After the initial discoveries by IRAS, the search and analysis of Vega-like
disks in the infrared has received a substantial boost with the availability of
data from the Infrared Space Observatory (ISO, Kessler et al. 1996). ISO has
improved on IRAS in several important ways. Firstly, the number of bands has
been increased and the infrared wavelength coverage has been extended to 200
m; secondly, the detection limits have been lowered; and, thirdly, imaging
and spectroscopy have been made possible on arcsec scales. One major ISO
finding, based on a statistical study, is that the detection of a debris dust
disk depends strongly on the age of a star: the probability of detecting a
Vega-like disk comes close to unity for main-sequence dwarfs of less than 400
Myr (Habing et al. 1999; Habing et al. 2001, hereafter Paper I). Disks
around older main-sequence stars are much less frequent, but they still exist.
The precise mechanism that prevents these older disks from dissipating is still
an open question (Jourdain de Muizon et al. 2001).
The search for new Vega-like stars has been based either on the analysis of the
infrared colours using the IRAS database (Fajardo-Acosta et al. 2000; Mannings
& Barlow 1998, and references therein for previous IRAS surveys), or by
comparing the far-infrared flux with a prediction based on a photospheric model
or extrapolation from optical photometry. In most cases the surveys rely on the
measurements at 60 m, because the excess emission is high compared to the
photospheric flux and the background confusion is low compared to observations
at longer wavelengths.
A significant excess at shorter wavelengths is a signature of warm debris material, presumably closer to the star than the particles emitting at the longer wavelengths. Detecting such an excess generally requires a high photometric accuracy due to the relatively large contribution of the photospheric emission.
In this paper we analyse photometric data at 25 m of a sample of 81
main-sequence stars in order to determine the fraction of the stars which have
significant infrared excess at 25 m.
In Sect. 2 we describe the sample, the different data sets and the
processing of the ISOPHOT data. In order to achieve the highest possible
photometric accuracy we merged the ISO and IRAS data taking into account the
systematic differences in calibration between the two data sets. The merging of
the data sets and the extraction of the stars with 25 m excess are
described in Sect. 3. The results are analysed in
Sect. 4. In Sect. 5 we discuss the properties of the
excess stars. The conclusions are stated in Sect. 6.
Copyright ESO 2002