6418(  3) - 01/31/96 11:13  - [  1]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      6418
                                                       Version: 3
                                                       Check-in Date: 31-Jan-1996
                                                                      11:07:04

1.Proposal Title:
Small-Scale ISM Structure: The Remarkable Sightline Toward Mu Cru
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2. Scientific Category   3. Proposal For  4. Cycle
INTERSTELLAR MEDIUM         GO               6
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5. Investigators
                                                                                     Contact?
    PI: David M. Meyer                    Northwestern University
   CoI: J. Chris Blades                   Space Telescope Science Institute             N

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6. Abstract

We propose to use the GHRS on HST to accurately measure and compare the interstellar
Zn II, Cr II, Mg I, and C I absorption line profiles toward both members of the
binary Mu Cru. Recent optical observations of Na I at very high spectral resolution
(Deltav = 0.3 km s^-1) toward this system reveal line strength variations in at
least 4 velocity components indicative of ISM structure on scales less than the
projected binary separation of 6600 AU. Based on similar observations toward other
binaries, such structure now appears to be a common feature of the cold diffuse gas
in the local ISM and detectable toward systems with projected separations as small
as 1000 AU. The Mu Cru sightline provides an ideal laboratory to study the nature of
this small-scale ISM structure with high S/N GHRS echelle observations of key
interstellar ions. In particular, comparisons between the optically thin Zn II and
Cr II lines will allow us to assess how the clumpiness of the gas is reflected in
the dust. Observations of the C I fine -structure excitation will provide good
estimates of the pressures in the cold clumps whose small scale lengths suggest high
gas densities in a very diffuse ambient medium. In summary, our proposed GHRS
observations of Mu Cru will provide the first quantitative steps toward
understanding how small- scale structure forms and evolves in the diffuse ISM.
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                                                6418(  3) - 01/31/96 11:13  - [  2]

  Observations Description
  ------------------------

     Our goal is to obtain high S/N GHRS echelle observations of the interstellar Zn
     II, Cr II, Mg I, and C I absorption line profiles toward both members of the
     binary Mu Cru. All observations will be made using the SSA and a sampling
     strategy of four substeps per diode in order to maximize the spectral
     resolution needed for good profile comparisons. The observations of Zn II
     LambdaLambda2026.14, 2062.66, Cr II LambdaLambda2062.23, 2066.16, and Mg I
     LambdaLambda2026.48, 2852.96 will involve three ECH-B wavelength setups per
     star. Based on the IUE fluxes for Mu Cru and accounting for the effects of the
     ECH-B blaze efficiency as well as the reduced SSA throughput on the
     sensitivities listed in the Cycle 6 GHRS Handbook, we can achieve spectra of
     all of these lines at a S/N ratio of 150 in a total integration time of 39 min.
     (3 exposures) on Mu^1 Cru and 160 min. (9 exposures) on Mu^2 Cru. We plan to
     use the FP-Split option in all of our exposures in order to better assess and
     remove the effects of fixed pattern noise. In the case of C I, the multiplets
     centered near 1277 and 1280 Angstrom\\ can both be covered with one ECH-A
     wavelength setup per star. Since the oscillator strengths and wavelength
     separations of the various J = 0, 1, 2 transitions in these multiplets are
     quite different, it will be possible to make an accurate comparison of the C I
     fine-structure excitation as a function of velocity in the Mu Cru line
     profiles. We can obtain a C I spectrum with a S/N ratio of 150 toward Mu^1 Cru
     in a total integration time of 19 min. (1 exposure) and one with a S/N ratio of
     100 toward Mu^2 Cru in a time of 64 min. (4 exposures). The C I sensitivity
     goal is somewhat lower for Mu^2 Cru because this star is appreciably fainter in
     the far- UV and the key component (-9 km s^-1) to be studied is in the Mu^1 Cru
     sightline. Allowing for all of the acquisition and exposure overheads
     (including the 45 min. GHRS side change from ECH-A to ECH-B which can occur
     mostly during occultation), our proposed observations will require 2 orbits on
     Mu^1 Cru and 6 orbits on Mu^2 Cru given a visibility window of 57 min. per
     orbit.

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

     None. None.

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


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

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  Data Distribution

  Media:           8MM
  Blocking Factor: 10
  Ship To:         PI_Address
  Ship Via:        UPS
  Email:                                                              ,
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                                                6418(  3) - 01/31/96 11:13  - [  3]

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   HD112092     STAR, B0-B2 V-IV        RA=12H 54M 35.64S +/- 0.03S,     J2000             V = 4.03
                                         DEC=-57D 10' 39.8" +/- 0.5"                        B-V = -0.17

2   HD112091     STAR, BE                RA=12H 54M 36.91S +/- 0.03S,     J2000             V = 5.17
                                         DEC=-57D 10' 06.7" +/- 0.5"                        B-V = -0.12

                                                6418(  3) - 01/31/96 11:13  - [  4]

Visit: 01
     Visit Requirements:  <none>
     On Hold Comments:    <none>
     Additional Comments: <none>


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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1        HD112092    HRS    ACQ     2.0     MIRROR-N1        BRIGHT=RETURN,S 1   1.8S    ONBOARD ACQ FOR 2
                                                             EARCH-SIZE=3
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2        HD112092    HRS    ACQ/PEA 0.25    MIRROR-N1        SEARCH-SIZE=5   1   5.0S    ONBOARD ACQ FOR 3-6
                            KUP
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3        HD112092    HRS    ACCUM   0.25    ECH-A44  1278.7  FP-             2   544.0S
                                                             SPLIT=STD,STEP-
                                                             PATT=7
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4        HD112092    HRS    ACCUM   0.25    ECH-B28  2026.0  FP-             4   652.8S
                                                             SPLIT=STD,STEP-
                                                             PATT=7
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5        HD112092    HRS    ACCUM   0.25    ECH-B27  2063.0  FP-             3   544.0S
                                                             SPLIT=STD,STEP-
                                                             PATT=7
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6        HD112092    HRS    ACCUM   0.25    ECH-B20  2853.0  FP-             2   435.2S
                                                             SPLIT=STD,STEP-
                                                             PATT=7
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                                                6418(  3) - 01/31/96 11:13  - [  5]

Visit: 02
     Visit Requirements:  <none>
     On Hold Comments:    <none>
     Additional Comments: <none>


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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7        HD112091    HRS    ACQ     2.0     MIRROR-N1        BRIGHT=RETURN,S 1   1.8S    ONBOARD ACQ FOR 8
                                                             EARCH-SIZE=3
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8        HD112091    HRS    ACQ/PEA 0.25    MIRROR-N1        SEARCH-SIZE=5   1   5.0S    ONBOARD ACQ FOR 9-12
                            KUP
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9        HD112091    HRS    ACCUM   0.25    ECH-A44  1278.7  FP-             4   979.2S
                                                             SPLIT=STD,STEP-
                                                             PATT=7
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10       HD112091    HRS    ACCUM   0.25    ECH-B28  2026.0  FP-             4   870.4S
                                                             SPLIT=STD,STEP-
                                                             PATT=7
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11       HD112091    HRS    ACCUM   0.25    ECH-B27  2063.0  FP-             3   870.4S
                                                             SPLIT=STD,STEP-
                                                             PATT=7
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12       HD112091    HRS    ACCUM   0.25    ECH-B20  2853.0  FP-             2   979.2S
                                                             SPLIT=STD,STEP-
                                                             PATT=7
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                                                6418(  3) - 01/31/96 11:13  - [  6]

            Summary Form for Proposal  6418

  Item                         Used in this proposal
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  Configurations               HRS
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  Opmodes                      ACQ ACQ/PEAKUP ACCUM
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  Optional Parameters          BRIGHT=RETURN SEARCH-SIZE=3 SEARCH-SIZE=5 STEP-PATT=7 FP-SPLIT=STD
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  Proposal Category            GO
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  Scientific Category          INTERSTELLAR MEDIUM
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  Special Requirements         ONBOARD ACQ FOR 2 ONBOARD ACQ FOR 3-6 ONBOARD ACQ FOR 8 ONBOARD ACQ FOR 9-12
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  Spectral Elements            MIRROR-N1 ECH-A44 ECH-B28 ECH-B27 ECH-B20
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  Target Names                 HD112092 HD112091
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