Dipping Our Toes in the Water: First Models of GD-1 as a Stream

TL;DR

This paper introduces a new, efficient method for modeling tidal streams from disrupting satellites in arbitrary potentials, applying it to the GD-1 stream to infer Galactic parameters more accurately than previous orbit-fitting techniques.

Contribution

The paper presents the first direct 6-D stream modeling method that accounts for stream morphology rather than assuming orbit fitting, enabling detailed analysis of GD-1.

Findings

Recovered Galactic potential parameters consistent with previous estimates.

Demonstrated the method's ability to match observed stream data.

Showed orbit fitting remains accurate for this cold stream in a logarithmic potential.

Abstract

We present a model for producing tidal streams from disrupting progenitors in arbitrary potentials, utilizing the idea that the majority of stars escape from the progenitor's two Lagrange points. The method involves releasing test particles at the Lagrange points as the satellite orbits the host and dynamically evolving them in the potential of both host and progenitor. The method is sufficiently fast to allow large-dimensional parameter exploration using Monte Carlo methods. We provide the first direct modelling of 6-D stream observations -- assuming a stream rather than an orbit -- by applying our methods to GD-1. This is a kinematically cold stream spanning $60^{\circ}$ of the sky and residing in the outer Galaxy $\approx 15$ kpc distant from the centre. We assume the stream moves in a flattened logarithmic potential characterised by an asymptotic circular velocity $v_0$ and a flattening $q$. We recover values of normalisation $v_0$ = $227.2^{+15.6}_{-18.2}$ kms$^{-1}$ and flattening $q$ = $0.91^{+0.04}_{-0.1}$, if the stream is assumed to leading, and $v_0$ = $226.5^{+17.9}_{-17.0}$ kms$^{-1}$, $q$ = $0.90^{+0.05}_{-0.09}$, if it is assumed to be trailing. This can be compared to the values $v_0 = 224 \pm 13$ kms$^{-1}$ and $q= 0.87^{+0.07}_{-0.04}$ obtained by Koposov et al (2010) using the simpler technique of orbit fitting. Although there are differences between stream and orbit fitting, we conclude that orbit fitting can provide accurate results given the current quality of the data, at least for this kinematically cold stream in this logarithmic model of the Galaxy.