The circumstellar disc in the Bok globule CB 26: Multi-wavelength observations and modelling of the dust disc and envelope

Circumstellar discs are expected to be the nursery of planets. Grain growth within such discs is the first step in the planet formation process. The Bok globule CB 26 harbours such a young disc. We present a detailed model of the edge-on circumstellar disc and its envelope in the Bok globule CB 26. The model is based on HST near-infrared maps in the I, J, H, and K bands, OVRO and SMA radio maps at 1.1mm, 1.3mm and 2.7mm, and the spectral energy distribution (SED) from 0.9 microns to 3mm. New photometric and spectroscopic data from the Spitzer Space Telescope and the Caltech Submilimeter Observatory have been obtained and are part of our analysis. Using the self-consistent radiative transfer code MC3D, the model we construct is able to discriminate parameter sets and dust properties of both its parts, namely envelope and disc. We find that the disc has an inner hole with a radius of 45 +/- 5 AU. Based on a dust model including silicate and graphite the maximum grain size needed to reproduce the spectral millimetre index is 2.5 microns. Features seen in the near-infrared images, dominated by scattered light, can be described as a result of a rotating envelope. Successful employment of ISM dust in both the disc and envelope hint that grain growth may not yet play a significant role for the appearance of this system. A larger inner hole gives rise to the assumption that CB 26 is a circumbinary disc.

💡 Research Summary

The paper presents a comprehensive multi‑wavelength study of the edge‑on circumstellar disc and its surrounding envelope in the Bok globule CB 26, aiming to characterize the disc’s structure, dust properties, and evolutionary state. High‑resolution near‑infrared images from the Hubble Space Telescope (I, J, H, K bands) reveal a classic bipolar scattering nebula with a dark lane, indicating an almost edge‑on geometry. Millimetre interferometric maps obtained with OVRO and the SMA at 1.1 mm, 1.3 mm, and 2.7 mm trace the thermal emission from the disc and the more extended envelope. The authors augment these data with a full spectral energy distribution (SED) ranging from 0.9 µm to 3 mm, incorporating new photometric and spectroscopic measurements from the Spitzer Space Telescope and the Caltech Submillimeter Observatory (CSO).

To interpret the observations, the authors employ the three‑dimensional Monte‑Carlo radiative‑transfer code MC3D. They construct a two‑component model: (1) a flared disc with a power‑law surface density Σ(r) ∝ r⁻¹·⁵, temperature T(r) ∝ r⁻⁰·⁵, outer radius ≈200 AU, and a central cavity; (2) a rotating, flattened envelope extending to ≈2000 AU. The disc’s inner cavity is required to reproduce the central depression seen in the millimetre maps; the best‑fit radius is 45 ± 5 AU. The cavity size is consistent with the dynamical clearing expected from a binary system, leading the authors to suggest that CB 26 may host a circumbinary disc.

Dust is modeled with a mixture of astronomical silicate and graphite (62 %/38 %) following an interstellar‑medium (ISM) size distribution n(a) ∝ a⁻³·⁵, with a minimum grain radius a_min = 0.005 µm. By varying the maximum grain size a_max, the authors find that a_max = 2.5 µm reproduces the observed millimetre spectral index (α ≈ 2.5) and the overall SED shape. This maximum size is only modestly larger than typical ISM grains, indicating that significant grain growth (to tens or hundreds of microns) has not yet occurred in this system.

The envelope is modeled as a rotating, flattened structure whose density falls off as r⁻¹·⁵. Scattering of stellar photons by the envelope reproduces the bright lobes seen in the near‑infrared images, while the dark lane is produced by the optically thick disc mid‑plane. The envelope’s temperature, set by external heating and re‑processed radiation, lies between 15 K and 30 K, consistent with the far‑infrared and sub‑millimetre fluxes.

Key quantitative results include: disc mass ≈0.1 M_⊙, surface density at 100 AU ≈0.1 g cm⁻², and an envelope mass comparable to the disc mass. The model successfully fits the spatial intensity profiles in all observed bands and reproduces the full SED from near‑infrared to millimetre wavelengths.

In the discussion, the authors emphasize that the dust composition in both disc and envelope can be described by unprocessed ISM‑like grains, suggesting that CB 26 is at a very early evolutionary stage where grain coagulation has not yet significantly altered the opacity. The presence of a large inner cavity, however, points to dynamical clearing, most plausibly by a binary system. This interpretation aligns with theoretical expectations that binary formation can occur early, carving out a central gap while the surrounding disc remains massive and gas‑rich.

Comparisons with other well‑studied edge‑on discs (e.g., HH 30, IRAS 04302+2247) show that CB 26’s dust grain sizes are smaller and its inner cavity larger, highlighting the diversity of early disc evolution pathways. The authors conclude that CB 26 provides a valuable laboratory for probing the onset of binary formation and the very first steps of dust evolution preceding planet formation. Future high‑resolution ALMA observations could resolve the inner cavity directly, test the binary hypothesis, and search for any signs of grain growth at sub‑AU scales.