The Orbit of the Orphan Stream
  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 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) deg., 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]=-2.1. The orbit is best fit to a halo potential with
a halo plus disk mass of about 2.6x10^11 Solar masses, integrated to 60 kpc
from the Galactic center. Our best fit is found with a logarithmic halo speed
of v_halo=73+/-24 km/s, a disk+bulge mass of M(R< 60 kpc) = 1.3x10^11 Solar
masses, and a halo mass of M(R< 60 kpc) = 1.4x10^11 Solar masses. The Orphan
Stream is 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 (truncated).
