Stellar rotation in open clusters

I use open clusters to calibrate the evolution of stellar rotation. Open clusters are collections of stars that all share the same age. Because a cluster includes stars of many masses, we can define its age fairly well. I observe stars in many open clusters to see how rotation (and other properties) change over time. In particular, I focus on calibrating the evolution of rotation itself, and how magnetic fields and binary companions alter a star’s rotational evolution.

I gather data from a variety of space- and ground-based observatories. I measure rotation periods in open clusters using space-based missions like Kepler/K2 and TESS. I measure stellar activity using spectra gathered at observatories like MDM or WIYN at KPNO, and various archival sources like the SDSS. And I look for binary companions two different ways: using high-resolution spectra from observatories like MMTFLWO, and Magellan at Las Campanas Observatory; and using adaptive optics or speckle imaging at Keck or Gemini.

The impact of companions on stellar rotation

Models for stellar angular momentum evolution assume that stars are born and evolve in isolation (among other simplifying assumptions). However, just under half of all Sun-like stars and a quarter of low-mass stars have a stellar companion. We must therefore account for the influence of binaries on cluster and field rotation period distributions.

There are two primary ways for a binary companion to influence stellar angular momentum evolution: tidal interaction or disk disruption. A few percent of stars exist in very close binaries, where tidal forces alter both stars’ rotation periods while circularizing their orbit. The presence of a disk is generally invoked to explain slowly rotating stars in the youngest clusters—the disks act as an angular momentum sink, allowing the stars to contract without increasing their rotation rate too much. Binary companions can disrupt these disks, and the stars will subsequently become rapid rotators. These stars do eventually spin down and join their single brethern on the slow rotator sequence, but if binaries comprise most or all rapid rotators at young ages, this drastically changes the necessary initial conditions for angular momentum evolution models.

I primarily study binaries using two methods: spectroscopy and direct imaging. To search for close binaries, which appear as a single point of light in most images, I use spectroscopy. As the stars orbit each other, their spectral lines shift back and forth periodically. I measure this shift over time to determine the binary period (and other information about the orbit). Advanced imaging techniques such as adaptive optics and speckle imaging can reveal companions that would otherwise be washed out by or blended with the image of the primary star.

Magnetic Activity

Stellar rotation is tied to magnetism in two ways. First, stars are made up of charged particles, which generate a magnetic field if they move in a circle—this is called the dynamo effect. Second, the large-scale magnetic field carries away mass via a stellar wind, and in the right configuration that leads to angular momentum loss as well. So rotation creates the magnetic field, but that also leads to slower rotation.

We care about stellar magnetism because it affects planets around that star. Here on Earth, Solar stoms yield beautiful aurorae, but also interfere with communications. A big enough storm could disable satellites and the power grid. Much smaller stars have even stronger magnetic fields, and the increased rate of flares and CMEs could prevent life from ever forming there.

It’s nearly impossible to measure magnetic fields directly, so I look for indirect evidence of stellar magnetic field strength. Magnetic fields heat stellar atmospheres, and this excess energy is radiated away through particular spectral lines. I mostly focus on the Balmer-alpha line of Hydrogen, or H-alpha. Young and/or low-mass stars in particular show high levels of H-alpha emission, and a stronger H-alpha line is often correlated with faster rotation. By studying the evolution of stellar activity through H-alpha, X-ray, and other emission signatures, I aim to constrain how the high-energy environments of young planets evolves with time.