Spitzer Space Telescope - Guaranteed Time Observer Proposal #30180


An Enigmatic Source Towards The LMC

Principal Investigator: Giovanni Fazio
           Institution: Harvard-Smithsonian Astrophysical Observatory

Co-Investigators: 
Brian Patten, Smithsonian Astrophysical Observatory                   
Joseph Hora, Smithsonian Astrophysical Observatory                    
Matthew Ashby, Smithsonian Astrophysical Observatory                  
Nitya Kallivayalil, Harvard University                                
Pauline Barmby, Smithsonian Astrophysical Observatory                 

Science Category: extragalactic stellar studies 
 Observing Modes: IrsStare 
  Hours Approved: 1.0 

Abstract:
We are proposing to invest a small amount of Spitzer/IRS time to
acquire a low resolution spectrum from 5.2-38 microns as part of a
program to ascertain the physical nature of a 24-micron bright/IRAC
faint enigmatic object seen in the direction of the LMC.  This object
just discovered while investigating the infrared SED of gravitational
microlensing event MACHO-LMC-4, as a part of GTO program PID 118
"Macho Search".  We serendipitously noted an object in both the IRAC
and MIPS fields-of-view that was very bright at 24 microns (9 mJy),
but had very low flux levels in all IRAC bandpasses (0.93 mJy at 8
microns).  Because of the shape of the broadband SED as observed by
IRAC and MIPS, we favor two basic possibilities as being the most
likely explanation for the nature of his object:  (1) A z~2 starburst
galaxy, where the strong 24 micron emission is produced by 8 micron
rest-frame PAH emission redshifted into the MIPS 24 micron bandpass or
(2) A dust enshrouded luminous blue variable (LBV) in a pre-planetary
nebula (pre-PN) evolutionary state.  We will use the IRS in standard
staring mode to acquire spectra with both low resolution modules in
the range of 5.2 - 38 microns.  A IRS low-resolution spectrum will
reveal a more detailed picture of the shape of its SED.  We estimate
that no more than 1 hour will be required to obtain spectra with
sufficient signal-to-noise to determine whether this object is a
distant starburst galaxy (via a PAH emission peak) or a more nearby
pre-PN LBV (a rising continuum that flattens off towards the mid-IR).
The proposed observation represents the first stage of investigating
nature of this object.