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==> DISCRETIONARY TIME PROPOSAL
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One of the exciting discoveries made using IRAS was the dust disc
around Vega. So far, only Beta Pic has had its disc confirmed
optically, and for many of the Vega-like candidates the disc is
only visible between around 20um and 80um. Due to the low spatial
resolution of IRAS, relatively little is known about the immediate
environment of Vega-like stars/candidates in the infrared. ISOPHOT
is being used to make oversampled maps (PHT32 at 60um and 90um) for
some Vega-like stars and candidates.
The reduction of the OTF from 10 to 2 arcsec has made high resolution
scans possible. Recently the P3 detector was used to scan Vega at 60um,
using an aperture below the diffraction limit and with a very small step
size, giving a high resolution scan across the disc. This showed the extent
of the disc very clearly when compared to the point source Gamma Dra
(HR6705). This new observing technique, resulting from ISO's excellent
pointing accuracy, means that the Airy disc can be sampled with around
7 to 8 positions across it, revealing the extended dust disc with ease.
We propose to use this method on five new targets where the oversampled
map (or other data) suggests a resolved disc at 60um. The five new targets
are Beta Pic (HR2020), Alpha PsA (HR8728), and three stars from the Walker
and Wolstencroft (1988) list, namely HD34700, HD142666 and HD169142. We
propose 2 scans across Beta Pic and Alpha PsA, in orthogonal directions
to investigate the disc orientation. We also request a scan across Gamma
Dra (HR6705) when the scan across Beta Pic made, for confirmation of the
behaviour of the detector for a genuine point source (from the influence
of the tripod), and a second scan across Vega (HR7001), in the orthogonal
direction to the initial test scan, to confirm that the disc is face-on.