Spinning-down RU Lup. Constraints on the physics of the outflow from high-resolution spectroscopy

TL;DR

This study uses ultra-high-resolution ESPRESSO spectra to dissect RU Lup's outflow into multiple components, combining forbidden-line kinematics with a rotating-absorber ring model to trace wind launching regions and angular-momentum removal. It finds a stratified, warm disk wind launched from an inner annulus $r_0 \lesssim 6.76\,R_{\star}$, comprising a low-velocity component (LVA) and a higher-velocity component (MVA) with distinct ionization and toroidal speeds, plus a large-scale HVC/jet knot. The LVC-BC traces a conical wind from the inner disk near the truncation radius, while the HVA links to the outer jet; together these components indicate substantial mass loading and angular-momentum extraction, potentially balancing accretion spin-up. The results disfavor a pure X-wind scenario and support a picture in which episodic, magnetized ejections from the inner disk regulate the stellar rotation, with MVA capable of removing a sizable fraction of the accretion torque depending on ionization.

Abstract

Magnetic winds are a key mechanism for angular momentum removal in young stars. In this work, we aim at characterizing the multi-component outflow of RU Lup. The unprecedented high resolution of the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) enabled 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 wind launching radius, magnetic lever arm, and mass-loss rate. We identified 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 showed that the discrete absorption components trace spatially and physically distinct regions of the outflow. The medium and low velocity components are launched from the inner disk (< 6.76 stellar radii) with low lever arms indicative of warm, highly mass-loaded streamlines. However, the two components differ mainly in vertical velocity. The low velocity absorption is consistent with an outer absorbing shell, while 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. In conclusion, our work shows that 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 amount of the stellar spin-up torque. The observations disfavor an X-wind scenario.

Figures (17)

• Figure 1: Photospheric subtracted profiles of the [O i] 5577, [O i] 6300, and [S ii] 6731 lines in the ES 22.5 spectrum of RU Lup, and best-fitting combinations of Gaussian functions.
• Figure 2: Ca ii H & K and Na i D$_2$ & D$_1$ doublets in the ES 22.5 spectrum of RU Lup. The Ca ii H & K lines are plotted in velocity relative to their rest wavelengths, marked with a vertical dashed line. The Na i doublet is plotted in velocity relative to the rest wavelength of the D$_2$ line. The line centers of each line are indicated by the vertical dashed lines.
• Figure 3: Discrete absorption components observed for the Na i D$_2$ (green) and Ca ii K (blue) lines in the ES 22.5 spectrum of RU Lup. The shaded areas mark the velocity ranges where absorption is observed.
• Figure 4: Variability of the Ca ii K and Na i D$_2$ lines in the high resolution ESPRESSO spectra (Table \ref{['tab:log_specobs']}). The shaded areas mark the velocity ranges where absorption is observed. These ranges are the same as in Fig. \ref{['fig:CaII_vs_NaI']} for the spectra from 2022, while the velocity range of the MVA is different in 2021, being $[-150, -110] ~ \rm{km~s^{-1}}$.
• Figure 5: Comparison between the Ca ii K (blue) and [S ii] 6731 (magenta) lines for a selection of spectra of RU Lup. From the top panel to the bottom panel, the spectra are from UVES, ESPaDOnS, and ESPRESSO. The Ca ii K lines were scaled by $1/40$. The shaded areas trace the evolution of the two components, while the vertical lines mark the peak $v_{\rm rad}$ of the HVC.