We have analyzed nearly all images of the Taurus star-forming region at 3.6 , 4.5 , 5.8 , 8.0 , and 24 µm that were obtained during the cryogenic mission of the Spitzer Space Telescope ( 46 deg ^ { 2 } ) and have measured photometry for all known members of the region that are within these data , corresponding to 348 sources , or 99 % of the known stellar population .
By combining these measurements with previous observations with the Spitzer Infrared Spectrograph and other facilities , we have classified the members of Taurus according to whether they show evidence of circumstellar disks and envelopes ( classes I , II , and III ) .
Through these classifications , we find that the disk fraction in Taurus , N ( II ) /N ( II+III ) , is \sim 75 % for solar-mass stars and declines to \sim 45 % for low-mass stars and brown dwarfs ( 0.01–0.3 M _ { \odot } ) .
This dependence on stellar mass is similar to that measured for Chamaeleon I , although the disk fraction in Taurus is slightly higher overall , probably because of its younger age ( 1 Myr vs. 2–3 Myr ) .
In comparison , the disk fraction for solar-mass stars is much lower ( \sim 20 % ) in IC 348 and \sigma Ori , which are denser than Taurus and Chamaeleon I and are roughly coeval with the latter .
These data indicate that disk lifetimes for solar-mass stars are longer in star-forming regions that have lower stellar densities .
Through an analysis of multiple epochs of Spitzer photometry that are available for \sim 200 Taurus members , we find that stars with disks exhibit significantly greater mid-infrared variability than diskless stars , which agrees with the results of similar variability measurements for a smaller sample of stars in Chamaeleon I .
The variability fraction for stars with disks is higher in Taurus than in Chamaeleon I , indicating that the IR variability of disks decreases with age .
Finally , we have used our data in Taurus to refine the observational criteria for primordial , evolved , and transitional disks .
The ratio of the number of evolved and transitional disks to the number of primordial disks in Taurus is 15/98 for spectral types of K5–M5 , indicating a timescale of 0.15 \times \tau _ { primordial } \sim 0.45 Myr for the clearing of the inner regions of optically thick disks .
After applying the same criteria to older clusters and associations ( 2–10 Myr ) that have been observed with Spitzer , we find that the proportions of evolved and transitional disks in those populations are consistent with the measurements in Taurus when their star formation histories are properly taken into account .

ERRATUM : In Table 7 , we inadvertently omitted the spectral type bins in which class II sources were placed in Table 8 based on their bolometric luminosities ( applies only to stars that lack spectroscopic classifications ) .
The bins were K6–M3.5 for FT Tau , DK Tau B , and IRAS 04370+2559 , M3.5–M6 for IRAS 04200+2759 , IT Tau B , and ITG 1 , and M6–M8 for IRAS 04325+2402 C. In addition , the values of K _ { s } - [ 3.6 ] in Table 13 and Figure 26 for spectral types of M4–M9 are incorrect .
We present corrected versions of Table 13 and Figure 26 .