Please see my ADS results for papers that go more in depth.
Stars in pairs can change each other’s lives
How many stars in the sky are more than what they seem? Open clusters are groups of stars all born at the same time and out of the same material. This makes open clusters excellent laboratories for learning about stellar evolution, and also provides opportunities to find the stars that defy our expectations of how we believe stars should evolve.
When we take a complete census of old open clusters we find that 25% of the evolved stars look odd. These stars have colors and luminosities that do not make sense given their age, given how we know single stars should evolve. We know that something must have happened in the histories of these stars to create the colors and luminosities we observe today.
If 25% of evolved stars follow some alternative pathway in stellar evolution they aren’t weird or anomalous at all, but in fact are a key subgroup of evolved stellar populations! My work focuses on figuring out the formation and future evolution of these stellar products so that we can move toward a more comprehensive understanding of stellar evolution.
Blue Straggler Stars
Blue stragglers are stars that appear younger than they actually are. We can tell the age of an open cluster by looking at the highest mass star that is still burning hydrogen. More massive stars evolve faster than less massive stars because they run through their fuel more quickly. Stars as massive as blue stragglers should have run out of hydrogen in their cores and evolved to become red giant stars (the fate of our Sun in ~5 billion years), but they have not! So, blue stragglers look young and are “straggling” behind their expected evolution. How could this happen?
Essentially, blue stragglers form by taking a main sequence star in a cluster and adding mass to it in some way. But the real question is how that mass is added. The first discovery of blue stragglers occurred over 60 years ago, but we’re just now starting to address this issue in detail.
My study of the blue straggler stars in the old open cluster NGC 188 revealed that two-thirds of the blue stragglers formed through mass transfer. This means almost every blue straggler in a wide binary system has a mass transfer formation history. In general, an asymptotic giant branch star (one of the last stages of stellar evolution) transferred material onto a main sequence companion (like our Sun), leaving behind a white dwarf (the core of the asymptotic giant branch star) with a blue straggler. We detected the white dwarf companions with the Hubble Space Telescope using far-ultraviolet photometry.
We also know the mass transfer happened quite recently, so we can place important constraints on the binary systems before mass transfer started. These constraints will help improve theoretical models of mass transfer, and also inform our understanding of the physics involved in creating these systems.
Sub-subgiant Stars
Subgiant stars are stars that have left the main sequence and are on their way to becoming red giant stars. Sub-subgiant stars are subgiants that are a bit too faint and a bit too red than we expect them to be, sitting below the subgiant branch in a cluster color-magnitude diagram. Most sub-subgiants are in relatively close binary systems and have active magnetic fields because of their close companions. Those active magnetic fields cause the surface of the sub-subgiants to be impacted by starspots.
Our Sun has starspots from time to time, but we think that these sub-subgiants may have over 30% of their entire surface covered with spots at once! By studying the amount of spots we can better understand the impact of the magnetic activity on the stellar structure and evolution of these stars.