Stellar Groups, IV. The Groombridge 1830 Group of High Velocity Stars and Its Relation to the Globular Clusters

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Abstract
The available proper motions and radial velocity data have been used to establish the existence of a moving group of subdwarfs (Groombridge 1830 group) which includes RR Lyrae. On the basis of the relationship between the observed ultra-violet excess and displacement below the normal main sequence, the subdwarfs in the Groombridge 1830 group are identified with main sequence stars in the globular clusters. This identification gives a modulus of m – M = 14 m .2 for the globular cluster M13 with the result that M V ∼ + 0 m .5 for the 2 RR Lyrae variables and M V = − 2 m .3 for the brightest stars in the cluster. RR Lyrae itself, for which we derive M V ∼ + 0 m .8 from the moving cluster parallax, is shown to obey the period-amplitude relation for the variables in M3 and to be reddened by 0 m .05 with respect to those variables. By equating the luminosity of RR Lyrae to the mean of the variables in M3 we obtain a modulus of m – M = 15 m .0 for the cluster. We have not derived this modulus in the logical way of fitting the M3 main sequence to the main sequence of the Groombridge 1830 group because the colour observations of the M3 main sequence stars may contain a systematic error. Because the presence of RR Lyrae variables in stellar groups may provide the only accurate calibration of the luminosities of these stars, it is important to make a systematic search for such groups. We have tabulated the available information for (1) all stars classified as subdwarfs on the basis of their spectral features, (2) stars that are probably extreme subdwarfs, on the basis of their observed ultra-violet excesses, although classified as normal main sequence objects, and (3) stars that may be globular cluster giants, subgiants or horizontal branch stars. Space motions of objects in these three categories have been computed from parallaxes derived by fitting them to the ( M V , B – V )-diagram of M13, which was calibrated by the Groombridge 1830 group. It is shown that there is a high probability that several of these stars are travelling together in space and that, when additional material becomes available, several groups similar to the Groombridge 1830 group will be found. Also, the space motions of 4 RR Lyrae variables with relatively well-determined proper motions are identified with those for some nearby subdwarfs with a resulting M V ∼ + 0 m .6 for the four variables.

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