Spitzer Space Telescope - General Observer Proposal #20159


Grain Composition in the Southern Coalsack

Principal Investigator: B-G Andersson
           Institution: The Johns Hopkins University

Co-Investigators: 
Peter G. Wannier, Jet Propulsion Laboratory                           
Stephan R. McCandliss, The Johns Hopkins University                   

Science Category: ISM 
 Observing Modes: IrsStare 
  Hours Approved: 7.1 

Abstract:
We propose to use the Spitzer IRS SL1 & SL2 spectrographs to observe
the solid state features due to carbonaceous/PAH and silicate grains
in absorption towards four stars behind the Southern Coalsack dark
cloud.  Our goal is to determine the composition of the small grains,
by contrasting sightlines, in the same cloud, showing nominal and
enhanced grain destruction.  Combined optical and NIR photometry shows
distinct values of the total to selective extinction (R_V) for
different parts of the cloud, indicating varying grain size
distributions.  Together with optical, FUV and X-ray observations, the
R_V measurements indicate that shock-induced grain destruction is
taking place in parts of the Coalsack.  By comparing the dust
absorption on sightlines with and without enhanced grain destruction,
and by combining these with tracers of the gas phase abundance, we can
determine the composition of the small grains.  The target stars have
been chosen because:  1) they have well determined distances, 2) they
probe only absorption by Coalsack material, 3) they have FUSE and
HST/STIS (or IUE) UV and FUV data already in hand to provide gas phase
abundances and extinction curves and 4) they provide two examples each
of sightlines with enhanced and nominal R_V.