White dwarfs are the dying embers of collapsed stars that are slowly cooling and fading away. They occur when a star has burned through all of its helium and expands to more than 100 times its original size, into a red giant.
"Because the red giants are so big, the gravity at their surface is quite low, making it easy for matter to escape," said Fontaine in an email.
The outer shell, or envelope, of the star is ejected, forming a nebula, and the core of the star contracts, forming a white dwarf.
A white dwarf contains about half the original mass of a star, but has collapsed into a dense object about the size of the Earth. A golf ball-sized piece of a white dwarf would weigh more than a tonne.
Fontaine and his colleagues observed a white dwarf star about 1,400 light years away in the constellation Virgo, called PG 1159-035, and measured its oscillations and vibrations to probe its internal structure, similar to the way geologists use sound waves to determine if there's, for instance, oil under a certain patch of ground.
The technique is called astroseismology, and using it, Fontaine and his colleagues found that the white dwarf spins at a rate of once every 34 hours, from its surface all the way down to its core.
"We have been able to measure the rotation profile inside the star, covering more than 97 per cent of its mass in the process," said Fontaine.
"To my knowledge, this is the first time, with the exception of the Sun, that the internal rotation profile of a star has been mapped with high accuracy," he said.
The low speed and uniformity of its spin is raising new questions about the life cycle of stars, how they form, and how they collapse and die.
Stars have a large mass and they all spin, which means they all have a lot of angular momentum. Angular momentum is the phenomenon that keeps a spinning top upright.
A collapsing red giant star loses a lot of its angular momentum when its outer envelope is expelled.
But because the white dwarf is the contracted, dense core of the star, you would expect that they would spin rapidly, for the same reason that a ice skater spins more quickly when he pulls in his arms: conservation of angular momentum.
However, observations of white dwarf stars have shown that they spin quite slowly, completing a rotation in a few hours or a few decades, depending on the star.
Where did momentum go?
One explanation proposed to explain why the small stars were spinning so slowly was that they could be "hiding" some of their angular momentum: their insides could be spinning much faster than their surfaces.
Fontaine's study, published this week in the journal Nature, shows that this white dwarf doesn't have a rapidly spinning core, so it's not hiding any angular momentum.
"With a rigid rotation period of 33.6 hours, the star's residual rotational energy corresponds to a mere 0.000001 per cent of its thermal energy responsible for its luminosity," said Fontaine.
"In other words, the star has lost essentially all of its angular momentum," he said.
The question of where all that momentum has gone remains.
In an accompanying article in Nature, German astrophysicist Sung-Chul Yoon, who was not involved in the research, says interactions between the star's core and its outer layers could slow down a white dwarf star.
Yoon wrote that theoretical mechanisms involving a star's magnetic field or gravitational waves, a phenomenon predicted by Einstein's theory of general relativity, could explain the "braking."
As for Fontaine, he prefers the magnetic approach.
"I believe that models involving magnetic torques are more promising than others in terms of providing a strong coupling for angular momentum transfer between the core and the outer envelope in a star," he said.
As always, the answer lies in more research, wrote Yoon.
"Future observations of white dwarfs whose traits — for example mass and chemical composition — differ at different evolutionary stages would greatly improve our understanding of the problem," wrote Yoon. ( cbc.ca )
No comments:
Post a Comment