Contents of: VI/111/./abstract/SVWB_SBDSK7_A.abs

The following document lists the file abstract/SVWB_SBDSK7_A.abs from catalogue VI/111.
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SCIENTIFIC ABSTRACT
These observations will survey a sample of approximately 100 stars
between the ages of $10^6$ and $10^9$ years to determine the
evolutionary characteristics of circumstellar disks.  The observations
will help establish:
  a. The timescale for dissipation or coagulation of the particulate
     material in the disks.
  b. The timescale for cessation of accretion onto the disks and the
     stars.
  c. The timescale for development of gaps in the distribution
     of disk matter.
  d. The influence of a variety of stellar and environmental
     characteristics on the formation and subsequent evolution of the
     disks.
Far infrared continuum emission from these stars originates in
circumstellar disks; the photospheric emission is well below the
sensitivity of ISOPHOT, and so only disks will be detected.  It is
expected that disks will be detected around approximately 50% of the
youngest stars and few if any of the older stars, depending on the
rate at which disks evolve.  The main goal of the proposal is to
ascertain in exactly what age group the disk emission begins to
disappear signaling the depletion of small particles in the disks.

OBSERVATION SUMMARY
Each star will be observed at 60 and 25 microns with PHT to determine
the infrared excess radiation from cold and warm disk matter;
supplemental ground-based observations at 10 microns of the same
sample will detect the hotter, inner disk radiation.  The sample size
must be of order 100 objects to reach statistically meaningful
conclusions; the sensitivity must be sufficient to detect passively
heated disks around these stars which are optically thick at 60
microns, typical of young disks.  The required sensitivity is achieved
after approximately 1 minute of integration time at both wavelengths.
Observations of nearby sky positions will allow us to subtract ambient
background emission from galactic cirrus and zodiacal light.  All
stars have been checked for cirrus emission from the IRAS database,
and the objects have been selected to minimize this emission.  We
expect to detect flux densities of approximately 20 mJy at 60 microns
and 10 mJy at 25 microns.  In most cases, the sensitivity at 60
microns will be limited by background subtraction and at 25 microns by
detector noise.

All stars are in compact clusters and may, therefore, be observed as
concactenated observations in sparse maps (PHT17, PHT18, & PHT19).
Concatenation increases the overall observing efficiency from a few
percent to over 50%.  Observation of a series of stars consists of
observing each star and a nearby sky position in a single filter (for
example, 60 microns) with 64 seconds of integration time, changing the
filter (to 25 microns, in this example), and reobserving all stars and
sky positions in reverse order.  All observations will be made with an
aperture of 52 seconds of arc.



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