Birth and fate of hot-Neptune planets

This paper presents a consistent description of the formation and the subsequent evolution of gaseous planets, with special attention to short-period, low-mass hot-Neptune planets characteristic of μ Ara-like systems. We show that core accretion, including migration and disk evolution, and subsequent evolution, taking irradiation and evaporation into account, provides a viable formation mechanism for this type of strongly irradiated light planets. At an orbital distance a ≃ 0.1 AU, this revised core accretion model leads to the formation of planets with total masses ranging from ~14 M_⊕ (0.044 M_J) to ~400 M_⊕ (1.25 M_J). The newly born planets have a dense core of ~6 M_⊕, independent of the total mass, and heavy element enrichments in the envelope, M_Z,env/M_env , varying from 10% to 80% from the largest to the smallest planets. We examine the dependence of the evolution of the born planet on the evaporation rate due to the incident XUV stellar flux. In order to reach a μ Ara-like mass (~14 M_⊕) after ∼ 1 Gyr, the initial planet mass must range from 166 M_⊕ (~0.52 M_J) to about 20 M_⊕, for evaporation rates varying by 2 orders of magnitude, which corresponds to 90% to 20% mass loss during evolution. The presence of a core and heavy elements in the envelope affects the structure and the evolution of the planet appreciably and yields ∼ 8%-9% difference in radius compared to coreless objects of solar composition for Saturn-mass planets. These combinations of evaporation rates and internal compositions translate into different detection probabilities and thus into different statistical distributions for hot-Neptunes and hot-Jupiters. These calculations provide an observable diagnostic, namely a mass-radius-age relationship to distinguish between the present core-accretion-evaporation model and the alternative colliding core scenario for the formation of hot-Neptunes.