The VLT Interferometer - ESO

2.3. DERIVED DESIRED PROPERTIES FOR THE VLTI 39
spots swept along by stellar rotation and possibly coupled with the disc?
The typical variation time scale of T Tau stars ranges wi thin a few days.
• Stars: Late Type Giants and Supergiants. The determination of statistical
properties (mean size, mean contrast in temperature) of the turbulent
eddies at the surface of these stars can be done without imaging; but
imaging possibilities are required to measure the mean lifetime (perhaps a
few days or tens days, in fact totally unknown because of the too primitive
treatment of the convection in stellar mantles). Also mapping is required
to survey large spots activity, related to the magnetic field behaviour;
again, the typical timescale of these phenomena is not known.
• Stars: Flare stars. The rising time of flare is steep, typically 1 minute; the
mean lifetime of a flare is 20 minutes. So, images, possibly by differential
interferometry, during flares will have very few pixels. But to be able to
observe a star during an eruption will require: (i) monitoring the star
photometry with a small telescope, the array being working on another
program, and (ii) adapting very rapidly the array configuration to shift
to the observation of the flare star as soon as the eruption· rises.
• Exotic objects. The expected size of the accretion disc of SS433 ranges
from 1 to a few marcsec. SS433 varies according to two periods: 13 and
164 days, and the flux varies from cycle to cycle at given phase. Furthermore,
who knows about those exotic objects which could be discovered
by HST, by ISO, or by the VLT ?
• Solar system. We have mentioned above the interest to survey planetary
surfaces. This should be done at least in two cases: Neptune (period =
15.8 hours) and 10 (period = 1.8 day). Volcanic eruptions change rapidly
on 10 surface, so that a survey of 10 will bring information about the
inner structure of that planet, about tidal interactions, and about the
solar system formation. Also very exciting is the survey of Triton, which
could have active giant geysers, from Voyager 2.
At infrared wavelengths, we are in the so called bright source case, due to thermal
noise. We need short integrations in the (u,v) plane, but many of them to
obtain many pixels and to increase the dynamical range of images. The hypersynthesis
technique proposed by [Vivekanand et al 1988] would provide that.
It may also be more desirable for the user to reconfigure the array in this way
within a night or so, instead of within several weeks.