- Title: Capella (α Aurigae) Revisited: New Binary Orbit, Physical Properties, and Evolutionary State
- Authors: Guillermo Torres, Antonio Claret, Kresimir Pavlovski, and Aaron Dotter
- First Author’s Institution: Harvard-Smithsonian Center for Astrophysics, USA
- Paper Status: Accepted for publication in The Astrophysical Journal
Like many familiar stars in the night sky, Capella is not exactly what it seems. This twinkling dot is not one, not two, but four individual stars! In 1936, Stearns discovered a faint pair of stars orbiting the bright Capella binary, forming a hierarchical quadruple.
While you might expect astronomers to already have a complete understanding of conspicuous stars like Capella—especially the brighter of the two binaries!—observations of this pair of giant stars have historically failed to agree with stellar evolution theories. Today’s paper by Torres et al. revisits the giants of Capella with a new suite of observations to finally address this longstanding mismatch.
Thwarted by a spinning star
To know a star’s mass is to know its fate. Unfortunately, precise masses for the two giant stars in Capella’s bright binary have eluded astronomers despite a wealth of observations. The problem boils down to the fact that the secondary star is spinning rapidly on its axis, which makes it very tricky to measure how fast it is moving as it orbits. As a result, previous studies haven’t been able to nail down its mass, evolutionary state, or age. A further complication is metallicity. While mass is the main thing that dictates the a star’s life path, chemical composition is a secondary factor, and this is also difficult to measure when a star is spinning rapidly.
To confront these problems, Torres et al. use more than 400 new spectroscopic observations over a wide range of wavelengths. The sheer number of observations and the ability to compare brightness at different wavelengths enables them to measure radial velocities and metallicity for the two giant stars much more accurately than before. This means that the bright binary’s nearly-circular 104-day orbit is finally well-constrained, and the authors determine both stars’ masses with impressive 0.3% precision. They are both about 2.5 times as massive as our Sun, but close to 10 times as large.
Capella’s life story
Now that we know the two giant stars’ masses and chemical compositions, as well as their sizes and other important properties that come from mapping orbits, it’s time to see how these measurements stack up. Torres et al. use a few different techniques to model both giant stars’ past, present, and future, one of which is shown below. The more massive (and thus more evolved) primary star is in the so-called “red clump” phase of life, where it is just beginning to fuse the helium in its core. The less massive secondary star has not yet ignited the helium in its core and is powered instead by a shell of fusing hydrogen. Both stars formed about 650 million years ago.
So, is that it? Have we uncovered all of Capella’s secrets? Not quite. One thing the authors can’t quite reconcile is why one star is spinning so much faster than the other. In addition, the observations are consistent with both stars are spinning in the same orientation as their orbit, but they can’t tell for sure. The authors also can’t determine if the orbital eccentricity is exactly zero or just close to it. (Given 650 million years, tidal forces should have had enough time to circularize the orbit, but not enough time to line up the spin and orbit axes.)
In an era when we are discovering and characterizing distant exoplanets by the dozen, it’s humbling to remember there are still some cosmic mysteries visible to the naked eye.