10645(  1) - 04/05/05 12:15  - [  1]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      10645
                                                       Version: 1
                                                       Check-in Date: 05-Apr-2005
                                                                      16:14:49

1.Proposal Title:
Si III Absorption as a Sensitive Probe of the High-Velocity Cloud Population
------------------------------------------------------------------------------------
2. Proposal For  3. Cycle  4. Parallel Pointing Tolerance
AR               14        <none>
------------------------------------------------------------------------------------
5. Investigators
                                                                                     Contact?
    PI: Dr. Joseph A. Collins             University of Colorado at Boulder
   CoI: Prof. J. Michael Shull            University of Colorado at Boulder             N
   CoI: Prof. Mark L. Giroux              East Tennessee State University               N

------------------------------------------------------------------------------------
6. Abstract

We propose an archival HST program, surveying Si III absorption from high-velocity
clouds (HVCs) in STIS E140M and G140M data for 70 AGN/QSO sight lines. The
interstellar Si III line at 1206.50 Angstroms is the strongest metal line in the
HST-STIS bandpass for HVC studies and is not often contaminated by IGM interlopers.
At present, large surveys have been carried out for only the highest-column-density
HVCs through studies of H I 21cm emission. This program will map the distribution,
covering factor, and kinematics of HVCs down to a sensitivity of logN(H I)=15-16, an
improvement of approximately 3 orders of magnitude over previous HVC surveys in H I
emission. In addition, we will explore the ionization characteristics of the Si III-
detected HVCs in order to assess models of HVCs and their role in structure
formation scenarios. Despite the problems with current UV instruments and
uncertainty surrounding future missions, this program presents new science through
the use of existing HST data.
------------------------------------------------------------------------------------
                                                10645(  1) - 04/05/05 12:15  - [  2]

            Summary Form for Proposal  10645

  Item                         Used in this proposal
------------------------------------------------------------------------------------------------------------------------------------
  Proposal Category            AR
------------------------------------------------------------------------------------------------------------------------------------