Dipping our toes in the water: first models of GD-1 as a stream

We present a model for producing tidal streams from disrupting progenitors in arbitrary potentials, utilizing the idea that themajority 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 6D stream observations - assuming a stream rather than an orbit - by applying our methods to GD-1. This is a kinematically cold stream spanning 60. of the sky and residing in the outer Galaxy approximate to 15 kpc distant from the centre. We assume the stream moves in a flattened logarithmic potential characterized by an asymptotic circular velocity v(0) and a flattening q. We recover values of normalization v(0) = 227.2(-18.2)(+15.6) and flattening q = 0.90(-0.1)(+0.04), if the stream is assumed to be leading, and v(0) = 226.5(-17.0)(+17.9) km s(-1), q = 0.90(-0.09)(+0.05), if it is assumed to be trailing. This can be compared to the values v(0) = 224 +/- 13 km s(-1) and q = 0.87(-0.4)(+0.07) obtained by Koposov et al. 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.