The age structure of stellar populations in the solar vicinity Clues of a two-phase formation history of the Milky Way disk

By M. Haywood, Paola Di Matteo, M. Lehnert, D. Katz, Ana G'omez. 2013

394 citations recorded by the source corpus.

We analyze a sample of solar neighborhood stars that have high-quality abundance determinations and show that there are two distinct regimes of [α/Fe] versus age, which we identify as the epochs of the thick and thin disk formation. A tight correlation between metallicity and [α/Fe] versus age is clearly identifiable for thick disk stars, implying that this population formed from a well mixed interstellar medium, probably initially in starburst and then more quiescently, over a time scale of 4−5 Gyr. Thick disk stars have vertical velocity dispersions which correlate with age, with the youngest objects of this population having small scale heights similar to those of thin disk stars. A natural consequence of these two results is that a vertical metallicity gradient is expected in this population. We suggest that the youngest thick disk set the initial conditions from which the inner thin disk started to form about 8 Gyr ago, at [Fe/H] in the range of (−0.1, +0.1) dex and [α/Fe] ∼ 0.1 dex. This also provides an explanation for the apparent coincidence between the existence of a step in metallicity at 7−10 kpc in the thin disk and the confinement of the thick disk within R < 10 kpc. We suggest that the outer thin disk developed outside the influence of the thick disk, giving rise to a separate structure, but also that the high alphaenrichment of those regions may originate from a primordial pollution of the outer regions by the gas expelled from the forming thick disk. Metal-poor thin disk stars ([Fe/H] < −0.4 dex) in the solar vicinity, whose properties are best explained by them originating in the outer disk, are shown to be as old as the youngest thick disk (9−10 Gyr). This implies that the outer thin disk started to form while the thick disk was still forming stars in the inner parts of the Galaxy. Hence, while the overall inner (thick+thin) disk is comprised of two structures with different scale lengths and whose combination may give the impression of an inside-out formation process, the thin disk itself probably formed its first stars in its outskirts. Moreover, we point out that, given the tight age−metallicity and age–[α/Fe] relations that exist in the thick disk, an inside-out process would give rise to a radial gradient in metallicity and α-elements in this population, which is not observed. Finally, we argue that our results leave little room for radial migration (in the sense of churning) either to have contaminated the solar vicinity, or, on a larger scale, to have redistributed stars in significant proportion across the solar annulus.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/11ed0e7522bc9379866bbdd3c66352116a16a0d3

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

In Instrumentation

Citations in the corpus

Chemodynamical evolution of the Milky Way disk - I. The solar vicinity (2012, result): “Thin-disk stars with [Fe/H] < −0.4, referred to as the metal-poor thin disc (e.g., Haywood et al. 2013), appear to originate mostly from beyond the solar radius, in agreement with the conclusions by Haywood et al. (2013).”
Impacts of a flaring star-forming disc and stellar radial mixing on the vertical metallicity gradient (2016, methodology): “We then run the simulations for 8 Gyr which roughly corresponds to the age of the thin disc of the Milky Way (e.g. Haywood et al. 2013).”
Measuring the vertical age structure of the Galactic disc using asteroseismology and SAGA (2015, background): “…with [Fe/H] > −1, implying that this overdensity is associated with disc stars, rather than the halo, and it could be the signature e.g. associated with the formation of the thick disc or enhanced star formation in the early Galaxy (cf. Haywood et al. 2013; Robin et al. 2014; Snaith et al. 2014).”
PLATO as it is: a legacy mission for Galactic archaeology (2017, neutral): “…et al. 2013, 2014; Snaith et al. 2015), where current data suggest that the thick disk formation started at z ∼ 3.5 (12 Gyear ago) while the thin disk began to form at z ∼ 1.5 (8 Gyear ago) (e.g., Bensby et al. 2014; Bergemann et al. 2014; Fuhrmann 2011; Haywood et al. 2013; Robin et al. 2014).”
TRACING CHEMICAL EVOLUTION OVER THE EXTENT OF THE MILKY WAY'S DISK WITH APOGEE RED CLUMP STARS (2014, background): “The Haywood et al. (2013) data (see their Figure 10) indicate a significant overlap in age between the metal-rich end of the high-α sequence and the metal-poor end of the low-α sequence.”
The formation and assembly history of the Milky Way revealed by its globular cluster population (2018, background): “Similarly, we identify six disc stars from the Haywood et al. (2013) sample in Figure 4 that have metallicities [Fe/H] < −0.65 and ages τ < 11 Gyr, making them a natural extension of the GC satellite branch.”
Dissecting simulated disc galaxies - I. The structure of mono-age populations (2014, background): “…so that the youngest ones are also the coldest and the most radially extended (assuming that age can be matched with chemical abundance, as has been shown to be very likely by Haywood et al. 2013 for a sample of solar neighbourhood stars, and by Stinson et al. 2013 for a simulated galaxy).”
The Gaia-ESO Survey : radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk (2014, methodology): “The most recent in-depth analysis of the age-metallicity relation in the solar neighbourhood is that by Haywood et al. (2013) who arrive to similar conclusions.”
Reconstructing the star formation history of the Milky Way disc(s) from chemical abundances (2014, methodology): “There is considerably more scatter in the data in the age-[O/Fe] plot than in the Haywood et al. (2013) sample, and it was not possible to identify a specific inner and outer disc using oxygen abundances.”
The biggest splash (2019, result): “4 shows that the thick disc formation likely continued until 8 Gyr ago, i.e. 1–2 Gyr after the Splash, in good agreement with previous studies (see e.g. Haywood et al. 2013).”
Co-formation of the disc and the stellar halo (2018, background): “…disc is bracketed to have happened between ∼8 and ∼11 Gyr ago, based on white dwarf cooling ages (see Oswalt et al. 1996; Leggett et al. 1998; Knox et al. 1999; Kilic et al. 2017), isochrone modelling (e.g. Haywood et al. 2013; Martig et al. 2016) and nucleocosmochronology (del Peloso et al. 2005).”
THE STELLAR POPULATION STRUCTURE OF THE GALACTIC DISK (2015, background): “Because high-[α/Fe] populations are likely to be the oldest populations in the disk (see discussion in Bovy et al. 2012c and Haywood et al. 2013; Bergemann et al. 2014; Martig et al. 2015), this implies that similar physical conditions existed throughout the disk at early times, a conclusion also…”
StarHorse : a Bayesian tool for determining stellar masses, ages, distances, and extinctions for field stars (2017, positive): “If stellar ages are available as well, they can be used to probe the chemo-dynamical evolution of different Galactic components much more directly (e.g., Zoccali et al. 2003; Haywood et al. 2013; Mitschang et al. 2014; Anders et al. 2017; Mackereth et al. 2017).”
The GALAH survey: scientific motivation (2015, background): “However high-resolution abundance studies (e.g. Fuhrmann 2008; Haywood et al. 2013; Bensby et al. 2014) show that thick disc stars are enhanced in α-elements relative to thin disc stars of similar [Fe/H].”
The structure and dynamical evolution of the stellar disc of a simulated Milky Way-mass galaxy (2016, background): “…at high redshift from chaotic gas accretion during hierarchical assembly (Brook et al. 2004) or in a turbulent, gas-rich disk (Bournaud et al. 2009; Haywood et al. 2013), (3) radial migration of stars from the inner disk to the outer disk (Schönrich & Binney 2009b; Loebman et al. 2011), and (4)…”
The Lives of Stars: Insights from the TGAS–RAVE–LAMOST Data Set (2018, methodology): “Similar endeavors were undertaken using the High-Accuracy Radial velocity Planetary Searcher (Mayor et al. 2003) for about 1,000 stars (see for example Haywood et al. 2013).”
High-precision abundances of elements in solar twin stars - Trends with stellar age and elemental condensation temperature (2015, background): “…mixing of stars born at different galactocentric distances (RG) provided that there is a strong radial gradient in [Fe/H] around the solar distance (Haywood et al. 2013), but then the very small dispersion in [X/Fe] at a given age requires that the star formation history is independent ofRG…”
CHEMICAL CARTOGRAPHY WITH APOGEE: METALLICITY DISTRIBUTION FUNCTIONS AND THE CHEMICAL STRUCTURE OF THE MILKY WAY DISK (2015, background): “…two distinct stellar populations in the solar neighborhood (e.g., Fuhrmann 1998; Prochaska et al. 2000; Reddy et al. 2006; Adibekyan et al. 2012; Haywood et al. 2013; Anders et al. 2014; Bensby et al. 2014; Nidever et al. 2014; Snaith et al. 2014), with one track having roughly solar-[α/Fe]…”