Testing the fossil field hypothesis: could strongly magnetized OB stars produce all known magnetars?

Abstract

Stars of spectral types O and B produce neutron stars (NSs) after supernova explosions. Most of NSs are strongly magnetized including normal radio pulsars with B ∝ 1012 G and magnetars with B ∝ 1014 G. A fraction of 7–12 per cent of massive stars are also magnetized with B ∝ 103 G and some are weakly magnetized with B ∝ 1 G. It was suggested that magnetic fields of NSs could be the fossil remnants of magnetic fields of their progenitors. This work is dedicated to study this hypothesis. First, we gather all modern precise measurements of surface magnetic fields in O, B, and A stars. Secondly, we estimate parameters for lognormal distribution of magnetic fields in B stars and found μB = 2.83 ± 0.1 log10 (G), σB = 0.65 ± 0.09 for strongly magnetized and μB = 0.14 ± 0.5 log10 (G), $\sigma =0.7_{-0.27}^{+0.57}$ for weakly magnetized. Third, we assume that the magnetic field of pulsars and magnetars have 2.7-dex difference in magnetic fields and magnetars represent 10 per cent of all young NSs and run population synthesis. We found that it is impossible to simultaneously reproduce pulsars and magnetars populations if the difference in their magnetic fields is 2.7 dex. Therefore, we conclude that the simple fossil origin of the magnetic field is not viable for NSs.

Evolution of Neutron Star Magnetic Fields

“…Modern MHD simulations [22] showed that strong stellar magnetic fields could be formed as a result of stellar merger. Authors [22] found that a star that is born as a result of a merger has a magnetic field of ≈9 kG, which is compatible with values for some strongly magnetised massive stars [21].…”

“…In general, the absence of magnetars and M7-like sources with P 20 s is indirect evidence of field decay since with an initial surface field B 0 ∼ 10 14 G, a NS can spin down to a period of a few hundred seconds in a few hundred thousand years (a typical age of the M7 sources) if the field is not decaying. Recent examples of population studies of magneto-rotational evolution of initially highly magnetised NSs can be found in [21,119]. These studies demonstrated that rapid field decay in magnetars prevents the formation of bright sources with spin periods longer than those already observed for these objects.…”

“…If magnetic flux is conserved during the supernova explosion, the field is amplified by a factor (R OB /R NS ) 2 ≈ 5 × 10 11 [19]. Some 7-12% of O and B stars are strongly magnetised, with field strengths reaching 10 kG, which could then easily produce NS magnetic fields up to ≈5 × 10 15 G [20,21].…”

“…For magnetars, it has been suggested that the impurity parameter Q could be 1 [51]. This hypothesis is motivated by the observed absence of magnetars and their supposed descendants (such as the Magnificent Seven NSs; see below) with spin periods longer than 12 s. Population synthesis studies [21,52] with small values of Q ≤ 1 predicted that a large number of magnetars should exist with spin periods P > 20 s, contrary to observations. A lack of magnetars with long spin periods could also occur, due to a different reason.…”

Magnetic Archaeology of Early-Type Stellar Dynamos

“…Secondly, it was recently noted that there are toofew strongly-magnetized massive stars for simple flux conservation to explain the observed magnetar population (Makarenko et al 2021). A similar challenge exists at lower masses: ∼ 10 % of A/B star show strong surface magnetic fields (Power et al 2007) while ∼ 20 % of their White Dwarf remnants have the 10 5−6 G fields those would produce under flux conservation (Landstreet & Bagnulo 2019).…”