  We have conducted an N-band survey of 14 young stars in the ~30 Myr-old
Tucana-Horologium Association to search for evidence of warm, circumstellar
dust disks. Using the MIRAC-BLINC camera on the Magellan I (Baade) 6.5-m
telescope, we find that none of the stars have a statistically significant
N-band excess compared to the predicted stellar photospheric flux. Using three
different sets of assumptions, this null result rules out the existence of the
following around these post-T Tauri stars: (a) optically-thick disks with inner
hole radii of ~<0.1 AU, (b) optically-thin disks with masses of >10^{-6}
M_Earth (in ~1-micron-sized grains) within ~<10 AU of these stars, (c)
scaled-up analogs of the solar system zodiacal dust cloud with >4000X the
emitting area. Our survey was sensitive to dust disks in the terrestrial planet
zone with fractional luminosity of log(L_{dust}/L_{star}) ~ 10^{-2.9}, yet none
were found. Combined with results from previous surveys, these data suggest
that circumstellar dust disks become so optically-thin as to be undetectable at
N-band before age ~20 Myr. We also present N-band photometry for several
members of other young associations and a subsample of targets that will be
observed with Spitzer Space Telescope by the Formation and Evolution of
Planetary Systems (FEPS) Legacy Science Program. Lastly, we present an absolute
calibration of MIRAC-BLINC for four filters (L, N, 11.6, and Q_s) on the
Cohen-Walker-Witteborn system.
