Abstract
Abstract Early-type stars show a bimodal distribution of magnetic field strengths, with some showing very strong fields (≳1 kG) and others very weak fields (≲10 G). Recently, we proposed that this reflects the processing or lack thereof of fossil fields by subsurface convection zones. Stars with weak fossil fields process these at the surface into even weaker dynamo-generated fields, while in stars with stronger fossil fields magnetism inhibits convection, allowing the fossil field to remain as is. We now expand on this theory and explore the timescales involved in the evolution of near-surface magnetic fields. We find that mass loss strips near-surface regions faster than magnetic fields can diffuse through them. As a result, observations of surface magnetism directly probe the frozen-in remains of the convective dynamo. This explains the slow evolution of magnetism in stars with very weak fields: these dynamo-generated magnetic fields evolve on the timescale of the mass loss, not that of the dynamo.
Highlights
Massive stars play host to some of the most unusual stellar phenomena, including rapid mass loss (Smith 2014), time-varying UV emission (Ramiaramanantsoa et al 2014), strong magnetic fields (Sundqvist et al 2013), stochastic low-frequency variability (Bowman 2020), rotational mixing (Maeder & Meynet 2000), opacity-driven pulsations (Dziembowski & Pamiatnykh 1993), and massive outbursts (Humphreys & Davidson 1994)
This explains the slow evolution of magnetism in stars with very weak fields: these dynamogenerated magnetic fields evolve on the time-scale of the mass loss, not that of the dynamo
The hierarchy of timescales in (5) shows that it should be difficult to catch a star in the act of losing its near-surface fossil field, because the relevant timescale for that is at most the mass-stripping time τM, and more likely of order the convective turnover time, both of which are very short compared with the mainsequence stellar lifetime
Summary
Massive stars play host to some of the most unusual stellar phenomena, including rapid mass loss (Smith 2014), time-varying UV emission (Ramiaramanantsoa et al 2014), strong magnetic fields (Sundqvist et al 2013), stochastic low-frequency variability (Bowman 2020), rotational mixing (Maeder & Meynet 2000), opacity-driven pulsations (Dziembowski & Pamiatnykh 1993), and massive outbursts (Humphreys & Davidson 1994). We find that the convective turnover time is always the shortest of these, followed by mass-loss and magnetic diffusion. This ordering motivates a story which we describe.
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