Spitzer Space Telescope - Archive Research Proposal #20198


Searching for the Most Massive Stars in Starbursts with Archival
Spectra

Principal Investigator: Jean Turner
           Institution: UCLA

Co-Investigators: 
Sara Beck, Tel Aviv University                                        

Science Category: starburst galaxies 
Dollars Approved: 28490.0 

Abstract:
We believe that the upper mass end of the IMF in extragalactic
starbursts has not yet been measured.  It is crucial to know the
population of the most massive stars;  they are the most energetic and
provide pressure, metals, winds and radiation tothe host galaxy.  In
most cases they cannot be observed directly, only through the infrared
and radio nebulae they excite.  The middle-infrared ionic emission
lines can probe the nebulae and find the stellar types, but the
validity of the results depends on the spatial structure of the
starburst and the resolution of the observations.  We have obtained
infrared and radio images and spectra, with sub-arcsecond resolution,
of a sample of 19 galaxies that contain very compact star formation
sources.  These sources are excited by embedded star clusters which
are very young, contain thousands of O stars, and are the best place
to look for the most massive stars.  There are infrared spectra of
these galaxies including all the ionic emission lines in the SPITZER
archive, from which we can form the line ratios used to calculate the
types of the exciting stars, but the emission from the compact sources
must be distinguished from other components or their effects will be
diluted and the massive stars will not be detected.  We propose to use
our high-resolution information to model the contributions of the
different components off the starburst, and to use the IRS results and
these models to find the stellar types.  With these data, and the
excellent libraries of stellar atmospheres and photoionization now
available, we may finally be able to see the most massive stars in
starbursts.