Oral Presentation Astronomical Society of Australia Annual Scientific Meeting including HWWS 2013

Adopting the Aquarius Stream: Is the Aquarius Stream the result of a disrupted classical Globular Cluster?  (#63)

Andrew Casey 1 , Stefan Keller 1 , Alan Alves-Brito 1 , Anna Frebel , Gary Da Costa 1 , Amanda Karakas 1 , Kevin Schlaufman 1 , Heather Jacobson , QinQin Yu , David Yong 1 , Ken Freeman 1
  1. RSAA, ANU, Canberra, ACT, Australia

Stellar streams occur from the tidal disruption of a stellar system as it orbits the galaxy, providing ongoing evidence for the hierarchical assembly of galaxies. Usually these streams are distant, and their parent system -- typically a globular cluster or a dwarf galaxy -- are identified before any associated stream. The Aquarius Stream was discovered serendipitously by the RAVE survey and in these respects it is quite distinct: it is relatively close (within 10 kpc) and has no associated parent stellar system. Follow-up spectroscopic data from Wylie de-Boer et al. (2012) suggested that stars in the stream have the same metallicity, and that these stars exhibit chemical abundances that are synonymous with globular cluster stars. Thus, they propose the Aquarius Stream is the result of a disrupted globular cluster. Since the stream is close, the parent ought to be a close by -- and presumably already known -- globular cluster. However, no such parent globular cluster has yet been associated.

We have performed a detailed chemical abundance analysis on five stream members with high S/N, high-resolution spectra taken using the MIKE spectrograph on the Magellan Clay telescope. While our velocities match those taken with low-resolution spectra, our data indicates the stream is not chemically coherent: we find a large range of metallicities even over this small sample. Additionally, we find no strong chemical evidence for globular cluster origin. No Na-O anti-correlation is present, and we find a positive Mg-Al abundance relationship, both inconsistent with that expected from stars enriched in a globular cluster environment. Chemically, from our data the Aquarius Stream appears indistinguishable from the Milky Way thick disk population. Despite that, the stream members do have similar velocities. We propose and show evidence for a non-accreted origin where the Aquarius Stream has resulted from a perturbation or some minor merger with the Milky Way's thick disk, demonstrating that the structure of the Milky Way is more complex than we think.