# The Initial Magnetic Field Distribution in AB Stars

**Authors:** Eoin Farrell, Adam S. Jermyn, Matteo Cantiello, Daniel Foreman-Mackey  
**arXiv:** [2210.11180](https://arxiv.org/abs/2210.11180)  
**Date:** 2022-10-20  
**Summary page:** https://www.summarizepaper.com/en/paper/2210.11180v1/

## Key points

- Stars are born with magnetic fields, but the distribution of their initial field strengths is uncertain.
- A recent study by Farrell et al. used observations and theoretical models to infer the initial distribution of magnetic fields for AB stars in the mass range of 1.6 - 3.4 M$_{\odot}$.
- The most-favored distribution was found to be Gaussian, with a mean value of 𝜇 = 770 G and standard deviation 𝜎 = 146 G.
- The trapezoidal form was explored but found to be less likely than the Gaussian or triangular forms.
- Independent approaches suggest values closer to 2 - 3 kG, creating some discrepancy that could suggest either a mass-dependent and bimodal initial field distribution or an alternative theoretical picture for the origin of these magnetic fields.
- The posterior distribution for the Gaussian form has mean 𝜇 = 770 ± 44 G and standard deviation 𝜎 = 145 ± 77 G.
- There is still uncertainty regarding whether all stars were born with some non-zero initial magnetic field or if there is a possibility that the IFD itself is bimodal, with a second mode at very weak field strengths.
- This study provides important insights into the initial distribution of magnetic fields in AB stars and highlights the need for further research to better understand the origin and evolution of these fields.

## Summary

Stars are born with magnetic fields, but the distribution of their initial field strengths has remained uncertain. In a recent study, Farrell et al. combined observations with theoretical models of magnetic field evolution to infer the initial distribution of magnetic fields for AB stars in the mass range of 1.6 - 3.4 M$_{\odot}$. The authors tested a variety of distributions with different shapes and found that a distribution with a mean of $\sim$800 G and a full width of $\sim$600 G is most consistent with the observed fraction of strongly magnetized stars as a function of mass. The most-favored distribution was found to be Gaussian, with a mean value of 𝜇 = 770 G and standard deviation 𝜎 = 146 G. 

The trapezoidal form was also explored, which favored relatively high values at both ends and low values in between, resulting in two additional free parameters (𝐵min, 𝐵max). However, it was found to be less likely to be found in nature than the Gaussian or triangular forms. Independent approaches to measure the typical field strength suggest values closer to 2 - 3 kG, which creates some discrepancy that could suggest either a mass-dependent and bimodal initial field distribution or an alternative theoretical picture for the origin of these magnetic fields. The posterior distribution for the Gaussian form has mean 𝜇 = 770 ± 44 G and standard deviation 𝜎 = 145 ± 77 G. These are anti-correlated such that distributions with higher means tend to be narrower while those with higher variance tend to have lower means. Although all three forms obtained similar initial field distributions (IFDs), features like mean and width were considered robust. It is worth noting that there is still uncertainty regarding whether all stars were born with some non-zero initial magnetic field or if there is a possibility that the IFD itself is bimodal, with a second mode at very weak field strengths. Observations of young stars suggest the latter possibility. Overall, this study provides important insights into the initial distribution of magnetic fields in AB stars and highlights the need for further research to better understand the origin and evolution of these fields.