Feeling the pull, a study of natural Galactic accelerometers. II: kinematics and mass of the delicate stellar stream of the Palomar 5 globular cluster

TL;DR

This study uses spectroscopic and photometric data to analyze the kinematics, mass, and stellar populations of the Palomar 5 stellar stream, confirming its narrow structure and low-mass star depletion, and providing updated structural parameters.

Contribution

The paper provides new spectroscopic surveys and an updated structural model of the Palomar 5 stream, including mass estimates and stellar population analysis, enhancing understanding of its dynamics and evolution.

Findings

High-probability stream stars are confined to a narrow sky region.

Palomar 5's stellar stream shows no significant fanning, indicating a non-chaotic orbit.

The stream's stellar population is similar to the core, lacking low-mass stars.

Abstract

We present two spectroscopic surveys of the tidal stellar stream of the Palomar 5 globular cluster, undertaken with the VLT/FLAMES and AAT/AAOmega instruments. We use these data in conjunction with photometric data presented in the previous contribution in this series to classify the survey stars in terms of their probability of belonging to the Palomar 5 stellar stream. We find that high-probability candidates are only found in a very narrow spatial interval surrounding the locus of the stream on the sky. PanSTARRS RRLyrae stars in this region of sky are also distributed in a similar manner. The absence of significant "fanning" of this stellar stream confirms that Palomar 5 does not follow a chaotic orbit. Previous studies have found that Palomar 5 is largely devoid of low-mass stars, and we show that this is true also of the stellar populations along the trailing arm out to $6\deg$. Within this region, which contains 73\% of the detected stars, the population is statistically identical to the core, implying that the ejection of the low-mass stars occurred before the formation of the stream. We also present an updated structural model fit to the bound remnant, which yields a total mass of $4297\pm98 {\rm\,M_\odot}$ and a tidal radius $0.145\pm0.009$ kpc. We estimate the mass of the observed system including the stream to be $12200\pm400 {\rm\,M_\odot}$, and the initial mass to have been $\sim47000\pm1500 {\rm\,M_\odot}$. These observational constraints will be employed in our next study to model the dynamics of the system in detail.