Radiation-Hydrodynamic Models of X-Ray & EUV Photoevaporating Protoplanetary Discs (0909.4309v1)

Abstract: (Abridged) We present the first radiation-hydrodynamic model of a protoplanetary disc irradiated with an X-EUV spectrum. In a model where the total ionizing luminosity is divided equally between X-ray and EUV luminosity, we find a photoevaporation rate of 1.4e-8 M_sun/yr, which is two orders of magnitude greater than the case of EUV photoevaporation alone. Thus it is clear that the X-rays are the dominant driving mechanism for photoevaporation. This can be understood inasmuch as X-rays are capable of penetrating much larger columns (~1e22 cm^-2) and can thus effect heating in denser regions and at larger radius than the EUV can. The radial extent of the launching region of the X-ray heated wind is 1-70AU compared with the pure EUV case where the launch region is concentrated around a few AU. When we couple our wind mass-loss rates with models for the disc's viscous evolution, we find that, as in the pure EUV case, there is a photoevaporative switch, such that an inner hole develops at ~ 1 AU at the point that the accretion rate in the disc drops below the wind mass loss rate. At this point, the remaining disc material is quickly removed in the final 15-20% of the disc's lifetime. This is consistent with the 1e5 yr transitional timescale estimated from observations of T-Tauri stars. We also caution that although our mass-loss rates are high compared to some accretion rates observed in young stars, our model has a rather large X-ray luminosity of 2e30 erg/s; further modeling is required in order to investigate the evolutionary implications of the large observed spread of X-ray luminosities in T-Tauri stars.

• The paper combines radiative transfer calculations with 2D hydrodynamic simulations to model the effects of X-ray and EUV irradiation on disc photoevaporation.
• The paper finds a high mass-loss rate of 1.4 × 10⁻⁸ M☉ yr⁻¹, underscoring the dominant role of X-rays in driving disc dispersal.
• The paper demonstrates that when the accretion rate drops below the wind mass-loss rate, a photoevaporative switch quickly clears the disc, matching observed T-Tauri star timescales.

Overview of Radiation-Hydrodynamic Models of X-ray and EUV Photoevaporating Protoplanetary Discs

The paper by Owen et al. presents a detailed analysis of the radiation-hydrodynamic processes governing the photoevaporation of protoplanetary discs under X-ray and Extreme Ultraviolet (EUV) radiation. This paper is significant in understanding disc dispersal mechanisms and provides insights into the conditions essential for planet formation.

Key Concepts and Results

1. Methodology: The authors combine radiative transfer calculations with two-dimensional hydrodynamic simulations to model the effects of X-ray and EUV irradiation on protoplanetary discs. They utilize the Monte Carlo photoionization code, MOCASSIN, to compute gas temperatures, which are then integrated into a hydrodynamic framework to achieve a self-consistent flow solution.
2. Findings on Photoevaporation Rates: A notable outcome of this research is the high mass-loss rate due to photoevaporation, estimated at $1.4 \times 10^{-8}$ M$_\odot$yr$^{-1}$. This rate is significantly higher compared to that driven by EUV radiation alone, highlighting the predominance of X-rays in the photoevaporation process. The enhanced penetration of X-rays facilitates heating over a larger radial extent (1-70 AU) than EUV photons, which are constrained to a few AU.
3. Disc Evolution and Dispersal: The simulations demonstrate a transition in the disc's state, characterized by a photoevaporative "switch." This transition opens an inner cavity when the accretion rate falls below the wind mass-loss rate, precipitating an accelerated clear-out of the surviving disc material. This phenomenon supports the observationally derived transitional timescale of $\sim 10^5$ years in T-Tauri stars.
4. Implications for Pre-Main-Sequence Stars: The paper's results align with observed average accretion rates in young stars, addressing discrepancies in prior models that underestimated the impact of X-ray irradiation. The large spread in X-ray luminosities observed in T-Tauri stars necessitates further modeling to explore evolutionary implications fully.

Theoretical and Practical Implications

• Understanding Disc Lifetimes: The insights from this research suggest that X-ray irradiation is a critical factor in defining the longevity of protoplanetary discs. This has implications for estimating the time available for planet formation and the frequency of observed disc-less young stars.
• Impact on Planet Formation: The extended mass-loss profile due to X-ray irradiation underscores the significant role X-rays play in determining the inner disc environment, potentially influencing planet formation processes and the architecture of resulting planetary systems.
• Future Directions: The paper suggests future efforts should focus on extending these models across a broader range of X-ray luminosities to consider the full spectrum of potential disc evolutionary paths. Additionally, observational studies are encouraged to test model predictions by examining spectral lines indicative of X-ray irradiation effects, such as [Ne II] emission.

In summary, the research by Owen et al. advances our understanding of protoplanetary disc evolution under the influence of X-ray and EUV radiation. The findings prompt a reevaluation of previously held notions about the disc dispersal processes, emphasizing the significant contribution of X-rays and calling for expanded observational correlates. The work provides a foundational platform for future explorations into the complexities of disc environments and their role in shaping planetary systems.