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Individual Stars in Andromeda’s Bulge

1er juin 2001

Astronomers are always anxious to push their observations to the limit. With the advent of the refurbished Hubble Space Telescope (HST), it has become possible to study individual stars at the distance of M31. Resolving stars at greater distance will require the next generation of ground or space based telescopes.

A team of astronomers (including Pascale Jablonka, from Meudon Observatory) took this opportunity to study with the Hubble Space Telescope at visible (WFPC2) and infrared (NICMOS) wavelengths, metal-rich stellar populations in globular clusters and fields close to or in the bulge of M31. These analyses pioneered the field of studies in extremely crowded environments. They give the first precise picture of a galaxy bulge besides the one of our own Galaxy.

With these images, they are able to :

  • Derive estimates for metallicities and ages for stellar clusters
  • Compare the star cluster properties with those of their surrounding field stars
  • Deduce some features of the spiral galaxy formation scenario
Cliquez sur l’image pour la voir en pleine grandeur

The figure above shows the visible and infrared images of two bright globular clusters, G170 and G177, projected on the bulge of M31. The field of view of WFPC2 and NICMOS cameras allows one to see the bulge stars around the clusters. An example of the colour-magnitude diagrams derived for the field stars is presented. The result of a new image deconvolution technique on G170 is also shown.

Conclusions could be made that the globular clusters seen from the ground in projection on M31 bulge are genuine bulge clusters which formed from the same gaseous material as the bulge field stars. No intermediate age population has been detected. The clusters’ very high chemical enrichment and old age indicate that they were formed, as was the bulge, in the very early stages of the galaxy history. The bulge field stellar population presents a large dispersion in abundances and an apparent sharp cut at low metallicity, very similar to what is seen in the bulge of our own Galaxy.

Publications

  • A.W. Stephens, J.A. Frogel, W. Freedman, C. Gallart, P. Jablonka, S. Ortolani, A. Renzini, -* R.M. Rich, R. Davies, 2000, astro-ph/0011047 and astro-ph/0011045
  • Jablonka, P. ; Courbin, F. ; Meylan, G. ; Sarajedini, A. ; Bridges, T. J. ; Magain, P 2000, A&A, 359, 131
  • Jablonka, P. ; Bridges, T. J. ; Sarajedini, A. ; Meylan, G. ; Maeder, A. ; Meynet, G., 1999, ApJ, 518, 627

Contact :

  • P. Jablonka
    DAEC, Observatoire de Paris-Meudon
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