Theoretical astrophysics and plasma physics at RPC

Abstract:

We report the first experimental detection of a zero-frequency fluctuation that is pumped by an Alfvén mode in a magnetically confined plasma. Core-localized Alfvén modes of frequency inside the toroidicity-induced gap (and its harmonics) exhibit three-wave coupling interactions with a zero-frequency fluctuation. The observation of the zero-frequency fluctuation is consistent with theoretical and numerical predictions of zonal modes pumped by Alfvén modes, and is correlated with an increase in the deep core ion temperature, temperature gradient, confinement factor H_{89,P}, and a reduction in the main ion heat diffusivity. Despite the energetic particle transport induced by the Alfvén eigenmodes, the generation of a zero-frequency fluctuation that can suppress the turbulence leads to an overall improvement of confinement.

Abstract:

We outline the rationale for holding a Discussion Meeting thus titled at the Royal Society, London during 15 and 16 April 2024, and summarize what we learnt there. This article is part of the discussion meeting issue ‘Challenging the standard cosmological model’.

Abstract:

<jats:p><jats:italic>Context</jats:italic>. Many Milky Way globular clusters (GCs) host multiple stellar populations, challenging the traditional view that GCs are single-population systems. It has been suggested that second-generation stars could form in a disk from gas lost by first-generation stars or from external accreted gas. Understanding how these multiple stellar populations evolve under a time-varying Galactic tidal field is crucial for studying internal mixing, the rotational properties, and mass loss of GCs over cosmological timescales.</jats:p> <jats:p><jats:italic>Aims</jats:italic>. We investigated how the introduction of a second stellar generation affects mass loss’ internal mixing, and rotational properties of GCs in a time-varying Galactic tidal field and different orbital configurations.</jats:p> <jats:p><jats:italic>Methods</jats:italic>. We conducted direct <jats:italic>N</jats:italic>-body simulations of GCs on three types of orbits derived from the observed Milky Way GCs using state-of-the-art stellar evolution prescriptions. We evolved the clusters for 8 Gyr in the time-varying Galactic potential of the IllustrisTNG-100 cosmological simulation. After 2 Gyr, we introduced a second stellar generation, comprising 5% of the initial mass of the first generation, as a flattened disk of stars. For comparison, we ran control simulations using a static Galactic potential and isolated clusters.</jats:p> <jats:p><jats:italic>Results</jats:italic>. We present here the mass loss, structural evolution, and kinematic properties of GCs with two stellar generations, focusing on tidal mass’ half-mass radii, velocity distributions, and angular momentum. We also examine the transition of the second generation from a flattened disk to a spherical shape.</jats:p> <jats:p><jats:italic>Conclusions</jats:italic>. Our results show that the mass loss of GCs depends primarily on their orbital parameters, with tighter orbits leading to higher mass loss. The growth of the Galaxy led to tighter orbits’ implying that the GCs lost much less mass than if the Galaxy had always had its current mass. The initially flattened second-generation disk became nearly spherical within one relaxation time. However, whether its distinct rotational signature was retained depends on the orbit: for the long radial orbit, it vanished quickly; for the tube orbit' it lasted several billion years for the circular orbit' rotation persisted until the present day.</jats:p>