6590( 5) - 11/05/96 11:13 - [ 0] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6590 Version: 5 Check-in Date: 05-Nov-1996 11:10:59 1.Proposal Title: The metallicity of high-velocity cloud complex C ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle INTERSTELLAR MEDIUM GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Bart Wakker University of Wisconsin CoI: Limin Lu Caltech N CoI: Blair Savage University of Wisconsin N CoI: Ulrich Schwarz Kapteyn Astronomical Institute N CoI: Hugo van Woerden Kapteyn Astronomical Institute N ------------------------------------------------------------------------------------ 6. Abstract Our requested observation will be used to determine the intrinsic heavy element abundance of the largest high-velocity cloud (HVC), complex C. This measurement can only be done with HST, as the required spectral lines are inaccesible by ground- based observations, while IUE was not sensitive enough. Knowing the metallicity of HVCs is essential for discriminating between different theories for their origin. Suggestions vary from a galactic fountain and superbubbles/supernova-remnants to a polar ring around our galaxy. Using GHRS, we will observe the interstellar S II absorption associated with complex C, in the spectrum of the extra-galactic background object Mark 290. Sulphur occurs mostly in the gas phase and is thus representative of the intrinsic metal abundance. We will be able to measure a Sulphur abundance in complex C as low as 0.07 solar (for a 3Sigma detection). Using the replacement for GHRS, STIS, would not improve our program, as all the interesting S II lines fall within a 10 Angstrom\ wavelength range, while the sensitivity is expected to be comparable. ------------------------------------------------------------------------------------ Observations Description ------------------------ We will observe the S II lines in the spectrum of the Seyfert galaxy Mark 290, projected on HVC complex C. The source will be put in the LSA, using the G160M grating of GHRS, with a central wavelength of 1250 Angstrom. This will produce a spectrum ranging from 1233 to 1267 Angstrom\\ at a resolution of 19kms. The spectrum will include the three S II lines of interest at 1250, 1253 and 1259 Angstrom. As a bonus we will be able to look at the N V lines at 1238 and 1243 Angstrom, for which galactic absorption at low velocities may exist. The high- velocity component of the Si II line at 1260 Angstrom\\ will partly blend with the low-velocity S II 1259 Angstrom\\ component, and may not yield useful information. Another possible bonus will be the presence of low redshift LyAlpha lines from intervening gas at Lambda<1252. Such absorption could blend with S II Lambda1250, but will not be a problem for S II LambdaLambda 1253 and 1259. All measurements will be obtained with a substep pattern of two samples per diode (pattern 4), which optimizes the actual signal-to-noise ratio of the spectrum. FP-SPLIT scanning will be used to reduce photocathode and detector window fixed- pattern noise. With this setup, 11\\% of the observing time will be spent to regularly sample with the full diode array the changing detector background during each orbit. The integration time is calculated from an ultra- violet flux of 12tdex-14 \\fu\\ at 1250 Angstrom, as observed for Mark 290 with IUE (Fig. 2). The GHRS Handbook (v6.0, June 1995) gives the sensitivities of the gratings, from which we estimate the expected count rate at 1250 Angstrom\\ to be 0.069 c/s/diode. With our selected step pattern there are two samples per diode, and thus the desired S/N of 15 in the continuum corresponds to S/N=21 per diode. The required on-source integration time is then 120 minutes. Since in the first orbit of a set of consecutive orbits about 35 minutes of on- source integration time is available (57 minutes of visibility of which 8 minutes go to guide-star acquisition, 11 to target acquisition and 3 to the wavelength calibration), and on subsequent orbits about 50 minutes, we can achieve our required S/N of 15 in a total of 3 orbits. Real Time Justification ----------------------- None. A major problem in converting absorption-line column densities to abundances can be that they are obtained along the (almost) infinitesimal pencil beams toward the background probes, while H I column densities are based on 21-cm data obtained with large radio beams (often >10'). Wakker & Schwarz (1991) showed that in HVC cores N(H I) can vary by a factor five over a few arcminutes. Outside HVC cores the variations are less (Wakker, Schwarz, van Woerden, work in progress), and 9'\\ resolution Effelsberg spectra may suffice. A high-resolution Westerbork map will be obtained for Mark 290. We thus can constrain the abundances to within a factor of at most two. Even in the worst case, this uncertainty will not prevent a conclusion about the possible origin of complex C, as different hypotheses may vary by more than a factor 5 in their predictions for the intrinsic heavy element abundance. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6590( 5) - 11/05/96 11:13 - [ 1] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 MARK290 GALAXY,SEYFERT,ISM,HIGH RA = 15H 35M 52.4S +/- 0.1S, DEC J2000 Z=0.030 V = 14.96 VELOCITY = 57D 54' 09.6" +/- 0.2", PLATE- B-V = 0.60 CLOUD,INTERMEDIATE ID = 01OK U-B = -0.62 VELOCITY CLOUD F-CONT(1250)=12e-14 6590( 5) - 11/05/96 11:13 - [ 2] 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 MARK290 HRS ACQ 2.0 MIRROR-N2 MAP=DEF,SEARCH-SIZE=3 1 14.4 S ONBOARD ACQ FOR 2-4 Comments: STEP-TIME=1.6S ------------------------------------------------------------------------------------------------------------------------------------ 2 MARK290 HRS ACCUM 2.0 G160M 1250 FP-SPLIT=STD,STEP-PATT=4 8 230.4 S Comments: STEP-TIME=0.2S ------------------------------------------------------------------------------------------------------------------------------------ 3 MARK290 HRS ACCUM 2.0 G160M 1250 FP-SPLIT=STD,STEP-PATT=4 12 230.4 S Comments: STEP-TIME=0.2S ------------------------------------------------------------------------------------------------------------------------------------ 4 MARK290 HRS ACCUM 2.0 G160M 1250 FP-SPLIT=STD,STEP-PATT=4 11 230.4 S Comments: STEP-TIME=0.2S ------------------------------------------------------------------------------------------------------------------------------------ 6590( 5) - 11/05/96 11:13 - [ 3] Summary Form for Proposal 6590 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters MAP=DEF SEARCH-SIZE=3 STEP-PATT=4 FP-SPLIT=STD ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category INTERSTELLAR MEDIUM ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 2-4 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-N2 G160M ------------------------------------------------------------------------------------------------------------------------------------ Target Names MARK290 ------------------------------------------------------------------------------------------------------------------------------------