FAUST

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

¹ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Region Metropolitana de Santiago, Chile
² Dipartimento di Fisica e Astronomia “Augusto Righi” Viale Berti Pichat 6/2, Bologna, Italy
³ Institut d’Estudis Espacials de Catalunya (IEEC), c/Gran Capita 24, 08034 Barcelona, Catalonia, Spain
⁴ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁵ National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
⁶ RIKEN Cluster for Pioneering Research, 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan
⁷ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁸ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Munchen, Germany
¹⁰ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autonóma de México, Apartado Postal 3-72, Morelia 58090, Michoacán, Mexico
¹¹ Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
¹² David Rockefeller Center for Latin American Studies, Harvard University, 1730 Cambridge Street, Cambridge, MA 02138, USA
¹³ NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada
¹⁴ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada
¹⁵ Department of Physics, National Sun Yat-Sen University, No. 70, Lien-Hai Road, Kaohsiung City 80424, Taiwan, ROC
¹⁶ Center of Astronomy and Gravitation, National Taiwan Normal University, Taipei 116, Taiwan
¹⁷ Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyoku, Tokyo 113-0033, Japan
¹⁸ Steward Observatory, 933 N Cherry Avenue, Tucson, AZ 85721, USA
¹⁹ Graduate School of Informatics and Engineering, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
²⁰ National Astronomical Observatory of Japan, Osawa 2-21-1, Mitakashi, Tokyo 181-8588, Japan
²¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²² Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
²³ Center for Gravitational Physics, Yukawa Institute for Theoretical Physics, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto-shi, Kyoto-fu 606-8502, Japan
²⁴ Department of Astronomy, Xiamen University, Xiamen, Fujian 361005, PR China
²⁵ Materials Science and Engineering, College of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan
²⁶ Institut de Radioastronomie Millimétrique, 38406 Saint-Martin d’Héres, France
²⁷ Department of Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang, Shanghai 200240, PR China
²⁸ IRAP, Université de Toulouse, CNRS, CNES, UPS, Toulouse, France
²⁹ University of Texas at Austin, Department of Astronomy, 2515 Speedway, Stop C1400, Austin, TX 78712-1205, USA
³⁰ Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstr. 1, 85741 Garching, Germany
³¹ Department of Physics and Astronomy, Rice University, 6100 Main Street, MS-108, Houston, TX 77005, USA
³² Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Straße 2, 69120 Heidelberg, Germany
³³ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
³⁴ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133, Rome, Italy
³⁵ SOKENDAI, Shonan Village, Hayama, Kanagawa 240-0193, Japan

^★ Corresponding author.

Received: 19 March 2025
Accepted: 6 June 2025

Abstract

Context. The (sub-)millimetre dust opacity spectral index (β) is a critical observable for constraining 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. Because of observational and analytical challenges, however, quite a gap exists in following the evolution of dust in the interstellar medium (ISM): we lack measures of the dust properties in the envelopes that feed newborn protostars and their disks.

Aims. To fill this gap, we used sensitive dust continuum emission data at 1.2 and 3.1 mm from the ALMA FAUST Large Program and constrained the spectral index of the submillimetre dust opacity for a sample of protostars.

Methods. Our high-resolution data, along with a method that was more refined than the methods in previous efforts, allowed us to distinguish the contributions from the disk and envelope in the uv-plane, and thus, to measure spectral indices for the envelopes that are not contaminated by the optically thick emission of the inner disk regions.

Results. The FAUST sources (n = 13) include a variety of morphologies in continuum emission: compact young disks, extended collapsing envelopes, and dusty outflow cavity walls. Firstly, we found that the young disks in our sample are small (down to < 9 au) and optically thick. Secondly, we measured the dust opacity spectral index β at envelope scales for n = 11 sources: The β of n = 9 of these sources were not constrained before. We effectively doubled the number of sources for which the dust opacity spectral index β has been measured at these scales. Thirdly, by combining the available literature measurements with our own (a total n = 18), we showed the distribution of the envelope spectral indices between ISM-like and disk-like values. This bridges the gap in the inferred dust evolution. Finally, we statistically confirmed a significant correlation between β and the mass of protostellar envelopes, as previously suggested in the literature.

Conclusions. Our findings indicate that the optical dust properties smoothly vary from the ISM (≫ 0.1 parsec) through envelopes (∼ 500–2000 au) to protoplanetary disks (< 200 au). Multi-wavelength surveys including longer wavelengths and in controlled starforming regions are needed to further this study and make more general claims about the dust evolution in its pathway from the cloud to disks.