The Origin and Universality of the Stellar Initial Mass Function

Abstract

We review current theories for the origin of the Stellar Initial Mass Function (IMF) with particular focus on the extent to which the IMF can be considered universal across various environments. To place the issue in an observational context, we summarize the techniques used to determine the IMF for different stellar populations, the uncertainties affecting the results, and the evidence for systematic departures from universality under extreme circumstances. We next consider theories for the formation of prestellar cores by turbulent fragmentation and the possible impact of various thermal, hydrodynamic and magneto-hydrodynamic instabilities. We address the conversion of prestellar cores into stars and evaluate the roles played by different processes: competitive accretion, dynamical fragmentation, ejection and starvation, filament fragmentation and filamentary accretion flows, disk formation and fragmentation, critical scales imposed by thermodynamics, and magnetic braking. We present explanations for the characteristic shapes of the Present-Day Prestellar Core Mass Function and the IMF and consider what significance can be attached to their apparent similarity. Substantial computational advances have occurred in recent years, and we review the numerical simulations that have been performed to predict the IMF directly and discuss the influence of dynamics, time-dependent phenomena, and initial conditions.Comment: 24 pages, 6 figures. Accepted for publication as a chapter in Protostars and Planets VI, University of Arizona Press (2014), eds. H. Beuther, R. S. Klessen, C. P. Dullemond, Th. Hennin

Submillimetre Studies of Prestellar and Starless Cores in the Ophiuchus, Taurus and Cepheus Molecular Clouds

“…we consider it to be real, as the onset of the flattening occurs at ∼ 0.2 M ⊙ , significantly above our 80 percent completeness limit of 0.051 M ⊙ , and because the flattening is consistent with both the expected behaviour of the low-mass core mass function (e.g. Chabrier 2003;Offner et al 2014) and the observed behaviour of the core mass function determined in previous studies of Ophiuchus (MAN98; S08).…”

“…The mass at which the CMF turns over is typically ∼ 3 times that of the IMF, suggesting that, if there is a direct link between the CMF and the IMF, the process of conversion of a prestellar core into a main-sequence star is ∼ 33% efficient (Offner et al 2014).…”

“…Offner et al 2014 for a recent review). Various theoretical models for the functional form of the CMF have been .…”

What physics determines the peak of the IMF? Insights from the structure of cores in radiation-magnetohydrodynamic simulations

“…The origin of the stellar initial mass function (IMF) is one of the outstanding problems in contemporary theoretical astrophysics (Krumholz 2014;Offner et al 2014). The observed IMF displays a characteristic peak at ∼ 0.2 − 0.3 M , with a decline in the number of objects on either side of this plateau (Kroupa 2002;Chabrier 2003Chabrier , 2005Bastian, Covey & Meyer 2010;Parravano, McKee & Hollenbach 2011;Offner et al 2014).…”

“…The observed IMF displays a characteristic peak at ∼ 0.2 − 0.3 M , with a decline in the number of objects on either side of this plateau (Kroupa 2002;Chabrier 2003Chabrier , 2005Bastian, Covey & Meyer 2010;Parravano, McKee & Hollenbach 2011;Offner et al 2014). This peak appears to be universal or nearly so within the Milky Way, and even in the most extreme environments to which we have access its location is different by at most a factor of ∼ 2 − 3 (e.g., van Dokkum & Conroy 2010, mark.krumholz@anu.edu.au 2011; Cappellari et al 2012;Spiniello et al 2012Spiniello et al , 2015Conroy et al 2013).…”