Astronomers know that while large stars can end their lives as violently
cataclysmic supernovae, smaller stars end up as planetary nebulae –
colourful, glowing clouds of dust and gas. In recent decades these
nebulae, once thought to be mostly spherical, have been observed to
often emit powerful, bipolar jets of gas and dust. But how do spherical
stars evolve to produce highly aspherical planetary nebulae?
In a theoretical paper published this week in the Monthly Notices of
the Royal Astronomical Society, a University of Rochester professor and
his undergraduate student conclude that only “strongly interacting”
binary stars – or a star and a massive planet – can feasibly give rise
to these powerful jets.
When these smaller stars run out of hydrogen to burn they begin to
expand and become Asymptotic Giant Branch (AGB) stars. This phase in a
star’s life lasts at most 100,000 years. At some point some of these AGB
stars, which represent the distended last spherical stage in the lives
of low mass stars, become “pre-planetary” nebula, which are aspherical.
“What happens to change these spherical AGB stars into non-spherical
nebulae, with two jets shooting out in opposite directions?” asks Eric
Blackman, professor of physics and astronomy at Rochester. “We have been
trying to come up with a better understanding of what happens at this
stage.”
For the jets in the nebulae to form, the spherical AGB stars have to
somehow become non-spherical and Blackman says that astronomers believe
this occurs because AGB stars are not single stars but part of a binary
system. The jets are thought to be produced by the ejection of material
that is first pulled and acquired, or “accreted,” from one object to the
other and swirled into a so-called accretion disk. There are, however, a
range of different scenarios for the production of these accretion
disks. All these scenarios involve two stars or a star and a massive
planet, but it has been hard to rule any of them out until now because
the “core” of the AGBs, where the disks form, are too small to be
directly resolved by telescopes. Blackman and his student, Scott
Lucchini, wanted to determine whether the binaries can be widely
separated and weakly interacting, or whether they must be close and
strongly interacting.
By studying the jets from pre-planetary and planetary nebulae,
Blackman and Lucchini were able to connect the energy and momentum
involved in the accretion process with that in the jets; the process of
accretion is what in effect provides the fuel for these jets. As mass is
accreted into one of the disks it loses gravitational energy. This is
then converted into the kinetic energy and momentum of the outflowing
jets, which is the mass that is expelled at a certain speed. Blackman
and Lucchini determined the minimum power and minimum mass flows that
these accretion processes needed to produce to account for the
properties of the observed jets. They then compared the requirements to
specific existing accretion models, which have predicted specific power
and mass flow rates.
They found that only two types of accretion models, both of which involve the most strongly interacting binaries, could create these jetted pre-planetary nebulae. In the first type of model, the “Roche lobe overflow,” the companions are so close that the AGB stellar envelope gets pulled into a disk around the companion. In the second type of models, or “common envelope” models, the companion is even closer and fully enters the envelope of the AGB star so that the two objects have a "common" envelope. From within the common envelope, very high accretion rate disks can either form around the companion from the AGB star material, or the companion can be shredded into a disk around the AGB star core. Both of these scenarios could provide enough energy and momentum to produce the jets that have been observed.
The name planetary nebulae originally came from astronomer William
Herschel, who first observed them in the 1780s, and thought they were
newly forming gaseous planets. Although the name has persisted, now we
know that they are in fact the end states of low mass stars, and would
only involve planets if a binary companion in one of the accretion
scenarios above were in fact a large planet. “Pre-planetary” and
“planetary” nebulae are different in the nature of the light they
produce; pre-planetary nebulae reflect light, whereas mature planetary
nebulae shine through ionisation (where atoms lose or gain electrons).
Pre-planetary nebulae shoot out two jets of gas and dust, the latter
forming in the jets as the outflows expand and cool. This dust reflects
the light produced by the hotter core. In planetary nebulae, thought to
be the evolved stage of pre-planetary nebula, the core is exposed and
the hotter radiation it emits ionises the gas in the now weaker jets,
which in turn glow.
The research was supported by the NSF grant AST-1109285.
Media contacts
Leonor Sierra
University of Rochester
United States
Tel: +1 585 276 6264
lsierra@rochester.edu
Robert Massey
Royal Astronomical Society
United Kingdom
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
rm@ras.org.uk
Image and caption
An image is available from https://www.ras.org.uk/images/stories/press/Rotteneggnebula.jpg
Caption: A Hubble Space Telescope image of the Rotten Egg Nebula, a
pre-planetary nebula 5000 light years away in the constellation of
Puppis. Credit: NASA/ESA & Valentin Bujarrabal (Observatorio
Astronomico Nacional, Spain)
Further information
The new work appears in “Using kinematic properties of pre-planetary
nebulae to constrain engine paradigms”, Eric G. Blackman and Scott
Luchini, Monthly Notices of the Royal Astronomical Society, Oxford
University Press.
The paper is available from http://mnrasl.oxfordjournals.org/content/early/2014/01/30/mnrasl.slu001
A preprint of the paper is available from http://arxiv.org/pdf/1312.5372.pdf
Notes for editors
The University of Rochester (www.rochester.edu)
is one of the leading private universities in the United States.
Located in Rochester, New York, the University gives students
exceptional opportunities for interdisciplinary study and close
collaboration with faculty through its unique cluster-based curriculum.
Its College, School of Arts and Sciences, and Hajim School of
Engineering and Applied Sciences are complemented by its Eastman School
of Music, Simon School of Business, Warner School of Education,
Laboratory for Laser Energetics, School of Medicine and Dentistry,
School of Nursing, Eastman Institute for Oral Health, and the Memorial
Art Gallery.
The Royal Astronomical Society (RAS, www.ras.org.uk), founded in 1820, encourages and promotes the study of astronomy,
solar-system science, geophysics and closely related branches of
science. The RAS organizes scientific meetings, publishes international
research and review journals, recognizes outstanding achievements by the
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astronomy nationally and internationally. Its more than 3800 members
(Fellows), a third based overseas, include scientific researchers in
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