A "blue ring nebula" surrounding a thousands of years old stellar merger
  Stellar mergers are a brief-lived but common phase in the evolution of binary
star systems. Among the many astrophysical implications of these events include
the creation of atypical stars (e.g. magnetic stars, blue stragglers, rapid
rotators), interpretation of stellar populations, and formation channels of
LIGO-detected compact object mergers. Although stellar mergers are thus
commonly invoked phenomena, observations of these events and details of their
evolution remain elusive. While a handful of stellar mergers have been directly
observed in recent years, the central remnants of these events remain shrouded
by an opaque shell of dust and molecules, making it impossible to observe their
final state (e.g. as a single merged star or a tighter surviving binary). Here
we report observations of an unusual, ring-shaped ultraviolet nebula and the
star at its center, TYC 2597-735-1. The nebula shows two opposing fronts,
suggesting a bipolar outflow from TYC 2597-735-1. TYC 2597-735-1's spectrum and
proximity above the Galactic plane suggest it is an old star, yet it shows
abnormally low surface gravity and a detectable long-term luminosity decay,
uncharacteristic for its evolutionary stage. TYC 2597-735-1 also exhibits
H-alpha emission, radial velocity variations, enhanced ultraviolet radiation,
and excess infrared emission, common signposts of dusty circumstellar disks,
stellar activity, and accretion. The combined observations, paired with stellar
evolution models, suggests TYC 2597-735-1 merged with a lower-mass companion
several thousand years ago. TYC 2597-735-1 provides a look at an unobstructed
stellar merger found at an evolutionary stage between its dynamic onset and the
theorized final equilibrium state, directly inferring how two stars merge into
a single star.
