A Young, Low-Density Stellar Stream in the Milky Way Disk: Theia 456
  Our view of the variety of stellar structures pervading the local Milky Way
has been transformed by the application of clustering algorithms to the Gaia
catalog. In particular, several stellar streams have been recently discovered
that are comprised of hundreds to thousands of stars and span several hundred
parsecs. We analyze one such structure, Theia 456, a low-density stellar stream
extending nearly 200 pc and 20$^{\circ}$ across the sky. By supplementing Gaia
astrometric data with spectroscopic metallicities from LAMOST and photometric
rotation periods from the Zwicky Transient Facility (ZTF) and the Transiting
Exoplanet Survey Satellite (TESS), we establish Theia 456's radial velocity
coherence, and we find strong evidence that members of Theia 456 have a common
age ($\simeq$175 Myr), common dynamical origin, and formed from chemically
homogeneous pre-stellar material ([Fe/H] = $-$0.07 dex). Unlike well-known
stellar streams in the Milky Way, which are in its halo, Theia 456 is firmly
part of the thin disk. If our conclusions about Theia 456 can be applied to
even a small fraction of the remaining $\simeq$8300 independent structures in
the Theia catalog, such low-density stellar streams may be ubiquitous. We
comment on the implications this has for the nature of star-formation
throughout the Galaxy.
