Abstract
ABSTRACT In this paper, we describe an analysis of the MiMeS (Magnetism in Massive Stars) survey of O-type stars to explore the range of dipolar field strengths permitted by the polarization spectra that do not yield a magnetic detection. We directly model the Stokes V profiles with a dipolar topology model using Bayesian inference. The noise statistics of the Stokes V profiles are in excellent agreement with those of the null profiles. Using a Monte Carlo approach, we conclude that a model in which all the stars in our sample were to host 100 G, dipolar magnetic field can be ruled out by the MiMeS data. Furthermore, if all the stars with no detection were to host a magnetic field just below their detection limit, the inferred distribution in strength of these undetected fields would be distinct from the known distribution in strength of the known magnetic O-type stars. This indicates that the 'initial magnetic field function' (IBF) is likely bimodal – young O-type stars are expected to have either weak/absent magnetic fields or strong magnetic fields. We also find that better upper limits, by at least a factor of 10, would have been necessary to rule out a detection bias as an explanation for the apparent lack of evolved main-sequence magnetic O-type stars reported in the literature, and we conclude that the MiMeS survey cannot confirm or refute a magnetic flux decay in O-type stars.
Highlights
We describe an analysis of the Magnetism in Massive Stars (MiMeS) Survey of O-type stars to explore the range of dipolar field strengths permitted by the polarisation spectra that do not yield a magnetic detection
We find that better upper limits, by at least a factor of 10, would have been necessary to rule out a detection bias as an explanation for the apparent lack of evolved main-sequence magnetic O-type stars reported in the literature, and we conclude that the MiMeS survey cannot confirm or refute a magnetic flux decay in O-type stars
We here test whether this result holds using our sample of O-type stars for which magnetic detection limits are known, by determining (i) if stars near the TAMS have been studied to detect magnetic fields, and (ii) whether these stars have been observed with a magnetic detection threshold sufficient to allow for detection of the expected surface field strength under the hypothesis of magnetic flux conservation
Summary
Magnetic fields are routinely detected at the surfaces of a sub-sample of OB stars. Their rarity, along with the lack of observed short-term (days – decades) evolution, suggests that these fields are not generated by a contemporaneous dynamo mechanism, as is the case for solar-type stars, but are rather a relic from an event or an These so-called fossil fields are increasingly well characterised thanks to the advent of large surveys performed with high-resolution spectropolarimetric instrumentation (e.g. Wade et al 2016; Fossati et al 2015). We directly model the Stokes V profiles with the Bayesian inference procedure developed by Petit & Wade (2012) for a dipolar field topology Such a study allows us to address several questions concerning the distribution of magnetic fields in O-type stars, described in the following two sections. It provides an independent evaluation of the polarisation signature of the magnetic candidate stars, as well as the potential to identify additional magnetic candidates
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