The Low-mass Stellar Population in L1641: Evidence for Environmental
  Dependence of the Stellar Initial Mass Function
  We present results from an optical photometric and spectroscopic survey of
the young stellar population in L1641, the low-density star-forming region of
the Orion A cloud south of the Orion Nebula Cluster (ONC). Our goal is to
determine whether L1641 has a large enough low-mass population to make the
known lack of high-mass stars a statistically-significant demonstration of
environmental dependence of the upper mass stellar initial mass function (IMF).
Our spectroscopic sample consists of IR-excess objects selected from the
Spitzer/IRAC survey and non-excess objects selected from optical photometry. We
have spectral confirmation of 864 members, with another 98 probable members; of
the confirmed members, 406 have infrared excesses and 458 do not. Assuming the
same ratio of stars with and without IR excesses in the highly-extincted
regions, L1641 may contain as many as ~1600 stars down to ~0.1 solar mass,
comparable within a factor of two to the the ONC. Compared to the standard
models of the IMF, L1641 is deficient in O and early B stars to a 3-4 sigma
significance level, assuming that we know of all the massive stars in L1641.
  With a forthcoming survey of the intermediate-mass stars, we will be in a
better position to make a direct comparison with the neighboring, dense ONC,
which should yield a stronger test of the dependence of the high-mass end of
the stellar initial mass function upon environment.
