On the Evolution of the Snow Line in Protoplanetary Discs (1207.4284v1)

Abstract: We model the evolution of the snow line in a protoplanetary disc. If the magneto-rotational instability (MRI) drives turbulence throughout the disc, there is a unique snow line outside of which the disc is icy. The snow line moves closer to the star as the infall accretion rate drops. Because the snow line moves inside the radius of the Earth's orbit, the formation of our water-devoid planet is difficult with this model. However, protoplanetary discs are not likely to be sufficiently ionised to be fully turbulent. A dead zone at the mid-plane slows the flow of material through the disc and a steady state cannot be achieved. We therefore model the evolution of the snow line also in a time-dependent disc with a dead zone. As the mass is accumulating, the outer parts of the dead zone become self gravitating, heat the massive disc and thus the outer snow line does not come inside the radius of the Earth's orbit, contrary to the fully turbulent disc model. There is a second, inner icy region, within the dead zone, that moves inwards of the Earth's orbit after a time of about 10^6 yr. With this model there is sufficient time and mass in the disc for the Earth to form from water-devoid planetesimals at a radius of 1 AU. Furthermore, the additional inner icy region predicted by this model may allow for the formation of giant planets close to their host star without the need for much migration.

Evolution of the Snow Line in Protoplanetary Discs

The paper "On the Evolution of the Snow Line in Protoplanetary Discs" by Rebecca G. Martin and Mario Livio provides a detailed analysis of the dynamics affecting the snow line within protoplanetary discs, taking into account the role of magneto-rotational instability (MRI) and the presence of dead zones. This work is particularly relevant for understanding the formation conditions of terrestrial planets, such as Earth, and the implications on their compositional characteristics, specifically concerning water content.

Overview of Key Concepts

The snow line, defined as the radial distance within a protoplanetary disc where the temperature is low enough for water to condense into ice, plays a crucial role in shaping the composition of forming planets. The paper investigates the position and evolution of this snow line under the influence of MRI-driven turbulence, which causes the snow line to shift closer to the central star over time as accretion rates decline. However, such a turbulence model suggests a snow line that falls within Earth's orbit, conflicting with Earth's current low water content.

Turbulence and Dead Zones

The authors challenge the fully turbulent disc model by considering the effect of dead zones—regions with suppressed turbulence due to low ionization, inhibiting MRI activity. These zones alter the conventional turbulence-driven flow, resulting in a time-dependent disc model that diverges from a steady-state form. The dead zone allows material to accumulate, potentially causing self-gravitation and disc heating that, contrary to the fully turbulent model, could ensure the snow line remains beyond Earth's orbit at critical formation times, thus supporting the formation of water-deprived terrestrial planets.

Numerical and Theoretical Insights

The paper employs a one-dimensional disc model simulating the evolution of the disc structure. In their analysis, the authors highlight several critical results:

• In a scenario where a dead zone exists, the snow line remains outside of Earth's orbit significantly longer than predicted by models assuming complete turbulence.
• An additional inner icy region can form within the dead zone, moving inward over time but allowing for water-devoid planetesimal formation at 1 AU.
• The snow line evolution, in the presence of a dead zone, presents conditions aligned with Earth's water content and provides a basis for the presence of large icy bodies or gas giants closer to their host star without extensive inward migration.

Implications and Future Directions

The implications of this model are substantial both theoretically and practically. It suggests that the consideration of dead zones within disc models could bridge the gap between theoretical predictions and observational data concerning terrestrial planet formation. Furthermore, the existence of an additional inner icy region brings forth the possibility of forming gas giants closer to their stars than previously theorized, potentially altering our understanding of planetary system architectures.

In summary, the authors present a compelling argument for re-evaluating traditional fully turbulent disc models by integrating dead zones to better align with observed planetary characteristics. Future research should aim to refine these models, investigate the competing processes within dead zones, and validate predictions with detailed numerical simulations and observational comparisons. This work lays a foundation for exploring the complex interplay between disc turbulence, snow line positioning, and the resultant planetary compositions within emerging solar systems.