Surface effects and turbulent pressure

The application of the full potential of stellar seismology is made difficult by the improper modelling of the upper-most layers of solar-like stars and their influence on the modelled frequencies. Our knowledge of these so-called ‘surface effects’ has improved thanks to the use of 3D hydrodynamical simulations, however, the calculation of eigenfrequencies relies on empirical models for the description of the Lagrangian perturbation of turbulent pressure, namely: the reduced-Γ1 model (RGM) and the gas-Γ1 model (GGM). Starting from the fully compressible turbulence equations, we derived both the GGM and RGM models by using a closure to model the flux of turbulent kinetic energy. We find that both models originate from two terms: the source of turbulent pressure due to compression produced by the oscillations and the divergence of the flux of turbulent pressure. We also demonstrate that they are both compatible with the adiabatic approximation and, additionally, that they imply a number of questionable assumptions, mainly with respect to mode physics. Among other hypotheses, it is necessary to neglect the Lagrangian perturbation of the dissipation of turbulent kinetic energy into heat and the Lagrangian perturbation of buoyancy work.

The PLATO Solar-like Light-curve Simulator

Context. ESA’s PLATO space mission, to be launched by the end of 2026, aims to detect and characterise Earth-like planets in their habitable zone using asteroseismology and the analysis of the transit events. The preparation of science objectives will require the implementation of hare-and-hound exercises relying on the massive generation of representative simulated light-curves. Aims. We developed a light-curve simulator named the PLATO Solar-like Light-curve Simulator (PSLS) in order to generate light-curves representative of typical PLATO targets, that is showing simultaneously solar-like oscillations, stellar granulation, and magnetic activity. At the same time, PSLS also aims at mimicking in a realistic way the random noise and the systematic errors representative of the PLATO multi-telescope concept. Methods. To quantify the instrumental systematic errors, we performed a series of simulations at pixel level that include various relevant sources of perturbations expected for PLATO. From the simulated pixels, we extract the photometry as planned on-board and also simulate the quasi-regular updates of the aperture masks during the observations. The simulated light-curves are then corrected for instrumental effects using the instrument point spread functions reconstructed on the basis of a microscanning technique that will be operated during the in-flight calibration phases of the mission. These corrected and simulated light-curves are then fitted by a parametric model, which we incorporated in PSLS. Simulation of the oscillations and granulation signals rely on current state-of-the-art stellar seismology. Results. We show that the instrumental systematic errors dominate the signal only at frequencies below ∼20 μHz. The systematic errors level is found to mainly depend on stellar magnitude and on the detector charge transfer inefficiency. To illustrate how realistic our simulator is, we compared its predictions with observations made by Kepler on three typical targets and found a good qualitative agreement with the observations. Conclusions. PSLS reproduces the main properties of expected PLATO light-curves. Its speed of execution and its inclusion of relevant stellar signals as well as sources of noises representative of the PLATO cameras make it an indispensable tool for the scientific preparation of the PLATO mission.

3. Stars

Most of what we know about the Universe has been gleaned from the study of stars, and a major achievement of 20th-century science was to understand how stars work and their lifecycles from birth to death. ‘Stars’ describes this lifecycle beginning with star formation when a cloud of interstellar gas suffers a runaway of its central density. It then considers nuclear fusion, key stellar masses, and life after the main sequence when the star burns its core helium. The surfaces of stars are described along with stellar coronae and exploding stars—both core-collapse and deflagration supernovae. Finally, globular star clusters, solar neutrinos, stellar seismology, and binary stars are discussed.

Near Continuous Photometry with the Whole Earth Telescope (WET)

The Whole Earth Telescope (WET) saw first light in 1988. It was invented by scientists from the Astronomy Department, University of Texas at Austin. The idea was to generate a world-wide network of cooperating astronomical observatories to obtain uninterrupted time-series measurements of some variable stars. The technological goal was to resolve the multi-periodic oscillations in these objects into their individual components; the scientific goal was to construct accurate theoretical models of the target objects, constrained by their observed behavior, from which fundamental astrophysical parameters could be derived. This approach has been extremely successful, and has placed stellar seismology at the forefront of stellar astrophysics. The network is run as a single astronomical instrument with many operators, and the collaboration includes scientists from all continents on our planet, taking part in the observations, data reduction, analysis and theoretical interpretation. The expertise of Lithuanian astronomers in photometry, and their access to the observing station Mt. Maidanak in Uzbekistan, has been important for the success of the network.

The Whole Earth Telescope: An International Adventure in Asteroseismology

Today, we are beginning to probe the interior of stars through the new science of stellar seismology. Certain stars, ranging from our own Sun to white dwarfs, undergo natural vibrations that can be detected with sensitive time-series photometry and/or spectroscopy. Since the signal we seek is an unbroken time-series to allow determination of the vibration frequencies, data from a single-site is usually incapable of uniquely identifying the pulsation modes, no matter how large the telescope being used. In many cases, the observational goals can be achieved using small-ish telescopes in well-coordinated global networks. Here, I briefly describe the work of one such international network of observatories and scientists known as the Whole Earth Telescope (WET). With the WET, we have sounded out the interiors of a large number of nonradially pulsating stars. Over the past 14 years, WET has observed dozens of stars in 20 separate observing campaigns. Our team has wide span of interests, and has observed several other classes of objects such as delta Scuti stars, CV stars, pulsating sdB stars, and rapidly oscillating Ap stars.