Disconnecting the Dots: Re-Examining the Nature of Stellar "Strings" in the Milky Way

TL;DR

This study critically examines stellar "strings" in the Milky Way, revealing they are mostly not physical structures but rather chance alignments, challenging previous interpretations of their significance in galactic structure.

Contribution

The paper provides a comprehensive analysis showing stellar strings are unbound, dispersing, and not indicative of common origins, questioning their role in galactic formation history.

Findings

Strings are gravitationally unbound.

Spatial dispersion does not improve with better data.

Chemical homogeneity can be mimicked by random star samples.

Abstract

Recent analyses of Gaia data have resulted in the identification of new stellar structures, including a new class of extended stellar filaments called stellar "strings", first proposed by Kounkel and Covey 2019. We explore the spatial, kinematic, and chemical composition of strings to demonstrate that these newfound structures are largely inconsistent with being physical objects whose members share a common origin. Examining the 3D spatial distribution of string members, we find that the spatial dispersion around the claimed string spine does not improve in the latest Gaia DR3 data release -- despite tangible gains in the signal-to-noise of the parallax measurements -- counter to expectations of a bona fide structure. Using the radial velocity dispersion of the strings (averaging $\rm\sigma_{V_r} = 16\;km\;s^{-1}$) to estimate their virial masses, we find that all strings are gravitationally unbound. Given the finding that the strings are dispersing, the reported stellar ages of the strings are typically $120\times$ larger than their measured dispersal times. Finally, we validate prior work that stellar strings are more chemically homogeneous than their local field stars, but show it is possible to obtain the same signatures of chemical homogeneity by drawing random samples of stars from spatially, temporally, and kinematically unrelated open clusters. Our results show that while some strings may be composed of real sub-structures, there is no consistent evidence for larger string-like connections over the sample. These results underline the need for caution in over-interpreting the significance of these strings and their role in understanding the star formation history of the Milky Way.