Evidence for Environmental Dependence of the Upper Stellar Initial Mass
  Function in Orion A
  We extend our previous study of the stellar population of L1641, the
lower-density star-forming region of the Orion A cloud south of the dense Orion
Nebula Cluster (ONC), with the goal of testing whether there is a statistically
significant deficiency of high-mass stars in low-density regions. Previously,
we compared the observed ratio of low-mass stars to high-mass stars with
theoretical models of the stellar initial mass function (IMF) to infer a
deficiency of the highest-mass stars in L1641. We expand our population study
to identify the intermediate mass (late B to G) L1641 members in an attempt to
make a more direct comparison with the mass function of the nearby ONC. The
spectral type distribution and the K-band luminosity function of L1641 are
similar to those of the ONC (Hillenbrand 1997; Muench et al. 2002), but
problems of incompleteness and contamination prevent us from making a detailed
test for differences. We limit our analysis to statistical tests of the ratio
of high-mass to low-mass stars, which indicate a probability of only 3% that
the ONC and the southern region of L1641 were drawn from the same population,
supporting the hypothesis that the upper mass end of the IMF is dependent on
environmental density.
