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We present the combination of ALMA-IMF and single-dish continuum images from the MUSTANG-2 Galactic Plane Survey (MGPS90) at 3 mm and the Bolocam Galactic Plane Survey (BGPS) at 1 mm. Six and 10 out of the 15 ALMA-IMF fields are combined with MGPS90 and BGPS, respectively. The combination is made via the feathering technique. We used thedendrogramalgorithm throughout the combined images, and performed further analysis in the six fields with the combination in both bands (G012.80, W43-MM1, W43-MM2, W43-MM3, W51-E, W51-IRS2). In these fields, we calculated spectral index maps and used them to separate regions dominated by dust or free–free emission, and then performed further structural analysis. We report the basic physical parameters of the dust-dominated (column densities, masses) and ionized (emission measures, hydrogen ionization photon rates) structures. We also searched for multiscale relations in the dust-dominated structures across the analyzed fields, finding that the fraction of mass in dendrogram leaves (which we label leaf mass efficiency (LME)) as a function of molecular gas column density follows a similar trend: a rapid, exponential-like growth, with maximum values approaching 100% in most cases. The observed behavior of the LME with the gas column is tentatively interpreted as an indicator of large star formation activity within the ALMA-IMF protoclusters. W51-E and G012.80 stand out as cases with comparatively large and reduced potential for further star formation, respectively.

 

Abstract Crescent-shaped structures in transition disks hold the key to studying the putative companions to the central stars. The dust dynamics, especially that of different grain sizes, is important to understanding the role of pressure bumps in planet formation. In this work, we present deep dust continuum observation with high resolution toward the Oph IRS 48 system. For the first time, we are able to significantly trace and detect emission along 95% of the ring crossing the crescent-shaped structure. The ring is highly eccentric with an eccentricity of 0.27. The flux density contrast between the peak of the flux and its counterpart along the ring is ∼270. In addition, we detect a compact emission toward the central star. If the emission is an inner circumstellar disk inside the cavity, it has a radius of at most a couple of astronomical units with a dust mass of 1.5 × 10 −8 M ⊙ , or 0.005 M ⊕ . We also discuss the implications of the potential eccentric orbit on the proper motion of the crescent, the putative secondary companion, and the asymmetry in velocity maps. 

The size of dust grains, a, is key to the physical and chemical processes in circumstellar discs, but observational constraints of grain size remain challenging. (Sub)millimetre continuum observations often show a per cent-level polarization parallel to the disc minor axis, which is generally attributed to scattering by ${\sim}100\, \mu{\rm m}$-sized spherical grains (with a size parameter x ≡ 2$\pi$a/λ < 1, where λ is the wavelength). Larger spherical grains (with x greater than unity) would produce opposite polarization direction. However, the inferred size is in tension with the opacity index β that points to larger mm/cm-sized grains. We investigate the scattering-produced polarization by large irregular grains with a range of x greater than unity with optical properties obtained from laboratory experiments. Using the radiation transfer code, RADMC-3D, we find that large irregular grains still produce polarization parallel to the disc minor axis. If the original forsterite refractive index in the optical is adopted, then all samples can produce the typically observed level of polarization. Accounting for the more commonly adopted refractive index using the DSHARP dust model, only grains with x of several (corresponding to ∼mm-sized grains) can reach the same polarization level. Our results suggest that grains in discs can have sizes in the millimetre regime, which may alleviate the tension between the grain sizes inferred from scattering and other means. Additionally, if large irregular grains are not settled to the mid-plane, their strong forward scattering can produce asymmetries between the near and far side of an inclined disc, which can be used to infer their presence.

 

Telescopes are now able to resolve dust polarization across circumstellar discs at multiple wavelengths, allowing the study of the polarization spectrum. Most discs show clear evidence of dust scattering through their unidirectional polarization pattern typically at the shorter wavelength of $\sim 870 \, \mu$m. However, certain discs show an elliptical pattern at ∼3 mm, which is likely due to aligned grains. With HL Tau, its polarization pattern at ∼1.3 mm shows a transition between the two patterns making it the first example to reveal such transition. We use the T-matrix method to model elongated dust grains and properly treat scattering of aligned non-spherical grains with a plane-parallel slab model. We demonstrate that a change in optical depth can naturally explain the polarization transition of HL Tau. At low optical depths, the thermal polarization dominates, while at high optical depths, dichroic extinction effectively takes out the thermal polarization and scattering polarization dominates. Motivated by results from the plane-parallel slab, we develop a simple technique to disentangle thermal polarization of the aligned grains T0 and polarization due to scattering S using the azimuthal variation of the polarization fraction. We find that, with increasing wavelength, the fractional polarization spectrum of the scattering component S decreases, while the thermal component T0 increases, which is expected since the optical depth decreases. We find several other sources similar to HL Tau that can be explained by azimuthally aligned scattering prolate grains when including optical depth effects. In addition, we explore how spirally aligned grains with scattering can appear in polarization images.