Is there a more easily measured property that might correlate with surface gravity, and thus be applicable to a larger sample of stars? Bastien et al.’s affirmative answer relies on the fact that stars similar to or smaller than our Sun transport energy in much of their interiors via convection. Pockets of hot gas are buoyed towards the surface, where the gas then cools and sinks. These cyclic convective motions lead to two important phenomena: granulation, a speckled appearance on the surface of a star (Figure 1), and stellar oscillations. Both lead to variations in brightness. It turns out that surface gravity has an important effect on stellar oscillations: the higher the gravity, the faster the rate of oscillations (this is why asteroseismology is able to measure gravity). Since both oscillations and granulation have their root in convection, perhaps we can learn about surface gravity by measuring the brightness variations associated with granulation?Enter KeplerIn order to detect the tiny decrease in brightness from an Earth-sized transiting exoplanet, Kepler was optimized for high-precision photometry – up to a few parts per million change in a star’s light curve. It is thus relatively easy for Kepler to detect the “mere” parts per thousand variations that are a result of granulation. It is more difficult, however, to disentangle these variations from the myriad other effects that can cause changes in flux, such as rotation and magnetic fields. Bastien et al. tackle this problem using three variables that characterize brightness variations in a novel way that isolates the role of surface gravity (Figure 2).
- Range represents the difference between maximum and minimum brightness. Younger, more active stars have larger ranges because they have bigger and more sunspots that rotate in and out of view.
- X0, “number of zero crossings”, measures the approximate rate at which variations occur. High X0 values occur when fast changes dominate (such as those due to granulation).
- F8, “8-hour flicker”, is the statistic that measures the variation in brightness on granulation timescales of several hours.
The stars in this figure have asteroseismically-determined surface gravities, as shown by the color scale. Aside from stars with high “range” values, F8 correlates well with surface gravity. The authors fit this relation with a polynomial and then use it to estimate surface gravities for 1000 Kepler stars (Figure 3). The scatter about the relation gives the uncertainty in the predicted surface gravity – only about 25%, as good as the spectroscopic method, but applicable to many more stars!
The UpshotWith more accurately determined surface gravities, we will now know much more about not just these stars, but about the planets that orbit many of them. Planet properties (mass, radius) are directly tied to stellar properties, and so knowing a host star’s surface gravity might allow us to distinguish between an Earth-sized and a Neptune-sized planet orbiting it, for example.Beyond its direct contribution, one of the most exciting aspects of this research is that Bastien et al. discovered this correlation almost by accident. They were exploring the Kepler database using Filtergraph, an online data visualization tool, to compare various observable quantities. Good data goes a long way, but a good way to look at it is just as important. And it is also clear that Kepler is far from done revolutionizing our understanding of astronomy.