The impact of pre-main sequence stellar evolution on mid-plane snowline locations and C/O in planet forming discs

Abstract

ABSTRACT We investigate the impact of pre-main sequence stellar luminosity evolution on the thermal and chemical properties of disc mid-planes. We create template disc models exemplifying initial conditions for giant planet formation for a variety of stellar masses and ages. These models include the 2D physical structure of gas as well as 1D chemical structure in the disc mid-plane. The disc temperature profiles are calculated using fully physically consistent radiative transfer models for stars between 0.5 and 3 M⊙ and ages up to 10 Myr. The resulting temperature profiles are used to determine how the chemical conditions in the mid-plane change over time. We therefore obtain gas and ice-phase abundances of the main carbon and oxygen carrier species. While the temperature profiles produced are not markedly different for the stars of different masses at early stages (≤1 Myr), they start to diverge significantly beyond 2 Myr. Discs around stars with mass ≥1.5 M⊙ become warmer over time as the stellar luminosity increases, whereas low-mass stars decrease in luminosity leading to cooler discs. This has an observable effect on the location of the CO snowline, which is located >200 au in most models for a 3 M⊙ star, but is always within 80 au for 0.5 M⊙ star. The chemical compositions calculated show that a well-defined stellar mass and age range exists in which high C/O gas giants can form. In the case of the exoplanet HR8799b, our models show that it must have formed before the star was 1 Myr old.