.5e-14 TYPE=O-B F-CONT-BKG(6563)=2.0+/-1.0e-14 F-CONT-BKG(2200)=5.0+/-1.0e-14 A(V) = 0.3+/-0.3 Coordinate Source: HST_IMAGE Comments: The flux values refer to the brightest compact source in the field, NGC 4214-I-As, which is resolved for the the wavelength and pixel size of interest. The background values refer to the whole field. Extinction is highly variable across the region. This is the same region as that of target 1 but with a different orientation. 3 NGC4214-II- EXT-CLUSTER, STAR RA=12H 15M 41.0S +/- 1", DEC=36D J2000 V = 291 V = 16.7+/-0.5 ANGLE1 FORMING REGION 19' 9.0" +/- 1", PLATE-ID=00Y2 F-CONT(6563)=1.5+/-0.5e-16 TYPE=O-B F-CONT-BKG(6563)=8.0+/-2.0e-15 A(V) = 0.8+/-0.8 Coordinate Source: HST_IMAGE Comments: The flux value refers to the brightest compact source in the field, NGC 4214-II-B, which is resolved for the the wavelength and pixel size of interest. The background value refers to the whole field. Extinction is highly variable across the region. This is the same region as that of target 4 but with a different orientation. 9096( 8) - 03/06/03 13:22 - [ 6] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC4214-II- EXT-CLUSTER, STAR RA=12H 15M 41.0S +/- 1", DEC=36D J2000 V = 291 V = 16.7+/-0.5 ANGLE2 FORMING REGION 19' 9.0" +/- 1", PLATE-ID=00Y2 F-CONT(6563)=1.5+/-0.5e-16 TYPE=O-B F-CONT-BKG(6563)=8.0+/-2.0e-15 A(V) = 0.8+/-0.8 Coordinate Source: HST_IMAGE Comments: The flux value refers to the brightest compact source in the field, NGC 4214-II-B, which is resolved for the the wavelength and pixel size of interest. The background value refers to the whole field. Extinction is highly variable across the region. This is the same region as that of target 3 but with a different orientation. 5 NGC604- EXT-CLUSTER, STAR RA=1H 34M 32.8S +/- 1", DEC=30D J2000 V = -226 V = 16.2+/-0.1 ANGLE1 FORMING REGION 47' 3" +/- 1" TYPE=O-B F-CONT-BKG(1700)=2.4+/-0.5e-13 A(V) = 0.8+/-0.8 Coordinate Source: Other_Source: HST image, but see comments. Comments: The flux value refers to the brightest star in the field. The background value refers to the whole field. Extinction is highly variable across the region. This is the same region as that of target 6 but with a different orientation. The position was derived from WFPC2 data set u2c60b01t. No plate ID could be determined because no _jif.fits exists in the archive for that data set. 6 NGC604- EXT-CLUSTER, STAR RA=1H 34M 32.8S +/- 1", DEC=30D J2000 V = -226 V = 16.2+/-0.1 ANGLE2 FORMING REGION 47' 3" +/- 1" TYPE=O-B F-CONT-BKG(1700)=2.4+/-0.5e-13 A(V) = 0.8+/-0.8 Coordinate Source: Other_Source: HST image, but see comments. Comments: The flux value refers to the brightest star in the field. The background value refers to the whole field. Extinction is highly variable across the region. This is the same region as that of target 5 but with a different orientation. The position was derived from WFPC2 data set u2c60b01t. No plate ID could be determined because no _jif.fits exists in the archive for that data set. 9096( 8) - 03/06/03 13:22 - [ 7] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type STIS-ALONG-SLIT Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.1353 Coordinate_Frame POS-TARG Pattern_Orient 90 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.1914 Coordinate_Frame POS-TARG Pattern_Orient 45 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 9096( 8) - 03/06/03 13:22 - [ 8] Visit: 01 Visit Priority: Visit Requirements: AFTER 02 BY 15 D TO 90 D 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 NGC4214-I-A STIS/N ACCUM F25SRF2 PRISM 2125 1 12 M SHADOW PATTERN 1 1 NGLE1 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4214-I-A STIS/N ACCUM F25CN18 MIRROR 1 4 M PATTERN 2 2 NGLE1 UV-MAM 2 A ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC4214-I-A STIS/N ACCUM F25SRF2 PRISM 2125 1 12 M SHADOW PATTERN 1 3 NGLE1 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC4214-I-A STIS/N ACCUM F25CN27 MIRROR 1 4 M PATTERN 2 4 NGLE1 UV-MAM 0 A ------------------------------------------------------------------------------------------------------------------------------------ 9096( 8) - 03/06/03 13:22 - [ 9] Visit: 02 Visit Priority: Visit Requirements: ORIENT 40D TO 120D FROM 01 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 NGC4214-I-A STIS/N ACCUM F25SRF2 PRISM 2125 1 12 M SHADOW PATTERN 1 1 NGLE2 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4214-I-A STIS/N ACCUM F25CN18 MIRROR 1 4 M PATTERN 2 2 NGLE2 UV-MAM 2 A ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC4214-I-A STIS/N ACCUM F25SRF2 PRISM 2125 1 12 M SHADOW PATTERN 1 3 NGLE2 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC4214-I-A STIS/N ACCUM F25CN27 MIRROR 1 4 M PATTERN 2 4 NGLE2 UV-MAM 0 A ------------------------------------------------------------------------------------------------------------------------------------ 9096( 8) - 03/06/03 13:22 - [ 10] Visit: 03 Visit Priority: Visit Requirements: AFTER 04 BY 15 D TO 90 D 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 NGC4214-II- STIS/N ACCUM F25SRF2 PRISM 2125 1 12 M SHADOW PATTERN 1 1 ANGLE1 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4214-II- STIS/N ACCUM F25CN18 MIRROR 1 4 M PATTERN 2 2 ANGLE1 UV-MAM 2 A ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC4214-II- STIS/N ACCUM F25SRF2 PRISM 2125 1 12 M SHADOW PATTERN 1 3 ANGLE1 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC4214-II- STIS/N ACCUM F25CN27 MIRROR 1 4 M PATTERN 2 4 ANGLE1 UV-MAM 0 A ------------------------------------------------------------------------------------------------------------------------------------ 9096( 8) - 03/06/03 13:22 - [ 11] Visit: 04 Visit Priority: Visit Requirements: ORIENT 46D TO 120D FROM 03 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 NGC4214-II- STIS/N ACCUM F25SRF2 PRISM 2125 1 12 M SHADOW PATTERN 1 1 ANGLE2 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4214-II- STIS/N ACCUM F25CN18 MIRROR 1 4 M PATTERN 2 2 ANGLE2 UV-MAM 2 A ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC4214-II- STIS/N ACCUM F25SRF2 PRISM 2125 1 12 M SHADOW PATTERN 1 3 ANGLE2 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC4214-II- STIS/N ACCUM F25CN27 MIRROR 1 4 M PATTERN 2 4 ANGLE2 UV-MAM 0 A ------------------------------------------------------------------------------------------------------------------------------------ 9096( 8) - 03/06/03 13:22 - [ 12] Visit: 05 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 NGC604-ANGL STIS/N ACCUM F25SRF2 PRISM 2125 1 12 M SHADOW PATTERN 1 1 E1 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC604-ANGL STIS/N ACCUM F25CN18 MIRROR 1 4 M PATTERN 2 2 E1 UV-MAM 2 A ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC604-ANGL STIS/N ACCUM F25SRF2 PRISM 2125 1 12 M SHADOW PATTERN 1 3 E1 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC604-ANGL STIS/N ACCUM F25CN27 MIRROR 1 4 M PATTERN 2 4 E1 UV-MAM 0 A ------------------------------------------------------------------------------------------------------------------------------------ 9096( 8) - 03/06/03 13:22 - [ 13] Visit: 06 Visit Priority: Visit Requirements: ORIENT 60D TO 120D FROM 05 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 NGC604-ANGL STIS/N ACCUM F25SRF2 PRISM 2125 1 12 M SHADOW PATTERN 1 1 E2 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC604-ANGL STIS/N ACCUM F25SRF2 MIRROR 1 4 M PATTERN 2 2 E2 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC604-ANGL STIS/N ACCUM F25SRF2 PRISM 2125 1 12 M SHADOW PATTERN 1 3 E2 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC604-ANGL STIS/N ACCUM F25SRF2 MIRROR 1 4 M PATTERN 2 4 E2 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 9096( 8) - 03/06/03 13:22 - [ 14] Summary Form for Proposal 9096 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F25SRF2 F25CN182 F25CN270 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 02 BY 15 D TO 90 D SHADOW PATTERN 1 1 PATTERN 2 2 SHADOW PATTERN 1 3 PATTERN 2 4 ORIENT 40D TO 120D FROM 01 AFTER 04 BY 15 D TO 90 D ORIENT 46D TO 120D FROM 03 ORIENT 60D TO 120D FROM 05 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements PRISM MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC4214-I-ANGLE1 NGC4214-I-ANGLE2 NGC4214-II-ANGLE1 NGC4214-II-ANGLE2 NGC604-ANGLE1 NGC604-ANGLE2 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 2125 ------------------------------------------------------------------------------------------------------------------------------------