Astronomers commonly refer to massive stars as the chemical factories of the
Universe. They generally end their lives in spectacular supernovae, events
that forge many of the elements on the periodic table. How elemental nuclei
mix within these enormous stars has a major impact on our understanding of
their evolution prior to their explosion. It also represents the largest
uncertainty for scientists studying their structure and evolution.
A team of astronomers led by May Gade Pedersen, a postdoctoral scholar at UC
Santa Barbara's Kavli Institute for Theoretical Physics, have now measured
the internal mixing within an ensemble of these stars using observations of
waves from their deep interiors. While scientists have used this technique
before, this paper marks the first time this has been accomplished for such
a large group of stars at once. The results, published in Nature Astronomy,
show that the internal mixing is very diverse, with no clear dependence on a
star's mass or age.
Stars spend the majority of their lives fusing hydrogen into helium deep in
their cores. However, the fusion in particularly massive stars is so
concentrated at the center that it leads to a turbulent convective core
similar to a pot of boiling water. Convection, along with other processes
like rotation, effectively removes helium ash from the core and replaces it
with hydrogen from the envelope. This enables the stars to live much longer
than otherwise predicted.
Astronomers believe this mixing arises from various physical phenomena, like
internal rotation and internal seismic waves in the plasma excited by the
convecting core. However, the theory has remained largely unconstrained by
observations as it occurs so deep within the star. That said, there is an
indirect method of peering into stars: asteroseismology, the study and
interpretation of stellar oscillations. The technique has parallels to how
seismologists use earthquakes to probe the interior of the Earth.
"The study of stellar oscillations challenges our understanding of stellar
structure and evolution," Pedersen said. "They allow us to directly probe
the stellar interiors and make comparisons to the predictions from our
stellar models."
Pedersen and her collaborators from KU Leuven, the University of Hasselt,
and the University of Newcastle have been able to derive the internal mixing
for an ensemble of such stars using asteroseismology. This is the first time
such a feat has been achieved, and was possible thanks only to a new sample
of 26 slowly pulsating B-type stars with identified stellar oscillations
from NASA's Kepler mission.
Slowly pulsating B-type stars are between three and eight times more massive
than the Sun. They expand and contract on time scales of the order of 12
hours to 5 days, and can change in brightness by up to 5%. Their oscillation
modes are particularly sensitive to the conditions near the core, Pedersen
explained.
"The internal mixing inside stars has now been measured observationally and
turns out to be diverse in our sample, with some stars having almost no
mixing while others reveal levels a million times higher," Pedersen said.
The diversity turns out to be unrelated to the mass or age of the star.
Rather, it's primarily influenced by the internal rotation, though that is
not the only factor at play.
"These asteroseismic results finally allow astronomers to improve the theory
of internal mixing of massive stars, which has so far remained uncalibrated
by observations coming straight from their deep interiors," she added.
The precision at which astronomers can measure stellar oscillations depends
directly on how long a star is observed. Increasing the time from one night
to one year results in a thousand-fold increase in the measured precision of
oscillation frequencies.
"May and her collaborators have really shown the value of asteroseismic
observations as probes of the deep interiors of stars in a new and profound
way," said KITP Director Lars Bildsten, the Gluck Professor of Theoretical
Physics. "I am excited to see what she finds next."
The best data currently available for this comes from the Kepler space
mission, which observed the same patch of the sky for four continuous years.
The slowly pulsating B-type stars were the highest mass pulsating stars that
the telescope observed. While most of these are slightly too small to go
supernova, they do share the same internal structure as the more massive
stellar chemical factories. Pedersen hopes insights gleaned from studying
the B type stars will shed light on the inner workings of their higher mass,
O type counterparts.
She plans to use data from NASA's Transiting Exoplanet Survey Satellite
(TESS) to study groups of oscillating high-mass stars in OB associations.
These groups comprise 10 to more than 100 massive stars between 3 and 120
solar masses. Stars in OB associations are born from the same molecular
cloud and share similar ages, she explained. The large sample of stars, and
constraint from their common ages, provides exciting new opportunities to
study the internal mixing properties of high-mass stars.
In addition to unveiling the processes hidden within stellar interiors,
research on stellar oscillations can also provide information on other
properties of the stars.
"The stellar oscillations not only allow us to study the internal mixing and
rotation of the stars, but also determine other stellar properties such as
mass and age," Pedersen explained. "While these are both two of the most
fundamental stellar parameters, they are also some of the most difficult to
measure."
Reference:
May G. Pedersen et al, Internal mixing of rotating stars inferred from
dipole gravity modes, Nature Astronomy (2021). DOI:
10.1038/s41550-021-01351-x
Tags:
Space & Astrophysics