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13 May 2003

Information from the European Southern Observatory

Sharper and Deeper Views with MACAO-VLTI

Part 2 of 2

MACAO-VLTI will now allow a gain of a factor 100 in the injected
light flux - this will be tested in detail when two VLT Unit
Telescopes, both equipped with MACAO-VLTI's, work together. However,
the very good performance actually achieved with the first system
makes the engineers very confident that a gain of this order will
indeed be reached. This ultimate test will be performed as soon as
the second MACAO-VLTI system has been installed later this year.

MACAO-VLTI First light

After one month of installation work and following tests by means of
an artificial light source installed in the Nasmyth focus of KUEYEN,
MACAO-VLTI had "First Light" on April 18 when it received "real"
light from several astronomical obejcts.

During the preceding performance tests to measure the image
improvement (sharpness, light energy concentration) in near-infrared
spectral bands at 1.2, 1.6 and 2.2 microns, MACAO-VLTI was checked by
means of a custom-made Infrared Test Camera developed for this
purpose by ESO. This intermediate test was required to ensure the
proper functioning of MACAO before it is used to feed a corrected
beam of light into the VLTI.

After only a few nights of testing and optimizing of the various
functions and operational parameters, MACAO-VLTI was ready to be used
for astronomical observations. The images below were taken under
average seeing conditions and illustrate the improvement of the image
quality when using MACAO-VLTI.

MACAO-VLTI - First Images

Here are some of the first images obtained with the test camera at
the first MACAO-VLTI system, now installed at the 8.2-m VLT KUEYEN
telescope. 

PR Photos 12b-c/03 show the first image in the infrared K-band
(wavelength 2.2 microns) of a star (visual magnitude 10) obtained
without and with image corrections by means of adaptive optics.

PR Photo 12d/03 displays one of the best images obtained with
MACAO-VLTI during the early tests. It shows a Strehl ratio (measure
of light concentration) that fulfills the specifications according to
which MACAO-VLTI was built. This enormous improvement when using AO
techniques is clearly demonstrated in PR Photo 12e/03, with the
uncorrected image profile (left) hardly visible when compared to the
corrected profile (right).

PR Photo 11f/03 demonstrates the correction capabilities of
MACAO-VLTI when using a faint guide star. Tests using different
spectral types showed that the limiting visual magnitude varies
between 16 for early-type B-stars and about 18 for late-type M-stars.

Astronomical Objects seen at the Diffraction Limit

The following examples of MACAO-VLTI observations of two well-known
astronomical objects were obtained in order to provisionally evaluate
the research opportunities now opening with MACAO-VLTI. They may well
be compared with space-based images.

The Galactic Center

The center of our own galaxy is located in the Sagittarius
constellation at a distance of approximately 30,000 light-years. PR
Photo 12h/03 shows a short-exposure infrared view of this region,
obtained by MACAO-VLTI during the early test phase.

Recent AO observations using the NACO facility at the VLT provide
compelling evidence that a supermassive black hole with 2.6 million
solar masses is located at the very center, cf. ESO PR 17/02. This
result, based on astrometric observations of a star orbiting the
black hole and approaching it to within a distance of only 17
light-hours, would not have been possible without images of
diffraction limited resolution.

Eta Carinae

Eta Carinae (PR Photo 12i/03) is one of the heaviest stars known,
with a mass that probably exceeds 100 solar masses. It is about 4
million times brighter than the Sun, making it one of the most
luminous stars known.

Such a massive star has a comparatively short lifetime of about 1
million years only and - measured in the cosmic timescale- Eta
Carinae must have formed quite recently. This star is highly unstable
and prone to violent outbursts. They are caused by the very high
radiation pressure at the star's upper layers, which blows
significant portions of the matter at the "surface" into space during
violent eruptions that may last several years.  The last of these
outbursts occurred between 1835 and 1855 and peaked in 1843. Despite
its comparaticely large distance - some 7,500 to 10,000 light-years -
Eta Carinae briefly became the second brightest star in the sky at
that time (with an apparent magnitude -1), only surpassed by Sirius.

Frosty Leo

Frosty Leo (PR Photo 12j/03) is a magnitude 11 (post-AGB) star
surrounded by an envelope of gas, dust, and large amounts of ice
(hence the name). The associated nebula is of "butterfly" shape
(bipolar morphology) and it is one of the best known examples of the
brief transitional phase between two late evolutionary stages,
asymptotic giant branch (AGB) and the subsequent planetary nebulae
(PNe).

For a three-solar-mass object like this one, this phase is believed
to last only a few thousand years, the wink of an eye in the life of
the star.  Hence, objects like this one are very rare and Frosty Leo
is one of the nearest and brightest among them.

Contacts

Norbert Hubin
ESO
Garching, Germany
Phone: +49 89 3200-6517
email: nhubin{at}eso.org

Robin Arsenault
ESO
Garching, Germany
Phone: +49 89 3200-6524
email: rarsenau{at}eso.org

Markus Kasper
ESO
Garching, Germany
Phone: +49 89 3200-6359
email: mkasper{at}eso.org

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