VLT/SPHERE and NACO image of the star L2 Puppis and its surroundings
The star L2 Puppis in the constellation of Puppis
Wide-field view of the sky around the red giant star L2 Puppis
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SPHERE reveals earliest stage of planetary nebula formation
Some of the sharpest images ever made
with ESO’s Very Large Telescope (VLT) have, for the first time, revealed
what appears to be an ageing star giving birth to a butterfly-like
planetary nebula. These observations of the red giant star L2 Puppis,
from the ZIMPOL mode of the newly installed SPHERE instrument, also
clearly showed a close companion. The dying stages of stars continue to
pose astronomers with many riddles, and the origin of such bipolar
nebulae, with their complex and alluring hourglass figures, doubly so.
This new imaging mode means that the VLT is currently the sharpest
astronomical direct imaging instrument in existence.
At about 200 light-years away, L2 Puppis is one of the closest red giants to Earth known to be entering its final stages of life. The new observations with the ZIMPOL mode of SPHERE
were made in visible light using extreme adaptive optics, which
corrects images to a much higher degree than standard adaptive optics,
allowing faint objects and structures close to bright sources of light
to be seen in greater detail. They are the first published results from
this mode and the most detailed of such a star.
ZIMPOL can produce images that are three times sharper than those from the NASA/ESA Hubble Space Telescope, and the new observations show the dust that surrounds L2 Puppis in exquisite detail [1]. They confirm earlier findings, made using NACO,
of the dust being arranged in a disc, which from Earth is seen almost
completely edge-on, but provide a much more detailed view. The
polarisation information from ZIMPOL also allowed the team to construct a
three dimensional model of the dust structures [2].
The astronomers found the dust disc to begin about 900
million kilometres from the star — slightly farther than the distance
from the Sun to Jupiter — and discovered that it flares outwards,
creating a symmetrical, funnel-like shape surrounding the star. The team
also observed a second source of light about 300 million kilometres —
twice the distance from Earth to the Sun — from L2 Puppis. This very
close companion star is likely to be another red giant of slightly lower
mass, but less evolved.
The combination of a large amount of dust surrounding a
slowly dying star, along with the presence of a companion star, mean
that this is exactly the type of system expected to create a bipolar
planetary nebula. These three elements seem to be necessary, but a
considerable amount of good fortune is also still required if they are
to lead to the subsequent emergence of a celestial butterfly from this
dusty chrysalis.
Lead author of the paper, Pierre Kervella, explains: “The
origin of bipolar planetary nebulae is one of the great classic
problems of modern astrophysics, especially the question of how,
exactly, stars return their valuable payload of metals
back into space — an important process, because it is this material
that will be used to produce later generations of planetary systems.”
In addition to L2 Puppis’s flared disc, the team found two
cones of material, which rise out perpendicularly to the disc.
Importantly, within these cones, they found two long, slowly curving
plumes of material. From the origin points of these plumes, the team
deduces that one is likely to be the product of the interaction between
the material from L2 Puppis and the companions star’s wind and radiation
pressure, while the other is likely to have arisen from a collision
between the stellar winds from the two stars, or be the result of an
accretion disc around the companion star.
Although much is still to be understood, there are two
leading theories of bipolar planetary nebulae, both relying on the
existence of a binary star system [3].
The new observations suggest that both of these processes are in action
around L2 Puppis, making it appear very probable that the pair of stars
will, in time, give birth to a butterfly.
Pierre Kervella concludes: “With the companion star
orbiting L2 Puppis only every few years, we expect to see how the
companion star shapes the red giant’s disc. It will be possible to
follow the evolution of the dust features around the star in real time —
an extremely rare and exciting prospect.”
Notes
[1] SPHERE/ZIMPOL use extreme adaptive optics to create diffraction-limited
images, which come a lot closer than previous adaptive optics
instruments to achieving the theoretical limit of the telescope if there
were no atmosphere. Extreme adaptive optics also allows much fainter
objects to be seen very close to a bright star. These images are also
taken in visible light — shorter wavelengths than the near-infrared
regime, where most earlier adaptive optics imaging was performed. These
two factors result in significantly sharper images than earlier VLT
images. Even higher spatial resolution has been achieved with VLTI, but
the interferometer does not create images directly.
[2] The dust in the disc was very efficient at scattering the stars’ light towards Earth and polarising
it, a feature that the team could use to create a three-dimensional map
of the envelope using both ZIMPOL and NACO data and a disc model based
on the RADMC-3D radiative transfer modeling tool, which uses a given set of parameters for the dust to simulate photons propagating through it.
[3] The first theory is that the dust produced by the primary, dying star’s stellar wind is confined to a ring-like orbit about the star by the stellar winds and radiation pressure
produced by the companion star. Any further mass lost from the main
star is then funneled, or collimated, by this disc, forcing the material
to move outwards in two opposing columns perpendicular to the disc.
The second holds that most of the material being ejected by the dying star is accreted by its nearby companion, which begins to form an accretion disc and a pair of powerful jets.
Any remaining material is pushed away by the dying star’s stellar
winds, forming an encompassing cloud of gas and dust, as would normally
occur in a single star system. The companion star’s newly created
bipolar jets, moving with much greater force than the stellar winds of
the dying star, then carve dual cavities through the surrounding dust,
resulting in the characteristic appearance of a bipolar planetary
nebula.
More Information
This research was presented in a paper entitled “The dust disk and
companion of the nearby AGB star L2 Puppis”, by P. Kervella, et al., to
appear in the journal Astronomy & Astrophysics on 10 June 2015.
The team is composed of P. Kervella (Unidad Mixta Internacional
Franco-Chilena de Astronomía, CNRS/INSU, France; Departamento de
Astronomía, Universidad de Chile, Santiago, Chile; Observatoire de
Paris, LESIA, France; Université Paris-Diderot, Meudon, France), M.
Montargès (LESIA, France; Institut de Radio-Astronomie Millimétrique,
St Martin d’Hères, France), E. Lagadec (Laboratoire Lagrange, Université
de Nice-Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, Nice,
France), S. T. Ridgway (National Optical Astronomy Observatories,
Tucson, Arizona, USA), X. Haubois (ESO, Santiago, Chile), J. H. Girard
(ESO, Chile), K. Ohnaka (Instituto de Astronomía, Universidad Católica
del Norte, Antofagasta, Chile), G. Perrin (Observatoire de Paris, LESIA,
France) and A. Gallenne (Universidad de Concepción, Departamento de
Astronomía, Concepción, Chile).
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Links
- Research paper
- Photos of the VLT
- ESOcast on ZIMPOL/SPHERE and polarimetry
- More information about SPHERE
Contacts
Pierre Kervella
Departamento de Astronomía, Universidad de Chile
Santiago, Chile
Cell: +33 628 076 550
Email: pierre.kervella@obspm.fr
Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Source: ESO
