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We present results from an optical photometric and spectroscopic survey ofthe young stellar population in L1641, the low-density star-forming region ofthe Orion A cloud south of the Orion Nebula Cluster (ONC). Our goal is todetermine whether L1641 has a large enough low-mass population to make theknown lack of high-mass stars a statistically-significant demonstration ofenvironmental dependence of the upper mass stellar initial mass function (IMF).Our spectroscopic sample consists of IR-excess objects selected from theSpitzer/IRAC survey and non-excess objects selected from optical photometry. Wehave spectral confirmation of 864 members, with another 98 probable members; ofthe confirmed members, 406 have infrared excesses and 458 do not. Assuming thesame ratio of stars with and without IR excesses in the highly-extinctedregions, 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 standardmodels of the IMF, L1641 is deficient in O and early B stars to a 3-4 sigmasignificance 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 abetter 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 ofthe stellar initial mass function upon environment.

The Low-mass Stellar Population in L1641: Evidence for Environmental Dependence of the Stellar Initial Mass Function