Pulsation timing is a cool technique where pulsations of a star are leveraged as an astrophysical clock. If the pulsator is orbited by an unseen companion, the light arrival time from the pulsator will change as it moves towards and away from the observer. As this happens, the pulsations will arrive early or late. With the pulsation phase, we can then deduce the existence and the orbit of the companion! It's like getting radial velocities for free -- except this technique is actually is biased *towards* longer-periods. I applied this technique to TESS photometry to see what we could find.
We used four years of TESS photometry of δ Scuti stars. δ Scuti stars are A or F type stars -- hotter than our Sun -- and very regular pulsators. From this search, we discovered 53 new binaries! We get precise orbital parameters, including mass functions, orbital periods, eccentricities for 24 of these systems with a new JAX-based forward model. Here are some cool examples:
The y axis represent how early or late the pulsations arrive, and the x axis is time. These binaries span the spectrum from circular (lower left) to eccentric orbits (top two), and there's even a PB2 system, where *both* stars in the binary are pulsating (lower right). When one star's pulsations arrive early, the other's arrives late (and vice versa). For this system we can obtain each star's mass individually, and for the rest of the stars we can obtain a mass function, which is related to the true mass.
One highlight in our sample is α Pictoris -- one of the brightest stars in the night sky! We present the first ever orbit for this system, incorporating Hipparcos astrometry in a novel joint fit. The α Pic system has an orbital period of 1316±2 days and α Pic B has a mass of 1.05±0.05 M☉.
Some stars in our sample appear to be much older than δ Scutis should be able to live. For binaries discovered from this technique by Kepler, we found 4 stars that are over 10 billion years old, when A or F type stars only last 2 billion years. We think this is a fascinating category of object called a blue straggler. The pulsating (primary) star was once a cooler, longer-lived star which had a massive companion. When that massive companion reached the end of its lifetime, it swelled to massive proportions and began tranferring all if its mass to the primary. The primary got "rejuvenated" by this process as it gained mass until it became an A or F type δ Scuti star. If this hypoethesis is true, we expect the companion to be a white dwarf now -- which we can test with follow up observations.