A gallery of bipolar planetary nebulae
Bipolar planetary nebula Hubble 12
Astronomers have used ESO's New
Technology Telescope and the NASA/ESA Hubble Space Telescope to explore
more than 100 planetary nebulae in the central bulge of our galaxy. They
have found that butterfly-shaped members of this cosmic family tend to
be mysteriously aligned — a surprising result given their different
histories and varied properties.
The final stages of life for a star like our Sun result in the star
blowing its outer layers out into the surrounding space, forming objects
known as planetary nebulae in a wide range of beautiful and striking
shapes. One type of such nebulae, known as bipolar planetary nebulae,
create ghostly hourglass or butterfly shapes around their parent stars.
All these nebulae formed in different places and have different
characteristics. And neither the individual nebulae, nor the stars that
formed them, would have interacted with other planetary nebulae.
However, a new study by astronomers from the University of Manchester,
UK, now shows surprising similarities between some of these nebulae:
many of them line up in the sky in the same way [1].
"This really is a surprising find and, if it holds true, a very important one," explains Bryan Rees of the University of Manchester, one of the paper's two authors. "Many
of these ghostly butterflies appear to have their long axes aligned
along the plane of our galaxy. By using images from both Hubble and the
NTT we could get a really good view of these objects, so we could study
them in great detail."
The astronomers looked at 130 planetary nebulae in the Milky Way's central bulge. They identified three different types [2], and peered closely at their characteristics and appearance.
"While two of these populations were completely randomly aligned
in the sky, as expected, we found that the third — the bipolar nebulae —
showed a surprising preference for a particular alignment," says the paper's second author Albert Zijlstra, also of the University of Manchester. "While any alignment at all is a surprise, to have it in the crowded central region of the galaxy is even more unexpected."
Planetary nebulae are thought to be sculpted by the rotation of the
star system from which they form. This is dependent on the properties of
this system — for example, whether it is a binary [3],
or has a number of planets orbiting it, both of which may greatly
influence the form of the blown bubble. The shapes of bipolar nebulae
are some of the most extreme, and are probably caused by jets blowing
mass from the binary system perpendicular to the orbit.
"The alignment we're seeing for these bipolar nebulae indicates something bizarre about star systems within the central bulge," explains Rees. "For
them to line up in the way we see, the star systems that formed these
nebulae would have to be rotating perpendicular to the interstellar
clouds from which they formed, which is very strange."
While the properties of their progenitor stars do shape these
nebulae, this new finding hints at another more mysterious factor. Along
with these complex stellar characteristics are those of our Milky Way;
the whole central bulge rotates around the galactic centre. This bulge
may have a greater influence than previously thought over our entire
galaxy — via its magnetic fields. The astronomers suggest that the
orderly behaviour of the planetary nebulae could have been caused by the
presence of strong magnetic fields as the bulge formed.
As such nebulae closer to home do not line up in the same orderly
way, these fields would have to have been many times stronger than they
are in our present-day neighbourhood [4].
"We can learn a lot from studying these objects," concludes Zijlstra. "If
they really behave in this unexpected way, it has consequences for not
just the past of individual stars, but for the past of our whole
galaxy."
Notes
[1] The "long axis" of a bipolar
planetary nebula slices through the wings of the butterfly, whilst the
"short axis" slices through the body.
[2] The shapes of the planetary nebula images were
classified into three types, following conventions: elliptical, either
with or without an aligned internal structure, and bipolar.
[3] A binary system consists of two stars rotating around their common centre of gravity.
[4] Very little is known about the origin and
characteristics of the magnetic fields that were present in our galaxy
when it was young, so it is unclear whether they have grown stronger
over time, or decayed.
More information
The research is presented in a paper
entitled "Alignment of the Angular Momentum Vectors of Planetary Nebulae
in the Galactic Bulge", to appear in the Monthly Notices of the Royal Astronomical Society.
The team is composed of B. Rees (University of Manchester, UK), and
A. A. Zijlstra (University of Manchester, UK). Bryan Rees came late to
research astronomy — he decided to do a PhD after his early retirement
and this work formed part of his thesis work.
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Links
Contacts
Albert ZijlstraUniversity of Manchester
Manchester, UK
Tel: +44 1613 063925
Email: a.zijlstra@manchester.ac.uk
Bryan Rees
University of Manchester
Manchester, UK
Tel: +44 1612 754145
Email: bryan.rees@manchester.ac.uk
Richard Hook
ESO, Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
