Disk evolution in the Orion OB1 association

We analyze multiband photometry of a subsample of low-mass stars in the associations Ori OB1a and 1b discovered during the Centro de Investigaciones de Astronomia (CIDA) Orion Variability Survey, which have ages of 7-10 and 3-5 Myr, respectively. We obtained UBVRCIC photometry at Mount Hopkins for six classical T Tauri stars (CTTSs) and 26 weak T Tauri stars (WTTSs) in Ori OB1a and for 21 CTTSs and two WTTSs in Ori OB1b. We also obtained L-band photometry for 14 CTTSs at Mount Hopkins and 10 and 18 mum photometry with OSCIR at Gemini for six CTTSs; of these, all six were detected at 10 pm, whereas only one was detected at 18 mum. We estimate mass accretion rates from the excess luminosity at U and find that they are consistent with determinations for a number of other associations, with or without high-mass star formation. The observed decrease of mass accretion rate with age is qualitatively consistent with predictions of viscous evolution of accretion disks, although other factors can also play a role in slowing accretion rates. We compare the excesses over photospheric fluxes in H - K, K - L, and K - N with the younger sample of Taurus and find an overall decrease of disk emission from Taurus to Ori OB1b to Ori OB1a. This decrease implies that significant grain growth and settling toward the midplane has taken place in the inner disks of Ori OB1. We compare the spectral energy distribution of the star detected at both 10 and IS mum with disk models for similar stellar and accretion parameters. We find that the low fluxes shortward of 18 mum of this Ori OB1b star cannot be due to the smaller disk radius expected from viscous evolution in the presence of the far-ultraviolet radiation fields from the OB stars in the association. Instead, we find that the disk of this star is essentially a flat disk, with little if any flaring, indicating a significant degree of dust settling toward the midplane, as expected from dust evolution in protoplanetary disks.