6542( 5) - 07/31/96 16:43 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6542 Version: 5 Check-in Date: 31-Jul-1996 17:40:12 1.Proposal Title: Interstellar Carbon Abundance in Low Density Gas ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle INTERSTELLAR MEDIUM GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: ULYSSES SOFIA VILLANOVA UNIVERSITY CoI: David M. Meyer Northwestern University N ------------------------------------------------------------------------------------ 6. Abstract We propose to obtain high signal-to-noise data (~1300) with the GHRS of the weak intersystem line of C II (Lambda2325 Angstrom) toward Kappa Ori and Lambda Ori to determine the limits on the gas-phase abundance of carbon, C/H, in environments where other elements show measurable return from dust to the gas-phase in comparison to moderately dense sight lines like Zeta Oph. In a recent study with the GHRS, we found that the abundance of carbon is remarkably constant in sight lines where the fraction of H in H_2, f(H_2), ranges between 10^-1 and 1. At the same time however, other elements exhibiting a similar bulk depletion (from solar) show a modest increase in their gas-phase abundance from high-to-low f(H_2). This increase is observed to continue to significantly lower values of f(H_2) of the order of 10^-5. Since we interpret this increase as the return of matter to the gas from the dust, the lack of variability in the observed C/H would seem to suggest that the carbon- based grains (and/or PAHs) are not easily disrupted. However, the existing C/H data in the moderate-to-low f(H_2) sight lines are not definitive. Determining the gas- phase C/H toward Kappa Ori and Lambda Ori f(H_2) ~eq 3times10^-5 and 3times10^-2, respectively will set practical limits on the variability of gas-phase C/H in low density gas and the resiliency of the carriers of carbon in non-atomic form. Given the high S/N requirements for this project, this is the last opportunity to achieve these results since such precision weak line work will not be possible with STIS. ------------------------------------------------------------------------------------ 6542( 5) - 07/31/96 16:43 - [ 2] Observations Description ------------------------ We propose to obtain Ech-B data in the LSA (2 samples/diode) on the C II line at Lambda2325.403 Angstrom\\ toward Kappa and Lambda Ori, stars sampling moderate-to-low f(H_2) values (see Fig. 1). We use the LSA to maximize the throughput (the 20\\% reduction in resolution over the SSA will have no impact on the science goals). Basic target information is shown in the table below. Since the C II line will be weak toward these two stars (see Fig. 2), accurate measures require very high S/N values at each diode substep. Accounting for guide star acquistion, target ACQ, ACQ/PEAKUP, and exposure overhead times, these values can be obtained in the time remaining in a single orbit (these estimates are based on the post-COSTAR LSA throughput data given in the GHRS Cycle 6 Handbook) for each star. 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. begintableh begintabularccccccccc Name & & Spectral Type & & V & & Log H_total^a & & Log f(H_2)^b , Kappa Ori (HD 38771) & & B0.5 Ia & & 2.09 & & 20.52 & & - 4.54 , Lambda Ori (HD 36861) & & O8 V & & 3.66 & & 20.83 & & -1.41 , endtabular beginraggedright, ^alog of the total column density of hydrogen nuclei, H_total = 2H_2 + H I, ^blog of the fractional abundance of hydrogen nuclei in H_2, f(H_2) = 2H_2/H_total, endraggedright endtable The targets proposed here have been previously observed with the GHRS (see Fig. 2). We plan on combining the higher S/N data obtained in this program with the existing data. This should yield a cumulative S/N ~ 1300 for each star (at each diode substep). We have already shown that such high S/N values are possible with the GHRS (Cardelli & Ebbets 1994; Meyer et al. 1994). Given that STIS will be unable to obtain such precise measures of weak lines, this represents the last foreseeable opportunity to do this type of science. Real Time Justification ----------------------- NONE NONE Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6542( 5) - 07/31/96 16:43 - [ 3] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HD57061 STAR, SUPERGIANT O RA=07H 18M 42.4S +/- 0.03S, DEC= J2000 V = 4.40 -24D 57' 15.2" +/- 0.5" B-V = -0.15 Comments: target previously observed with ghrs-these coordinates come from the hst observing log 6542( 5) - 07/31/96 16:43 - [ 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 HD57061 HRS ACQ 2.0 MIRROR-A2 BRIGHT=RETURN,SEARCH-SIZE=3 1 1.8 S ONBOARD ACQ FOR 2-4 Comments: STEP-TIME = 0.2s ------------------------------------------------------------------------------------------------------------------------------------ 2 HD57061 HRS ACCUM 2.0 ECH-B24 2324 FP-SPLIT=STD,STEP-PATT=6 3 6.72 M ------------------------------------------------------------------------------------------------------------------------------------ 3 HD57061 HRS ACCUM 2.0 ECH-B24 2325 FP-SPLIT=STD,STEP-PATT=6 3 6.72 M ------------------------------------------------------------------------------------------------------------------------------------ 4 HD57061 HRS ACCUM 2.0 ECH-B24 2326 FP-SPLIT=STD,STEP-PATT=6 3 6.72 M ------------------------------------------------------------------------------------------------------------------------------------ 6542( 5) - 07/31/96 16:43 - [ 5] Summary Form for Proposal 6542 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BRIGHT=RETURN SEARCH-SIZE=3 STEP-PATT=6 FP-SPLIT=STD ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category INTERSTELLAR MEDIUM ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 2-4 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-A2 ECH-B24 ------------------------------------------------------------------------------------------------------------------------------------ Target Names HD57061 ------------------------------------------------------------------------------------------------------------------------------------