Kinematics of the Palomar 5 stellar stream from RR Lyrae stars

TL;DR

This study uses Gaia data and RR Lyrae stars to measure the distance and kinematic properties of the Palomar 5 stellar stream, aiding in the development of more accurate dynamical models of the stream and dark matter distribution.

Contribution

It introduces a probabilistic method to identify RR Lyrae stars in the Pal 5 stream and measures their distances and proper motions, providing new data for modeling the stream's dynamics.

Findings

Detected distance and proper motion gradients along the stream.

Measured the Pal 5 cluster distance as 20.6 kpc with high precision.

Identified 27 RR Lyrae stars as probable members of the cluster and stream.

Abstract

Thin stellar streams, formed from the tidal disruption of globular clusters, are important gravitational tools, sensitive to both global and small-scale properties of dark matter. The Palomar 5 stellar stream (Pal 5) is an exemplar stream within the Milky Way: Its $\sim 20^\circ$ tidal tails connect back to the progenitor cluster, and the stream has been used to study the shape, total mass, and substructure fraction of the dark matter distribution of the Galaxy. However, most details of the phase-space distribution of the stream are not fully explained, and dynamical models that use the stream for other inferences are therefore incomplete. Here we aim to measure distance and kinematic properties along the Pal 5 stream in order to motivate improved models of the system. We use a large catalog of RR Lyrae-type stars (RRLs) with astrometric data from the Gaia mission to probabilistically identify RRLs in the Pal 5 stream. RRLs are useful because they are intrinsically-luminous standard candles and their distances can be inferred with small relative precision ($\sim3\%$). By building a probabilistic model of the Pal 5 cluster and stream in proper motion and distance, we find 27 RRLs consistent with being members of the cluster (10) and stream (17). Using these RRLs, we detect gradients in distance and proper motion along the stream, and provide an updated measurement of the distance to the Pal 5 cluster using the RRLs, $d = 20.6 \pm 0.2~\textrm{kpc}$. We provide a catalog of Pal 5 RRLs with inferred membership probabilities for future modeling work.