Environmental Dependence of the Stellar Initial Mass Function.

Abstract

I present an in-depth study of how the stellar initial mass function (IMF) depends on the environmental density from both theoretical and observational aspects. In my theoretical work, I used Gadget-2, an SPH code with sink particles, to test the applicability of competitive accretion in an initially non-clustered environment and see how the resulting IMF depends on the environment. The results show that in a sheet-like geometry, as well as a uniform sphere, the accretion rates of individual sinks follow the Bondi-Hoyle accretion at high masses, resulting in continual flattening of the slope towards an asymptotic form Gamma = 1. The asymptotic limit is most rapidly reached when starting from a broad distribution of initial sink masses. In general the higher sink masses are found in simulations with flatter slopes. Although these simulations are highly idealized, the results suggest that competitive accretion may be relevant in a wider variety of environments than previously considered. In my observational work, I surveyed the stellar population in the L1641 region, the distributed star-forming region south of the Orion Nebula Cluster (ONC). 864 low-mass members of L1641 are identified through optical photometry and spectroscopy. Overall, L1641 may contain up to 1600 stars. Compared to the standard IMFs, L1641 is deficient in O and early B stars to a 3-4 sigma significance level. I then searched for high-to-intermediate mass members of L1641 to make a direct comparison with the ONC. I compared the spectral type distribution to that of the ONC and the K-band luminosity function to the Trapezium cluster. Based on Fisher's exact test, there is only 3% probability that the ONC and the southern region of L1641 were drawn from the same distribution. This supports the hypothesis that the uppermass IMF depends on the environmental density. Additionally, in an attempt to characterize the highly-extincted population in L1641 and ensure that the luminous members in high extinction regions are not missing, I observed 115 members of L1641 with the MMIRS Spectrograph in H and K band. The IR spectra, along with 2MASS photometry, are used to characterize their accretion luminosities, spectral-types and ages.PHDAstronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/99878/1/wenhsin_1.pd