Spitzer IRAC and JHK_S observations of h and _χ persei: Constraints on protoplanetary disk and massive cluster evolution at ∼10⁷ years

We describe IRAC 3.6-8 μm observations and ground-based near-IR JHK _S photometry from Mimir and 2MASS of the massive double cluster h and _χ Persei complete to J = 15.5 (M ∼ 1.3 M_⊙). Within 25′ of the cluster centers we detect ∼11,000 sources with J ≤ 15.5, ∼7000 sources with [4.5] ≤ 15, and ∼5000 sources with [8] ≤ 14.5. In both clusters the surface density profiles derived from the 2MASS data decline with distance from the cluster centers as expected for a bound cluster. Within 15′ of the cluster centers, ∼50% of the stars lie on a reddened ∼13 Myr isochrone; at 15′-25′ from the cluster centers, ∼40% lie on this isochrone. Thus, the optical/2MASS color-magnitude diagrams indicate that h and _χ Per are accompanied by a halo population with roughly the same age and distance as the two dense clusters. The double cluster lacks any clear IR excess sources for J ≤ 13.5 (∼2.7 M_⊙). Therefore, disks around high-mass stars disperse prior to ∼10⁷ yr. At least 2%-3% of the fainter cluster stars have strong IR excess at both [5.8] and [8]. About 4%-8% of sources slightly more massive than the Sun (∼1.4 M_⊙) have IR excesses at [8]. Combined with the lack of detectable excesses for brighter stars, this result suggests that disks around lower mass stars have longer lifetimes. The IR excess population also appears to be larger at longer IRAC bands ([5.8], [8]) than at shorter IRAC/2MASS bands (K_S, [4.5]), a result consistent with an inside-out clearing of disks.