Press Release

ESO Retraction: ADONIS Discovers Dust Disk around a Star with a Planet

By SpaceRef Editor
September 11, 2001
Filed under , ,

Note: Text online at:
http://www.eso.org/outreach/press-rel/pr-2000/phot-27-00.html

In October 2000, an announcement was made about a dust disk around the star
“iota Horologii”. Subsequent observations with the same instrument have
failed to confirm the reality of this disk. It has since been possible to
trace the problem to the calibration of this difficult observation. The
details and a formal retraction of “PR Photo 27/00”, together with the
original text and images, are now available at:

http://www.eso.org/outreach/press-rel/pr-2000/phot-27-00.html

For immediate release: August 2001

ESO Press Photo 27/00

ADONIS Discovers Dust Disk around a Star with a Planet
Zodiacal Light in the iota Horologii Extrasolar Planetary System

Retraction (August 2001)

The presumed detection of a dust disk around iota Horologii, presented in
this Press Release, has been found to be due to an instrumental artefact and
is therefore not real. This is the conclusion reached by the astronomers
working with the ADONIS Adaptive Optics instrument, following a series of
extremely careful tests.

In September 2000, iota Horologii — a star which was found earlier by radial
velocity measurements to be orbited by a planetary companion — was observed
by ADONIS, the adaptive optics system at the ESO 3.6-m telescope. These
observations showed an elliptically shaped excess emission around this star
when comparing with two reference stars. Such an excess, if real, can only
be explained by the presence of a dust disk around this star. A series of
new observations — again with the ADONIS system — have not confirmed the
previous results.

The search for dust disks around stars is a very difficult observational
task, because the star is much brighter than the faint extended emission of
a dust disk, It is therefore necessary to block the light of the star by
means of a so-called coronographic mask. But even this is not sufficient –
it is also necessary to subtract the remaining “light-wings” from the star
(i.e., the pattern that results from the stray light in the telescope and
camera). For this purpose, a “reference” star assumed to be without excess
emission is always observed before and after each observation of the
“target” star. The light pattern observed for the “reference” star (the
standard Point-Spread-Function) is then used to remove numerically the
pattern of the “target” star, thereby isolating any additional light that
may come from a circumstellar disk. For this analysis, it is crucial that
the Point-Spread-Function remains unchanged during the observation of the
reference and target stars. In fact, for each observation of the target
star, at least two different “reference” stars are observed in order to
verify this assumption.

Following this observational methodology carefully, H-band observations
of iota Horologii showed an excess emission which was interpreted as the
signature of a circumstellar dust disk. Recognising the uncertainties
inherent in this kind of observation, the astronomers performed the
observations again several months later, this time in other filter bands
and with other reference stars, and were unable to confirm the extended
emission.

In order to investigate this unexpected result, new observations were made
to verify the basic assumption that the Point-Spread-Function remains
unchanged for reference stars of slightly different brightness (within
half a magnitude). They showed that substantial changes in the
Point-Spread-Function of the ADONIS system can occur for reference stars
in the brightness interval employed for the iota Horologii observations.
Indeed, observations of two reference stars with no circumstellar material
and application of the standard analysis technique appeared to indicate an
excess emission in a pattern ressembling that found around iota Horologii.

The conclusion is clear: the presumed dust disk around iota Horologii is
an artefact, resulting from an underestimation of the calibration
uncertainties in this type of delicate observation.

The observers and the ESO EPR Dept. regret the incorrect announcement made
in ESO PR Photo 27/00.

SpaceRef staff editor.