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From thermal to magnetic driving: spectral diagnostics of simulation-based magnetothermal disc wind models

Michael L. Weber, Eleftheria Sarafidou, Christian Rab, Oliver Gressel, Barbara Ercolano

TL;DR

This work investigates whether spectral diagnostics can distinguish magnetically driven from thermally driven disc winds in protoplanetary systems. By post-processing three self-consistent magnetothermal wind models (β4, β6) and a purely photoevaporative case (PE) with radiative transfer, the authors generate synthetic emission maps and line profiles for [OI] 6300 Å, [NeII] 12.81 μm, and o-H2 2.12 μm, comparing them to observations. They find that strongly magnetised inner winds produce broader, more blueshifted low-velocity components, while weakly magnetised or purely photoevaporative winds reproduce the majority of observed NLVCs; some line-profile diagnostics are not robust discriminants. The study concludes that most observed NLVCs are compatible with low magnetisation or photoevaporative winds, and that a combination of line kinematics with emission morphology offers meaningful constraints on wind-driving physics.

Abstract

Disc winds driven by thermal and magnetic processes are thought to play a critical role in protoplanetary disc evolution. However, the relative contribution of each mechanism remains uncertain, particularly in light of their observational signatures. We investigate whether spatially resolved emission and synthetic spectral line profiles can distinguish between thermally and magnetically driven winds in protoplanetary discs. We modelled three disc wind scenarios with different levels of magnetisation: a relatively strongly magnetised wind ($β$4), a rather weakly magnetised wind ($β$6), and a purely photoevaporative wind (PE). Using radiative transfer post-processing, we generated synthetic emission maps and line profiles for [OI] 6300 Å, [NeII] 12.81 $\mathrmμ$m, and o-H2 2.12 $\mathrmμ$m, and compared them with observations. The $β$4 model generally produces broader and more blueshifted low-velocity components across all tracers, consistent with compact emission regions and steep velocity gradients. The $β$6 and PE models yield narrower profiles with smaller blueshifts, in better agreement with most observed narrow low-velocity components (NLVCs). We also find that some line profile diagnostics, such as the inclination at maximum centroid velocity, are not robust discriminants. However, the overall blueshift and full-width at half-maximum (FWHM) of the low-velocity components provide reliable constraints. The $β$4 model reproduces the most extreme blueshifted NLVCs in observations, while most observed winds are more consistent with the $β$6 and PE models. Our findings reinforce previous conclusions that most observed NLVCs are compatible with weakly magnetised or purely photoevaporative flows. The combination of line kinematics and emission morphology offers meaningful constraints on wind-driving physics.

From thermal to magnetic driving: spectral diagnostics of simulation-based magnetothermal disc wind models

TL;DR

This work investigates whether spectral diagnostics can distinguish magnetically driven from thermally driven disc winds in protoplanetary systems. By post-processing three self-consistent magnetothermal wind models (β4, β6) and a purely photoevaporative case (PE) with radiative transfer, the authors generate synthetic emission maps and line profiles for [OI] 6300 Å, [NeII] 12.81 μm, and o-H2 2.12 μm, comparing them to observations. They find that strongly magnetised inner winds produce broader, more blueshifted low-velocity components, while weakly magnetised or purely photoevaporative winds reproduce the majority of observed NLVCs; some line-profile diagnostics are not robust discriminants. The study concludes that most observed NLVCs are compatible with low magnetisation or photoevaporative winds, and that a combination of line kinematics with emission morphology offers meaningful constraints on wind-driving physics.

Abstract

Disc winds driven by thermal and magnetic processes are thought to play a critical role in protoplanetary disc evolution. However, the relative contribution of each mechanism remains uncertain, particularly in light of their observational signatures. We investigate whether spatially resolved emission and synthetic spectral line profiles can distinguish between thermally and magnetically driven winds in protoplanetary discs. We modelled three disc wind scenarios with different levels of magnetisation: a relatively strongly magnetised wind (4), a rather weakly magnetised wind (6), and a purely photoevaporative wind (PE). Using radiative transfer post-processing, we generated synthetic emission maps and line profiles for [OI] 6300 Å, [NeII] 12.81 m, and o-H2 2.12 m, and compared them with observations. The 4 model generally produces broader and more blueshifted low-velocity components across all tracers, consistent with compact emission regions and steep velocity gradients. The 6 and PE models yield narrower profiles with smaller blueshifts, in better agreement with most observed narrow low-velocity components (NLVCs). We also find that some line profile diagnostics, such as the inclination at maximum centroid velocity, are not robust discriminants. However, the overall blueshift and full-width at half-maximum (FWHM) of the low-velocity components provide reliable constraints. The 4 model reproduces the most extreme blueshifted NLVCs in observations, while most observed winds are more consistent with the 6 and PE models. Our findings reinforce previous conclusions that most observed NLVCs are compatible with weakly magnetised or purely photoevaporative flows. The combination of line kinematics and emission morphology offers meaningful constraints on wind-driving physics.

Paper Structure

This paper contains 19 sections, 10 equations, 11 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Methods
  3. Magnetothermal wind models
  4. Equations of motion
  5. Initial disc model
  6. X-Ray heating prescription
  7. Synthetic observables
  8. Results
  9. Density and velocity structure
  10. Temperature and ionisation structure
  11. Line luminosities & emission regions
  12. Spectral line profiles
  13. Profiles
  14. Multi-Gaussian Fits
  15. Correlation with inclination
  16. ...and 4 more sections

Figures (11)

  • Figure 1: Number density maps for the three models $\beta$4, $\beta$6 and PE (columns from left to right). Top row: The streamlines represent the velocity structure, and their colour represents the speed. White dashed lines are column density contours for the values (from top to bottom) 10$^{20}$, 10$^{21}$, 10$^{22}, 2.5\cdot10^{22}$ cm . Bottom row: White dashed lines are density contours for the values (from top to bottom) $10^6$, $5\cdot10^6$, $10^7$, and $5\cdot10^7$ c m .
  • Figure 2: Comparison of the velocity streamlines in the models $\beta$4 (red) and $\beta$6 (green).
  • Figure 3: mocassin post-processing results. Top panel: Gas temperature. The white dashed lines are contours at 100, 200, 1000, 4000, and 8000 K, smoothed with a Gaussian filter with $\sigma = 4$ to filter Monte-Carlo noise. Bottom panels: Ionisation fraction $n_\mathrm{e} / n_\mathrm{HI}$ and contours for values of $10^{-2}$, $10^{-1}$, 0.5, and 1, smoothed with a Gaussian filter with $\sigma = 2$.
  • Figure 4: $\mathrm{[OI]}\,6300\,\mathrm{\text{\normalfont\AA}\xspace}$ and [NeII] 12.81 µ m luminosities against the accretion rate in the models compared with the observations (grey symbols) by pascucci2020. Triangles indicate upper limits.
  • Figure 5: Emissivity maps of the $\mathrm{[OI]}\,6300\,\mathrm{\text{\normalfont\AA}\xspace}$ and [NeII] 12.81 µ m lines. The white dashed lines are contours enclosing the region where 80% of the total line flux is emitted in a 3D axisymmetric disc.
  • ...and 6 more figures