Abstract
Using IRAC instrument aboard the SPITZER Space Telescope, we have obtained images in the 3.6, 4.5 5.8 and 8.0 μm bands in two regions of the sky of ∼ 1 deg^{2} each one, both located in the Orion OB1 Association: the Orion 1b (age ∼ 4^{.}10^6 years) and Orion 1a (age ∼ 10^{.}10^6 years) sub-associations. Based on first data of the CIDA Variability Survey of Orion OB1 (Briceño et al. 2005), we carried out aperture photometry to 19 objects in OB 1a and to 45 objects in OB 1b. We have constructed with this data two colors diagrams: [3.6]-[4.5] vs [5.8]-[8.0] and [3.6]-[4.5] vs [4.5]-[5.8]. We have found that a fraction of 2/19 stars in OB 1a and a fraction of 9/45 stars in OB 1b subassociations show evidences of circumstellar disks. We have detected 6 stars with IRAC colors which suggests that dust in the inner region of their disks had experimented an appreciable evolution, consistent with predictions of planetary formation models. The stars in these sub-associations have shown in general an IRAC colors deficit with respect to the Taurus objects, which suggests that dissipation processes of fine dust could develope on time scales of ∼ 2-8 millions years.
