[Paper Review] Extended stellar systems in the solar neighborhood - II. Discovery of a nearby 120° stellar stream in Gaia DR2

Stefan Meingast, J. Alves+1 more|arXiv (Cornell University)|Jan 18, 2019

Stellar, planetary, and galactic studies27 references35 citations

TL;DR

The paper reports the discovery and characterization of a large, dynamically cold, coeval stellar stream at ≈100 pc, traced in Gaia DR2 data, with a 120° sky extent and 400 pc length.

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 $\left[ \mathrm{Fe/H} ight] = -0.04$. 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.

Motivation & Objective

• Identify dynamically cold stellar structures in the solar neighborhood beyond known clusters.
• Characterize the morphology, kinematics, and coeval nature of a newly found stream.
• Estimate age, metallicity, and total mass to assess origin and potential as a Galactic disk mass probe.

Proposed method

• Construct a 3D velocity space histogram (v_r, v_phi, v_z) for stars within 300 pc of the Sun.
• Apply a wavelet decomposition to identify overdensities at ~1.5 km/s scales.
• Extract a velocity peak and perform 6D clustering with DBSCAN on (r, phi, z, v_r, v_phi, v_z).
• Impose spatial density filtering to derive a final stream member list, and cross-check with HR diagrams and isochrones.
• Estimate contamination via mirrored phase-space sampling across the Galactic plane.
• Cross-match stream members with LAMOST for metallicity estimation.

Experimental results

Research questions

• RQ1 What is the existence and extent of dynamically cold stellar streams in the solar neighborhood beyond known clusters?
• RQ2 What are the kinematic, structural, and stellar population properties (age, metallicity, mass) of the newly identified stream?
• RQ3 How does this disk-associated stream inform our understanding of the Galaxy's mass distribution?

Key findings

• Identified a large, elongated stream about 400 pc long with a vertical extent of ≈50 pc and an axial ratio of 8.4:2.7:1.0.
• Measured a remarkably low 3D velocity dispersion of 1.3 km/s across the structure.
• Found the stream located ≈75–100 pc below the Galactic plane and moving away from it but not escaping to the halo.
• Observed a clear main sequence in the observational HR diagram, supporting a single coeval population with an age around 1 Gyr.
• Estimated a present-day mass function implying a minimum total mass ≳2000 Msun, larger than most nearby clusters/associations.
• Mean metallicity from LAMOST cross-match: [Fe/H] ≈ -0.04 ± 0.15.