A tell-tale tracer for externally irradiated protoplanetary disks: Comparing the [C I] 8727 Å line and ALMA observations in proplyds^★

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
² Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
³ Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
⁴ Space Science Institute, Boulder, CO 80301, USA
⁵ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
⁶ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁷ Department of Physics, Institute for Computational Cosmology, University of Durham, South Road, Durham DH1 3LE, UK
⁸ Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA
⁹ Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
¹⁰ Instituto de Astrofísica e Ciências do Espaco, Universidade de Lisboa, OAL, Tapada da Ajuda, 1349-018 Lisboa, Portugal
¹¹ Université Cote d’Azur, Observatoire de la Cote d’Azur, CNRS, Laboratoire Lagrange, 06300 Nice, France
¹² Universi´te Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹³ NASA Headquarters, 300 E Street SW, Washington, DC 20546, USA
¹⁴ Imperial Astrophysics, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK

^★★ Corresponding author; mariliis.aru@eso.org

Received: 31 July 2024
Accepted: 25 October 2024

Abstract

The evolution of protoplanetary disks in regions with massive OB stars is influenced by externally driven winds that deplete the outer parts of these disks. The winds have previously been studied via forbidden oxygen emission lines, which also arise in isolated disks in low-mass star-forming regions (SFRs) with weak external UV fields in photoevaporative or magnetic (internal) disk winds. It is crucial to determine how to disentangle external winds from internal ones. Here, we report a proxy for unambiguously identifying externally driven winds with a forbidden line of neutral atomic carbon, [C I] 8727 Å. We compare for the first time the spatial location of the emission in the [O I] 5577 Å, [O I] 6300 Å, and [C I] 8727 Å lines traced by VLT/MUSE-NFM with the ALMA Band 7 continuum disk emission in a sample of 12 proplyds in the Orion Nebula Cluster (ONC). We confirm that the [O I] 5577 Å emission is co-spatial with the disk emission, whereas that of [O I] 6300 Å is emitted both on the disk surface and on the ionization front of the proplyds. We show for the first time that the [C I] 8727 Å line is also co-spatial with the disk surface in proplyds, as seen in the MUSE and ALMA data comparison. The peak emission is compatible with the stellar location in all cases, apart from one target with high relative inclination with respect to the ionizing radiation, where the peak emission is located at the disk edge in the direction of the ionizing radiation. To verify whether the [C I] 8727 Å line is detected in regions where external photoevaporation is not expected, we examined VLT/X-Shooter spectra for young stars in low-mass SFRs. Although the [O I] 5577 Å and 6300 Å lines are well detected in all these targets, the total detection rate is ≪10% in the case of the [C I] 8727 Å line. This number increases substantially to a ∼40% detection rate in σ-Orionis, a region with higher UV radiation than in low-mass SFRs, but lower than in the ONC. The spatial location of the [C I] 8727 Å line emission and the lack of its detection in isolated disks in low-mass SFRs strongly suggest that this line is a tell-tale tracer of externally driven photoevaporative winds, which agrees with recent excitation models.