We have measured the mean three-dimensional kinematics of stars in Kapteyn's
Selected Area (SA) 76 (l=209.3, b=26.4 degrees) that were selected to be
Anticenter Stream (ACS) members on the basis of their radial velocities, proper
motions, and location in the color-magnitude diagram. From a total of 31 stars
ascertained to be ACS members primarily from its main sequence turnoff, a mean
ACS radial velocity (derived from spectra obtained with the Hydra multi-object
spectrograph on the WIYN 3.5m telescope) of V_helio = 97.0 +/- 2.8 km/s was
determined, with an intrinsic velocity dispersion sigma_0 = 12.8 \pm 2.1 km/s.
The mean absolute proper motions of these 31 ACS members are mu_alpha cos
(delta) = -1.20 +/- 0.34 mas/yr and mu_delta = -0.78 \pm 0.36 mas/yr. At a
distance to the ACS of 10 \pm 3 kpc, these measured kinematical quantities
produce an orbit that deviates by ~30 degrees from the well-defined swath of
stellar overdensity constituting the Anticenter Stream in the western portion
of the Sloan Digital Sky Survey footprint. We explore possible explanations for
this, and suggest that our data in SA 76 are measuring the motion of a
kinematically cold sub-stream among the ACS debris that was likely a fragment
of the same infalling structure that created the larger ACS system. The ACS is
clearly separated spatially from the majority of claimed Monoceros ring
detections in this region of the sky; however, with the data in hand, we are
unable to either confirm or rule out an association between the ACS and the
poorly-understood Monoceros structure.Comment: Accepted to ApJ. 48 pages, 20 figures, preprint forma
