Massive stars like Zeta Puppis
are relatively rare, but play a very important role in recycling
materials in the Universe. They burn their nuclear fuel much more
rapidly than stars like the Sun, living only for millions of years
before exploding as a supernova and returning most of their matter to
space. But during their brief lives, they lose a significant fraction of
their mass through fierce winds of gas driven off their surfaces by the
intense light emitted from the star. The fierce wind from a giant star
like Zeta Puppis, an extreme blue supergiant, one of the most luminous
stars in the Milky Way, 12,500 times more powerful than the Sun, is not
a uniform breeze but is fragmented into hundreds of thousands of
pieces, according to a study by the ESA’s XMM-Newton space observatory.
Zeta Puppis also goes by the name Naos,
which in antiquity was the name given to the innermost sanctuary of a
temple, accessible to only a few people; thanks to XMM-Newton,
scientists have been able to unlock the secrets of this mysterious
stellar object.
The winds from massive stars are at least a hundred million times
stronger than the solar wind emitted by our own Sun and can
significantly shape their surrounding environment. They might trigger
the collapse of surrounding clouds of gas and dust to form new stars or,
conversely, blast the clouds away before they have the chance to get
started.
Despite their important role, however, the detailed structure of the
winds from massive stars remains poorly understood. Astronomers have now
gained a detailed glimpse into this wind structure by taking
observations with XMM-Newton spread over a decade to study variability
in the X-ray emission from zeta Puppis. One of the nearest massive stars
to Earth, it is bright enough to be seen with the naked eye in the
constellation of Puppis, in the southern hemisphere.
The X-rays arise from collisions between slow- and fast-moving clumps
in the wind, which heats them to a few million degrees. As individual
colliding clumps in the wind are heated and cooled, the strength and
energy of the emitted X-rays vary. If only a small number of large
fragments are present, variations in the combined emission could be
large. Conversely, as the number of fragments grows, a change in the
X-ray emission from any given fragment becomes less important, and the
overall variability decreases.
In zeta Puppis, the X-ray emission was found to be remarkably stable
over short timescales of just a few hours, pointing to a very large
number of fragments. There must still be clumps in the wind to make
X-rays in the first place, but there must be many of them to yield such
low variability. However, unexpected variation in the emission was seen
on the order of several days, implying the presence of a few very large
structures in the wind, possibly spiral-arm-like features superimposed
on the highly fragmented wind co-rotating with the star.
“Studies at other wavelengths had already hinted that the winds from
massive stars are not simply a uniform breeze, and the new XMM-Newton
data confirm this, but also reveal hundreds of thousands of individual
hot and cool pieces,” says Yaël Nazé, Université de Liège, Belgium, who led the study’s analysis.“This is the first time constraints have been placed on the number of fragments in a stellar wind of an adult massive star, a number which far exceeds theoretical predictions.”
To fully understand these observations, improved models of stellar
winds will be needed, taking into account both the large-scale emission
structures and the highly fragmented wind, in order to understand how
they affect mass-loss in stellar giants.
“This long-term XMM-Newton study of zeta Puppis has provided the
first constraints on the number of fragments in a stellar wind from a
massive star – there is no dataset with comparable sensitivity or time
and or spectral coverage currently available for any other massive
star,” says Norbert Schartel, ESA’s XMM-Newton project scientist.
Source: The Daily Galaxy via ESA
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