Photochemical-dynamical models of externally FUV irradiated protoplanetary disks

Photochemical-dynamical models of externally FUV irradiated protoplanetary discs

Authors:

Haworth et al

Abstract:

There is growing theoretical and observational evidence that protoplanetary disc evolution may be significantly affected by the canonical levels of far ultraviolet (FUV) radiation found in a star forming environment, leading to substantial stripping of material from the disc outer edge even in the absence of nearby massive stars. In this paper we perform the first full radiation hydrodynamic simulations of the flow from the outer rim of protoplanetary discs externally irradiated by such intermediate strength FUV fields, including direct modelling of the photon dominated region (PDR) which is required to accurately compute the thermal properties. We find excellent agreement between our models and the semi-analytic models of Facchini et al. (2016) for the profile of the flow itself, as well as the mass loss rate and location of their "critical radius". This both validates their results (which differed significantly from prior semi-analytic estimates) and our new numerical method, the latter of which can now be applied to elements of the problem that the semi--analytic approaches are incapable of modelling. We also obtain the composition of the flow, but given the simple geometry of our models we can only hint at some diagnostics for future observations of externally irradiated discs at this stage. We also discuss the potential for these models as benchmarks for future photochemical-dynamical codes.

LYα Emissions From FUV Irradiated Protoplanetary Disks

FUV IRRADIATED DISK ATMOSPHERES: LYα AND THE ORIGIN OF HOT H2 EMISSION

Authors:

Ádámkovics et al

Abstract:

Protoplanetary disks are strongly irradiated by a stellar FUV spectrum that is dominated by ${\rm{Ly}}\alpha $ photons. We investigate the impact of stellar ${\rm{Ly}}\alpha $ irradiation on the terrestrial planet region of disks (lesssim1 AU) using an updated thermal-chemical model of a disk atmosphere irradiated by stellar FUV and X-rays. The radiative transfer of ${\rm{Ly}}\alpha $ is implemented in a simple approach that includes scattering by H i and absorption by molecules and dust. Because of their non-radial propagation path, scattered ${\rm{Ly}}\alpha $ photons deposit their energy deeper in the disk atmosphere than the radially propagating FUV continuum photons. We find that ${\rm{Ly}}\alpha $ has a significant impact on the thermal structure of the atmosphere. Photochemical heating produced by scattered ${\rm{Ly}}\alpha $ photons interacting with water vapor and OH leads to a layer of hot (1500–2500 K) molecular gas. The temperature in the layer is high enough to thermally excite the ${{\rm{H}}}_{2}$ to vibrational levels from which they can be fluoresced by ${\rm{Ly}}\alpha $ to produce UV fluorescent ${{\rm{H}}}_{2}$ emission. The resulting atmospheric structure may help explain the origin of UV fluorescent ${{\rm{H}}}_{2}$ that is commonly observed from classical T Tauri stars.