Intrinsic Near-Infrared Excesses of T Tauri Stars: Understanding the Classical T Tauri Star Locus

Abstract

We re-examine the observed near-infrared properties of T Tauri stars and interpret them with the aid of accretion disk models. Based upon a careful analysis of the dereddened near-infrared colors, we find that T Tauri stars exhibit a surprisingly narrow range in (J-H)/(H-K) and (H-K)/(K-L) color-color diagrams. We find that accretion disk models with a range of accretion rates (10(-8) Msun yr(-1) < Mdot < 10(-6) Msun yr(-1) ), inner-disk radii (1-6 R_*), and viewing angles can account for the distribution of intrinsic near-infrared excesses. If the assumptions upon which our models are based are correct, we find that: i) the disk accretion rates needed to explain the observations are consistent with those inferred from optical spectroscopic studies; and ii) inner-disk holes are required in order to explain the range of observed intrinsic near-infrared excesses. Our model results suggest that a given near-IR excess requires a minimum disk accretion rate, though larger accretion rates can be accommodated with specific combinations of inner disk hole size and viewing inclinations. Further, we combine optical veiling measurements, knowledge of stellar SEDs and reddening, in order to estimate infrared photometric flux excesses for our sample of T Tauri stars. Assuming a distribution of mass accretion rates inferred from optical veiling studies, and a random distribution of viewing inclinations, we examine the plausible range of inner-disk hole sizes. The absence of inner-disk holes predicts near-IR excesses larger than those observed. Inner-disk holes exclusively > 8R_* are inconsistent with the observations, whereas hole sizes between 2-6 R_* are well-matched to the data. Finally we examine the impact of our results on attempts to characterize the accretion properties of embedded young star clusters.