Dust dissipation timescales in the intermediate and outer regions of protoplanetary disks

Dust and gas in protoplanetary disks dissipate as central stars evolve. In order to estimate the dust dissipation timescales in the protoplanetary disks, we stacked the WISE 12 and 22, and the AKARI 90 mu m survey images of known T Tauri stars and derived the average fluxes, well below the survey flux limit in the 90 mu m band. We classified 4783 T Tauri stars into three age groups, which are young (<2 Myr), mid-age (2-6 Myr), and old (>6 Myr) groups, and stacked the WISE 12 and 22 and the AKARI 90 mu m images in each group. The photometry of the stacked image shows the flux decay timescales of 1.4 +/- 0.2, 1.38 +/- 0.05, and 1.4(-0.5)(+0.6) Myr in the 12, 22, and 90 mu m bands, respectively. In optically thin disks with one-solar luminosity central stars, the 12 and 22 mu m fluxes are attributed to the emission from the intermediate (similar to 1 au) region and the 90 mu m flux corresponds to that from the outer (similar to 10 au) region in the disk. We hence conclude that the dust dissipation timescale is tau(med,dust) similar to 1.4 Myr in the intermediate disks and is tau(outer.dust) = 1.4(-0.5)(+0.6) Myr in the outer disks. The dust-dissipation time difference between the outer and intermediate disks is Delta tau(dust) = tau(outer.dust) - tau(med,dust) = 0.0(-0.5)(+0.6) Myr, indicating that the dust in the intermediate and outer disks dissipates on almost the same timescale.