Formation of the Galactic Bulge 

 Ongoing projects on the Galactic bulge now focus on the determination of ages in fields away from the minor axis, that was the focus of previous studies by the same team, having demonstrated that the Bulge is dominated by stars older than about 10 Gyr. Observations with the new near-IR camera Hawk-I at the VLT are under way for this purpose, along with ultra deep HST imaging to improve a previous measure of the Bulge IMF. In parallel, FLAMES+GIRAFFE and FLAMES+UVES high-resolution spectroscopic observations have been conducted securing spectra for almost 1000 stars, determining elemental abundances of Fe, O, Mg, Si, and other elements, that demonstrate that the abundance ratios O/Fe and alpha/Fe in bulge stars differ from those of disk stars. More similar observations with FLAMES are now planned, also extending the exploration to high-resolution near-IR spectroscopy with CRIRES at the VLT. Discovery of candidate transiting planets among stars in the direction of the Bulge was the result of a large program with HST. Some of the candidates were confirmed as plated via UVES spectroscopy. Using second epoch HST/ACS data a clean proper-motion sample of bulge stars has been obtained showing no trace of stellar populations younger than ∼10 Gyr in the explored field. Further observations are on hold waiting for the refurbished HST after SM4, for which a WFC3 Treasury program has been approved (PI T. Brown) aimed at measuring ages and metallicities of individual stars in several bulge fields, including the candidate planet hosts mentioned above.


People: A. Renzini

Collaboration: M. Zoccali (PUC, Chile), V. Hill (Meudon), B. Barbuy (Sao Paulo), B. Gustafsson (Uppsala), S. Ortolani (Padova Univ.), K. Sahu (STScI), et al.

Publications: Lecureur et al. (2007), A&A 465,799; Zoccali et al. (2008), A&A 486,177; Clarkson et al. (2008), ApJ 684,1110; Brown et al. (2009), AJ 137,3172.

Halo Stars in the Milky Way 

Figure 1:Besancon simulation for a direction in the Galaxy in the broad-band colors and the simulated CaK index. This diagnostic plot will be used to select candidate EMP stars.

Our knowledge of the stellar content of the Milky Way Halo has been completely changed in the last 15 years by the advent of large multi-wavelength photometric surveys. The picture of a smooth Halo has been substituted by a structured system, populated with ghostly streams of dead dwarf galaxies, dying globular clusters and "over-densities" of still-unknown-origin. Although we are starting to understand the recent past, merger history of the Galaxy we do not have yet a clear picture of the origin of the galactic Halo and of the real nature of its elusive and supposedly metal poor stellar population. We have recently started two research projects in which we try to address the following points:  

  • Search of extremely metal poor stars using photometric surveys.
    Extremely metal poor stars provide important clues to the chemical formation history of our Galaxy. Actually the known number of extremely metal poor (EMP) stars ([Fe/H]<-3.0) is of the order of 1 dozen. We are using the Wide Field Imager on the 2.2m telescope, equipped the unique narrow band filter centered on the Ca II H+K lines, in order to identify candidate EMP stars. The Survey (for which already 20 square degrees are available) is performed on SDSS fields with ugriz photometry. The combination of SDSS and narrow band filter will provide a metallicity discrimination down to [Fe/H]=-4.0.
  • Pencil beam deep spectroscopic survey.
    We are using the FLAMES facility to perform a pencil-beam deep stellar survey of the outer Halo in several directions of the Galaxy. This survey is allow us to explore in great detail the chemistry and the dynamics of the outer Galactic Halo. The analysis of the data for 2 deep fields is providing the metallicity distribution together with the kinematics of the stellar components. The high precision of the radial velocities of GIRAFFE will allow us to detect the presence of possible stellar streams.


People:S. Zaggia, Y. Al Momany, M. Gullieuszik 

 Collaboration:P. Bonifacio, E. Caffau (Observatoire Paris-Meudon) 



Il notiziario online dell'Istituto Nazionale di Astrofisica
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