The<i>Spitzer</i>c2d Survey of Weak‐Line T Tauri Stars. II. New Constraints on the Timescale for Planet Building

Abstract

One of the central goals of the Spitzer Legacy Project ``From Molecular Cores\nto Planet-forming Disks'' (c2d) is to determine the frequency of remnant\ncircumstellar disks around weak-line T Tauri stars (wTTs) and to study the\nproperties and evolutionary status of these disks. Here we present a census of\ndisks for a sample of over 230 spectroscopically identified wTTs located in the\nc2d IRAC (3.6, 4.5, 4.8, and 8.0 um) and MIPS (24 um) maps of the Ophiuchus,\nLupus, and Perseus Molecular Clouds. We find that ~20% of the wTTs in a\nmagnitude limited subsample have noticeable IR-excesses at IRAC wavelengths\nindicating the presence of a circumstellar disk. The disk frequencies we find\nin these 3 regions are ~3-6 times larger than that recently found for a sample\nof 83 relatively isolated wTTs located, for the most part, outside the highest\nextinction regions covered by the c2d IRAC and MIPS maps. The disk fractions we\nfind are more consistent with those obtained in recent Spitzer studies of wTTs\nin young clusters such as IC 348 and Tr 37. From their location in the H-R\ndiagram, we find that, in our sample, the wTTs with excesses are among the\nyounger part of the age distribution. Still, up to ~50% of the apparently\nyoungest stars in the sample show no evidence of IR excess, suggesting that the\ncircumstellar disks of a sizable fraction of pre-main-sequence stars dissipate\nin a timescale of ~1 Myr. We also find that none of the stars in our sample\napparently older than ~10 Myrs have detectable circumstellar disks at\nwavelengths &lt; 24 um. Also, we find that the wTTs disks in our sample exhibit a\nwide range of properties (SED morphology, inner radius, L_DISK/L*, etc) which\nbridge the gaps observed between the cTTs and the debris disk regimes.\n