Letters of Intent received in 2016

LoI 2018-1956
The Unstable HR Diagram: Asteroseismology at all masses and ages

Topics

Asteroseismology
Stellar structure and evolution
Stellar internal rotation
Stellar magnetism and activity
Space photometry missions
Spectroscopy and spectropolarimetry

Rationale

The diverse functions of asteroseismology include space technology. We are reaping the benefits of ultraprecise photometry and continuous time coverage from Kepler, K2, CoRoT and MOST, tasting the first fruits of BRITE Constellation, and planning for results yet to come from TESS, CHEOPS, and eventually PLATO. Results will be enhanced soon by new accurate stellar luminosity measurements from Gaia, supplemented by data from spectropolarimeters and precise radial velocity spectrometers operating on Earth. The asteroseismic tool is also equipped with powerful “apps” – new data analysis software, stellar models and spectral line databases, and metadata archives to be mined.

Although the techniques, applications and potentials of asteroseismology span a broad range, we have identified several theoretical and empirical challenges – and very exciting opportunities – which will merit particular focus two years from now, during the 2018 IAU General Assembly:

 In the 1990s, it was proposed to test pre-main-sequence (PMS) evolution models by measuring period changes in pulsating PMS stars. Two decades later, with a growing list of known pulsators monitored with space-based precise photometry, we are on the verge of such direct empirical tests of PMS evolutionary theory.

 Internal rotation of stars is finally an observational science, thanks to precision asteroseismology. The implications are profound for our understanding of stellar structure and evolution – from massive unevolved stars to red giants. We are also opening windows on the internal magnetic fields of stars, casting light on mysteries of the origins and evolution of stellar magnetic fields across the HR Diagram.

 While the interiors of stars are hidden from view, models of the detailed structures and phenomena at the surfaces of stars are surprisingly still a major limitation in our understanding of stellar astrophysics. How do `surface effects' and granulation affect observed pulsation frequencies and the fundamental parameters we hope to derive from them? What are the synergies between 1D and 3D numerical models of surface convection?

 Two decades ago, triggered by the impasse of no viable driving mechanisms to explain β Cephei pulsations, new OPAL and OP calculations of stellar opacities solved that mystery and other astrophysical paradoxes. The latest measurements of and model fits to pulsations in hot massive stars have brought us to another major impasse, where the best available opacities applied to stellar models cannot account for the data. Is asteroseismology forcing us to wage another revolution in opacity physics and stellar structure theory?

 K2 photometry of pulsating white dwarfs is transforming white dwarf seismology into a statistical science for the first time. By 2018, we’ll have a sample of a dozen, maybe dozens, of pulsating white dwarfs whose hydrogen envelope thicknesses have been measured based on their g-mode eigenspectra. Data will be in hand, with model fitting underway, to measure the C/O ratios in white dwarfs to test the predicted outcomes of the triple-alpha reaction in the cores of red giants.