Starquakes spring stellar surprises

Abstract

Don Kurtz, Simon Jeffrey and Conny Aerts describe discoveries in the new era of precision asteroseismology. We call it the “Tychonic Principle”: a revolutionary improvement in observational precision inevitably leads to discovery. For Tycho Brahe, the improvement was in precision of astromet- ric position; for us, over the decades since we were students, it has been orders of magnitude improvement in the precision of radial velocity and photometric measure- ments for stars. Three decades ago, stellar radial velocities were measured to 1 kms–1 precision; now, in the best cases they are measured to 10s of cms–1. Three decades ago, stellar light variability was measured to mmag precision (one part per thousand); now in the best cases it is measured to better than µmag precision (one part per million). Stellar astrophysics is being revo- lutionized by this new ultra-high precision.
The driving force has been the search for exoplanets. The two main techniques for exoplanet searches – radial velocity and transit measurements – require exquisite precision to detect either the tiny perturba- tive motion of a star being tugged about by a planet, or the slight drop in stellar bright- ness as a planet transits its star.
The improvement in radial velocity preci- sion came about from brilliant engineering:
1 Luminosity– effective temperature (Hertzsprung–Russell) diagram showing locations of major pulsating variables coloured roughly by spectral type, the zero-
age main sequence and horizontal branch, the Cepheid instability strip, and evolution tracks for model stars of various masses, indicated by small numbers (M⊙). Shadings represent heat-engine p modes (\\\), g modes (///) and strange modes (|||) and acoustically driven stochastic modes (≡). Rough spectral types are shown on the top axis. (Based on figures by J Christensen-Dalsgaard and then by CS Jeffery. See Jeffery & Saio 2016) of stellar apparent brightnesses – the µmag revolution – came with space missions that took photometers above the Earth’s atmos- phere. The highest precision ground-based photometry of one particular star reached 14 µmag – 14 parts per million (Kurtz et al. 2005) – in pulsation amplitude, but typically 1 mmag has been considered

old photoelectric photometers, or dozens with CCDs on the ground. The third – and for some purposes the most important
– benefit is the nearly continuous observa- tions for four years of about 150000 stars. For decades, groups of astronomers have organized campaigns to observe pulsating
stars contemporaneously

ground-based spectrographs have been placed in temperature-stabilized vacuums on vibrationally stable platforms with the light from the telescope fibre-fed to the ultra-stable spectrographs. For exoplanet

good. With the advent of the French-led ESA CoRoT mis- sion and the NASA Kepler mission, it has become pos- sible to reach µmag precision

“Stellar astrophysics is being revolutionized by this new ultra-high precision”

from observatories around the world to try to get con- tinuous measurement of the changes in stellar brightness; one project for this is called

searches the spectroscopic measurements have to provide true radial velocities to feed into Newton’s form of Kepler’s third law to determine the planets’ masses. This carries its own difficulties; at radial velocity preci- sion better than 1 ms–1 even the definition of radial velocity is interestingly complex (Lindegren & Dravins 2003). But, for astero- seismology, the fundamental data are the pulsation frequencies, with only secondary information coming from the amplitudes of the radial velocity excursions.
The improvement in the measurement

for thousands of stars simultaneously.
The planet hunters have built beautifully stable spectrographs and ultra-precise photometers in their search for exoplanets; we asteroseismologists have used those precision data for new stellar astrophysics.
Here, we are primarily showing results from the Kepler mission. For asteroseismol- ogy this mission provided multifold ben- efits over ground-based observations. The obvious one is the µmag precision; another is the observation of 200000 stars simulta- neously, instead of just one at a time with

the “Whole Earth Telescope”, (WET; see Provencal et al. 2014). But where WET can observe a single target, or a few targets, for several weeks with duty cycles (fraction of time observing out of the full time possible) of, say, 50%, Kepler observations have bet- ter than 90% duty cycles for four years for 150000 stars. For many asteroseismic tar- gets this unprecedented length of observing time has been the key to discovery.
So the planet hunters drove the technol- ogy development. We asteroseismologists have put that technology to excellent use. In