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تسجيل مستخدم جديدA Spectroscopic Survey for Omega Centauri Members at and beyond the Cluster Tidal Radius
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We have used the two-degree field (2dF) multi-fibre spectrograph of the Anglo-Australian Telescope to search for candidate members of the unusual globular cluster omega Centauri at and beyond the cluster tidal radius. Velocities with an accuracy of ~10 kms-1 were obtained for 4105 stars selected to lie in the vicinity of the lower giant branch in the cluster colour-magnitude diagram and which cover an area on the sky of ~2.4x3.9 deg2 centered on the cluster. Within the velocity interval 190-270 kms-1, the cluster member candidates have a steeply declining surface density distribution consistent with the adopted tidal radius of 57`. For stars in the sample beyond the tidal radius, an analysis of line-strengths from the spectra, as well as radial velocities, identifies only six stars as possible candidates for extra-tidal association with the cluster. If all six of these stars are indeed related to the cluster, then a maximum of 0.7 +/- 0.2 per cent of the total cluster mass is contained in the region between one and two tidal radii. Given this limit, we conclude that there is no compelling evidence for any significant extra-tidal population in omega Cen. The effects of tidal shocks on the outer parts of the cluster are consistent with this limit. Theories for the origin of omega Cen frequently suggest that the cluster is the former nucleus of a tidally stripped dwarf galaxy. Our results require that the stripping process must have been largely complete at early epochs, consistent with dynamical models of the process. The stripped former dwarf galaxy stars are therefore now widely distributed around the Galaxy.
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a$ Centauri. Recent analysis of the second data release of Gaia had revealed the Fimbulthul stellar stream in the halo of the Milky Way. It had been proposed that the stream is associated with the $omega$ Centauri, but this proposition relied exclusively upon the kinematics and metallicities of the stars to make the association. In this work, we find our two new members of the stream to be metal-poor stars that are enhanced in sodium and aluminium, typical of second population globular cluster stars, but not otherwise seen in field stars. Furthermore, the stars share the s-process abundance pattern seen in $omega$ Centauri, which is rare in field stars. Apart from one star within 1.5 deg of $omega$ Centauri, we find no other stars observed by GALAH spatially near $omega$ Centauri or the Fimbulthul stream that could be kinematically and chemically linked to the cluster. Chemically tagging stars in the Fimbulthul stream to $omega$ Centauri confirms the earlier work, and further links this tidal feature in the Milky Way halo to $omega$ Centauri.
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