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Stellar Streams

The existence of stellar streams was first established from astronomical investigations dating as far back as 1869. They were originally thought to consist of previously clustered stars evolving from a common origin, that have been gradually dispersed by the dynamic processes of tidal forces, differences in galactic rotation, and encounters with other stars.

Increasingly comprehensive star catalogues published in the 1950's opened the way for more thorough analyses. Beginning in 1958, Olin Eggen produced a series investigations showing significantly increased membership counts and spatial extents of stellar streams, leading him to hypothesize a more protracted process of dissolution for star clusters. In Eggen’s scenario, as star clusters dissolve during their journeys around the Galaxy, they are stretched into tube-like formations, which were subsequently called superclusters.

The investigation of stellar streams received a major boost with the arrival of the precision astrometry afforded by the Hipparcos mission. A wide range of stellar ages was identified within stellar streams, challenging Eggen’s hypothesis of common origin. The search for other types of dynamical mechanisms to account for streams has been ongoing.

Stream Properties

Famaey et al., 2005, described six kinematic groups: three streams, Hyades/Pleiades, Sirius and Hercules, a group of young giants, high velocity stars and a smooth background distribution. Famaey found a total stream membership of over 25%, but using statistical analysis it is only possible to put least bounds on stream membership. After taking into consideration the fact that the velocity distribution is highly structured by colour, one sees that streams represent the bulk of the population.

We smoothed the velocity distribution by replacing each discrete point with a two-dimensional Gaussian function and finding the sum. A standard deviation of 1 gave a clear contour plot. We distinguish the Hyades and Pleiades streams, since the velocity distributions shows separate peaks, and because these streams contain different distributions of stellar types and ages. There is a large and well dispersed stream centred at (U, V) ≈ (25, −23) , which we have called the Alpha Ceti stream, after the brightest star we identified with this motion.

Large stars burn faster and hotter. So, blue stars are necessarily young. The bluest stars, with B − V < 0.04 (~B-A0) reflect recent star formation. The Pleiades stream consists largely of new-born stars, originating in our own spiral arm and with low eccentricities and typical orbits near to apocentre. We distinguish it from a stream with orbits close to pericentre which contains young as well as old stars, and which we have called the Alpha Lacertae stream. It appears that Famaey’s young giants belong to the Alpha Lacertae stream.

For 0.04 ≤ B − V < 0.16 (~A1-A5), the velocity distribution is concentrated in the Pleiades and Sirius streams.

The Hyades stream becomes more prominent than the Sirius stream for 0.16 ≤ B − V < 0.4 (~A6-A9).

The Hyades stream dominates the velocity distributions (by density, not by total population) for dwarves with 0.4 ≤ B − V < 0.56 (~F)

and 0.56 ≤ B − V < 0.8 (~G).

We restricted the population to mature stars, red giants and dwarfs aged over 2Gyrs, The distribution is is markedly different from that of the whole population. There is now no peak for the Pleiades stream, and the Sirius stream is also much less prominent.

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