Dead zones in protostellar discs: the case of Jet Emitting Discs

Planet formation and migration in accretion discs is a very active topic. Among the many aspects related to that question, dead zones are of particular importance as they can influence both the formation and the migration of planetary embryos. The ionisation level in the disc is the key element in determining the existence and the location of the dead zone. This has been studied either within the Standard Accretion Disc (SAD) framework or using parametric discs. In this paper, we extend this study to the case of Jet Emitting Discs (JED), the structure of which strongly differ from SADs because of the new energy balance and angular momentum extraction imposed by the jets. We make use of the (r,z) density distributions provided by self-similar accretion-ejection models, along with the JED thermal structure derived in a previous paper, to create maps of the ionisation structure of JEDs. We compare the ionisation rates we obtain to the critical value required to trigger the magneto-rotational instability. It is found that JEDs have a much higher ionisation degree than SADs which renders very unlikely the presence of a dead zone in these discs. As JEDs are believed to occupy the inner regions of accretion discs, the extension of the dead zones published in the literature should be re-considered for systems in which a jet is present. Moreover, since JEDs require large scale magnetic fields close to equipartition, our findings raise again the question of magnetic field advection in circumstellar accretion discs.

💡 Research Summary

The paper investigates whether the “dead zones” that are commonly predicted in the inner regions of protoplanetary disks—regions where the ionisation level is too low to sustain the magnetorotational instability (MRI) and thus where turbulent viscosity is quenched—also exist in Jet Emitting Discs (JEDs). JEDs differ fundamentally from the Standard Accretion Disc (SAD) paradigm because a substantial fraction of the disc’s angular momentum and energy is extracted vertically by a magnetocentrifugally driven jet. This changes the disc’s density, temperature, and magnetic field structure, and consequently its ionisation balance.

The authors adopt self‑similar Magnetised Accretion‑Ejection Structures (MAES) solutions that provide full (r, z) density and temperature profiles for a JED that launches a jet with realistic mass‑loading and velocity characteristics. The chosen solution has an ejection efficiency ξ≈0.04, a disc aspect ratio ε≈0.03, and a magnetic plasma‑β close to unity, implying a strong, near‑equipartition magnetic field. The JED is cooler, thinner, and less dense than an equivalent SAD for the same stellar mass and accretion rate.

Ionisation is assumed to be dominated by stellar X‑rays, because cosmic rays are likely excluded by the strong outflows that accompany JEDs. The X‑ray ionisation rate ζ_X is computed using the standard formulation that includes the source luminosity L_X, photon energy kT_X, geometric dilution (distance d), and an attenuation factor J(x₀, τ) that depends on the line‑of‑sight column density N_H and the energy‑dependent absorption cross‑section σ(kT_X). The authors adopt typical T Tauri X‑ray parameters (L_X≈10³⁰ erg s⁻¹, kT_X≈5 keV) and calculate τ from the MAES density structure. Because the upper layers of a JED are low‑density, X‑rays penetrate deeper than in a SAD, leading to ionisation rates that are several times to an order of magnitude higher at a given radius.

Recombination is treated only in the gas phase. The authors solve the coupled rate equations for electrons, molecular ions, and metal ions, using temperature‑dependent two‑body recombination coefficients β. Grain surface recombination is deliberately omitted: in the innermost disc (≲0.3 AU) where JEDs are expected, dust grains are either sublimated or have settled to the mid‑plane, making gas‑phase chemistry the dominant recombination channel.

Balancing ionisation and recombination yields the electron fraction x_e = n_e / n_H. For the JED models, x_e remains in the range 10⁻⁸–10⁻⁷ throughout the jet‑launching region (0.04–0.3 AU). This exceeds by many orders of magnitude the critical ionisation fraction required for MRI (≈10⁻¹³–10⁻¹²). By contrast, equivalent SAD calculations produce x_e values that fall below the critical threshold at similar radii, reproducing the classic dead‑zone picture.

The key insight is that the altered energy balance and vertical angular‑momentum extraction in JEDs raise the ionisation level sufficiently to suppress dead zones entirely. Consequently, any planet‑formation or migration scenario that relies on the presence of a low‑turbulence dead zone in the inner disc must be re‑examined for systems that host jets. The absence of a dead zone also implies that pressure bumps, Rossby‑wave vortices, or other structures that form at the dead‑zone edge will not arise in JED‑dominated regions, potentially affecting the efficiency of pebble concentration and core growth.

Finally, the authors note that JEDs require large‑scale magnetic fields close to equipartition, raising the longstanding problem of magnetic‑field advection in protoplanetary discs. Maintaining such fields against turbulent diffusion is non‑trivial, and the paper calls for further theoretical and numerical work to understand how discs can sustain the magnetic flux needed for jet launching while simultaneously remaining highly ionised.

In summary, the study demonstrates that jet‑emitting discs are intrinsically well‑ionised, lack dead zones, and therefore represent a distinct environment for early planet formation compared with the traditional viscous disc picture. This result has important implications for interpreting observations of young stellar objects, for modelling disc evolution, and for the broader theory of magnetic field transport in accretion discs.