6686( 6) - 08/01/96 16:13 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6686 Version: 6 Check-in Date: 01-Aug-1996 17:10:32 1.Proposal Title: On the Nature of Dust Mineralogy ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle INTERSTELLAR MEDIUM GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: ULYSSES SOFIA VILLANOVA UNIVERSITY CoI: Jason A. Cardelli Villanova University N ------------------------------------------------------------------------------------ 6. Abstract The Lambda1240 Angstrom transitions of Mg ii have been, and remain, the primary absorption lines used for determining the abundance of interstellar Mg, an element which represents an important constituent of interstellar dust. However, there are strong indications that the f-values (oscillator strengths) commonly used to derive column densities from these lines are too small by nearly a factor of 5. This has very important implications for dust mineralogy since Mg is one of the more abundant elements in interstellar dust grains. We propose to empirically derive the Mg ii Lambda1240 Angstrom f-values through an analysis of high signal-to-noise observations of these lines in comparison with those of the strong transition doublet near Lambda2800 Angstrom for which accurate f-values are available. Since the f-values for the Lambda1240 Angstrom doublet likely cannot be measured in the laboratory and theoretical determinations are extremely complex and unreliable, empirical analysis via Cycle 6 GHRS observations offer the last opportunity in the forseeable future to determine reliable oscillator strengths for these critical transitions. The results of this project will greatly benefit the analysis and understanding of previously acquired data from Copernicus, IUE, and the GHRS as well as future data obtained with STIS. ------------------------------------------------------------------------------------ 6686( 6) - 08/01/96 16:13 - [ 2] Observations Description ------------------------ We propose to obtain Ech-A and Ech-B data in the LSA (4 samples/diode) on the Mg ii doublet at Lambda1240 Angstrom and Lambda2800 Angstrom toward gamma Ara. We use the LSA to maximize the throughput (the 20\\% reduction in resolution over the SSA will have no impact on the science goals). Since our goal is to produce accurate empirical f- values for the Lambda1240 Angstrom doublet, we require very high S/N values at each diode substep for both observations. Accounting for guide star acquistion, target ACQ, and exposure overhead times, the observations require two orbits. The necessity for two orbits results from the fact that we using both side 1 and side 2 (the overhead required to side switch means that both observations cannot be obtained in a single orbit). However, to maximize the efficiency, we shall specify the observation sequence such that the side switch can be done during the unusable portion of the first orbit (the occultation). The required S/N values will be achieved through analysis of multiple grating position subexposures, each obtained with the FP- SPLIT command. We are well versed in both the development and use of the techniques for obtaining high S/N GHRS data -- Cardelli & Ebbets 1994, ``Attaining High Signal-to-Noise Data with the Goddard High Resolution Spectrograph", in Calibrating the Hubble Space Telescope, (STScI: Baltimore) and Cardelli 1995, ``High S/N Spectroscopy with the GHRS: Methods and Applications", in Calibrating HST in the Post- Costar Era (STScI: Baltimore), in press. Given that STIS will be unable to obtain such precise measures, this represents the only foreseeable opportunity to do this project. Real Time Justification ----------------------- NONE NONE Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6686( 6) - 08/01/96 16:13 - [ 3] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HD157246 STAR,B0-B2 III-I RA=17H 25M 23.63S +/-0.02S, DEC= J2000 V = 3.4+/-0.10 -56D 22' 39.5" +/-0.2" B-V = -0.14 E(B-V) = 0.06 TYPE=B1IB Comments: Coordinates are from Hipparcos INCA. This value is from the final version of the Hipparcos catalog and disagrees slightly in RA and Dec with the preliminary Hipparchos value listed in the HST Guide Star Catalog. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000.000 0.0002 -0.012 0.0 6686( 6) - 08/01/96 16:13 - [ 4] Visit: 01 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 HD157246 HRS ACQ 2.0 MIRROR-A2 BRIGHT=RETURN 1 1.8 S ONBOARD ACQ FOR 2-5 SEQ 1-3 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 HD157246 HRS ACCUM 2.0 ECH-B 2800.5 FP-SPLIT=STD 2 326.4 S ------------------------------------------------------------------------------------------------------------------------------------ 3 HD157246 HRS ACCUM 2.0 ECH-B 2799.5 FP-SPLIT=STD 2 326.4 S ------------------------------------------------------------------------------------------------------------------------------------ 4 HD157246 HRS ACCUM 2.0 ECH-A 1240.5 FP-SPLIT=STD 4 326.4 S ------------------------------------------------------------------------------------------------------------------------------------ 5 HD157246 HRS ACCUM 2.0 ECH-A 1239.5 FP-SPLIT=STD 3 326.4 S ------------------------------------------------------------------------------------------------------------------------------------ 6686( 6) - 08/01/96 16:13 - [ 5] Summary Form for Proposal 6686 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BRIGHT=RETURN FP-SPLIT=STD ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category INTERSTELLAR MEDIUM ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 2-5 SEQ 1-3 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-A2 ECH-B ECH-A ------------------------------------------------------------------------------------------------------------------------------------ Target Names HD157246 ------------------------------------------------------------------------------------------------------------------------------------