FAUST XXVI. The dust opacity spectral indices of protostellar envelopes bridge the gap between interstellar medium and disks

Abstract

The sub-millimetre dust opacity spectral index is a critical observable to constrain dust properties, such as the maximum grain size of an observed dust population. It has been widely measured at galactic scales and down to protoplanetary disks. However, because of observational and analytical challenges, quite a gap exists in measuring dust properties in the envelopes that feed newborn protostars and their disks. To fill this gap, we use sensitive dust continuum emission data at 1.2 and 3.1 mm from the ALMA FAUST Large Program and constrain the dust opacity millimetre spectral index around a sample of protostars. Our high-resolution data, along with a more refined methodology with respect to past efforts, allow us to disentangle disk and envelope contributions in the uv-plane, and thus measure spectral indices for the envelopes uncontaminated by the optically thick emission of the inner regions. First, we find that the young disks are small and optically thick. Secondly, we measure the dust opacity spectral index at envelope scales for n=11 sources: the beta of n=9 sources had never been constrained in the literature. We effectively double the number of sources for which the dust opacity spectral index beta has been measured at these scales. Third, combining the available literature measurements with our own (total n=18), we show how envelope spectral indices distribute between ISM-like and disk-like values, bridging the gap in the inferred dust evolution. Finally, we statistically confirm a significant correlation between beta and the mass of protostellar envelopes, previously suggested in the literature. Our findings indicate that the dust optical properties smoothly vary from the ISM, through envelopes and all the way down to disks. Multi-wavelength surveys are needed to further this study and make more general claims on dust evolution in its pathway from cloud to disks.

Accepted for publication in A&A (June 2025). Main text: 15 pp. (including 12 Fig., 4 Tab.); Appendix: 10 pp (Figures./Tables)

References in corpus (47)

• CASA, the Common Astronomy Software Applications for Radio Astronomy
• An Ice Age JWST inventory of dense molecular cloud ices
• Dust masses of young disks: constraining the initial solid reservoir for planet formation
• Four annular structures in a protostellar disk less than 500,000 years old
• The Radial Distribution of Dust Particles in the HL Tau Disk from ALMA and VLA Observations
• Early Planet Formation in Embedded Disks (eDisk). I. Overview of the Program and First Results
• The anomalously low (sub)millimeter spectral indices of some protoplanetary disks may be explained by dust self-scattering
• Grain growth in the envelopes and disks of Class I protostars
• Dust growth and evolution in protoplanetary disks
• Class 0 Protostars in the Perseus Molecular Cloud: A Correlation Between the Youngest Protostars and the Dense Gas Distribution
• Characterizing the dust content of disk substructures in TW Hya
• Subsonic Mechanical Alignment of Irregular Grains
• Low dust emissivities and radial variations in the envelopes of Class 0 protostars: a signature of early grain growth?
• Outflows, infall and evolution of a sample of embedded low-mass protostars. The William Herschel Line Legacy (WILL) survey
• Hot Corinos Chemical Diversity: Myth or Reality?
• Multi-wavelength continuum sizes of protoplanetary discs: scaling relations and implications for grain growth and radial drift
• Molecular outflow launched beyond the disk edge
• Probing changes of dust properties along a chain of solar-type prestellar and protostellar cores in Taurus with NIKA
• "Ash-fall" induced by molecular outflow in protostar evolution
• Effects of scattering, temperature gradients, and settling on the derived dust properties of observed protoplanetary disks
• Dust coagulation feedback on magnetohydrodynamic resistivities in protostellar collapse
• Porous Dust Particles in Protoplanetary Disks: Application to the HL Tau Disk
• FAUST II. Discovery of a Secondary Outflow in IRAS 15398-3359: Variability in Outflow Direction during the Earliest Stage of Star Formation?
• Population Synthesis Models Indicate a Need for Early and Ubiquitous Disk Substructures
• FAUST III. Misaligned rotations of the envelope, outflow, and disks in the multiple protostellar system of VLA 1623$-$2417
• Revealing the dust grain size in the inner envelope of the Class I protostar Per-emb-50
• A systematic study of radiative torque grain alignment in the diffuse interstellar medium
• Early planet formation in embedded protostellar disks: Setting the stage for the first generation of planetesimals
• FAUST VIII. The protostellar disk of VLA 1623-2417 W and its streamers imaged by ALMA
• FAUST IX. Multi-band, multi-scale dust study of L1527 IRS. Evidence for dust properties variations within the envelope of a Class 0/I YSO
• FAUST XVII: Super deuteration in the planet forming system IRS 63 where the streamer strikes the disk
• Chemical and Physical Characterization of the Isolated Protostellar Source CB68: FAUST. IV
• The centimeter emission from planet-forming disks in Taurus
• A CO (J=3-2) Outflow Survey of the Elias 29 Region
• Accuracy of ALMA estimates of young disk radii and masses. Predicted observations from numerical simulations
• The Mechanical Alignment of Dust (MAD) I: On the spin-up process of fractal grains by a gas-dust drift
• Mixing is easy: New insights for cosmochemical evolution from pre-stellar core collapse
• FAUST XX. The chemical structure and temperature profile of the IRAS 4A2 hot corino at 20-50 au
• IRAS4A1: Multi-wavelength continuum analysis of a very flared Class 0 disk
• FAUST VI. VLA 1623--2417 B: a new laboratory for astrochemistry around protostars on 50 au scale
• FAUST. XVIII. Evidence for annular substructure in a very young Class 0 disk
• The warm CO gas along the UV-heated outflow cavity walls: a possible interpretation for the Herschel/PACS CO spectra of embedded YSOs
• Early Planet Formation in Embedded Disks (eDisk) XVI: An asymmetric dust disk driving a multi-component molecular outflow in the young Class 0 protostar GSS30 IRS3
• FAUST X: Formaldehyde in the Protobinary System [BHB2007] 11: Small Scale Deuteration
• Dust evolution during the protostellar collapse: influence on the coupling between the neutral gas and the magnetic field
• Dust dynamics in current sheets within protoplanetary disks. I. Isothermal models including ambipolar diffusion and Ohmic resistivity
• Seeing the unseen: a method to detect unresolved rings in protoplanetary disks

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• FAUST. XXVIII. High-Resolution ALMA Observations of Class 0/I Disks: Structure, Optical Depths, and Temperatures
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