Spinning-down RU Lup

Constraints on the physics of the outflow from high-resolution spectroscopy^★

¹ Institut für Astronomie und Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
² INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
³ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁵ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal
⁷ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monte Porzio Catone, Italy
⁸ ASI, Italian Space Agency, via del Politecnico snc, 00133 Roma, Italy

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 March 2025
Accepted: 26 October 2025

Abstract

Context. Magnetic winds are a key mechanism for angular momentum removal in young stars.

Aims. We aim to characterize the multi-component outflow of RU Lup, link discrete absorption components in resonance lines to forbidden-line emission, and quantify the mass loading, lever arms, and torque carried by the wind.

Methods. The high resolution of the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) allowed us to perform a detailed study of the forbidden emission lines and the blueshifted absorption in the lines of the Na I and Ca II doublets, which we resolved in three discrete absorption components at low, medium, and high velocities. We developed a method that disentangles vertical and toroidal velocities in the absorption components and infers the launching radius r₀, magnetic lever arm λ, and Ṁ_wind.

Results. We identify a low-velocity broad component in the [O I] 5577 line, consistent with a rotating magnetohydrodynamic disk wind launched near the disk truncation radius. We show that the discrete absorption components trace spatially and physically distinct regions of the outflow. The high-velocity absorption component is connected to the high-velocity component of the forbidden lines and it is formed in a knot in the large-scale, low-density jet. The medium- and low-velocity components are launched from the inner disk (r₀ ≲ 6.76 R_⋆) with low lever arms indicative of warm, highly mass-loaded streamlines. The two components differ mainly in their vertical velocity. The low-velocity absorption is consistent with an outer absorbing shell, whereas the medium-velocity absorption forms near the disk truncation radius. Its higher vertical velocity is compatible with either a slightly larger lever arm or additional heating at the base of the flow. For plausible ionization levels in the inner disk, this outflow component removes a substantial fraction of the accretion spin-up torque.

Conclusions. RU Lup hosts a stratified, rotating, warm disk wind launched across a narrow annulus near the disk truncation radius, which is sufficiently mass-loaded to extract a large fraction of the stellar spin-up torque. These observations disfavor an X-wind scenario.