The Evolution of the MW and its Clusters

 The Milky Way is a giant spiral galaxy, with a significant central bulge or more likely a bar. Its evolution might be typical of moderately massive galaxies in relatively low density environments, where both dissipation collapse and accretion events on different scales have to be considered.

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Figure 1:The spiral galaxy NGC 4501, quite similar to the Milky Way (image taken at the TNG telescope)

Since the classical studies by Walter Baade during the second world war, the Milky Way is known to possess different stellar populations, which trace different phases of its evolution. After an early chaotic collapse phase, originating the halo, an early dissipation disk formed (thick disk). However, this early phase was interrupted after a few billion years, perhaps by a major accretion event, probably the accretion of a satellite galaxy with a mass of about 1/10th of the Milky Way at that epoch. After a phase in which star formation was at low level or even unexisting, at least at ∼8 kpc from the galactic centre (the Sun location), star formation started again in a thin disk, that was quite undisturbed in the last 8-10 billion years. The recognition of the hiatus between the thick and thin disk formation is a recent acquisition, dating back only a dozen years. There are still various uncertainties in this general picture, the most important concerning the phase at which the central bulge or bar is formed. However, the central bulge or bar certainly is old. The formation and evolution of the halo is also very complex, with different fragments having their own distinct evolution, possibly extending over several billion years or even over whole Hubble time: some of them are still recognizable as distinct dwarf (spheroidal) galaxies or at least as streams of stars formed in from their disruption due to the tides caused by the Milky Way.  

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Figure 2:The globular cluster NGC 5024 (image taken at the TNG telescope)

 Stellar clusters play an important role in the study of the Milky Way, because they are easily recognizable even at large distances, and their ages can be quite accurately determined. Two main classes of stellar clusters are known, globular and open: the first ones, more massive and centrally concentrated, are more likely related to more violent phases of evolution, while open clusters are more typical of quiet star formation in disks. Astronomers in Padova, both at Astronomical Observatory and Astronomical Department are very active in this field. Very important contributions of our group include:

  • The construction of a scale of metallicity for globular cluster, basic in the determination of their parameters
  • The recognition of the presence of a hiatus in star formation between thick and thin disk phases, from the run of the overabundances of O and Mg with respect to Fe
  • The determination of accurate ages for a few globular clusters, allowing to link the evolution of the Milky Way to the cosmological framework
  • The clarification of the role of mixing episodes in stellar evolution, which was fundamental in order to correctly interpret abundances determined in stars in globular clusters
  • The recognition that also globular clusters have a short but significant chemical evolution, which we studies fo some important cases (omega Centauri)
  • The discovery that stars that are extremely metal-poor but rich in carbon are in binary systems, can be used to constrain the initial mass function of extremely metal-poor stars

The main current activities concerns:

  • extensive studies of the internal evolution of globular clusters
  • reconstruction of the evolution of thin disk using open clusters
  • study of extremely metal poor stars in the Halo to probe the early Galactic chemical evolution
  • constraints on the evolution of extremely metal-poor intermediate mass stars that can be drawn from the analysis of extremely metal poor carbon stars
  • determination of the multiplicity as a function of metallicity among stars in the low end of the Galactic metallicity distribution

 

People: R. Gratton, S. Lucatello 

Collaboration: G. Piotto, S. Ortolani (Padova Univ.), E. Carretta, A. Bragaglia, M. Tosi, (INAF OA Bologna), F. D'Antona (INAF OA Roma), P. Bonifacio (Observatoire de Paris-Meudon and INAF OA Trieste), L. Pasquini, F. Primas (ESO), P. Francois (Observatoire de Paris-Meudon), F. Ferraro (Bologna Univ.), C. Sneden (Texas Univ.), M. Bolte (California Santa Cruz Univ.), J. Johnson (Ohio State Univ.), T. Beers (Michigan State Univ.), N. Christlieb (Uppsala Univ.), F. Herwig (Keele Univ.) 

Publications: Gratton et al. (2007), A&A 464,953


Multiple Stellar Populations in GCs

The ongoing project aims at surveying with HST the most massive globular clusters in the Milky Way, following the discovery that many (most) of them harbor multiple stellar populations, some of which are extremely enriched in helium. This was found for omega Cen, NGC 2808, NGC 1851, NGC 6388 and NGC 6441, and a current Cycle 15 program is dedicated to observe several more massive globulars. Reductions of these data are under way, while one waits for the HST refurbished mission to start an approved program with WFC3 (PI G. Piotto). The main aim is the characterization in terms of age and composition (including helium) of the multiple stellar populations. Besides the HST program, VLT+FLAMES high resolution spectroscopy of stars in the different populations within each cluster has been and is being carried on with the aim at measuring the abundance of iron and other elements (CNO, s-process, etc.), which can provide hints on the formation of such multiple stellar populations and their helium enrichment. The discovery of ubiquitous multiple stellar populations in massive globular clusters has been a major, unexpected result in this field of research.  

People: A. Renzini

Collaboration: G. Piotto (Padova Univ.), I. King (Seattle), L.R. Bedin ( STScI), S. Cassisi (OATE), A. Sweigart (Goddard), et al.

Publications: Piotto et al. (2007), ApJ 661,L53; Villanova et al. (2007), ApJ 663,296; Renzini (2007), astro-ph/0708.1243; Renzini (2008), MNRAS 391,354

News – MEDIA INAF

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