Galactic archaeology: mapping and dating stellar populations with asteroseismology of red-giant stars

Abstract

Our understanding of how the Galaxy was formed and evolves is severely hampered by the lack of precise constraints on basic stellar properties such as distances, masses, and ages. Here, we show that solar-like pulsating red giants represent a well-populated class of accurate distance indicators, spanning a large age range, which can be used to map and date the Galactic disc in the regions probed by observations made by the CoRoT † and Kepler space telescopes. When combined with photometric constraints, the pulsation spectra of such evolved stars not only reveal their radii, and hence distances, but also provide well-constrained estimates of their masses, which are reliable proxies for the ages of the stars. As a first application we consider red giants observed by CoRoT in two different parts of the Milky Way, and determine precise distances for ∼2000 stars spread across nearly 15,000 pc of the Galactic disc, exploring regions which are a long way from the solar neighbourhood. We find significant differences in the mass distributions of these two samples which, by comparison with predictions of synthetic models of the Milky Way, we interpret as mainly due to the vertical gradient in the distribution of stellar masses (hence ages) in the disc. In the future, the availability of spectroscopic constraints for this sample of stars will not only improve the age determination, but also provide crucial constraints on age-velocity and age-metallicity relations at different Galactocentric radii and heights from the plane.

Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields

“…Second, comparisons with stellar models suggest that Eq. (2) for stars similar to those in this study is accurate to ∼3% (see White et al 2011;Miglio et al 2012Miglio et al , 2013, and the analysis presented in Mosser et al 2013). To check the effect of such a systematic uncertainty on log g, we have increased/decreased the observed Δν by 2.5%, while we consider it as the only seismic constraint.…”

“…The discovery of the solar-like oscillations in red giants (e.g., De Ridder et al 2009) and their full exploitation as asteroseismic tools (e.g., Miglio et al 2009Miglio et al , 2013 was one of the most relevant. Therefore, very high signal-to-noise ratio (S /N > ∼ 200) spectra were obtained during the period 2007−2012 to perform an accurate determination of both the stellar parameters (T eff and log g) and the chemical abundances.…”

“…Amongst the most exciting results achievable by asteroseismology of red-giant stars are the possibility of inferring their evolutionary status (e.g., Montalbán et al 2010;Bedding et al 2011;Mosser et al 2011), constraining their rotation profile (e.g., Beck et al 2012;Deheuvels et al 2012), or determining the detailed properties of the core in He-burning stars (Mosser et al 2012, Montalbán et al 2013. In addition, their global seismic properties can provide a high level of accuracy of their fundamental properties, such as masses, radii, and distances, which may then be used to map and date stellar populations in our Galaxy (e.g., Miglio et al 2009Miglio et al , 2013.…”

Asteroseismology of red giants: From analysing light curves to estimating ages

“…With the advent of the Kepler (Borucki et al 2010) and CoRoT (Baglin et al 2006) space missions, time series measuring stellar variability of very-high quality have become widely available. Analysis of these time series can deliver precise estimates of stellar ages Lebreton et al 2014;Meibom et al 2015;Metcalfe et al 2015;Miglio et al 2013b;Silva Aguirre et al 2015) -a quantity critical for reconstructing the history of the Milky Way. With the re-purposed Kepler mission K2 (Howell et al 2014) currently capturing solar-like oscillations Stello et al 2015) in a number of different galactic directions, and the future missions of TESS (Ricker et al 2014) and PLATO (Rauer et al 2014) adding to this, ages for many thousands of stars in many different galactic distances and directions present an exciting possibility.…”

Asteroseismic age determination for dwarfs and giants

“…Thanks to the detection of oscillating modes in thousands of stars across the HRD, stellar properties can now be determined for large samples of targets with an unprecedented level of precision. This opens the exciting possibility of accurately characterising stellar populations in different regions of the Milky Way to constraint the history of formation and evolution of our Galaxy (e.g., Casagrande et al 2014;Miglio et al 2013). Of particular importance in this endeavour are precise age determinations for large cohorts of dwarfs and giants (Casagrande et al 2015;Chaplin et al 2014), which can further help the usual kinematic and chemical dissection of the Galactic disc which has so far driven most of the comparisons between observations and simulations of our Galaxy (e.g., Adibekyan et al 2012;Edvardsson et al 1993;Haywood et al 2013;Schönrich & Binney 2009, just to name a few).…”