GASPS - a Herschel survey of gas and dust in Protoplanetary Disks:
  Summary and Initial Statistics
  GASPS is a far-infrared line and continuum survey of protoplanetary and young
debris disks using PACS on the Herschel Space Observatory. The survey includes
[OI] at 63 microns, as well as 70, 100 and 160um continuum, with the brightest
objects also studied in [OI]145um, [CII]157um, H2O and CO. Targets included T
Tauri stars and debris disks in 7 nearby young associations, and a sample of
isolated Herbig AeBe stars. The aim was to study the global gas and dust
content in a wide disk sample, systemically comparing the results with models.
In this paper we review the main aims, target selection and observing strategy.
We show initial results, including line identifications, sources detected, and
a first statistical study.
  [OI]63um was the brightest line in most objects, by a factor of ~10.
Detection rates were 49%, including 100% of HAeBe stars and 43% of T Tauri
stars. Comparison with published dust masses show a dust threshold for [OI]63um
detection of ~1e-5 M_solar. Normalising to 140pc distance, 32% with mass 1e-6 -
1e-5 M_solar, and a small number with lower mass were also detected. This is
consistent with moderate UV excess and disk flaring. In most cases, continuum
and line emission is spatially and spectrally unresolved, suggesting disk
emission. ~10 objects were resolved, likely from outflows. Detection rates in
[OI]145um, [CII]157um and CO J=18-17 were 20-40%, but [CII] was not correlated
with disk mass, suggesting it arises instead from a compact envelope.
  [OI] detection rates in T Tauri associations of ages 0.3-4Myr were ~50%. ~2
stars were detectable in associations of 5-20Myr, with no detections in
associations of age >20Myr. Comparing with the total number of young stars, and
assuming a ISM-like gas/dust ratio, this indicates that ~18% of stars retain a
gas-rich disk of total mass >1M_Jupiter for 1-4Myr, 1-7% keep such disks for
5-10Myr, and none remain beyond 10-20Myr.
