Pebbles in an embedded protostellar disk: the case of CB 26

Abstract

Context. Planetary cores are thought to form in proto-planetary disks via the growth of dusty solid material. However, it is unclear how early this process begins. Aims. We study the physical structure and grain growth in the edge-on disk that surrounds the ≈1 Myr old low-mass (≈0.55 M⊙) protostar embedded in the Bok globule CB26 to examine how much grain growth has already occurred in the protostellar phase. Methods. We combine the spectral energy distribution between 0.9 μm and 6.4 cm with high-angular-resolution continuum maps at 1.3, 2.9, and 8.1 mm and use the radiative transfer code RADMC-3D to conduct a detailed modeling of the dust emission from the disk and envelope of CB 26. Results. Given the presence of a central disk cavity, we infer inner and outer disk radii of 16−8+37 and 172 ± 22 au, respectively. The total gas mass in the disk is 7.610−2 M⊙, which amounts to ≈14% of the mass of the central star. The inner disk contains a compact free-free emission region, which could be related to either a jet or a photoevaporation region. The thermal dust emission from the outer disk is optically thin at millimeter wavelengths, while the emission from the inner disk midplane is moderately optically thick. Our best-fit radiative transfer models indicate that the dust grains in the disk have already grown to pebbles with diameters on the order of 10 cm in size. Residual 8.1 mm emission suggests the presence of even larger particles in the inner disk. For the optically thin millimeter dust emission from the outer disk, we derive a mean opacity slope of βmm ≈ 0.7 ± 0.4, which is consistent with the presence of large dust grains. Conclusions. The presence of centimeter-sized bodies in the CB 26 disk indicates that solids are already growing rapidly during the first million years in a protostellar disk. It is thus possible that Class II disks are already seeded with large particles and may even contain planetesimals.