8344( 9) - 05/05/99 13:15 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8344 Version: 9 Check-in Date: 05-May-1999 13:15:09 1.Proposal Title: UV Spectral Properties of O and B Stars Versus Metallicity ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Frederick Bruhweiler Catholic University of America CoI: Dr. Alex de Koter University of Amsterdam N CoI: Dr. Margaret Smith Neubig The Catholic University of America N CoI: Dr. Thierry Lanz AURA/Goddard Space Flight Center N CoI: Dr. Nolan Walborn Space Telescope Science Institute N ------------------------------------------------------------------------------------ 5. Abstract Metallicity is the most fundamental parameter governing both stellar and galactic evolution. Yet, little is known observationally about how metallicity affects the physics of massive stars. To explore this connection, we will use the STIS to obtain slitless UV spectroscopy of bright OB associations in three Local Group galaxies that span a factor of 30 in metallicity. Our targets are star-forming regions in M31, NGC 6822, and Sextans A. Nebular O/H abundances indicate Sextans A is the most metal deficient galaxy in the Local Group, with a metallicity 30x below the Galaxy and 3x below the SMC. The STIS UV slitless spectra will resolve numerous O and B stars in a single exposure. This study will use the unique spectral imaging capabilities of STIS to probe stellar populations spanning the widest range of metallicity possible. Theoretical synthetic spectra will be constructed in an attempt to match the strengths and shapes of both strong photospheric and wind features. In th is way we will derive approximate abundances, mass loss rates, and terminal velocities. We will also apply our new UV stellar classification criteria and probe the variation with metallicity of mass-loss characteristics through the H- R diagram. Our goal is to answer one of the most fundamental questions in astrophysics: ``What is the impact of chemical abundance upon stellar mass loss?" ------------------------------------------------------------------------------------ 8344( 9) - 05/05/99 13:15 - [ 2] Observations Description ------------------------ Each of our three targets will be observed with the same observational setup using STIS. Our three targets in order of priority are: 1.) The SE region of the most metal-deficient galaxy in the Local Group, the irregular Sextans A. The 25MAMA STIS aperture will be positioned to maximize UV spectral coverage of the bright H II regions (Numbers 16-20 as denoted by Hodge et al. 1994). The total HAlpha observed luminosity for this region, uncorrected for extinction, implies the equivalent of at least 40 O6 main sequence stars are needed to explain the Balmer emission. Our target is the cluster seen in archival WFPC2 imagery (Dohm- Palmer et al. 1997a, 1997b) corresponding to H II region Number 16. About 1/2 of the above HAlpha emission appears to originate from this cluster. Our position is based upon our own analysis of imagery available through the STScI archive. 2.) The unusually rich OB association, OB78, in the nearby Andromeda galaxy (M31). M31, with a metallicity slightly enhanced relative to the Galaxy, is the most metal rich galaxy in the Local Group. This region denotes one of the most concentrated regions of OB stars in M31 and has been extensively studied from the ground by Massey et al. (1995). 3.) A bright H II region in the outskirts of NGC 6822. This region is quite rich and shows a pronounced ring structure. It has an O/H ratio metallicity intermediate to the LMC and SMC. However, stellar spectra imply metallicities below the SMC, comparable to Sextans A. Details of STIS observations. We will observe each region with one pointing of the STIS/G140L in the slitless mode. (however, since Sextans A requires 7 orbits, we will need to split the observation into 2 distinct visits. these observations need to be obtained with the same ORIENTs. The pointings will be appropriately adjusted to provide complete spectral coverage from 1220Angstrom-1570Angstrom (350Angstrom) in the rest frame of the individual stars observed. This is necessary to adequately sample all three of the mass loss profiles, N V, Si IV, and C IV. This can be done for a 10" x 25" region of the STIS 25" x 25" open aperture. This ``effective FOV" is due to the 0.024"/pixel scale for the MAMA 1024 x 1024 pixel array and the 0.6Angstrom/ pixel dispersion of the G140L. Outside the 10" region, parallel to the dispersion axis, we will not obtain the complete required spectral coverage because part of the UV spectra will fall off the edge of the detector. As seen in Figure 1, Ly-Alpha background is a s er ious consideration. Since we will be obtaining our STIS observations in slitless mode, observations made during the day will be significantly contaminated by Ly-Alpha. Thus, we are requesting that all of our observations be made during shadow. We have estimated STIS exposure times for our three targets based upon a previous FOS/BL G140L spectrum of Sk -65 44, an O9 V star with E(B-V) = 0.09 in the LMC (50 Kpc). In our flux calculations, we used the measured fluxes at N V (6.0x10^-13 ergs/s/cm^2/Angstrom) and C IV (4.25x10^-13 ergs/s/cm^2/Angstrom) for this star and adjusted the flux estimates for stars in our target galaxies according to the adopted distances of M31 (680 Kpc), NGC 6822 (600 Kpc), and Sextans A (1.4 Mpc). In each case we have also assumed there was a Ly-Alpha background that was equivalent to 10\ To achieve a S/N ratio of 10 per pixel at C IV 1550 for individual, slightly-reddened O9 V stars, we require exposure times of 10, 200 s for Sex A, 2100 s for M 31, and 1500 s for NGC 6822. These exposures would go 1.5 tim es (0.44 magnitudes) fainter than our STIS exposure of NGC 604 (Fig. 1). In calculating these exposure times we have used the sensitivities as quoted in the new STIS Instrument Handbook. The S/N per pixel was calculated using the formula as given on page 77 of the handbook. In our orbit request we include 6 minutes for each guide star acquisition and 5 minutes for each reacquisition on each orbit. We also include the 6 minutes for setup for each STIS spectroscopic exposure. However, since we need to observe our targets during shadow, we have only 25 minutes of exposure time per orbit. This determines our orbit request. To accomplish our scientific goals we need 7 orbits for Sextans A, 1 orbit for NGC 6822 , and 2 orbits for M31. Thus, our total request is 10 orbits. noindent Priorities: The most metal-deficient galaxy of the Local Group, Sextans A, is our highest priority object. Our second priority target, M31-OB78, is the highest metallicity galaxy in the Local Group. O ur third priority target is NGC 6822-A, a galaxy where the nebular O/H metallicity appears to be in sharp contrast with stellar abundances (Massey et al. 1998). Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- My prayer wheel will be going full-tilt until these observations are completed. ------------------------------------------------------------------------------------ 8344( 9) - 05/05/99 13:15 - [ 3] Data Distribution Paper Products: YES Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8344( 9) - 05/05/99 13:15 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SEXTANS-A EXT-CLUSTER, OB RA=10H 11M 4.6S +/- 0.5", DEC=- J2000 V = +324 V = 18.2218+/-0.05 ASSOCIATION, EXT-STAR, 04D 42' 44.2" +/- 0.5", PLATE- B = 17.8680+/-0.05 STAR FORMING REGION ID=02NC Comments: Brightest star in the field. At least 7 stars brighter than V=20.6. Seven stars appear to be exciting stars of HII regions. 2 NGC6822-A EXT-CLUSTER, OB RA=19H 44M 34.7S +/- 0.5", DEC=- J2000 V = 60 V = 18.53+/-0.05 ASSOCIATION, EXT-STAR, 14D 42' 23.4" +/- 0.5", PLATE- B-V = 0.26+/-0.05 STAR FORMING REGION ID=00CN Comments: Several O stars in field slightly fainter than magnitude listed here. 3 M31-OB78 EXT-CLUSTER, OB RA=00H 40M 29.6S +/- 0.05", J2000 V = -295 V = 18.63+/-0.05 ASSOCIATION, EXT-STAR, DEC=40D 45' 28.6" +/- 0.05" B-V = -0.19+/-0.05 STAR FORMING REGION Comments: Magnitudes for O8.5 I star. There are other stars of comparable magnitude both brighter and fainter. See Massey et al. (1995: AJ, 110, 2715) 8344( 9) - 05/05/99 13:15 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENT 330D TO 350D On Hold Comments: Additional Comments: Allowed orientation angles are 330 to 350 degrees. Optimum orientation at (or near) 350 degrees. Visit 1 and Visit 2 must have same orientation angle. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 SEXTANS-A STIS/F ACCUM 25MAMA G140L 1425 1 2467 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 20 SEXTANS-A STIS/F ACCUM 25MAMA G140L 1425 1 2879 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 30 SEXTANS-A STIS/F ACCUM 25MAMA G140L 1425 1 2879 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 31 SEXTANS-A STIS/F ACCUM 25MAMA G140L 1425 1 2879 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 40 SEXTANS-A STIS/F ACCUM 25MAMA G140L 1425 1 2879 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8344( 9) - 05/05/99 13:15 - [ 6] Visit: 02 Visit Priority: Visit Requirements: SAME ORIENT AS 1 On Hold Comments: Additional Comments: Allowed orientation angles are 330 to 350 degrees. Optimum orientation at (or near) 350 degrees. Visit 1 and Visit 2 must have same orientation angle. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 SEXTANS-A STIS/F ACCUM 25MAMA G140L 1425 1 2467 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8344( 9) - 05/05/99 13:15 - [ 7] Visit: 03 Visit Priority: Visit Requirements: ORIENT 218D TO 230D On Hold Comments: Additional Comments: Orientation range from 218 to 230 degrees, with optimum orientation at 225 degrees. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC6822-A STIS/F ACCUM 25MAMA G140L 1425 1 2467 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC6822-A STIS/F ACCUM 25MAMA G140L 1425 1 2764 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8344( 9) - 05/05/99 13:15 - [ 8] Visit: 04 Visit Priority: Visit Requirements: ORIENT 35D TO 46D On Hold Comments: Additional Comments: Allowed orientation angles from 35 degrees to 46 degrees. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 M31-OB78 STIS/F ACCUM 25MAMA G140L 1425 1 2590 S GS ACQ SCENARIO SINGLE UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 20 M31-OB78 STIS/F ACCUM 25MAMA G140L 1425 1 2887 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8344( 9) - 05/05/99 13:15 - [ 9] Summary Form for Proposal 8344 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 25MAMA ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 330D TO 350D SAME ORIENT AS 1 ORIENT 218D TO 230D ORIENT 35D TO 46D GS ACQ SCENARIO SINGLE ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names SEXTANS-A NGC6822-A M31-OB78 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 ------------------------------------------------------------------------------------------------------------------------------------