Moving Groups in the Galaxy

A moving group is a group of stars with similar velocity; thus stars in a moving group probably have formed in the same gas cloud at the same time and would scatter along time. Although 2 obvious moving groups were detected nearby the Sun in 1869[1], the conception of "moving group" was first suggested by an American astronomer, Olin J. Eggen in the 1960s. Since then, moving groups have absorbed enormous attention of astronomers. In the 1990s, Hipparcos launched by the European Space Agency published astrometric data with high resolution, researches of moving groups forged ahead into a whole new era, with a large number of moving groups and candidates of sub-structure successively detected. Based on kernel estimation and the wavelet technique, we certified 22 candidates of moving groups nearby the Sun[2], with 11 of them never reported before.

Moving Groups

There are a lot of methods to detect moving groups, e.g., to resolve stellar distribution in the 3D velocity space into several of Gaussian distributions with SEM method[3], with each Gaussian distribution representing one moving group; to search for moving groups in the 3D velocity space with kernel estimation and non-parameter methods[4]; to detect the sub-structure of the velocity space with the maximum likelihood method[5]; to detect the sub-structure of the space of stellar angular momentum and eccentricity[6]; to detect the sub-structure in the space of (V,(U2+2V2)1/2) using 2D continuous wavelet tranform technique[7][8]. Different methods can achieve diffrent effects in detecting moving groups, with better ones detecting larger number of and more reliable candidates. For example in 2005, Famaey and his colleagues using the method of maximum likelihood and detected 3 independent moving groups out of more than 6000 giants[5]; later they used the wavelet technique and detected 4 independent moving groups from the same sample[9]. Therefore the method for detection is a crucial element in searching for moving groups.

We used two sample of over 14000 dwarfs and over 6000 giants respectively to search for moving groups. Since the number of giants in the Galaxy is far larger than that of giants, and therefore we made separate detections in the two samples with different stellar compositions, so as to avoid some small structures to submerge in the giant sample. The procedure of the detection is as follows: frist using kernel function to measure the distribution function of the probability density in the (U,V) space, where we used the radial basis function and carefully selected the smooth factor[10]. After obtaining the probability density function, we carried out 2D continuous wavelet transform and calculated the wavelet coefficients of every scale, and then selected a reasonable scale to search for sub-structure and to estimate the reliability of the sub-structure. Estimation of the reliability has considered the effect of errors and selection effect, using Monte Carlo method to obtain the reliable factor of each sub-structure candidate. Finally, for sub-structures detected in the dwarf sample, matellicities and average values and scatters of Zmax were calculated. We found out that results for the giant and dwarf sample agreed quite well, and the giant sample exhibited more sub-structures than the dwarf. In the 22 sub-structures we detected, 6 were widely known by astronomers, including the Hercules Stream, the Sirun-UMa Stream, the Caster Stream, the Hyades Stream, the Pleiades Stream, and the IC2391 Stream. Besides, the other 5 were already reported in other reference materials, while 11 newly candidates of moving groups were never reported before but showed obvious structures in our work.

See Also

References

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