The envelopes of low-mass stars: the impact of Coulomb effects on stellar oscillations Ana Brito 1),2) & Ilídio Lopes 2) 2) Centro de Astrofísica e Gravitação - CENTRA, Departamento de Física, Instituto Superior Técnico, IST, Universidade de Lisboa - UL, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal | ilidio.lopes@tecnico.ulisboa.pt 1) Departamento de Matemática, Instituto Superior de Gestão, Av. Marechal Craveiro Lopes, 1700-284, Lisboa, Portugal | anabrito@isg.pt I. Introduction V. Conclusion In this work, we explore the outer layers of stars less massive than the Sun. In particular, we have computed a set of stellar models raging from 0.4 to 0.9 solar masses with the aim at determining the impact on stellar oscillations of two physical processes occurring in the envelopes of these stars. Namely, the partial ionisation of chemical elements and the electrostatic interactions between particles in the outer layers. We find that alongside with partial ionisation, Coulomb effects also impact the acoustic oscillation spectrum. The influence of Coulomb interactions on the sound-speed gradient profile produces a strong oscillatory behaviour with diagnostic potential for the future. We find that, around the value of 0.6 solar masses, the peak/dip in the first adiabatic exponent/acoustic potential corresponding to helium ionisation zones (Fig. 1) vanishes and, consequently, does not produce an oscillatory signature in the seismic diagnostic β(ν). This is thus the reference value, for models with solar metallicity, around which occurs a transition in the characteristics of the oscillatory signal imprinted in the oscillation frequencies by two distinct physical processes. The acoustic spectrum of stars more massive than 0.6 solar masses is mostly impacted by partial ionisation processes, whereas the acoustic spectrum of stars less massive than 0.6 solar masses is predominantly impacted by Coulomb interactions. II. Thermodynamic properties of the turbulent outer regions The behaviour of the first adiabatic The behaviour of the first adiabatic exponent around its monatomic ideal exponent around its monatomic ideal gas value, 5/3, is influenced by two gas value, 5/3, is influenced by two fundamental aspects. First, the increase fundamental aspects. First, the increase of the degree of ionisation decreases of the degree of ionisation decreases the Γ the Γ1 value, causing a known value, causing a known depression in the radial profile of Γ depression in the radial profile of Γ1, , particularly throughout the partial particularly throughout the partial ionisation zones of light elements, ionisation zones of light elements, hydrogen and helium. The second hydrogen and helium. The second aspect that influences the behaviour of aspect that influences the behaviour of the first adiabatic exponent relates to the first adiabatic exponent relates to the electrostatic interactions between the electrostatic interactions between charged particles, known generally as charged particles, known generally as Coulomb effects. Coulomb effects Coulomb effects. Coulomb effects increase the value of Γ increase the value of Γ1 causing it to causing it to become larger than 5/3 in some regions become larger than 5/3 in some regions of the convective zone. Also, the of the convective zone. Also, the acoustic potential is determined by the acoustic potential is determined by the model structure and, therefore, will be model structure and, therefore, will be impacted by physical processes like impacted by physical processes like partial ionisation of atomic species and partial ionisation of atomic species and electrostatic interactions between electrostatic interactions between charged particles. Fig. 1 shows these charged particles. Fig. 1 shows these two profiles for all the models. two profiles for all the models. Fig. 2 The ionic plasma interaction parameter, Γi , as a function of temperature. Also plotted in this figure, with grey colour, is the sound- speed gra