Measuring the vertical age structure of the Galactic disc using asteroseismology and SAGA

L. Casagrande*, V. Silva Aguirre, K. J. Schlesinger, D. Stello, D. Huber, A. M. Serenelli, R. Schönrich, S. Cassisi, A. Pietrinferni, S. Hodgkin, A. P. Milone, S. Feltzing, M. Asplund

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    121 Citations (Scopus)

    Abstract

    The existence of a vertical age gradient in the Milky Way disc has been indirectly known for long. Here, wemeasure it directly for the first time with seismic ages, using red giants observed by Kepler. We use Strömgren photometry to gauge the selection function of asteroseismic targets, and derive colour and magnitude limits where giants with measured oscillations are representative of the underlying population in the field. Limits in the 2MASS system are also derived. We lay out a method to assess and correct for target selection effects independent of Galaxy models. We find that low-mass, i.e. old red giants dominate at increasing Galactic heights, whereas closer to theGalactic plane they exhibit a wide range of ages and metallicities. Parametrizing this as a vertical gradient returns approximately 4 Gyr kpc-1 for the disc we probe, although with a large dispersion of ages at all heights. The ages of stars show a smooth distribution over the last ≃10 Gyr, consistent with a mostly quiescent evolution for the Milky Way disc since a redshift of about 2. We also find a flat age-metallicity relation for disc stars. Finally, we show how to use secondary clump stars to estimate the present-day intrinsic metallicity spread, and suggest using their number count as a new proxy for tracing the ageing of the disc. This work highlights the power of asteroseismology for Galactic studies; however, we also emphasize the need for better constraints on stellar mass-loss, which is a major source of systematic age uncertainties in red giant stars.

    Original languageEnglish
    Pages (from-to)987-1007
    Number of pages21
    JournalMonthly Notices of the Royal Astronomical Society
    Volume455
    Issue number1
    DOIs
    Publication statusPublished - 1 Jan 2016

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