Extended stellar systems in the solar neighborhood - II. Discovery of a
  nearby 120{\deg} stellar stream in Gaia DR2
  We report the discovery of a large, dynamically cold, coeval stellar stream
that is currently traversing the immediate solar neighborhood at a distance of
only 100 pc. The structure was identified in a wavelet decomposition of the 3D
velocity space of all stars within 300 pc to the Sun. Its members form a highly
elongated structure with a length of at least 400 pc, while its vertical extent
measures only about 50 pc. Stars in the stream are not isotropically
distributed but instead form two parallel lanes with individual local
overdensities, that may correspond to a remnant core of a tidally disrupted
cluster or OB association. Its members follow a very well-defined main sequence
in the observational Hertzsprung-Russel diagram and also show a remarkably low
3D velocity dispersion of only 1.3 km s$^{-1}$. These findings strongly suggest
a common origin as a single coeval stellar population. An extrapolation of the
present-day mass function indicates a total mass of at least 2000 M$_\odot$,
making it larger than most currently known clusters or associations in the
solar neighborhood. We estimated the stream's age to be around 1 Gyr based on a
comparison with a set of isochrones and giant stars in our member selection and
find a mean metallicity of $\left[ \mathrm{Fe/H} \right] = -0.04$. This
structure may very well represent the Galactic disk counterpart to the
prominent stellar streams observed in the Milky Way halo. As such, it
constitutes a new valuable probe to constrain the Galaxy's mass distribution.
