Disentangling the Arcturus stream

TL;DR

This study analyzes the kinematic and chemical properties of the Arcturus stream and other stellar groups using Gaia DR2 data, suggesting they are remnants of an ancient merger rather than a dissolved cluster or bar-related structures.

Contribution

It provides a detailed kinematic and chemical analysis of multiple stellar streams, proposing that the Arcturus stream is likely a phase-space wave from an ancient merger event.

Findings

Arcturus, AF06, and KFR08 streams are low-velocity, thick disk-like structures.

Streams are detected in steps of 20-30 km/s in azimuthal velocity.

Chemical diversity indicates they are not dissolved open clusters.

Abstract

The Arcturus stream is an over-density of stars in velocity space and its origin has been much debated recently without any clear conclusion. Resolving the nature of the Arcturus stream can provide clues to the formation history of the Milky Way and its stellar populations. The space velocities, angular momenta and actions for a sample of more than 5.8 million stars, composed from Gaia DR2, are analysed with a wavelet transform. The kinematic characteristics of each identified group is used to select possible members of the groups from the GALAH and APOGEE spectroscopic surveys to further study and constrain their chemical properties. In the velocity and angular momentum spaces the already known Sirius, Pleiades, Hyades, Hercules, AF06, Arcturus and KFR08 streams are clearly identified. The Hercules stream appears to be a mixture of thin and thick disk stars. The Arcturus stream, as well as the AF06 and KFR08 streams, are low-velocity and low-angular momentum structures with chemical compositions similar to the thick disk. These three groups extend further from the Galactic plane compared to the Hercules stream. The detections of all the groups were spaced by approximately 20-30 km/s in azimuthal velocity. A wide spread of chemical abundances within the Arcturus stream indicates that the group is not a dissolved open cluster. Instead the Arcturus stream, together with the AF06 and KFR08 streams, are more likely to be part of a phase-space wave, that could have been caused by an ancient merger event. This conclusion is based on that the different structures are detected in steps of 20-30 km/s in azimuthal velocity, that the kinematic and chemical features are different from what is expected for bar-originated structures, and that the lower-velocity streams extend further from the disk than bar-originated structures.