HD 45166: we finally find the first magnetar precursor star

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Magnetars are some of the most extreme objects we know of . They have magnetic fields so strong that chemistry becomes impossible in their vicinity . They are neutron stars with a superfluid interior that includes charged particles, so a magnetic dynamo is kept there to support that field. But it's hard to fully understand what makes that dynamo go in the first place.

The main idea, which benefits from its simplicity, is that a magnetar inherits its ma Phone Number List gnetic field from the star that exploded, in a supernova, to create it. The original magnetic field, when squashed to match the tiny size of the resulting neutron star, would provide a huge kick of energy to start the magnetar. There is only one problem with this idea: we had not detected any of the highly magnetized precursor stars that this hypothesis requires .

Ars Technica

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It was right under our noses.
It turns out that we have been observing one of them for years. It just looked like something completely different, and it took a more careful analysis, published today in Science , to understand what we've been observing.

HD 45166 , at some levels, appears to be a relatively simple binary star system, composed of a normal star and a very hot Wolf-Rayet star; both orbit at close range, with the light showing a periodicity of 1.6 days, presumably due to the orbit.

But even at that level of understanding, some things seemed strange about the Wolf-Rayet part of the system. Typically, Wolf-Rayet are massive, hot, helium-rich stars that expelled most of their hydrogen through violent eruptions . But HD 45166 is only four times as massive as the Sun, half the mass of the smallest example we've seen anywhere else. It also has a lot of carbon, oxygen and nitrogen present, which is rare; and its spectral lines have some unusual characteristics.



The star's rotation axis also appeared to be oriented in the direction of its orbit, which is also unlikely to occur. So there were several things that were difficult to explain about the system, even before the new observations. In some ways, the updated data makes the system easier to understand; otherwise it makes it worse.

The key finding was that the star's light spectrum indicated that the 1.6-day periodicity is likely due to a regular physical pulse from the normal star in the HD 45166 system . That finding prompted researchers to examine other periodic changes in the light of HD 45166. The most likely orbital signal suggests that the orbit takes about 8,200 days, a fairly radical difference of 1.6 days. It puts the stars much further apart and means that none of them are likely to orbit in the direction of their axis of rotation.

The greater separation, in turn, requires a revision of the masses that were estimated based on their orbital interactions. The new estimate reduces the mass of the Wolf-Rayet star by half, making it only twice the mass of the Sun.