# Piercing the Milky Way: an all-sky view of the Orphan Stream

## TL;DR

This study maps the Orphan Stream across the entire sky using Gaia DR2 data, revealing its extended structure, shape twists, and evidence of interaction with a massive perturber through detailed astrometric and photometric analysis.

## Contribution

It provides the first all-sky map of the Orphan Stream with multi-tracer data, uncovering its full extent, shape features, and kinematic anomalies indicating external interactions.

## Key findings

- Stream extends approximately 150 kpc in length.
- Detected shape twists and changes in tangential motion.
- Evidence of interaction with a massive perturber.

## Abstract

We use astrometry, broad-band photometry and variability information from the Data Release 2 of ESA's Gaia mission (GDR2) to identify members of the Orphan Stream (OS) across the whole sky. The stream is traced above and below the celestial equator and in both Galactic hemispheres, thus increasing its visible length to ~ 210 degrees equivalent to ~150 kpc in physical extent. Taking advantage of the large number of RR Lyrae stars in the OS, we extract accurate distances and proper motions across the entire stretch of the tidal debris studied. As delineated by the GDR2 RR Lyrae, the stream exhibits two prominent twists in its shape on the sky which are accompanied by changes in the tangential motion. We complement the RR Lyrae maps with those created using GDR2 Red Giants and the DECam Legacy Survey Main Sequence Turn-Off stars. The behavior of the OS track on the sky is consistent across all three tracers employed. We detect a strong non-zero motion in the across-stream direction for a substantial portion of the stream. Such a misalignment between the debris track and the streaming velocity cannot be reproduced in a static gravitational potential and signals an interaction with a massive perturber.