  Spitzer Space Telescope infrared measurements are presented for 24 members of
the TW Hydrae association (TWA). High signal-to-noise 24-micron (um) photometry
is presented for all of these stars, including 20 stars that were not detected
by IRAS. Among these 20 stars, only a single object, TWA 7, shows excess
emission at 24um and at the level of only 40% above the star's photosphere. TWA
7 also exhibits a strong 70um excess that is a factor of 40 brighter than the
stellar photosphere at this wavelength. At 70um, an excess of similar magnitude
is detected for TWA 13, though no 24um excess was detected for this binary. For
the 18 stars that failed to show measurable IR excesses, the sensitivity of the
current 70um observations does not rule out substantial cool excesses at levels
10-40x above their stellar continua. Measurements of two T Tauri stars, TW Hya
and Hen 6-300, confirm that their spectacular IR spectral energy distributions
(SEDs) do not turn over even by 160um, consistent with the expectation for
their active accretion disks. In contrast, the Spitzer data for the luminous
planetary debris systems in the TWA, HD 98800B and HR 4796A, are consistent
with single-temperature blackbody SEDs. The major new result of this study is
the dramatic bimodal distribution found for the association in the form of
excess emission at a wavelength of 24um, indicating negligible amounts of warm
(>100 K) dust and debris around 20 of 24 stars in this group of very young
stars. This bimodal distribution is especially striking given that the four
stars in the association with strong IR excesses are >100x brighter at 24um
than their photospheres.
