LIGHT-ECHO DETECTION OF CIRCUMSTELLAR DISKS AROUND FLARING STARS

Light echoes can be used to detect and characterize disks around flari ng stars. Such disks are thought to be a hallmark of planet formation but are very difficult to detect by ordinary means. Dwarf emission-lin e M stars experience flares with luminosities comparable to their quie scent photospheres on time scales of minutes, less than the light trav el time across a disk many astronomical units in extent; they are thus ideal candidates for such a search. Bromley (1992, Publ. Astron. Soc. Pac. 104, 1049-1053) calculated that the detection of Jupiter-sized c ompanions using light echoes requires photometric accuracies better th an 1 part in 10(6). However, a disk consisting of grains or small bodi es will scatter a much larger fraction of the light than a planet of s imilar mass. I estimate the light echo amplitudes from plausible geome tries of circumstellar material and present simulated light curves. Th e light echo amplitudes are typically 1% of the flare and I conclude t hat such events will be detected best in cases where the flare is ecli psed by the star. An examination of the time scales associated with in ternal processes in a protoplanetary disks around dM stars indicates t hat any primordial disks may become undetectable in 10(4) years and wi ll have completely disappeared by 10(8) years, the estimated age of dM e stars in the solar neighborhood. However, searches for light echoes might constrain the amount of material continuing to fall into these y oung stellar systems in the form of comet-like objects. (C) 1994 Acade mic Press, Inc.