6500(  4) - 02/22/96 14:51  - [  1]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      6500
                                                       Version: 4
                                                       Check-in Date: 22-Feb-1996
                                                                      14:47:45

1.Proposal Title:
A NEW ABUNDANCE REFERENCE AT THE PRESENT EPOCH
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2. Scientific Category   3. Proposal For  4. Cycle
GALAXIES & CLUSTERS         GO               6
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5. Investigators
                                                                                     Contact?
    PI: Limin Lu                          Caltech
   CoI: Wallace Sargent                   Caltech                                       N
   CoI: Blair Savage                      University of Wisconsin                       N
   CoI: Kenneth Sembach                   Massachusetts Institute of Technology         N
   CoI: Bart Wakker                       University of Wisconsin                       N

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6. Abstract

Observations of damped Ly-alpha\ absorption systems in quasar spectra have provided
important clues about the chemical evolution of galaxies between 0.7<z<4.5. However
interpretations of these results are complicated by the possible presence of dust:
it is not clear if the observed elemental abundances are intrinsic (ie, direct
products of nucleosynthesis) or have been modified by dust depletion. The above
issue can be resolved if we know the intrinsic abundance patterns in astrophysical
sites that have comparable degrees of chemical enrichment as in damped Ly-alpha\
systems but are dust-free, since such abundance patterns can serve as a reference
for recognizing the effects of dust depletion in damped Ly-alpha\ systems. Various
evidence suggests that the high velocity cloud HVC287+22+240 in the direction of the
Seyfert galaxy NGC3783 may be a good candidate for such a dust -free site.
Additionally, our previous HST observations indicate a Sulphur abundance of 0.15
solar for the HVC, comparable to those in damped Ly-alpha\ systems. Here we seek a
small amount of HST time in order to determine if dust is indeed absent in this HVC.
If the results indicate there is not much dust in the HVC, we will propose to make a
detailed study of the elemental abundance patterns in the HVC in a future HST cycle
for the purposes stated above. Regardless of whether or not we find dust in the HVC,
the current observations will provide important clues to the origin and nature of
the HVC.
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  Observations Description
  ------------------------

     GHRS Large Science Aperture observations of the bright Seyfert galaxy NGC3783
     will be made with the G270M grating over the wavelength region 2337-2381
     Angstrom\\ in order to measure the Fe II column density from the HVC287+22+240.
     The Fe II lines covered include Lambda2344 and 2374. Since we know the S II
     column density in the HVC from previous observations of NGC3783, N(S II)=3.4*
     10^14 cm^-2, we can predict the strength of the Fe II lines for various Fe/S
     abundance ratios (note that S II and Fe II are the dominant ionization stages
     of S and Fe in H I regions so ionization corrections are likely to be
     unimportant). These predictions are shown in Figure 3 (previous page). The flux
     of NGC3783 at 2350 Angstrom\\ is 3* 10^-14 ergs s^-1 cm^-2 AA^-1 based on IUE
     observations, giving a count rate of 0.066 c/s/d. We request 4 spacecraft
     orbits (~ 100 minutes actual integration time) in order to achieve S/N=20:1 per
     diode. With this S/N, the combination of the Fe II LambdaLambda2344, 2374 lines
     (which differ in their Lambda f values by a factor of ~ 4) will allow us to
     probe any Fe/S ratio from solar to 1/50 solar (see Figure 3). This is
     sufficient to tell if a significant amount of dust is present in the HVC, which
     is our main goal here. The S/N ratio and number of spacecraft orbits are
     estimated using the Instrument Handbook for the Goddard High Resolution
     Spectrograph (v6.0, June 1995). Standard comb-addition and FP-split will be
     used to remove the effects of fixed pattern noise. The spectra will be sampled
     with 2 sub-steps per diode (Step Pattern 4). We emphasize that the IUE is not
     capable of obtaining the high resolution (FWHM~ 13 km/s), high S/N spectra that
     we need for NGC3783. The GHRS medium resolution is needed in order to separate
     the HVC absorption from Galactic absorption, and to assess the effects of line
     saturation.

  Real Time Justification
  -----------------------

     None We have already secured observing time on the Australia radio telescope to
     obtain very high resolution (1') 21-cm maps of the HVC in the direction of
     NGC3783. This will allow us to more accurately estimate the line-of-sight H I
     column density of the HVC toward NGC3783, therefore improving the accuracy of
     the absolute metallicity measurements. Note that uncertainties in the N (H I)
     have no effect on the analysis discussed in this proposal since the dust
     depletion analysis will be based solely on the relative abundance ratio of Fe
     to S.

  Calibration Justification
  -------------------------


  Additional Comments
  -------------------

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                                                6500(  4) - 02/22/96 14:51  - [  3]

  Data Distribution

  Media:           8MM
  Blocking Factor: 10
  Ship To:         the HRS team at*Goddard Space*Flight
                   Center
  Ship Via:        UPS
  Email:           hrsdata@cosmos.gsfc.nasa.gov                       ,
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                                                6500(  4) - 02/22/96 14:51  - [  4]

TARGET LIST    a) Fixed Targets
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Tar|   Target   |         Target        |            Target              |Coord   | Radial |             Flux data
No |    Name    |       Description     |           Position             |Eqnx    |  Vel.  |
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1   NGC3783      Galaxy, Seyfert         RA=11H 39M 1.662S+/-1", DEC=-37D J2000    Z=0.0096 V = 13.0
                                         44' 19.79"+/-1", PLATE-ID=00JJ                     F(2350)=3.0+/-0.3e-14
    Comments: Coordinates from GASP used in pervious HST
              observations

                                                6500(  4) - 02/22/96 14:51  - [  5]

Visit: 01
     Visit Requirements:  <none>
     On Hold Comments:    <none>
     Additional Comments: target schedulabe only in July and December of 1996 before the 2nd
                          Servicing Mission


Exposures
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Exposure|   Target  |Instr | Oper. | Aper  |Spectral|Central|   Optional    |Num|  Time |               Special
 Number |    Name   |Config| Mode  |or FOV |Element |Waveln.|  Parameters   |Exp|       |             Requirements
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101      NGC3783     HRS    ACQ     2.0     MIRROR-N2        SEARCH-         1   9.0S    ONBOARD ACQ FOR 102
                                                             SIZE=3,BRIGHT=R
                                                             ETURN
   Comments:  STEP-TIME=1.0s Expect 4000 count/sec
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102      NGC3783     HRS    ACCUM   2.0     G270M    2359    FP-             5   16.32M
                                                             SPLIT=STD,STEP-
                                                             PATT=4
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103      NGC3783     HRS    ACQ/PEA 2.0     MIRROR-N2                        1   51.0S   ONBOARD ACQ FOR 104
                            KUP
   Comments:  STEP-TIME=0.5s
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104      NGC3783     HRS    ACCUM   2.0     G270M    2359    FP-             5   15.36M
                                                             SPLIT=STD,STEP-
                                                             PATT=4
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            Summary Form for Proposal  6500

  Item                         Used in this proposal
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  Configurations               HRS
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  Opmodes                      ACQ ACCUM ACQ/PEAKUP
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  Optional Parameters          SEARCH-SIZE=3 BRIGHT=RETURN STEP-PATT=4 FP-SPLIT=STD
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  Proposal Category            GO
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  Scientific Category          GALAXIES & CLUSTERS
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  Special Requirements         ONBOARD ACQ FOR 102 ONBOARD ACQ FOR 104
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  Spectral Elements            MIRROR-N2 G270M
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  Target Names                 NGC3783
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