Binarity as a key factor in protoplanetary disk evolution:: Spitzer disk census of the η Chamaeleontis cluster

The formation of planets is directly linked to the evolution of the circumstellar (CS) disk from which they are born. The dissipation timescales of CS disks are therefore of direct astrophysical importance in evaluating the time available for planet formation. We employ Spitzer Space Telescope spectra to complete the CS disk census for the late-type members of the similar or equal to 8 Myr old eta Chamaeleontis star cluster. Of the 15 K- and M-type members, eight show excess emission. We find that the presence of a CS disk is anticorrelated with binarity, with all but one disk associated with single stars. With nine single stars in total, about 80% retain a CS disk. Of the six known or suspected close binaries, the only CS disk is associated with the primary of RECX 9. No circumbinary disks have been detected. We also find that stars with disks are slow rotators with surface values of specific angular momentum j= 2-15 j(circle dot). All high specific angular momentum systems with j=20-30j(circle dot) are confined to the primary stars of binaries. This provides novel empirical evidence for rotational disk locking and again demonstrates the much shorter disk lifetimes in close binary systems compared to single-star systems. We estimate the characteristic mean disk dissipation timescale to be similar to 5 and approximate to 9 Myr for the binary and single-star systems, respectively.