The impact of magnetic fields on the IMF in star-forming clouds near a supermassive black hole

Abstract

Star formation in the centers of galaxies is thought to yield massive stars with a possibly top-heavy stellar mass distribution. It is likely that magnetic fields play a crucial role in the distribution of stellar masses inside star-forming molecular clouds. In this context, we explore the effects of magnetic fields, with a typical field strength of 38 μG, such as in RCW 38, and a field strength of 135 μG, similar to NGC 2024 and the infrared dark cloud G28.34+0.06, on the initial mass function (IMF) near (≤10 pc) a 10 7 solar mass black hole. Using these conditions, we perform a series of numerical simulations with the hydrodynamical code FLASH to elucidate the impact of magnetic fields on the IMF and the star-formation efficiency (SFE) emerging from an 800 solar mass cloud. We find that the collapse of a gravitationally unstable molecular cloud is slowed down with increasing magnetic field strength and that stars form along the field lines. The total number of stars formed during the simulations increases by a factor of 1.5−2 with magnetic fields. The main component of the IMF has a lognormal shape, with its peak shifted to sub-solar (≤0.3 M ) masses in the presence of magnetic fields, due to a decrease in the accretion rates from the gas reservoir. In addition, we see a top-heavy, nearly flat IMF above ∼2 solar masses, from regions that were supported by magnetic pressure until high masses are reached. We also consider the effects of X-ray irradiation if the central black hole is active. X-ray feedback inhibits the formation of sub-solar masses and decreases the SFEs even further. Thus, the second contribution is no longer visible. We conclude that magnetic fields potentially change the SFE and the IMF both in active and inactive galaxies, and need to be taken into account in such calculations. The presence of a flat component of the IMF would be a particularly relevant signature for the importance of magnetic fields, as it is usually not found in hydrodynamical simulations.

The Origin and Universality of the Stellar Initial Mass Function

“…The authors attributed this difference to the suppression of intermediate mass stars from converging flows; magnetic support inhibited isolated collapse and instead stars formed predominantly within magnetized filaments, which tended to produce smaller mass stars. An anti-correlation between characteristic stellar mass and magnetic field strength was also demonstrated in simulations of star formation in the galactic center (Hocuk et al, 2012). In principle, a nonideal MHD treatment would allow the field to diffuse from magnetically subcritical regions, however, few simulations have included such effects and the impact on the shape of the IMF is unclear.…”

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

“…The former eject mass directly, while the latter lower accretion rates and inhibit the formation of massive collapsing regions Hocuk et al 2012). 1 The difficulty of teasing out the physics that is responsible for setting the IMF is partly driven by the fact that most simulations to date do not include all the possibly-important effects.…”

“…On top of all these processes, the characteristic mass scale can be shifted lower by both protostellar outflows (Hansen et al 2012;Krumholz, Klein & McKee 2012) and strong magnetic fields. The former eject mass directly, while the latter lower accretion rates and inhibit the formation of massive collapsing regions (Li et al 2010;Hocuk et al 2012). 1 The difficulty of teasing out the physics that is responsible for setting the IMF is partly driven by the fact that most simulations to date do not include all the possibly-important effects.…”

Star formation and accretion in the circumnuclear disks of active galaxies

“…Indeed, enhanced accretion has been observed in numerical simulations in the presence of highly supersonic turbulence (Hobbs et al 2011), and more realistic approaches aim to self-consistently inject turbulent energy via supernova-explosions (Wada et al 2009). Other attempts have focused on the impact of black hole feedback on nearby starforming clouds (Hocuk & Spaans 2011), including the potential effects of magnetic fields (Hocuk et al 2012).…”