A team of astronomers has detected a so-called ‘fossil’ magnetic field in both components of an early-type binary star system called Epsilon Lupi.
This image shows Epsilon Lupi (center). Image credit: Centre de Données astronomiques de Strasbourg / SIMBAD.
Epsilon Lupi, also known as HD 136504, is a bright binary located in the southern constellation Lupus.
The pair of stars is approximately 500 light years away, each have between 7 and 8 times the mass of the Sun, and combined together the pair is around 6,000 times as luminous as the Sun.
Astronomers have known for many years that Epsilon Lupi is a binary system, but had no idea that the two stars had magnetic fields.
“The origin of magnetism amongst massive stars is something of a mystery and this discovery may help to shed some light on the question of why any of these stars have magnetic fields,” said Matt Shultz of Queen’s University, Canada, a team member and the lead author of a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).
In cool stars, such as the Sun, magnetic fields are generated by ‘dynamos’ powered by strong convection in the outer layers of the star, where hot material rises, cools and falls back. But there is essentially no convection in the envelopes of massive stars, so there is no support for a magnetic dynamo. Nevertheless, about 10 percent of massive stars have strong magnetic fields.
Two explanations have been proposed for their origin, both variants on the idea of a ‘fossil’ magnetic field — a field generated at some point in the star’s past and then locked into the star’s surface.
The first hypothesis is that the magnetic field is generated while the star is being formed.
A second is that the magnetic field originates in dynamos driven by the violent mixing of material when two already-formed stars in a close binary merge.
The polarity of the star’s surface magnetic field, north or south, is indicated by red and blue respectively. Yellow lines indicate the magnetic field lines running from the stellar surfaces. Image credit: Volkmar Holzwarth / KIS, Freiburg.
“This discovery allows us to rule out the binary merger scenario,” Shultz said.
The research shows the strengths of the magnetic fields are similar in the two stars of Epsilon Lupi, however their magnetic axes are anti-aligned, with the south magnetic pole of one star pointing in approximately the same direction as the north pole of the other. It may even be that the two stars share a single magnetic field.
“We’re not sure why yet, but it probably points to something significant about how the stars are interacting with one another,” Shultz said.
The stars are close enough that their magnetospheres are likely to be interacting during the whole of their orbit around each other.
This means that their magnetic fields may even act as a giant brake, slowing down the stars.
In the long term, the two stars could be spiraling in towards each other.
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M. Shultz et al. 2015. Detection of magnetic fields in both B-type components of the ε Lupi system: a new constraint on the origin of fossil fields? MNRAS, accepted for publication; arXiv: 1507.05084
