The Orbit of the Orphan Stream

Abstract

We use recent SEGUE spectroscopy and SDSS and SEGUE imaging data to measure the sky position, distance, and radial velocities of stars in the tidal debris stream that is commonly referred to as the "Orphan Stream." We fit orbital parameters to the data, and find a prograde orbit with an apogalacticon, perigalacticon, and eccentricity of 90 kpc, 16.4 kpc and e = 0.7, respectively. Neither the dwarf galaxy UMa II nor the Complex A gas cloud have velocities consistent with a kinematic association with the Orphan Stream. It is possible that Segue-1 is associated with the Orphan Stream, but no other known Galactic clusters or dwarf galaxies in the Milky Way lie along its orbit. The detected portion of the stream ranges from 19 to 47 kpc from the Sun and is an indicator of the mass interior to these distances. There is a marked increase in the density of Orphan Stream stars near (l, b) = (253 • , 49 • ), which could indicate the presence of the progenitor at the edge of the SDSS data. If this is the progenitor, then the detected portion of the Orphan Stream is a leading tidal tail. We find blue horizontal branch (BHB) stars and F turnoff stars associated with the Orphan Stream. The turnoff color is (g−r) 0 = 0.22. The BHB stars have a low metallicity of [Fe/H] WBG = −2.1. The orbit is best fit to a halo potential with a halo plus disk mass of about 2.6 × 10 11 M ⊙ , integrated to 60 kpc from the Galactic center. Our fits are done to orbits rather than full N-body simulations; we show that if N-body simulations are used, the inferred mass of the galaxy would be slightly smaller. Our best fit is found with a logarithmic halo speed of v halo = 73 ± 24 km s −1 , a disk+bulge mass of M(R < 60 kpc) = 1.3 × 10 11 M ⊙ , and a halo mass of M(R < 60 kpc) = 1.4 × 10 11 M ⊙ . However, we can find similar fits to the data that use an NFW halo profile, or that have smaller disk masses and correspondingly larger halo masses. Distinguishing between different classes of models requires data over a larger range of distances. The Orphan Stream -2is projected to extend to 90 kpc from the Galactic center, and measurements of these distant parts of the stream would be a powerful probe of the mass of the Milky Way.

Exploring Halo Substructure With Giant Stars. Xiv. The Nature of the Triangulum-Andromeda Stellar Features

“…An apparent difference in the v GSR distributions is seen: TriAnd2 has a median near 40 km s −1 and a fairly cold dispersion of σ ∼ 25 km s −1 , whereas TriAnd1 stars show a prominent peak at v GSR = 50-60 km s −1 and a similar dispersion. These dispersions are colder than expected for a random distribution of halo stars, but hotter than the v GSR dispersion for the Sgr tidal stream (σ ∼ 10 km s −1 ; Majewski et al 2004) and the Orphan stream (σ ∼ 10 km s −1 ; Newberg et al 2010).…”

Target Selection for the SDSS-IV APOGEE-2 Survey

“…Gray points are other 2MASS sources in the field that were not targeted as LMC stars. Carlberg et al 2012), GD-1 (Koposov et al 2010), Orphan (Newberg et al 2010;Casey et al 2013Casey et al , 2014Sesar et al 2013), and the Sgr tail (Bellazzini et al 2003;Yanny et al 2009;Carrell et al 2012;Pila-Díez et al 2014). Each stream has one to five numbered fields placed along its length (e.g., GD1-1, GD1-2, etc.).…”

Modeling the Gravitational Potential of a Cosmological Dark Matter Halo with Stellar Streams

“…The knowledge of a stream's trajectory provides a constraint on the potential and thus on the matter distribution and shape of the dark halo (Eyre & Binney 2009;Willett et al 2009;Law & Majewski 2010;Newberg et al 2010;Sanders & Binney 2013).…”