SMHASH: Anatomy of the Orphan Stream using RR Lyrae stars
  Stellar tidal streams provide an opportunity to study the motion and
structure of the disrupting galaxy as well as the gravitational potential of
its host. Streams around the Milky Way are especially promising as phase space
positions of individual stars will be measured by ongoing or upcoming surveys.
Nevertheless, it remains a challenge to accurately assess distances to stars
farther than 10 kpc from the Sun, where we have the poorest knowledge of the
Galaxy's mass distribution. To address this we present observations of 32
candidate RR Lyrae stars in the Orphan tidal stream taken as part of the
Spitzer Merger History and Shape of the Galactic Halo (SMHASH) program. The
extremely tight correlation between the periods, luminosities, and
metallicities of RR Lyrae variable stars in the Spitzer IRAC $\mathrm{3.6 \mu
m}$ band allows the determination of precise distances to individual stars; the
median statistical distance uncertainty to each RR Lyrae star is $2.5\%$. By
fitting orbits in an example potential we obtain an upper limit on the mass of
the Milky Way interior to 60 kpc of $\mathrm{5.6_{-1.1}^{+1.2}\times 10^{11}\
M_\odot}$, bringing estimates based on the Orphan Stream in line with those
using other tracers. The SMHASH data also resolve the stream in line--of--sight
depth, allowing a new perspective on the internal structure of the disrupted
dwarf galaxy. Comparing with N--body models we find that the progenitor had an
initial dark halo mass of approximately $\mathrm{3.2 \times 10^{9}\ M_\odot}$,
placing the Orphan Stream's progenitor amongst the classical dwarf spheroidals.
