ESO Annual Report 2004

It turned out that this new and independently
derived value of the zero-point is
exactly the same as the one obtained during
previous work based on a large number
of relatively low-precision Cepheid distance
measurements by the ESA
Hipparcos astrometric satellite. The agreement
between these two independent,
geometrical calibrations is remarkable and
greatly increases the confidence in the
cosmic distance scale now in use.
20
ESO Annual Report 2004
With several 1.8-m Auxiliary Telescopes
soon to be ready on the VLTI platform,
astronomers will be able to observe many
more Cepheids with a precision at least
as good as the present high-precision
VINCI observations of L Car. In addition,
the future AMBER instrument will extend
the VLTI capabilities toward shorter
wavelengths (J- and H-bands), providing
even higher spatial resolution than
what is now possible with VINCI (K-band).
The combined effect of these two improvements
will be to extend significantly
the accessible sample of Cepheids. It
is expected that the distances to more
than 30 Cepheids will then be measurable
with a precision better than 5 %. This will
provide a high precision calibration of both
the reference point (down to ± 0.01 mag)
and the slope of the Galactic Cepheid Period-Luminosity.
Spectrum of a meteor (FORS1/VLT)
On May 12, 2002, the FORS1 instrument on the
VLT serendipitously recorded the spectrum of
a bright meteor. This is quite possibly the only
meteor spectrum recorded with a large telescope
and a modern spectrograph. The spectrum
covers the wavelength range from 637
to 1050 nm, which is dominated by emissions
from air atoms and molecules in the meteor
a
b
The Period-Luminosity
relation in the V-band, as
deduced from the interferometric
observations
of Cepheids and the HST
parallax measurement of
Delta Cep.
path and teaches us about the collision processes
in the wake of a meteoroid. Closer
inspection of the spectrum revealed about
20 tell-tale meteor emissions of oxygen and nitrogen
atoms and nitrogen molecules. The
ratio of atomic and molecular emissions could
be used as a “thermometer” to measure the
conditions in the meteor-induced hot gas in the
wake of the meteoroid by means of laboratory
measurements and meteor models that calibrate
the VLT data. The meteor spectrum also
provided a first view of such an object in
the near-infrared window between wavelengths
900 and 1050 nm. This spectral region contains
relatively strong lines of atomic carbon, but no
such emissions were detected. This observation
is important because it sets new constraints
on the efficiency of meteor-induced atmospheric
chemistry at the time when life began on
our planet.
Most remarkable is the fact that the meteor trail
was out of focus. The VLT is indeed focussed
at infinity, which is perfect for most astronomical
objects that it routinely observes. But not
for meteoroids entering the atmosphere above
Paranal. A point at 100 kilometres distance
will appear as a small circle with 15 arcsec of
diameter at the VLT focal plane. This corresponds
to roughly half of the maximum apparent
diameter of Mars in the evening sky!