A stellar relic filament in the Orion star forming region

TL;DR

The paper reports the discovery of the oldest stellar substructure in Orion, called the relic filament, which offers new insights into star formation processes in molecular clouds and their early evolution.

Contribution

It identifies and characterizes the Orion relic filament as a distinct stellar structure, providing evidence of its formation from a molecular cloud filament and its evolutionary state.

Findings

The Orion relic filament is approximately 430pc away from the Sun.

It is too old to be a single cluster or a tidal stream.

The structure formed from a molecular cloud filament and has retained its shape.

Abstract

We report the discovery of the oldest stellar substructure in the Orion star forming region (OSFR), the Orion relic filament. The relic filament is physically associated with the OSFR as demonstrated by Gaia DR2 photometry and astrometry, as well as targeted radial velocity follow-up observations of a bright sub-sample of proper-motion selected candidate members. Gaia DR2 parallaxes place the Orion relic filament in the more distant part of the OSFR, approx. 430pc from the Sun. Given its age, velocity dispersion, spatial extent, and shape, it is not possible for the Orion relic filament to have formed as a single stellar cluster, even taking into account residual gas expulsion. The relic filament is also too young to be a tidal stream, since Galactic tides act on much longer time scales of order 100 Myr. It therefore appears likely that the structure formed from a molecular cloud filament similar to Orion A in the OSFR and retained its morphology despite decoupling from its natal gas. Hence, the Orion relic filament bears witness to the short-lived evolutionary phase between gas removal and dispersion due to shears and tides, and provides crucial new insights into how stars are formed in molecular clouds.