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Radio Script
Intended Audience: Discovery (World Service); Leading Edge, Frontiers (Radio 4)
Astronomers have now been able to shed new light on the mysterious lack of
angular momentum exhibited by white dwarf stars, through implementing
techniques similar to those used to probe the earth’s interior. Our science
correspondent Malini Patel tells us more:
White dwarfs represent the end product of millions of years of evolution for
95% of the stars in our night sky. These small, compact descendents are
theoretically expected to rotate rapidly, completing rotations every few
seconds. However observations of the stars have shown that some take days
and even years to make one full turn.
Astronomers have now come closer to explaining the unusually slow rotational
periods, by taking a look at the interior of a pulsating white-dwarf star
labelled as PG 1159-035.
Previously, scientists were only able to make observations of the surfaces of
stars and the internal regions remained hidden from inspection. Therefore,
they had no way of knowing whether the stars slowed down in the
evolutionary stages before white-dwarf formation, or if in fact, they hid their
rotation at deeper levels.
Paul Brassard tells us:
"Prior to our investigation, there was no proof of momentum
transfer before the white-dwarf stage, as nobody had ever probed
the internal rotation state of evolved stars. Whilst observations of
the surface had recognized that they rotate slowly, this was only
limited only to the outermost layers. It was certainly possible to
imagine that the slowly rotating external layers were hiding a fast
rotating core containing the majority of the angular momentum of
the star. Our aim was to find out if this really was the case.”
The periods of the pulsations in PG 1159-035 contain fundamental data about
the global state of the star and can be used to infer information about the
internal structure. So by analysing the star’s rich pulsation spectrum,
astronomers were able to construct a picture of the inner regions. This
technique, called asteroseismology, is similar to that used by geologists to
determine the composition of the interior of our very own planet Earth.
Gill Fontaine, of the University of Montreal explains further:
"We devised a new method to decode the star’s pulsation signature
and, hence, composed an internal rotation profile. This is the first
time that this has been attempted for a star, except for the case of
our Sun."
Fontaine and his colleagues made two assumptions about the state of the star
that they would then go on to verify. Firstly they assumed that the star was a
slow rotator. This would mean that the pulsation periods of the star would be
much smaller than its rotation period.
To validate this assumption, they used mathematical theories to compute
estimates of pulsation periods for a given rotation speed. They then
compared their calculated values to those observed empirically, and varied
the assumed rotation until they found the best agreement between the two.
Their results showed that the rotational period of the star was indeed larger
than the observed pulsation periods, and hence could conclude that the star’s
surface rotated slowly with a period of approximately thirty four hours.
The team’s next postulate was that the star rotated as a rigid body. To
confirm this, the team constructed models of the star that consisted of two
zones: an outer zone, which rotated slowly with a period of approximately 34
hours, and an inner zone that was assumed to rotate with an arbitrary period.
Due to the fact that the pulsations occurring in the star were global modes
and depended on the whole vertical structure, the depth of the outer zone
was also set as a variable.
The next step was to use the stellar models to find the values of the inner
zone period and outer zone depth, which led to calculations of pulsation
periods that best fit the observational measurements.
What they found, was a clear result that would have a huge impact on
theories of angular momentum transfer in stars.
Fontaine: “The outcome of our investigation found that the star’s
interior rotates with the same slow speed as its surface. This is true,
through at least 90% of its depth and hence we can conclude that
the star rotates uniformly as a rigid body. The findings support ideas
that suggest the observed, slow rotational periods can be attributed
to the transferral of angular momentum to the outer reaches of a
star as it undergoes the transformation into a white-dwarf.”
With the team now planning to apply their method to more stars, we could be
close to solving the mystery of slow rotating white-dwarfs.