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

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 & 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 EDR3 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 \(\rm\sigma_{V_r} \approx 15\;km\;s^{-1}\)) 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 \(100\times\) 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.