By Fernanda Morais
Cover image credit: NASA Astrobiology
On February 25, Dr. Mark Popinchalk, a postdoctoral fellow at the American Museum of Natural History, visited the Amherst Physics & Astronomy Department for its weekly colloquium. Dr. Popinchalk is a member of the Brown Dwarfs in New York City (BDNYC) research group. Brown dwarfs are objects “in between” planets and stars; they are more massive than planets but not quite as massive as stars. Their defining characteristic is that they burn deuterium as fuel instead of the hydrogen typically burned by stars.
Along the blurry line between stars and brown dwarfs lie M-dwarfs, which Dr. Popinchalk researches. Some M-dwarfs are the most massive brown dwarfs, while others are the least massive stars. In the Milky Way, M-dwarfs are much more common than more massive stars like our Sun. They are faint, much colder than our Sun, and live longer than stars and brown dwarfs.
Like other stars, when M-dwarfs are born, they rotate. The speed at which they rotate decreases over time, meaning young M-dwarfs (and here, we mean 10-250 million years old) rotate faster than older M-dwarfs. This makes rotation speed a powerful tool for constraining their ages.
To measure this rotation, astronomers take advantage of colder, dimmer regions of an M-dwarf’s surface called starspots. When an M-dwarf rotates, its starspots will move in and out of view, changing the total brightness of the M-dwarf over time. Astronomers plot this brightness versus time to create light curves (LCs), which can reveal the rotation period of the M-dwarf. Dr. Popinchalk compared these LCs to a reverse lighthouse, where we measure how dim the M-dwarf luminosities become as these spots periodically face our telescopes.
These LCs have revealed a peculiar class of rapidly rotating young M-dwarfs with rapid dips in brightness that starspots cannot explain. These complex rotators represent about 1% of the known rapidly rotating M-dwarf population, and even more mysteriously, their anomalous behavior has been observed to last for several months at a time, suggesting some underlying physical mechanism at work.
Some possibilities have been ruled out by observations, such as protoplanetary disks (a rotating disk of dust from which planets are thought to form) around the M-dwarfs. The leading possibilities to account for this rapid dimming are dust or plasma interference, each with its pros and cons. While dust interference can account for all of the dimming, the absence of protoplanetary disks around these M-dwarfs makes the origin of this dust inconclusive. Plasma, meanwhile, can be linked to the generally stronger magnetic fields of M-dwarfs, but their interference may not block enough light to account for these dips.
Dr. Popinchalk’s talk left us with more questions than answers, which makes these complex rotators so fascinating. Are we seeing unaccounted-for dust or magnetic field interactions shaping the plasma around these M-dwarfs? This mystery continues to divide brown dwarf researchers to this day.