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

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

Stellar, planetary, and galactic studies27 references38 citations

TL;DR

This paper presents the discovery of a nearby, dynamically cold, coeval stellar stream in the solar neighborhood using Gaia DR2 data, identified via wavelet decomposition and DBSCAN clustering in 6D phase space. The stream, extending over 400 pc with a velocity dispersion of only 1.3 km s⁻¹ and a total mass of at least 2000 M⊙, is likely a disrupted disk-bound cluster or association, offering a new probe for Galactic mass modeling with a metallicity of [Fe/H] = −0.04 and an age of ~1 Gyr.

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

• To identify and characterize extended, dynamically cold stellar systems in the solar neighborhood using Gaia DR2 kinematic data.
• To investigate the presence of previously undetected stellar streams associated with the Galactic disk, distinct from halo streams.
• To determine the age, mass, and metallicity of a newly discovered stream to assess its origin and evolutionary history.
• To evaluate the potential of such disk streams as probes for constraining the Milky Way’s mass distribution, including dark matter.
• To compare the stream’s properties with known clusters like the Hyades and halo streams like Phlegethon to infer its origin and evolutionary state.

Proposed method

• Applied wavelet decomposition to 3D velocity space (vr, vφ, vz) of stars within 300 pc of the Sun to detect overdensities at scales of ~1.5 km s⁻¹.
• Used a 5-σ threshold to identify significant overdensities in the wavelet components, isolating a previously unknown structure at vr = 1.7 km s⁻¹, vφ = 227.7 km s⁻¹, vz = −11.4 km s⁻¹.
• Performed DBSCAN clustering in 6D phase space (r, φ, z, vr, vφ, vz), after standardizing parameters to zero mean and unit variance, with minPts = 50 and ϵ = 0.75.
• Applied a spatial density threshold requiring at least 7 neighbors within 30 pc to refine the stream member sample.
• Constructed observational Hertzsprung-Russell diagrams (HRDs) to verify coevality and used isochrone comparisons (e.g., 1 Gyr PARSEC models) to estimate age and metallicity.
• Estimated total mass via extrapolation of the initial mass function (IMF), applying a completeness correction based on Hyades data (70% completeness loss) to derive a lower limit of ~2000 M⊙.

Experimental results

Research questions

• RQ1 What is the kinematic and spatial structure of a newly discovered stellar stream in the solar neighborhood?
• RQ2 What is the age and metallicity of this stream, and how do they compare to known clusters like the Hyades?
• RQ3 What is the total mass of the stream, and what does it imply about its origin and evolutionary history?
• RQ4 How does the stream’s low velocity dispersion and coeval main sequence support a common origin?
• RQ5 How does this stream compare to halo stellar streams in terms of morphology, age, and metallicity?

Key findings

• The stream spans at least 400 pc in length with a vertical extent of only ~50 pc, forming two parallel lanes with local overdensities, suggesting a possible remnant core of a tidally disrupted system.
• The stream exhibits an exceptionally low 3D velocity dispersion of 1.3 km s⁻¹, indicating high dynamical coldness and strong kinematic coherence.
• Member stars form a well-defined main sequence in the Hertzsprung-Russell diagram, confirming a coeval population with an estimated age of ~1 Gyr, older than the Hyades.
• The total mass of the stream is estimated at a minimum of 2000 M⊙, based on completeness-corrected initial mass function extrapolation, exceeding most known nearby clusters.
• The mean metallicity is [Fe/H] = −0.04 ± 0.15, significantly higher than halo streams like Phlegethon ([Fe/H] ≈ −1.5), indicating a disk origin.
• The stream is located ~75 pc below the Galactic midplane and is moving away from it, but its vertical velocity is insufficient for escape into the halo, suggesting it remains bound to the disk.