Detection of a Stellar Stream Behind Open Cluster NGC 188: Another Part of the Monoceros Stream

TL;DR

This study identifies a stellar overdensity behind open cluster NGC 188, consistent with the Monoceros stream, using photometric and astrometric data, and suggests it is part of this galactic structure.

Contribution

The paper provides the first detection of a stellar stream behind NGC 188, supporting its association with the Monoceros stream through combined observational and theoretical analysis.

Findings

Detected a stellar overdensity behind NGC 188.

The overdensity's properties match Monoceros stream predictions.

Kinematically cold population consistent with Monoceros stream.

Abstract

We present results from a WIYN/OPTIC photometric and astrometric survey of the field of the open cluster NGC 188 ((l,b) = (122.8\arcdeg, 22.5\arcdeg)). We combine these results with the proper-motion and photometry catalog of Platais et al. and demonstrate the existence of a stellar overdensity in the background of NGC 188. The theoretical isochrone fits to the color-magnitude diagram of the overdensity are consistent with an age between 6 and 10 Gyr and an intermediately metal poor population ([Fe/H] = -0.5 to -1.0). The distance to the overdensity is estimated to be between 10.0 and 12.6 kpc. The proper-motions indicate that the stellar population of the overdensity is kinematically cold. The distance estimate and the absolute proper motion of the overdensity agree reasonably well with the predictions of the Pe\~{n}arrubia et al. model of the formation of the Monoceros stream. Orbits for this material constructed with plausible radial-velocity values, indicate that dynamically, this material is unlikely to belong to the thick disk. Taken together, this evidence suggests that the newly-found overdensity is part of the Monoceros stream.