The impact of the turbulent Mach number on star formation and the initial mass function

Abstract

Turbulence regulates star formation by influencing the density structure and fragmentation of molecular clouds, and therefore it is expected to play a key role in setting the initial mass function (IMF). We study how the strength of the turbulent shocks affects star formation and the IMF by comparing a series of magnetohydrodynamical (MHD) simulations of star cluster formation in clouds with three different rms Mach number values, $\mathcal {M}=2.5, 5$, and 10, but otherwise the same Alfvén Mach number ($\mathcal {M}_\mathrm{A}\sim 3$) and virial parameter (αvir ~ 0.5). All three simulations include stellar feedback in the form of protostellar jets/outflows and accretion heating. We find that the star formation rate per freefall time (SFRff) for the $\mathcal {M}=10$ model is higher by a factor of ~3 compared to the $\mathcal {M}=2.5$ and 5 cases, which have similar SFRff. In terms of the IMF, the $\mathcal {M}=5$ and 10 models produce almost similar distributions, resembling typical observed IMFs. The $\mathcal {M}=2.5$ model, on the other hand, produces a bimodal IMF with a primary peak at supersolar masses (~2 M⊙) and a secondary peak in the substellar regime (<0.1 M⊙). The $\mathcal {M}=2.5$ and 5 simulations producing significantly different IMFs while having similar SFRff, suggest that a distinct set of physical processes governs the SFR and IMF. Through a resolution study for the $\mathcal {M}=2.5$ case, we find our base models have reached near numerical convergence, potentially only slightly underestimating close-binary formation. We also see signatures of bimodality in the $\mathcal {M}=2.5$ model for the multiplicity fraction and stellar angular momentum distribution.