We have measured the mean three-dimensional kinematics of stars in Kapteyn ’ s Selected Area ( SA ) 76 ( l = 209.3 \arcdeg , b = 26.4 \arcdeg ) 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 The WIYN Observatory is a joint facility of the University of Wisconsin-Madison , Indiana University , Yale University , and the National Optical Astronomy Observatory . ) of V _ { helio } = 97.0 \pm 2.8 km s ^ { -1 } was determined , with an intrinsic velocity dispersion \sigma _ { o } = 12.8 \pm 2.1 km s ^ { -1 } .
The mean absolute proper motions of these 31 ACS members are \mu _ { \alpha } cos \delta = -1.20 \pm 0.34 mas yr ^ { -1 } , and \mu _ { \delta } = -0.78 \pm 0.36 mas yr ^ { -1 } .
At a distance to the ACS of 10 \pm 3 kpc , these measured kinematical quantities produce an orbit that deviates by \sim 30 \arcdeg 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 .