Letter to the Editor

FAUST

XXIV. Large dust grains in the protostellar outflow cavity walls of the Class I binary L1551 IRS5

¹ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
² National Radio Astronomy Observatory, PO Box O Socorro, NM 87801, USA
³ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Region Metropolitana de Santiago, Chile
⁴ Center for Astrochemical Studies, Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstr. 1, 85748 Garching, Germany
⁵ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶ RIKEN Cluster for Pioneering Research, 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan
⁷ Dipartimento di Fisica e Astronomia “Augusto Righi”, Viale Berti Pichat 6/2, Bologna, Italy
⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany
⁹ Graduate School of Informatics and Engineering, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
¹⁰ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹¹ Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Spain
¹² NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada
¹³ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada
¹⁴ Instituto de Radioastronomía y Astrofísica, UNAM, Apartado Postal 3-72, Morelia, 58090 Michoacán, Mexico
¹⁵ Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
¹⁶ David Rockefeller Center for Latin American Studies, Harvard University, 1730 Cambridge Street, Cambridge, MA 02138, USA
¹⁷ Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research, Wako, Saitama 351-0198, Japan
¹⁸ SOKENDAI (The Graduate University for Advanced Studies), Shonan Village, Hayama, Kanagawa 240-0193, Japan

^⋆ Corresponding author: giovanni.sabatini@inaf.it

Received: 25 March 2025
Accepted: 15 May 2025

Abstract

Context. Planet formation around young stars requires the growth of interstellar dust grains from micron-sized (μm-sized) particles to kilometre-sized (km-sized) planetesimals. Numerical simulations have shown that large (mm-sized) grains found in the inner envelope of young protostars could be lifted from the disc via winds. However, we are still lacking unambiguous evidence for large grains in protostellar winds and outflows.

Aims. We investigated dust continuum emission in the envelope of the Class I binary L1551 IRS5 in the Taurus molecular cloud, aiming to identify observational signatures of grain growth, such as variations in the dust emissivity index (β_mm).

Methods. In this context, we present new, high-angular resolution (50 au) observations of thermal dust continuum emission at 1.3 mm and 3 mm in the envelope (∼3000 au) of L1551 IRS5, obtained as part of the ALMA-FAUST Large Program.

Results. We analysed dust emission along the cavity walls of the CO outflow, extended up to ∼1800 au. We found an H₂ volume density > 2 × 10⁵ cm⁻³, a dust mass of ∼58 M_⊕, and β_mm ≲ 1, implying the presence of grains ∼10³ times larger than typical sizes for the interstellar medium (ISM).

Conclusions. We present the first spatially resolved observational evidence of large grains within an outflow cavity wall. Our results suggest that these grains have been transported from the inner disc to the envelope by protostellar winds and may subsequently fall back into the outer disc by gravity and/or via accretion streamers. This cycle provides longer time for grains to grow, demonstrating their crucial role in the formation of planetesimals.