Stephen E. Strom¹
¹Visiting Astronomer, Kitt Peak National Observatory, operated by AURA under contract to the National Science Foundation
Five College Astronomy Department, University of Massachusetts
GRC B-517G, Amherst, MA 01003
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at ages t << 1 Myr, the fraction of stars
in this mass range surrounded by disks approaches 100%.
Infrared photometric surveys of optically-revealed young stellar clusters
and associations provide important new constraints on the
lifetimes of circumstellar disks surrounding stars
of differing mass. By an age t ~ 1 Myr, stars with
masses M > 2 M show no evidence of infrared
excesses of a magnitude consistent with the presence
of circumstellar accretion disks.
However, for stars with M < 0.5 M, accretion disks
may persist for ages in excess of 10 Myr.
New techniques promise major advances in our ability
to trace the evolution of disks beyond the accretion phase.
ISO should have sufficient sensitivity
in the mid- and far- infrared to detect (1) emission
from the outer regions of disks in transition between massive
(M >> 0.01 M), optically thick accretion disks
and post-accretion structures; and (2) emission
from ``secondary'' disks, analogous to that surrounding
Pictoris. The recent discovery of small
(100 < r < 1000 AU) regions of ionized gas surrounding
young, low mass stars located in clusters associated with
O stars may provide another tool for identifying candidate
post-accretion disks. Recent observations suggest that
although these regions in most cases appear to be
alternative ``signposts'' for accretion disks manifest
as well through their infrared signatures, some
appear to be associated with pre-main sequence stars which
lack near-infrared excesses. Stars in this latter group
may be surrounded by gas-dominated disks,
in which nearly all solid material is in the form of bodies
much larger than micron-size grains.
Keywords: STARS: PRE-MAIN-SEQUENCE