Determine the effect of multidimensional radial interstellar irradiation

Determine the sign and magnitude of the effect of radially incident interstellar radiation-field photons on the outer-disk CO photodissociation front and neutral-carbon emission, including whether the resulting carbon is predominantly neutral or ionised.

Background

The external-irradiation models illuminate independent vertical columns and omit interstellar photons entering radially through the outer disk edge. Such photons could move the CO photodissociation front inward, but their effect on neutral-carbon emission is ambiguous: they could create a narrow outer neutral-carbon rim or ionise exposed carbon into C+ and thereby make the emission more compact. A genuinely multidimensional treatment including CO self-shielding and the C/C+/CO transitions is therefore unresolved.

References

Thus, including radial ISRF irradiation could either enhance a narrow outer rim or, if the exposed carbon becomes predominantly ionised, suppress the extended neutral-carbon emission and make the distribution more compact. Because our representative model overpredicts the radial extent of , this missing radial irradiation may act in the direction needed to reduce the outer emission, but a genuine two-dimensional treatment of the ISRF, including CO self-shielding and C/C$+$/CO transitions, is required to determine the sign and magnitude of the effect.

— Thermochemical constraints on a primordial-origin of gas-rich debris disks  (2608.22781 - Mitani et al., 24 Aug 2026) in Section 4.2, “Irradiation treatment”