Accretion in Evolved and Transitional Disks in Cep OB2: Looking for the Origin of the Inner Holes

TL;DR

This study measures accretion rates in solar-type stars within Cep OB2, revealing that most have evolved disks with lower accretion, and explores various processes responsible for inner disk holes in transitional objects.

Contribution

It provides the first large-scale accretion rate analysis in Cep OB2, comparing disk morphologies and accretion, and discusses different mechanisms behind inner disk clearing.

Findings

Accretion rates are about ten times lower than in younger regions.

Half of the transitional objects are non-accreting.

Multiple processes, including photoevaporation and planet formation, may cause inner disk holes.

Abstract

We present accretion rates for a large number of solar-type stars in the Cep OB2 region, based on U band observations. Our study comprises 95 members of the ~4 Myr-old cluster Tr 37 (including 20 "transition" objects; TO), as well as the only CTTS in the ~12 Myr-old cluster NGC 7160. The stars show different disk morphologies, with the majority of them having evolved and flattened disks. The typical accretion rates are about one order of magnitude lower than in regions aged 1-2 Myr, and we find no strong correlation between disk morphology and accretion rates. Although half of the TO are not accreting, the median accretion rates of normal CTTS and accreting "transition" disks are similar (~3 10^{-9} and 2 10^{-9} Msun/yr, respectively). Comparison with other regions suggests that the TO observed at different ages do not necessarily represent the same type of objects, which is consistent with the fact that the different processes that can lead to reduced IR excess/inner disk clearing (e.g., binarity, dust coagulation/settling, photoevaporation, giant planet formation) do not operate on the same timescales. Accreting TO in Tr 37 are probably suffering strong dust coagulation/settling. Regarding the equally large number of non-accreting TO in the region, other processes, like photoevaporation, the presence of stellar/substellar companions, and/or giant planet formation may account for their "transitional" SEDs and negligible accretion rates.