# The Ophiuchus stream progenitor: a new type of globular cluster and its possible Sagittarius connection

## TL;DR

This paper proposes that the Ophiuchus stream originated from a new, faint type of globular cluster, possibly linked to Sagittarius, with recent disruption evidenced by Gaia data and N-body simulations.

## Contribution

It introduces a new class of faint globular cluster progenitors for stellar streams, supported by Gaia data and N-body simulations, and suggests a possible recent interaction with Sagittarius.

## Key findings

- The stream is likely from a recently disrupted, faint globular cluster.
- Gaia proper motions resolve previous motion mismatches.
- Possible recent interaction between Sagittarius and Ophiuchus about 100 Myrs ago.

## Abstract

The Ophiuchus stream is a short arc-like stellar feature of uncertain origin located $\sim 5$ kpc North of the Galactic centre. New proper motions from the second $Gaia$ data release reconcile the direction of motion of stream members with the stream arc, resolving a puzzling mismatch reported in earlier work. We use N-body simulations to show that the stream is likely only on its second pericentric passage, and thus was formed recently. The simulations suggest that the entire disrupted progenitor is visible in the observed stream today, and that little further tidal debris lies beyond the ends of the stream. The luminosity, length, width, and velocity dispersion of the stream suggest a globular cluster (GC) progenitor substantially fainter and of lower surface brightness than estimated in previous work, and unlike any other known globulars in the Galaxy. This result suggests the existence of clusters that would extend the known GC population to fainter and more weakly bound systems than hitherto known. How such a weakly-bound cluster of old stars survived until it was disrupted so recently, however, remains a mystery. Integrating backwards in time, we find that the orbits of Sagittarius and Ophiuchus passed within $\sim 5$ kpc of each other about $\sim 100$ Myrs ago, an interaction that might help resolve this puzzle.