The importance of magnetic fields for the initial mass function of the first stars

Abstract: Magnetic fields play an important role for the formation of stars in both local and high-redshift galaxies. Recent studies of dynamo amplification in the first dark matter haloes suggest that significant magnetic fields were likely present during the formation of the first stars in the Universe at redshifts of 15 and above. In this work, we study how these magnetic fields potentially impact the initial mass function (IMF) of the first stars. We perform 200 high-resolution, three-dimensional (3D), magneto-hydrodynamic (MHD) simulations of the collapse of primordial clouds with different initial turbulent magnetic field strengths as predicted from turbulent dynamo theory in the early Universe, forming more than 1100 first stars in total. We detect a strong statistical signature of suppressed fragmentation in the presence of strong magnetic fields, leading to a dramatic reduction in the number of first stars with masses low enough that they might be expected to survive to the present day. Additionally, strong fields shift the transition point where stars go from being mostly single to mostly multiple to higher masses. However, irrespective of the field strength, individual simulations are highly chaotic, show different levels of fragmentation and clustering, and the outcome depends on the exact realisation of the turbulence in the primordial clouds. While these are still idealised simulations that do not start from cosmological initial conditions, our work shows that magnetic fields play a key role for the primordial IMF, potentially even more so than for the present-day IMF.

• The paper shows that strong magnetic fields suppress fragmentation, leading to fewer low-mass Population III stars.
• The paper finds that magnetic fields shift the mass threshold for multiple star formation to higher values.
• The paper demonstrates robust statistical evidence from 200 simulations, affirming the critical role of magnetic fields in early star formation.

The Role of Magnetic Fields in the Initial Mass Function of the First Stars

In the quest to understand the formation of the first stars, known as Population III stars, this study by Sharda et al. provides a comprehensive analysis of the impact of magnetic fields on the initial mass function (IMF) of these primordial stars. Utilizing 200 high-resolution, three-dimensional magnetohydrodynamic (MHD) simulations, the authors explore how different initial turbulent magnetic field strengths affect star formation processes in the early Universe. The numerical results reveal significant insights into the potential influences these magnetic fields may have had on the primordial star formation epoch.

The study builds on theoretical predictions and earlier numerical simulations, alluding to the likelihood of significant magnetic fields existing in the first dark matter halos. These fields are thought to be generated and amplified through processes such as turbulent dynamo action, which operates effectively even in the conditions of the early Universe. The simulations conducted include a range of initial magnetic field strengths, from zero magnetic field to fields that saturate a considerable fraction of kinetic energy, thus providing a robust framework to analyze the dynamical role of magnetic fields in primordial star formation.

Key Findings

1. Suppression of Fragmentation: The paper reports a clear suppression of fragmentation in cases with strong magnetic fields. These scenarios resulted in fewer low-mass stars, suggesting that magnetic fields played a more substantial role in setting the primordial IMF compared to contemporary star formation environments. This effect is apparent in the reduced number of sub-solar and solar-type stars emerging in simulations with stronger magnetic fields.
2. Impact on Stellar Multiplicity: While the overall multiplicity fraction (the number of singles, binaries, and higher-order systems formed) did not show significant variation across different magnetic field strengths, a detailed analysis indicated that the presence of magnetic fields did alter the mass at which stars transition from being mostly single to generally forming in multiples. Stronger fields shifted this transition to higher stellar masses.
3. Statistical Robustness: The use of 200 simulations allowed the authors to perform robust statistical analysis on the simulated star clusters. The study emphasizes the importance of large sample sizes for reliable determinations of the IMF, given the chaotic and stochastic nature of star formation processes.
4. Theoretical and Practical Implications: The findings have important implications for both the theoretical understanding of magnetic fields in star-forming regions and the practical models used to simulate star formation in cosmological settings. The result aligns with the hypothesis that magnetic fields were critical in shaping the properties of the first stars and subsequently influencing the evolution of galaxies and the chemistry of the Universe.

Implications for Future Research

The results underscore the necessity of incorporating magnetic fields in simulations of early Universe star formation to obtain an accurate depiction of primordial stellar populations and their mass distributions. Further developments in both computational power and techniques could allow for finer resolution simulations that might resolve additional complexities, such as non-ideality effects and radiation feedback, that could alter the role of magnetic fields. Additionally, linking this research with observational data, particularly from future telescopes that may detect remnants of Population III stars, could provide empirical tests for the predictions made.

Ultimately, understanding the first stars through the lens of their interaction with magnetic fields not only helps unravel the history of the early Universe but also provides insights into fundamental processes that continue to govern star formation in various astrophysical contexts. This study contributes significantly to the field, highlighting magnetic fields as a potentially dominant factor in the formation and evolution of the earliest stellar populations.