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 \SI 100pc .
The structure was identified in a wavelet decomposition of the 3D velocity space of all stars within \SI 300pc to the Sun .
Its members form a highly elongated structure with a length of at least \SI 400pc , while its vertical extent measures only about \SI 50pc .
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 \SI 1.3 \km \per \s .
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 \SI 2000M_ ⊙ , making it larger than most currently known clusters or associations in the solar neighborhood .
We estimated the stream’s age to be around \SI 1Gyr 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 .