GD-1: The Relic of an Old Metal-poor Globular Cluster

Abstract Combining data from Gaia DR2, SDSS DR14, and LAMOST DR6, we update the fit to model of the properties of the stellar stream GD-1 and find that it has an age of ∼13 Gyr, Fe/H of −2.2 ± 0.12, and a distance from the Sun of ∼8 kpc. We tabulate six-dimensional (6D) phase-space fiducial points along the GD-1 stream orbit over a 90° arc. The fitted orbit shows that the stream has an eccentricity e ∼ 0.3, perigalacticon of 14.2 kpc, apogalacticon of 27.0 kpc, and inclination i ∼ 40°. There is evidence along the arc for four candidate stellar overdensities, one candidate gap, two candidate stellar underdensities, and that the stream is cut off at ϕ 1 ∼ 2° (in the stream-aligned ( ϕ 1 , ϕ 2 ) coordinate system of Koposov et al.). The spur originating at ϕ 1 ∼ −40° implies stars were pulled away from the stream trace by an encounter (potentially a dark matter subhalo). The narrowest place (FWHM ∼ 44.6 pc) of the GD-1 trace is at ( ϕ 1 , ϕ 2 c ) ∼ (−14°, 0.°15), which is ∼(178.°18, 52.°19) in (R.A., decl.), where the progenitor is possibly located. We also find six blue horizontal branch and 10 blue stragglers spectroscopic stars in the GD-1 stream.