The Initial Mass Function of Stars: Evidence for Uniformity in Variable Systems

Abstract

The distribution of stellar masses that form in one star-formation event in a given volume of space is called the initial mass function (IMF). The IMF has been estimated from low-mass brown dwarfs to very massive stars. Combining IMF estimates for different populations in which the stars can be observed individually unveils an extraordinary uniformity of the IMF. This general insight appears to hold for populations including present-day star formation in small molecular clouds, rich and dense massive star-clusters forming in giant clouds, through to ancient and metal-poor exotic stellar populations that may be dominated by dark matter. This apparent universality of the IMF is a challenge for star formation theory because elementary considerations suggest that the IMF ought to systematically vary with star-forming conditions.The physics of star formation determines the conversion of gas to stars. The outcome of star formation are stars with a range of masses. Astrophysicists refer to the distribution of stellar masses as the stellar initial mass function. Together with the time-modulation of the star-formation rate, the IMF dictates the evolution and fate of galaxies and star clusters. The evolution of a stellar system is driven by the relative initial numbers of brown dwarfs (BDs, < ∼ 0.072 M ) that do not fuse H to He, very low-mass stars (0.072 − 0.5 M ), low-mass stars (0.5−1 M ), intermediate-mass stars (1−8 M ) and massive stars (m > 8 M ). Non-luminous BDs through to dim low-mass stars remove gas from the interstellar medium (ISM), lockingup an increasing amount of the mass of galaxies over cosmological time scales. Intermediate and luminous but short-lived massive stars expel a large fraction of their mass when they die and thereby enrich the ISM with elements heavier than H and He. They heat the ISM through radiation, outflows, winds and supernovae (1, 2). It is therefore of much importance to quantify the relative numbers of stars in different mass ranges and to find systematic variations of the 1

Do Low Luminosity Stars Matter?

“…The mass function (MF) for stars with masses 0.1M < M < 0.8M is shown in figure 6 (from (Bochanski et al (2009))). Compared with previous determinations (Kroupa (2002) ;Miller & Scalo (1979)), the MF determined by Bochanski et al (2009) presents a decline in the contribution of stars with masses below 0.27M , thus discarding the old idea that there could be huge numbers of low mass stars making an important contribution to the local baryonic dark matter.…”

Dusty star-forming galaxies at high redshift

“…Kruijssen & Longmore, 2013), some indications suggest bottom light or top heavy IMFs as well. Rieke et al (1993) and Förster Schreiber et al (2003) find potentially that the turnover mass may be a factor 2 − 6 larger than a traditional Kroupa (2002) IMF in M82. Similarly, Fardal et al (2007 examine the present day K-band luminosity density, cosmic background radiation, and cosmic star formation rate density, and suggest that there is an excess of intermediate mass stars.…”

The evolution of the oxygen abundance radial gradient in the Milky Way Galaxy disk

“…In this standard model (STD), the star and molecular cloud formation are calculated with two efficiencies for each galaxy model, which we vary simultaneously with values among 0 and 1. We have now an updated grid of models (Mollá et al, in preparation), where we use the stellar yields from Gavilán, Buell & Mollá (2005), Gavilán, Mollá & Buell (2006), Limongi & Chieffi (2003), Chieffi & Limongi (2004) and IMF from Kroupa (2002), selected as the best combination of stellar yields and IMF from 144 models in Mollá et al (2015). We have also revised the infall rates of gas, calculated to create disks as observed following Salucci et al (2007).…”