HD 45166 is a star that can one day go supernova — and should grow to be a magnetar. Within the background of this illustration lies HD 45166’s companion, a extra typical blue star. Credit score:
ESO/L. Calçada
Astronomers have found a brand new sort of star, and with it, a brand new clue to an previous thriller: how magnetars kind.
The invention is said to a peculiar star in a binary system some 3,000 light-years away within the southern constellation Monoceros. It has lengthy been generally known as a misfit, wealthy in helium and sporting unusually highly effective winds for its fairly humdrum measurement. However new observations have added a brand new piece of the puzzle: HD 45166 additionally has a very robust magnetic subject.
And this, in flip, could show to be the important thing to a good better thriller — the origin of magnetars, a uncommon class of stars which can be the strongest magnets within the universe. In keeping with fashions, HD 45166 has the appropriate mixture of mass and magnetic subject to at least one day go supernova and go away behind a extremely magnetic neutron star. That makes the star a doable magnetar progenitor, the primary of its form.
“We’ve by no means detected a magnetic subject in a large helium star that can bear core collapse,” says Shenar. “It’s actually a brand new sort of star.”
A star in Wolf-Rayet clothes
Though HD 45166 is a part of an unique household known as Wolf-Rayet stars — there are solely 500 recognized up to now — it’s a very uncommon member.
Wolf-Rayet stars are often as much as 25 occasions extra huge than the Solar, shining 1,000,000 occasions brighter. All of the hydrogen gas of their cores has lengthy since been transformed into helium. And so they have highly effective stellar winds, expelling 10 Suns value of their mass each million years. Wolf-Rayet stars typically finish their lives as explosive supernovae that grow to be ultra-dense neutron stars.
HD45166 is a relative light-weight at simply twice the mass of the Solar. But, it inexplicably produces torrential stellar winds similar to a normal Wolf-Rayet. It’s the sort of paradox that fascinates astronomers.
“I didn’t have a look at the star as a result of I used to be searching for a magnetar progenitor,” says Tomer Shenar, creator of the brand new examine. “That was a byproduct, a shock. The rationale I began engaged on this star, and was more and more obsessed, is that it is rather distinctive.”
A magnetar progenitor
HD 45166 is the one recognized Wolf-Rayet star with such a low mass. If there have been others on the market, Shenar thought, they need to have simply been discovered by now. “That made me actually skeptical in regards to the earlier analyses,” he says. “I used to be going via all of the literature questioning what can or not it’s, and keep in mind sitting at my desk and pondering, ‘Wait, what about magnetic fields?’”
Cautious assessment of the stellar spectra from earlier researchers all of the sudden reminded Shenar of stars with robust magnetic fields that he had as soon as studied. Shenar reached out to an knowledgeable on magnetic fields, Gregg Wade of the Royal Army School of Canada in Kingston, Ontario. Wade took some convincing, however they utilized for time on the Canada-France-Hawaii Telescope atop Maunakea, which is equipped with a spectropolarimeter, an instrument that may measure magnetic fields.
Wade’s observations confirmed Shenar’s suspicions: HD 45166 is extremely magnetic. Whereas regular stellar spectra current single bands that correspond to varied components, robust magnetic fields round a star produce a number of spectral bands for those self same components — a phenomenon known as Zeeman splitting. The space between such bands reveals the energy of the magnetic subject. For HD 45166, that is an astounding 43,000 gauss — about 100,000 occasions stronger than Earth’s magnetic subject, and probably the most magnetic huge star discovered up to now.

Nonetheless, the magnetic field of a magnetar is round 1014 gauss, round 10 billion occasions better. To grow to be a magnetar, HD 45166 should first explode as a supernova and collapse right into a neutron star. Like an ice skater pulling of their arms to spin sooner, when a Solar-sized star collapses to a neutron star measuring simply eight miles (13 kilometers) extensive, its magnetic flux is conserved and the sector energy is multiplied some 10 billion occasions.
Typical supernovae candidates (and potential magnetars) have the mass of eight Suns. Whereas HD 45166 is simply one-fourth that mass, it has one final ace to play. Shenar says that HD 45166 will grow to be what known as a Kind Ibc supernova, utterly hydrogen-free. About 20 % of supernovae don’t have any hydrogen in any respect, coming from stars already stripped of their shells and lowered to reveal helium cores, that are heavier by quantity than hydrogen.
Mergers and acquisitions
Shenar’s group additionally proposes a mannequin for a way HD 45166 got here to be such an uncommon star within the first place: They suppose it’s the product of a merger of two helium stars that advanced in shut proximity. One grew sooner, changing into so giant that materials began flowing to its nearest companion, which then surpassed the primary star in measurement, stripping it of its hydrogen shell and making a helium star.
With the added materials, the companion star advanced sooner than the primary, stripped star. Because the mass ratio turned extra excessive, the circulation of fuel between them reversed, embroiling them in a maelstrom of stellar trade that astronomers name a standard envelope stage. This phase triggered the space between the 2 stellar cores to shrink till they merged right into a single helium star.
The result’s a captivating object, says Shenar. “As a laboratory for very robust magnetic fields, this star provides us one thing that we merely can’t mimic or examine elsewhere.”