VLT Interferometer Gives Insight Into the Shape of Eta Carinae

Summary

Ever since 1841, when the until then inconspicuous southern star
Eta Carinae underwent a spectacular outburst, astronomers have
wondered what exactly is going on in this unstable giant star.
However, due to its considerable distance — 7,500 light-
years — details of the star itself were beyond observation.

This star is known to be surrounded by the Homunculus Nebula,
two mushroom-shaped clouds ejected by the star, each of which
is hundreds of times larger than our solar system.

Now, for the first time, infrared interferometry with the VINCI
instrument on ESO’s Very Large Telescope Interferometer (VLTI)
enabled an international team of astronomers [1] to zoom-in on
the inner part of its stellar wind. For Roy van Boekel, leader
of the team, these results indicate that "the wind of Eta
Carinae turns out to be extremely elongated and the star itself
is highly unstable because of its fast rotation."

Text with all links and the photos are available on the ESO Website at URL:
http://www.eso.org/outreach/press-rel/pr-2003/pr-31-03.html

Eta Carinae, the most luminous star known in our Galaxy, is by all
standards a real monster: it is 100 times more massive than our
Sun and 5 million times as luminous. This star has now entered the
final stage of its life and is highly unstable. It undergoes giant
outbursts from time to time; one of the most recent happened in
1841 and created the beautiful bipolar nebula known as the
Homunculus Nebula (see ESO PR Photo 32a/03). At that time, and
despite the comparatively large distance — 7,500 light-years —
Eta Carinae briefly became the second brightest star in the night
sky, surpassed only by Sirius.

Eta Carinae is so big that, if placed in our solar system, it
would extend beyond the orbit of Jupiter. This large size,
though, is somewhat arbitrary. Its outer layers are continually
being blown into space by radiation pressure — the impact of
photons on atoms of gas. Many stars, including our Sun, lose
mass because of such "stellar winds", but in the case of Eta
Carinae, the resulting mass loss is enormous (about 500
Earth-masses a year) and it is difficult to define the border
between the outer layers of the star and the surrounding
stellar wind region.

Now, VINCI and NAOS-CONICA, two infrared-sensitive instuments
on ESO’s Very Large Telescope (VLT) at the Paranal Observatory
(Chile), have probed the shape of the stellar wind region for
the first time. Looking down into the stellar wind as far as
possible, the astronomers could infer some of the structure of
this enigmatic object.

The astronomer team [1] first used the NAOS-CONICA adaptive
optics camera [2], attached to the 8.2-m VLT YEPUN telescope,
to image the hazy surroundings of Eta Carinae, with a spatial
resolution comparable to the size of the solar system, cf.
PR Photo 32a/03.

This image shows that the central region of the Homunculus
nebula is dominated by an object that is seen as a point-like
light source with many luminous "blobs" in the immediate
vicinity.

Towards the limit

In order to obtain an even sharper view, the astronomers then
turned to interferometry. This technique combines two or more
telescopes to achieve an angular resolution [3] equal to that
of a telescope as large as the separation of the individual
telescopes (cf. ESO PR 06/01 and ESO PR 23/01).

For the study of the rather bright star Eta Carinae the full
power of the 8.2-m VLT telescopes is not required. The
astronomers thus used VINCI, the VLT INterferometer
Commissioning Instrument [4], together with two 35-cm siderostat
test telescopes that served to obtain "First Light" with the VLT
Interferometer in March 2001 (see ESO PR 06/01).

The siderostats were placed at selected positions on the VLT
Observing Platform at the top of Paranal to provide different
configurations and a maximum baseline of 62 meters. During
several nights, the two small telescopes were pointed towards
Eta Carinae and the two light beams were directed towards a
common focus in the VINCI test instrument in the centrally
located VLT Interferometric Laboratory. It was then possible
to measure the angular size of the star (as seen in the sky)
in different directions.

Pushing the spatial resolution of this configuration to the
limit, the astronomers succeeded in resolving the shape of the
outer layer of Eta Carinae. They were able to provide spatial
information on a scale of 0.005 arcsec, that is about 11 AU
(1650 million km) at the distance of Eta Carinae, corresponding
to the full size of the orbit of Jupiter.

Scaled down to terrestial dimensions, this achievement compares
to making the distinction between an egg and a billiard ball at
a distance of 2,000 kilometers.

The VLTI observations brought the astronomers a surprise. They
indicate that the wind around Eta Carinae is amazingly elongated:
one axis is one-and-a-half times longer than the other! Moreover,
the longer axis is found to be aligned with the direction in
which the much larger mushroom-shaped clouds (seen on less sharp
images) were ejected.

Spanning a scale from 10 to 20-30,000 AU, the star itself and
the Homunculus Nebula are thus closely aligned in space.

VINCI was able to detect the boundary where the stellar wind from
Eta Carinae becomes so dense that it is no longer transparent.
Apparently, this stellar wind is much stronger in the direction
of the long axis than of the short axis.

According to mainstream theories, stars lose most mass around
their equator. This is because this is where the stellar wind
gets "lifting" assistance from the centrifugal force caused by
the star’s rotation. However, if this were so in the case of
Eta Carinae, the axis of rotation (through the star’s poles)
would then be perpendicular to both mushroom-shaped clouds.
But it is virtually impossible that the mushroom clouds are
positioned like spokes in a wheel, relative to the rotating
star. The matter ejected in 1841 would then have been
stretched into a ring or torus.

For Roy van Boekel, "the current overall picture only makes
sense if the stellar wind of Eta Carinae is elongated in the
direction of its poles. This is a surprising reversal of the
usual situation, where stars (and planets) are flattened at
the poles due to the centrifugal force.

The next supernova?

Such an exotic shape for Eta Carinae-type stars was predicted
by theoreticians. The main assumption is that the star itself,
which is located deep inside its stellar wind, is flattened at
the poles for the usual reason. However, as the polar areas of
this central zone are then closer to the centre where nuclear
fusion processes take place, they will be hotter. Consequently,
the radiation pressure in the polar directions will be higher
and the outer layers above the polar regions of the central
zone will get more "puffed up" than the outer layers at the
equator.

Assuming this model is correct, the rotation of Eta Carinae
can be calculated. It turns out that it should spin at over
90 percent of the maximum speed possible (before break-up).

Eta Carinae has experienced large outbursts other than the one
in 1841, most recently around 1890. Whether another outburst
will happen again in the near future is unknown, but it is
certain that this unstable giant star will not settle down.

At the present, it is losing so much mass so rapidly that
nothing will be left of it after less than 100,000 years. More
likely, though, Eta Carinae will destroy itself long before
that in a supernova blast that could possibly become visible
in the daytime sky with the naked eye. This may happen "soon"
on the astronomical time-scale, perhaps already within the
next 10-20,000 years.

More information

The research presented in this Press Release was published
as a Letter to the Editor in the European astronomy journal
Astronomy and Astrophysics ("Direct measurement of the size
and shape of the present-day stellar wind of Eta Carinae",
by Roy van Boekel et al., A&A 410, L37-L40).

Notes

[1]: The team is composed of Roy van Boekel (ESO and the
University of Amsterdam, The Netherlands), Pierre Kervella,
Francesco Paresce and Markus Schˆller (ESO), Wolfgang Brandner,
Tom Herbst and Rainer Lenzen (MPI for Astronomy, Heidelberg,
Germany), Alex de Koter and Rens Waters (University of
Amsterdam, The Netherlands), John Hillier (University of
Pittsburgh, USA), and Anne-Marie Lagrange (Observatoire de
Grenoble, France).

[2]: The Nasmyth Adaptive Optics System (NAOS) has been
developed by a French Consortium including the Office National
d’Etudes et de Recherches AÈrospatiales (ONERA), the Laboratoire
d’Astrophysique de Grenoble (LAOG) and Observatoire de Paris
(DESPA and DASGAL), in collaboration with ESO. The CONICA
Near-Infrared CAmera has been developed by the Max-Planck-
Institut f¸r Astronomie (MPIA, Heidelberg) and the Max-Planck-
Institut f¸r Extraterrestrische Physik (MPE, Garching), with
an extensive ESO collaboration. See ESO PR 25/01.

[3]: The achievable angular resolution is inversely proportional
to the aperture of a telescope for single telescope observation,
and to the length of the "baseline" between two telescopes for
an interferometric observation. However, interferometric
observations with two telescopes will improve the resolution
only in the direction parallel to this baseline, while the
resolution in the perpendicular direction will remain that of
a single telescope. Nevertheless, the use of other telescope
pairs with different baseline orientations "adds" resolution
in other directions.

[4]: The VINCI instrument was built under ESO contract at
the Observatoire de Paris (France) and the camera in this
instrument was delivered by the Max-Planck-Institute f¸r
Extraterrestrische Physik (Garching, Germany). The IR detector
and the IRACE detector electronics were supplied by ESO.