ALMA observations of the early stages of substellar formation in the Lupus 1 and 3 molecular clouds

¹ European Southern Observatory, Av. Alonso de Córdova 3107, Casilla, 19001 Santiago, Chile
e-mail: asantama@eso.org
² Instituto de Física y Astronomía, Universidad de Valparaíso, Av. Gran Bretaña 1111, Casilla, 5030 Valparaíso, Chile
³ Núcleo Milenio Formación Planetaria – NPF, Valparaíso, Chile
⁴ National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22903, USA
⁵ Dpto. Astrofísica, Centro de Astrobiología (INTA-CSIC), ESAC Campus, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, 28692 Villanueva de la Cañada, Spain
⁶ Joint ALMA Observatory, Av. Alonso de Córdova 3107, Casilla, 19001 Santiago, Chile
⁷ Departamento de Física, Universidad Técnica Federico Santa María, Av. España 1680, Valparaíso, Chile
⁸ CENTRA, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal
⁹ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, PO Box 3-72 58090 Morelia, Michoacán, Mexico
¹⁰ Herzberg Astronomy and Astrophysics Research Centre, 5071 West Saanich Rd., Victoria, BC V9E 2E7, Canada
¹¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

Received: 14 September 2020
Accepted: 17 November 2020

Abstract

Context. The dominant mechanism leading to the formation of brown dwarfs (BDs) remains uncertain. While the census of Class II analogs in the substellar domain continues to grow, the most direct keys to formation, which are obtained from younger objects (pre-BD cores and proto-BDs), are limited by the very low number statistics available.

Aims. We aim to identify and characterize a set of pre- and proto-BDs as well as Class II BDs in the Lupus 1 and 3 molecular clouds to test their formation mechanism.

Methods. We performed ALMA band 6 (1.3 mm) continuum observations of a selection of 64 cores previously identified from AzTEC/ASTE data (1.1 mm), along with previously known Class II BDs in the Lupus 1 and 3 molecular clouds. Surveyed archival data in the optical and infrared were used to complement these observations. We expect these ALMA observations prove efficient in detecting the youngest sources in these regions, since they probe the frequency domain at which these sources emit most of their radiation.

Results. We detected 19 sources from 15 ALMA fields. Considering all the pointings in our observing setup, the ALMA detection rate was ∼23% and the derived masses of the detected sources were between ∼0.18 and 124 M_Jup. We classified these sources according to their spectral energy distribution as 5 Class II sources, 2 new Class I/0 candidates, and 12 new possible pre-BD or deeply embedded protostellar candidates. We detected a promising candidate for a Class 0/I proto-BD source (ALMA J154229.778−334241.86) and inferred the disk dust mass of a bona fide Class II BD. The pre-BD cores might be the byproduct of an ongoing process of large-scale collapse. The Class II BD disks follow the correlation between disk mass and the mass of the central object that is observed at the low-mass stellar regime.

Conclusions. We conclude that it is highly probable that the sources in the sample are formed as a scaled-down version of low-mass star formation, although disk fragmentation may be responsible for a considerable fraction of BDs.