Sizing up solitary stars

For astronomers to compile an accurate description of an exoplanet, they have to determine the properties of the planet’s host star. Yet only a small fraction of the stars monitored by the Kepler planet-hunting spacecraft, for example, have the requisite brightness or binary partner to enable precise estimates of mass and radius. Most of the rest are described by the efficient but inexact photometric method.

Now Keivan Stassun at Vanderbilt University and colleagues have shown that stellar light curves from telescopes such as Kepler, combined with other publicly available data sources, offer a means of pinning down the radii and masses of millions of single stars. The researchers start by fitting a stellar atmosphere model to the star’s spectrum to determine its bolometric flux—that is, how much flux the star emits in the spectral waveband. Next the researchers find the blackbody spectrum that most closely matches the bolometric flux. Applying the Stefan–Boltzmann law yields the star’s angular diameter. Plugging in the star’s distance, which comes from parallax measurements by the star-surveying Gaia spacecraft, yields the star’s radius. The final piece of the puzzle relies on a 2013 discovery by Fabienne Bastien , then a member of Stassun’s group, that the magnitude of hours-long stellar brightness variations, a parameter known as flicker, can be used to estimate the surface gravity of stars. That property, when combined with a known radius, yields the mass.

Stassun and his team crunched the numbers for 525 bright, easily measured Kepler stars and found that their radius and mass estimates were precise to 10% and 25%, respectively, as shown in the graphs above. The researchers expect those percentages to drop to 3% and 10%, as a result of an upcoming Gaia data release and improved background modeling techniques for light-curve data. The team’s technique should prove useful for Kepler‘s successor, the Transiting Exoplanet Survey Satellite, which will search for exoplanets among 200 000 nearby stars once it launches next year. (K. G. Stassun et al., Astronomical Journal 155, 22, 2018 .)