page_title: CoRoT
section_title: Red giants and chemical evolution of our galaxy
hierarchical_section_title: CoRoT / Discoveries / Asteroseismology and stellar physics / Red giants and chemical evolution of our galaxy
caption_attribution_description: English: Histograms of theoretical and CoRoT red giant populations in our Galaxy
context_page_description: CoRoT was a space telescope mission which operated from 2006 to 2013. The mission's two objectives were to search for extrasolar planets with short orbital periods, particularly those of large terrestrial size, and to perform asteroseismology by measuring solar-like oscillations in stars. The mission was led by the French Space Agency in conjunction with the European Space Agency and other international partners.
  Among the notable discoveries was CoRoT-7b, discovered in 2009 which became the first exoplanet shown to have a rock or metal-dominated composition.
  CoRoT was launched at 14:28:00 UTC on 27 December 2006, atop a Soyuz 2.1b rocket, reporting first light on 18 January 2007. Subsequently, the probe started to collect science data on 2 February 2007. CoRoT was the first spacecraft dedicated to the detection of transiting extrasolar planets, opening the way for more advanced probes such as Kepler and TESS. It detected its first extrasolar planet, CoRoT-1b, in May 2007, just 3 months after the start of the observations. Mission flight operations were originally scheduled to end 2.5 years from launch but operations were extended to 2013.
context_section_description: Following exhaustion of hydrogen in the core, the overall stellar structure drastically changes. Hydrogen burning now takes place in a narrow shell surrounding the newly processed helium core. While the helium core quickly contracts and heats up, the layers above the hydrogen-burning shell undergo important expansion and cooling. The star becomes a red giant whose radius and luminosity increase in time. These stars are now located on the so-called red giant branch of the Hertzsprung–Russell diagram; they are commonly named RGB stars. Once their central temperature reaches 100 10⁶ K, helium starts burning in the core. For stellar masses smaller than about 2 Mʘ, this new combustion takes place in a highly degenerate matter and proceeds through a helium flash. The readjustment following the flash brings the red giant to the so-called red clump (RC) in the Hertzsprung-Russell diagram.
  Whether RGB or RC, these stars all have an extended convective envelope favorable to the excitation of solar-like oscillations. A major success of CoRoT has been the discovery of radial and long-lived non-radial oscillations in thousands of red giants in the exo field. For each of them, the frequency at maximum power νₘₐₓ in the frequency spectrum as well as the large frequency separation between consecutive modes Δν could be measured, defining a sort of individual seismic passport.
  Red giant population in our galaxy: Introducing these seismic signatures, together with an estimation of the effective temperature, in the scaling laws relating them to the global stellar properties, gravities (seismic gravities), masses and radii can be estimated and luminosities and distances immediately follow for those thousands of red giants. Histograms could then be drawn and a totally unexpected and spectacular result came out when comparing these CoRoT histograms with theoretical ones obtained from theoretical synthetic populations of red giants in our galaxy. Such theoretical populations were computed from stellar evolution models, with adopting various hypotheses to describe the successive generations of stars along the time evolution of our galaxy. Andrea Miglio and collaborators noticed that both types of histograms were spitting images of one another, as can be seen in the histograms picture. Moreover, adding the knowledge of the distances of these thousands of stars to their galactic coordinates, a 3D map of our galaxy was drawn. This is illustrated in the figure where different colors relate to different CoRoT runs and to Kepler observations (green points).
  Age-metallicity relation in our galaxy: The age of a red giant is closely related to its former main sequence lifetime, which is in turn determined by its mass and metallicity. Knowing the mass of a red giant amounts to knowing its age. If the metallicity is known the uncertainty in age does not exceed 15%! Observational missions such as APOGEE (Apache Point Observatoty Galactic Evolution Environment) whose goal is to measure metallicities for 100 000 red giants in our galaxy, GALAH (Galactic Archaeology with HERMES) and GAIA (Global Astrometric Interferometer for Astrophysics) could of course widely benefit from these seismic gravities with the ultimate output of establishing the age-metallicity relation in our galaxy. Asteroseismology has crossed the doorstep of the structure and chemical evolution of our galaxy.
  Seismic signatures and extension of mixed zones during central hydrogen and helium burning: Increasing even further the scrutiny in analyzing the CoRoT and Kepler frequency spectra of red giants brought new important discoveries. Small and subtle differences in seismic signatures allow us to distinguish RGB from RC stars notwithstanding their similar luminosities. This is now theoretically confirmed thanks to elaborate red giant modeling. The period spacings of gravity-dominated modes are expected to be especially meaningful. Their detection for a large number of red giants could give us clues to establishing the extent of the extra-mixed region above the convective core during core hydr
caption: Histograms of a synthetic red giant population (in red) and CoRoT red giant population (in orange). From Andrea Miglio and collaborators
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