Extended stellar systems in the solar neighborhood - II. Discovery of a nearby 120{\deg} stellar stream in Gaia DR2

TL;DR

A nearby stellar stream at 100 pc was discovered in Gaia DR2, characterized by its size, low velocity dispersion, and coeval population, providing insights into Galactic structure and stellar evolution.

Contribution

This paper reports the discovery of a new, nearby stellar stream with unique properties, expanding our understanding of stellar streams in the solar neighborhood.

Findings

Stream is at 100 pc, 400 pc long, 50 pc wide.

Members form a well-defined main sequence with low velocity dispersion.

Estimated age is around 1 Gyr with near-solar metallicity.

Abstract

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.