My latest paper was accepted for publication on December 31, announced on the astro-ph preprint arXiv on January 5, and will appear soon in The Astrophysical Journal [UPDATE Feb 11! Rawls et al. 2016, ApJ, 818, 108]. It was written online using Authorea. I also wrote several programs in python to analyze data for this paper.
Picture two nearly-identical stars orbiting each other. Something like this:
Even though they usually appear as a single dot of light, binary stars are one of the best tools astronomers have to measure stellar properties. Thanks to the math behind gravity, we can weigh pairs of stars using the relationship between how long an orbit takes and how far apart the things are doing the orbiting. Weighing stars accurately is important because a star’s mass seals its fate. So if every star in the night sky had a secret companion star (or exoplanet!), we could wrap this up pretty neatly and move on to deeper questions about stars’ lives.
Unfortunately, only about half of stars have orbiting companions, and many of those aren’t observable because they don’t happen to be edge-on like the case shown above. This is where starquakes come in.
Weighing stars from the inside out
Some stars, including our own Sun, ring like bells. Pressure waves are excited by convection inside stars, and the waves bounce around at resonant frequencies just enough to make them pulse, or oscillate. Because an oscillating star is changing brightness ever-so-slightly, we can use regular observations of brightness versus time (from Kepler, in my case) to pull out the frequencies of oscillation. Heavy stars oscillate differently from lighter ones, and big stars oscillate differently from smaller ones. Voila—a new technique for weighing stars that doesn’t require anything in orbit!
Of course, the story doesn’t end there. While the study of starquakes (more formally known as asteroseismology) is a powerful way to characterize many stars quickly, it remains relatively untested. We don’t know how accurate of a scale we’re using when we whip out asteroseismology to weigh stars. To address this, my colleagues and I identified about twenty binary systems containing red giants. That’s the kind of star our Sun will become when it runs out of fuel in billions of years. Red giants are convenient targets for asteroseismology because they are bright and oscillate slowly. Both properties make them easier to observe than Sun-like stars. And since our red giant stars all live in binaries, we should be able to weigh them in two independent ways and compare the results.
The case of the missing oscillations
In my paper, I present a case study of two red giant stars in an eclipsing binary. From binary modeling, I show that the stars are both a little more than two times as massive as the Sun, and over eight times as large. However, I am surprised to only find a single signature of starquakes in the observations. Two similar but not-quite-identical stars should, in principle, both oscillate. The oscillation modes, pictured above, are broader and weaker than expected, too. The same physical process could be fully stopping oscillations in one star and only partially suppressing them in the other.
By harnessing many observations (both images and spectra) and modeling techniques, I thoroughly characterize both stars and investigate why only one of them appears to oscillate. I measure each star’s mass, size, temperature, chemical composition, level of magnetic activity, and tidal force strength, among other things. Then I bring in asteroseismology to see if I can tell which star is oscillating and if its story checks out.
As it turns out, the two stars in this binary are similar enough that it’s impossible to say for sure which one the oscillations belong to. Recent work has shown that magnetic fields may suppress oscillations in stars, however, so I strongly suspect the oscillating star is the less magnetically active of the pair. There may be a weak second set of oscillations, but the signal is very noisy and doesn’t appear quite where it should. Either way, the single mass and radius derived from asteroseismology is consistent with that of both stars from binary modeling.
Fraternal twins: born together, but not identical
Because oscillations bounce around inside stars, they carry information about how stellar interiors are structured. Stars of different ages have very different things happening inside: younger red giants are still fueled by hydrogen, while older ones are fueled by helium. The oscillating star in this binary appears to be in an advanced helium-burning stage of its life called the horizontal branch or secondary red clump. I verify this with stellar evolution modeling, and confirm that the two stars most likely were born, grew up, and evolved together. They are about 940 million years old.
The next step is to do a similar analysis for the other red giant binaries my team identified. We are working on two fronts: comparing masses and radii from binary modeling and asteroseismology, and using those results to investigate why about a third of red giants don’t show any oscillation behavior. Our work has important implications for understanding the composition of our Milky Way galaxy, because bright red giants are often surveyed to better understand our galaxy’s history and structure. It’s important to get their stories right.