Dust properties and disk structure of evolved protoplanetary disks in Cep OB2: Grain growth, settling, gas and dust mass, and inside-out evolution

TL;DR

This study analyzes the dust properties and disk structures of evolved protoplanetary disks in Cep OB2, revealing significant grain growth, settling, and inside-out evolution, with implications for planet formation.

Contribution

It provides new insights into disk evolution by combining infrared spectra and millimeter observations, highlighting the prevalence of grain growth, settling, and inside-out evolution in protoplanetary disks.

Findings

Most disks show substantial grain growth and settling.

Half of the disks exhibit inside-out evolution with dust-cleared inner regions.

Transition disks contain strongly processed grains, indicating advanced dust evolution.

Abstract

We present Spitzer/IRS spectra of 31 TTS and IRAM/1.3mm observations for 34 low- and intermediate-mass stars in the Cep OB2 region. Including our previously published data, we analyze 56 TTS and the 3 intermediate-mass stars with silicate features in Tr 37 (~4 Myr) and NGC 7160 (~12 Myr). The silicate emission features are well reproduced with a mixture of amorphous (with olivine, forsterite, and silica stoichiometry) and crystalline grains (forsterite, enstatite). We explore grain size and disk structure using radiative transfer disk models, finding that most objects have suffered substantial evolution (grain growth, settling). About half of the disks show inside-out evolution, with either dust-cleared inner holes or a radially-dependent dust distribution, typically with larger grains and more settling in the innermost disk. The typical strong silicate features require nevertheless the presence of small dust grains, and could be explained by differential settling according to grain size, anomalous dust distributions, and/or optically thin dust populations within disk gaps. M-type stars tend to have weaker silicate emission and steeper SEDs than K-type objects. The inferred low dust masses are in a strong contrast with the relatively high gas accretion rates, suggesting global grain growth and/or an anomalous gas to dust ratio. Transition disks (TD) in the Cep OB2 region display strongly processed grains, suggesting that they are dominated by dust evolution and settling. Finally, the presence of rare but remarkable disks with strong accretion at old ages reveals that some very massive disks may still survive to grain growth, gravitational instabilities, and planet formation.