Candidate BHB stars in Ophiuchus stream

Recent theoretical work has shown that streams on chaotic orbits may abruptly fan out near their apparent ends; stars in these fans are dispersed in both position and velocity and may be difficult to associate with the stream.

Here we present the first evidence of such stream-fanning in the Ophiuchus stream, traced by four blue horizontal branch stars beyond the apparent end of the stream. These stars stand out from the background by their high velocities (v_los_>230km/s) against ~40 other stars: their velocities are comparable to those of the stream, but would be exceptional if they were unrelated halo stars. Their positions and velocities are, however, inconsistent with simple extrapolation of the observed cold, high-density portion of the stream.