Issue |
A&A
Volume 690, October 2024
|
|
---|---|---|
Article Number | A369 | |
Number of page(s) | 12 | |
Section | Galactic structure, stellar clusters and populations | |
DOI | https://doi.org/10.1051/0004-6361/202449820 | |
Published online | 22 October 2024 |
Chemical abundances in nearby Sun-like stars and the history of the Milky Way disc
ESA exoplanet team,
ESTEC – Postbus 299,
2200
AG
Noordwijk,
The Netherlands
★ Corresponding author; ph.gondoin.astro@gmail.com
Received:
29
February
2024
Accepted:
2
September
2024
Context. The properties of nearby stars bear the imprint of the formation and evolution of the Milky Way (MW). Reconstructing its history requires the determination of precise ages for large samples of stars over long periods.
Aims. The present study aims to address the evolution of the MW disc in the region where the Sun and nearby Sun-like stars formed.
Methods. The evolution of the disc composition in that region during the last 6 Gyr was inferred from the mean abundances of various chemical elements in nearby Sun-like stars. Their age was estimated from their mean chromospheric activity index using an empirical age–activity relationship derived from stellar rotation period measurements in intermediate-age open clusters. The mean abundances versus age of the sample stars were compared with chemical evolution models of metal-rich gaseous discs experiencing an infall of pristine gas after a quenching period of star formation.
Results. The chemical composition of the sample stars reveals two distinct evolutionary trends. Light α elements and iron-peak elements show increasing abundances relative to iron with age. In contrast, the abundance ratios of s-process elements decay with age. Models that best fit the mean abundances of the sample stars as a function of age concur to a gas infall and a concomitant burst of star formation that occurred between 6.2 and 5.5 Gyr ago.
Conclusions. This timeline is consistent with a scenario where the first close pericentric passage of the Sagittarius (Sgr) dwarf galaxy ~5.7 Gyr ago induced an infall of metal-poor gas onto the MW disc and a major burst of star formation. The most massive stars that formed in this event rapidly released α elements via type II supernovae explosions, while intermediate-mass stars returned s-process elements on much longer timescales. The first encounter of the Sgr galaxy with the MW played an important role in determining the long-term evolution of the interstellar medium (ISM) composition in the region of the disc where the Sun and Sun-like stars formed, thus explaining the observed correlations between their chemical abundances and their age.
Key words: stars: abundances / stars: chromospheres / stars: solar-type / Galaxy: disk / Galaxy: evolution / solar neighborhood
© The Authors 2024
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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1 Introduction
Studies of nearby stars provide major contributions to our understanding of the formation and evolution of the Milky Way (MW; Bland-Hawthorn 2016; Helmi 2020). Their properties bear the imprint of the Galaxy history, which can be reconstructed using different techniques. These include the fitting of chemical evolution models to measured stellar abundances (Matteucci 2021); this requires a precise determination of the ages of large sample of stars over long periods.
It has long been known that the emission reversal in the cores of the Ca II H&K Fraunhofer lines of late-type dwarfs is correlated with their age and is potentially an accurate indicator of age (e.g. Wielen 1974). This chromospheric emission is closely linked to their magnetic activity via heating mechanisms and energy transport into their outer atmospheres (Hall 2008). The dynamo generation of magnetic fields in their interiors results from the interplay between convection and stellar rotation (Pallavicini et al. 1981) that decays with age due to rotational braking by stellar winds (Mestel & Spruit 1987).
Barry (1988) was the first to suggest that the distribution of chromospheric emission among nearby Sun-like stars (Wilson 1968; Duncan et al. 1991; Baliunas et al. 1995; Henry et al. 1996) is the result of a recent increase in the stellar birth rate. Based on the age-activity relationship of Soderblom et al. (1991), Rocha-Pinto et al. (2000) later concluded that the disc of our Galaxy has experienced several enhanced episodes of star formation. Using a chromospheric activity catalogue compiled by Boro-Saikia et al. (2018) from various photometric surveys and archival spectra, Gondoin (2020) showed that the bimodal distribution of magnetic activity among local G- and early K-type dwarfs with a near-solar metallicity can be explained as the result of a recent burst of star formation that occurred ~1.9–2.6 Gyr ago in the thin disc and of an old event that could not be dated due to the limited accuracy of the data.
A few years ago, Gomes da Silva et al. (2021) published a new catalogue of Ca II H&K emission from nearby stars using more than 180 000 high-resolution spectra from the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph, as compiled in the AMBRE project (de Laverny et al. 2013). This homogeneous dataset provides a new opportunity to address the formation history of the MW disc using high-precision measurements of the time-averaged chromospheric emission of Sun-like stars. An analysis of this homogeneous dataset (Gondoin 2023) confirms that the bimodal distribution of magnetic activity among local G- and early K-type dwarfs with a near-solar metallicity can be explained as the result of a recent burst of star formation ~2.0 Gyr ago and a more prominent rise of star formation in the solar neighbourhood between 7 and 6 Gyr ago, with a maximum occurring ~5 Gyr ago. This finding is in line with the metallicity distribution in the solar vicinity that is known to peak at solar metallicity, with most of its stars being younger than 7–8 Gyr (e.g. Haywood et al. 2019). This age distribution enables the use of nearby Sun-like stars as probes of the interstellar medium (ISM) composition during the last 7 Gyr.
Various studies (Vergely et al. 2002; Bernard 2017) had previously reported a recent episode of star formation 2–3 Gyr ago in the Milky Way. In view of the long timescale and the large amount of mass involved, some authors (Cignoni et al. 2006; Mor et al. 2019) proposed that this event is not intrinsic to the disc, but was produced by an external perturbation, possibly by a recent merger with a gas-rich satellite galaxy that could have started several gigayears ago. Colour-magnitude diagram fitting of Gaia data (Ruiz-Lara et al. 2020) recently indicated a number of peaks in the star formation rate within about 2 kpc at ages similar to those in the age distribution of nearby Sun-like stars derived from their chromospheric emission. These authors interpret these peaks as being induced by the pericentre passages (Law & Majewski 2010) of the Sagittarius dwarf galaxy (Sgr) discovered by Ibata et al. (1994).
This spheroidal galaxy is the closest example of the ongoing disruption of a dwarf satellite into a large galaxy. The interaction contributes to the build-up of the MW halo in terms of dark matter, stars, and globular clusters (Majewski et al. 2003; Huxor & Grebel 2015; Hasselquist et al. 2019; Bellazzini et al. 2020). It also leaves imprints on the structure, kinematics, and star formation history of the MW disc (Laporte et al. 2019; Ruiz-Lara et al. 2020; Carr et al. 2022). Gondoin (2023) suggested that the turbulence induced during the initial encounter between Sgr and the MW disc may be partly responsible for the increased dispersion of the stellar velocity components of nearby Sun-like stars towards the age of maximum star formation. The author also conjectured that this encounter could explain the observed decay of the mean iron abundance of nearby Sun-like stars from a supersolar value ~6 Gyr ago to a sub-solar value ~4 Gyr ago and the subsequent increase of the [Fe/H] ratio. The gas infall from the satellite galaxy onto the MW disc would have first diluted the mean stellar metallicity after the first pericentric passage. A continuous metal enrichment of the disc would have then progressively compensated the decaying infall of low-metallicity gas leading to an increase in the mean stellar metallicity from ~4 Gyr ago onwards.
The purpose of the present study is to further explore this metal enrichment scenario of the MW disc in the region where the Sun and Sun-like stars formed. The approach consists of searching for additional correlations between chemical abundances and age among nearby Sun-like stars and assessing whether those correlations, if they exist, could be associated with a major star formation event possibly related to a pericentric passage of the Sgr galaxy. Section 2 describes a sample of Sun-like stars extracted from catalogues of nearby stars with measured chromospheric emission and chemical abundances. In Sect. 3, I estimate the mean abundances relative to iron of various chemical elements in coeval stars as a function of age. Section 4 compares these data with simulated abundance ratios derived from chemical evolution models. The results are discussed in Sec. 5 in the light of recent studies.
2 Sample selection
Gomes da Silva et al. (2021) measured the chromospheric emission in the Ca II H&K lines of 1674 FGK main-sequence, subgiant, and giant stars using more than 180 000 high-resolution spectra obtained with the HARPS high-resolution spectrometer between 2003 and 2019. They converted the fluxes to bolometric and photospheric corrected chromospheric emissions and derived the indices of the stars. This index is defined as follows (e.g. Linsky 1979): (1)
where and are the emission fluxes in the cores of the Ca II H&K lines and σ is the Stefan–Boltzmann constant. Stellar atmospheric parameters including iron abundance were retrieved from the literature or determined by the authors using a homogeneous method. The vast majority (≳99%) of the stars have an [Fe/H] ratio between −0.9 and 0.5 and are located within 500 pc from the Sun. The authors used apparent V magnitudes, parallaxes, effective temperature, and metallicity to calculate stellar masses using theoretical isochrones derived from the PAdova and tRieste Stellar Evolutionary Code (PARSEC; Bressan et al. 2012 and a Bayesian estimation method described in da Silva et al. (2006).
I extracted main-sequence stars with masses between 0.85 and 1.0 M⊙ and −0.2 ≤ [Fe/H] ≤ +0.2 from the catalogue of Gomes da Silva et al. (2021). I cross-matched the selected stars with the well-characterised catalogue of objects within 100 pc of the Sun produced from the Gaia Early Data Release 3 (Gaia collaboration 2021). The cross-correlated list contains 222 stars whose indices have been measured at least twice.
I correlated this parent sample of nearby Sun-like stars with a catalogue of chemical abundances (Adibekyan et al. 2012; Delgado Mena et al. 2017) derived from the observations of 1111 FGK stars with a high spectral resolution (~115 000) within the context of the HARPS-GTO programme (Major et al. 2003; Lo Curto et al. 2010; Santos et al. 2011). The authors derived the metal abundances of these stars relative to iron from a standard local thermodynamic equilibrium analysis using measured equivalent widths injected into the code MOOG (Sneden 1973) and a grid of Kurucz ATLAS9 atmospheres (Kurucz 1993). Adibekyan et al. (2012) first performed a uniform and detailed abundance analysis of 12 refractory elements. As a continuation of this work, Delgado Mena et al. (2017) later derived chemical abundances of heavier elements. The present study uses the Mg, Al, Si, Ca, Ti, Cu, Zn, Sr, Y, Zr, Ba, Ce, and Nd abundances of the sample stars extracted from the catalogue published by these authors in the VizieR database (Ochsenbein et al. 2000).
The cross-correlation of the chromospheric activity and chemical abundance catalogues provide the average indices, masses, and abundances of 157 nearby Sun-like stars with masses between 0.85 and 1.0 M⊙ located within 65 pc of the Sun. The left graph in Fig. 1 shows their Gaia colour-magnitude diagram obtained by calculating the stars absolute magnitude in the G band using MG = G + 5–5 log d, with d the distance in parsec. Since the sample stars are located within the local bubble (Lallement et al. 2003), the dimming and reddening of their observed light due to the presence of dust along the line of sight is negligible. Their positions in the colour-magnitude diagram are compared with the PARSEC isochrones (Bressan et al. 2012) projected into the Gaia eDR3 photometric system (Evans et al. 2018). Most stars are located between the 1 Gyr ([Fe/H] = −0.2) and 6 Gyr ([Fe/H] = 0.2) isochrones. The graph on the right in Fig. 1 shows the mean ⟨⟩ index distribution of the sample stars. The grey areas represent the ±1σ envelopes of 1000 realisations of the distribution assuming a Gaussian distribution of the average ⟨⟩ index of each star around the value given in the AMBRE-HARPS catalogue of indices (Gomes da Silva et al. 2021) with the uncertainty provided in the catalogue.
Figure 2 shows the chemical abundances relative to iron of the sample stars as a function of their average indices used as a proxy for age. These ratios include the abundances relative to iron of the Mg, Al, and Si α elements (upper row); the Ti, Cu, and Zn iron-peak elements (upper middle row); and the Sr, Y, Zr, Ba, Ce, and Nd s-process elements (lower rows). The [Ti/Fe] and [Zr/Fe] abundance ratios shown in Fig. 2 are values extracted from Ti II and Zr II lines, respectively. Abundance ratios issued from Ti I lines show a decreasing trend as a function of the ⟨⟩ index similar to that of the abundance ratios issued from the Ti II lines. The abundances derived from the Zr I lines are not considered in the present study due their small equivalent width in the spectra of the hottest stars and the associated error risks (Delgado Mena et al. 2017). Abundances of the Eu r-process element are also provided in the catalogue for a limited number of stars and were not considered in the study.
The graphs in Fig. 2 show that the dispersion of the abundance ratios at a given chromospheric activity level is sometimes larger than the measurement errors. In spite of this dispersion, correlations between the abundances ratios of the sample stars and their chromospheric activity indices are visible. The abundance ratios of the s-process elements increase with increasing ⟨⟩ index. This suggests that the most active stars, that is the youngest, were born in a medium that became richer over time in those elements relative to iron. In contrast, the abundance ratios of the α elements and of the iron peak elements show a decaying trend with increasing ⟨⟩ index. This decaying trend suggests that the less active stars, that is the oldest, were born in a medium where those elements were on average more abundant relative to iron in the past. These correlations between the chemical abundances of the sample stars and their chromospheric activity provide an insight into the time evolution of the interstellar medium (ISM) composition in the region of the Milky Way disc where the Sun and nearby Sun-like stars formed.
Fig. 1 Evolutionary status and magnetic activity distribution of sample stars. Left: colour-magnitude diagram of sample stars compared with the PARSEC isochrones (Bressan et al. 2012) projected into the Gaia eDR3 photometric system (Evans et al. 2018). Right: mean ⟨⟩ index distribution of sample stars. The grey areas represent the ± 1σ envelopes of 1000 realisations of the index distribution assuming a Gaussian distribution of the average ⟨⟩ index of each star around the value provided in the AMBRE-HARPS catalogue of indices with the uncertainty given in the catalogue. |
3 Mean abundance ratios and age estimates
In order to quantify this evolution, I estimated the ages of the sample stars from their masses and average ⟨⟩ indices. Beyond an age of ~600 Myr, stellar age and mass become the main parameters that determine the rotation and magnetic activity of 0.7–1.0 M⊙ stars with a near-solar metallicity (Gondoin 2017, 2018a,b). Gondoin (2020) derived an empirical model of the average index evolution of Sun-like stars combining an activity versus Rossby number relationship (Mamajek & Hillenbrand 2008) with rotation period measurements in intermediate-age open clusters (Meibom et al. 2015; Barnes et al. 2016; Curtis et al. 2019). For main-sequence stars older than 1 Gyr with masses between 0.7 and 1.1 M⊙ and iron abundance between −0.2 dex and +0.2 dex, the age A (in Gyr) of a Sun-like star can be expressed as a function of its mean chromospheric activity index ⟨⟩ and of its mass M (in solar unit) as follows (Gondoin 2023): (2)
where P0(M) is the rotation period of a 1 Gyr old dwarf with a mass M between 0.7 and 1.1 M⊙ (Gondoin 2020) and τc(M) its convective turnover time (Wright et al. 2011).
The determination of the mean chromospheric activity level of a star requires a large number of observations on a time span long enough to filter out short-term variability effects, including the evolution of active regions and activity cycles. The catalogue of Gomes da Silva et al. (2021) provides mean activity indices averaged on several measurements (up to a few hundred) distributed over a long time span (up to a decade or more). The number and precision of these data enable the ages of the sample stars to be closely constrained. The requirement concerning the number of measurements per star and the length of the measurement period is alleviated if, instead of one star, we consider a group of coeval stars.
Gondoin (2023) estimated to ±0.6 Gyr the accuracy of the age–activity relationship applied to a spectroscopic survey of solar-type stars in the solar-age and solar-metallicity open cluster M67 (Giampapa et al. 2006). Simulations of the age distributions of artificially created samples of coeval stars with the same masses, errors on masses, and errors on mean indices of the sample stars indicate that the precision of the age inversion method at a more than 88% confidence level increases from ~540 Myr around 2 Gyr to ~890 Myr at 6 Gyr due to the flattening of the activity versus age relationship.
Since Sun-like stars have the chemical composition of the ISM region where they formed at the epoch of their formation, I estimated the mean abundances of element i relative to iron ⟨[Qi/Fe]⟩(t) in that region as a function of time by averaging the abundance ratios [Qi/Fe]n of element i in star n over a subsample of N = 20 stars with consecutive ages; i.e.: (3)
where An is the age of star n estimated from its average index and its mass according to Eq. (2). In order to propagate the uncertainties on the masses, indices, and abundance ratio measurements, I performed 1000 Monte Carlo realisations of the mean ⟨[Qi/Fe]⟩(Age) distribution of the abundance ratios using the measurements and corresponding errors provided in the HARPS catalogues of chromospheric activity indices (Gomes da Silva et al. 2021) and chemical abundances (Delgado Mena et al. 2017). Figure 3 displays these mean abundance ratios in age bins of 20 stars as a function of the average age of the stars in those bins. The vertical bars represent the ± 1σ errors on the mean abundance values. The upper row shows the abundances relative to iron of the Mg, Al, and Si α elements as a function of age. The middle upper row provides those of the Ti, Cu, and Zn iron-peak elements. The lower rows provide the abundance ratios of the Sr, Y, Zr, Ba, Ce, and Nd s-process elements. The circle in each graph indicates the abundance ratio of the corresponding element in the Sun. The mean [Ti/Fe] ratios of the sample stars derived from the Ti I lines are systematically higher by ~0.04 dex than those derived from the Ti II lines. The later ones are similar to the solar value for stars with solar ages and were selected in the present study.
The analysis confirms the existence of correlations between the mean abundance ratios of the sample stars and their ages. As noted previously, the element abundances relative to iron exhibit two opposite evolutions with time. Light elements including the Mg, Al, and Si α elements; and the Ti, Cu, and Zn iron-peak elements show increasing abundance ratios with age, i.e. decreasing abundance ratios as a function of time. In contrast, the abundance ratios of the s-process elements decrease with stellar age and therefore increase with time. The remaining questions thus concern the origin of these correlations and anti-correlations between the averaged stellar abundances of coeval stars and their age and if those trends are related to the encounter between the Milky Way disc and the Sgr dwarf galaxy.
Fig. 2 Abundances of sample stars relative to iron as a function of their mean ⟨⟩ indices. The upper row provides the abundances of the Mg, Al, and Si α elements. The upper middle row provides the abundances of the Ti, Cu, and Zn iron-peak elements. The lower middle row and the lower row show the abundances of the Sr, Y, and Zr light; and the Ba, Ce, and Nd heavy s-process elements, respectively. |
Fig. 3 Mean abundance ratios of sample stars in age bins of 20 stars as a function of the average age of the stars in those bins. The grey areas represent the ±1σ errors on the mean abundance values. The upper row and the upper middle row provide the mean abundances of the Mg, Al, and Si α elements; and the Ti, Cu, and Zn iron-peak elements, respectively. The lower middle row and the lower row show the mean abundances of the Sr, Y, and Zr light; and the Ba, Ce, and Nd heavy s-process elements, respectively. The circles indicate the corresponding element abundances in the Sun. The dashed lines represent best-fit simulations of the mean abundance ratios with a chemical evolution model of a metal-rich gaseous disc with low initial surface density experiencing an infall of pristine gas (see Model A in Table 1). The continuous lines represent the best-fit simulations of the mean abundance ratios with a chemical evolution model of a metal-rich gaseous disc with a moderate initial surface density experiencing an infall of pristine gas (see Model B in Table 1). |
4 Simulated evolution of abundance ratios
4.1 Model description
According to Ruiz-Lara et al. (2020), the Sgr galaxy was stripped from its gas during its first encounter with the MW ~5.7 Gyr ago, and that is responsible for a major burst of star formation. Such an event is expected to modify the ISM chemical composition in the impacted region of the MW disc. Possible changes include the mixing of the initial chemical composition of the disc with metal-poor gas and the nucleosynthesis of various chemical elements by stars formed during the event. Those effects can be quantified using chemical evolution models of gaseous discs experiencing some gas infall. In the absence of stellar migration, the time derivative of the surface density σi(t) of any heavy element i with an abundance per mass Xi(t) in such discs can be expressed as follows (Matteucci 2021): (5)
where σgas(t) is the surface density of the disc material, Ψ(t) the surface density rate of star formation, and the rate of returned mass of element i by stars in the form of old and new material. The quantities and are the infall and outflow rates of element i expressed in time derivatives of surface densities.
The abundance [Qi/Fe] (t) of element i relative to iron is given by: (7)
where σi(t) and σFe(t) are the surface densities of element i and of iron, respectively; mi and mFe their atomic masses; and log ϵ⊙,i and log ϵ⊙,Fe their solar photospheric abundances (Asplund et al. 2009).
In order to minimise the number of free parameters, I considered models without gas outflow and with an infall of pristine gas; that is to say, with for any metal. I parametrised the surface density rate of star formation Ψ(t) as (8)
where ∑SFR is the integrated star formation during the event, τmax the date of maximum star formation, and 4σSFR the event duration at 95% of the integrated star formation.
The modelling of the mass return from different stellar populations requires the specification of an initial mass function (IMF) that can be conveniently written as a three-part power-law form (Kroupa et al. 2013): (9)
where k is a normalisation constant. The IMF reconstructed from the observation of individual stars in young clusters or in OB associations is generally consistent with a canonical IMF that describes the galactic-field average IMF derived from the solar neighbourhood field (Bastian et al. 2010). Its power-law indices are α1 ≈ −1.3±0.3 for 0.07 ≲ M/M⊙ < 0.5, α2 ≈ −2.3±0.3 for 0.5 ≤ M/M⊙ ≤ 1.0, and α3 = α2 for M/M⊙ > 1.0 (Jerabkova et al. 2018).
The physical processes that govern star formation are related to the interstellar gas properties by so-called star formation laws (e.g. Kennicutt & Evans 2012). Among various formulations, the most often used is the Schmidt-Kennicutt power law (Kennicutt 1989). While the original relation connects the star formation rate (SFR) to the total gas surface density, the relation appears tighter with a more consistent slope across diverse environments when using the surface density of molecular gas instead (e.g. Wong & Blitz 2002; Bigiel et al. 2011). Recent measurements of the SFR and the properties of the star-forming molecular gas on 1.5 kpc scales across 80 nearby galaxies show that the power-law slope of the Schmidt-Kennicutt relation is close to unity in local star-forming galaxies (Sun et al. 2023). As a result, the molecular gas depletion time tdep that characterises the star formation efficiency varies weakly, with typical values of 1–3 Gyr (e.g. Leroy et al. 2008; Saintonge & Catinella 2022). Regarding the molecular gas mass fraction fMG in the Milky Way, Koda et al. (2016) found that it varies predominantly in the radial direction starting from ~100% in the central region to ~50% in the disc’s midplane at solar radius. Based on these observations, I expressed the surface gas density in the solar neighbourhood as follows: (10)
with tdep ~ 2 Gyr and fDG ~ 50%. An initial gas density σq was added to the proportional relationship between the SFR and the molecular gas density to account for a quenching period of star formation prior to the gas infall event. According to several studies (Snaith et al. 2015; Mor et al. 2019; Nissen et al. 2020; Xiang & Rix 2022), such a period occurred ~8 Gyr ago at an epoch when the MW disc had already reached a super-solar metallicity (Haywood et al. 2019) but was not forming stars.
4.2 Nucleosynthesis prescription
The nucleosynthesis of chemical elements can be divided into contributions from low-mass stars (M < 1.3 M⊙), intermediate-mass stars (1.3 ≤ M/M⊙ ≤ 8), and massive stars (M > 8 M⊙). Low-mass stars lock up the gas after their formation, but they do not contribute to the chemical enrichment due to their long lifetimes. In contrast, intermediate-mass stars issued from a gas infall event less than 8 Gyr ago had the time to experience significant mass losses during their late AGB phases. They thus contributed to the ISM enrichment in particular with s-process elements such as Sr, Y, Ba, Ce, Sr, Y, and Nd (Travaglio et al. 1999). Their mass return can be expressed as (11)
where τ(M) is the main-sequence lifetime of a star of mass M (Maeder & Meynet 1989) and QAGB,i(M) its stellar yields (in solar mass units) derived from asymptotic giant branch models of intermediate-mass stars with a near-solar metallicity. In this study, I used the stellar yields from Cristallo et al. (2011, 2015) for low- and intermediate-mass stars. The evolution of each star is followed during the AGB phases with successive thermal pulses and dredge-ups. The FRUITY web page1 gives the stellar masses, core, and surface abundances in each step, as well as the net and total yields for 400 isotopes and ten metallicities. I used the total yields for Z = 0.02; that is, the total ejected mass (new, and already existing in the star when formed) of the most abundant isotope of each considered element.
Single massive stars have short lifetimes (1–10 Myr) and end their lives as type II supernovae. They are responsible for the formation of the bulk of α-elements including Mg, Al, and Si, plus some Fe and Fe-peak elements. Their mass return (including those of type Ib and Ic supernovae) was calculated as (12)
where the limiting mass Mup is a free parameter in the model that was arbitrarily set to 100 M⊙. The nucleosynthesis yields QSNeII,i(M) (in solar masses) for type II supernovae were interpolated from simulation results issued from a recent grid of pre-supernova models of massive stars (Limongi & Chieffi 2018) with solar metallicity. The authors calculated the same sets for three different values of the stellar rotation velocity. In this study, I used the yields of rapidly rotating (vrot = 300 km s−1) massive stars.
The last main contribution to the ISM chemical evolution is another type of supernovae: the Type Ia supernovae (SNe Ia). They result from the thermonuclear explosions of carbon-oxygen white dwarfs in close binaries. They are responsible for the production of the bulk of Fe and Fe peak elements and some α elements. Their explosion is triggered when a white dwarf reaches the Chandrasekhar mass by accretion (Nomoto et al. 1997; Hillebrandt & Niemeyer 2000) in two main candidate scenarios. In the single-degenerate scenario, the accretion is from a non-degenerate companion star (Whelan & Iben 1973; Nomoto et al. 1982). In the double-degenerate scenario, the event results from the merger of two white dwarfs (Iben & Tutukov 1984; Webbink 1984). Using a general formulation of the SNe Ia rate from any progenitor model (Greggio 2005), the returned mass of SNe Ia binary systems can be expressed as (Matteucci 2021) (13)
where τmin, τMax are the minimum and maximum times, respectively, for the SNe Ia explosion. ASNeIa,i is the IMF weighted mass return of binary systems possessing the right characteristics to produce SNe Ia events; that is, (14)
where QSNeIa,i is the nucleosynthesis yield of element i (in solar masses). There is a large variety of possible thermonuclear explosion models (Arcones & Thielemann 2023; Liu et al. 2023), which produce elements in different quantities. In this study, I used the nucleosynthesis yields from the benchmark model of Leung & Nomoto (2018) that represents a statistical average of the SNe Ia that are observed with solar metallicity, ~0.6 M⊙ of 56Ni, and a composition compatible with the solar abundance.
The function DTD(τ) in Eq. (13) describes the distribution of delay times τ before the explosion including the lifetime of the secondary star plus a possible gravitational time. Totani et al. (2008) measured the delay time distribution (DTD) of Type Ia supernovae by using the statistics of the faint variable objects detected in the systematic variable object survey performed as a part of the Subaru/XMM − Newton Deep Survey project. They found that the SNe Ia DTD from 0.1–11 Gyr is well described by a simple power-law DTD(τ) ∝ τα with α = −1.08±0.15. In the present study, I used α = −1.
An estimate of the overall mass return of element i from intermediate- and high-mass stars is obtained by summing up the contributions from the mass loss of intermediate stars in their AGB evolution phase and from supernovae. This calculation takes into account the fraction of intermediate-mass stars in the initial mass functions that produce SNe Ia events. Values from 0.8–1.7% (Manucci et al. 2005) and 14–40% (de Plaa et al. 2007) were compiled by Maoz (2008) after converting to a Salpeter (1955) IMF. Using a binary population synthesis code, Claeys et al. (2014) more recently estimated the percentage of binary systems with a primary mass between 3 and 8 M⊙ that result in an SNe Ia assuming various IMF formulations as 2.3–2.5%. Comparisons of stellar α enhancement predicted by hydrodynamical simulations of disc galaxies with observations supports a value of ~3% for the fraction of binary systems producing SNe Ia (Valentini et al. 2019).
4.3 Simulation results versus measurements
Assuming an infall of pristine gas and neglecting stellar migration and gas outflows, the abundance ratio evolution [Qi/Fe](t) of any heavy element i can be estimated from models described in Sec. 4.1 using the nucleosynthesis prescription described in Sec. 4.2. The free parameters of the model are the star formation parameters and the initial disc properties before the start of the gas infall. These include the initial disc surface density σq, the initial mass fraction of iron, and the initial abundance ratios [Qi/Fe]q relative to iron. The star formation parameters are the integrated star formation ∑star during the event, the age tMax of maximum star formation, and the duration σSFR of the star formation event folded with the age measurement accuracy of the sample stars.
I set the initial mass fraction of iron to 0.28; that is, twice the solar value based on the iron abundance [Fe/H]q = 0.3 reached by an old sequence of stars at this epoch (Nissen et al. 2020; see their Fig. 3). Regarding the initial surface density of gas in the disc prior to the encounter, I consider two cases; namely a low-density (σq = 0.1 M⊙ /pc2) model (hereafter Model A) and a model (hereafter Model B) with a higher density (10 M⊙ /pc2) comparable to the threshold above which star formation occurs in a typical nearby star-forming late-type galaxy (Saintonge & Catinella 2022). For the low surface density case, I assumed a moderate star formation efficiency with tdep = 2 Gyr and fMG= 50% as justified in Sec. 4.1. Since the star formation efficiency increases with the molecular gas density, I used a shorter depletion time (tdep = 0.7 Gyr) and a high fraction of molecular gas (fMG = 100%) (Wong & Blitz 2002; Bigiel et al. 2011) for Model B. These assumptions are made to encompass a wide range of plausible initial conditions compatible with a period of low star formation in the Milky Way disc around the age of 8 Gyr (Snaith et al. 2015; Nissen et al. 2020, Xian & Rix 2022). The assumed surface densities are lower than or comparable to the mean surface density of molecular gas in the inter-arm and outer-disc environments of nearby galaxies (Querejeta et al. 2021).
A best-fit exercise of the model to the estimated ISM abundance ratios as a function of age (see Fig. 3) was conducted. The initial abundance ratio [Qi/Fe]q of each element i and the star formation parameters ∑SFR, τMax, and σSFR were adjusted to minimise the deviations of the simulated abundances from the mean abundance value ⟨[Qi/Fe]⟩(age) of each element i in sub-samples of 20 coeval sample stars (see Sec. 3). A first set of simulations were conducted using the canonical IMF with α3 = α2 = −2.3. The results lead to significant discrepancies between the best-fit parameters derived from the α and Fe-peak elements on one side and those derived from the s-process elements on the other side. The former ones indicated older maximum star formation ages than the latter ones and larger values of the integrated star formation rates. In order to reduce those differences, I investigated the effect of steepening the IMF at high masses.
Such a steeper function deduced from kiloparsec-scale star counts in the solar neighbourhood was constrained by Mor et al. (2018) using the Besançon Galaxy Model fast-approximate-simulations technique, which combines photometric data with a spatial and kinematical model of the Galaxy. Their results are compatible with the canonical IMF for masses lower than 1.53 M⊙. The IMF slope at the high-mass range up to 4 M⊙ is steeper, yielding α3 = −2.9±0.2 or α3 = −3.7±0.2 depending on the used extinction map. These shapes are in line with the general finding that the IMFs reconstructed from individual star cluster and OB associations are consistent with the canonical IMF but steeper for more massive stars when derived from the Galactic field star count (Kroupa et al. 2021). The discrepancies between the best-fit parameters derived from the α and Fe-peak elements on one side and those derived from the s-process elements on the other side are reduced when using an IMF with α3 = −2.9. A good agreement in the star formation parameters derived from most elements is obtained modifying the canonical IMF with α3 = −3.7 for M/M⊙ > 2.6 and assuming that 1% of intermediate-mass stars produce SNeIa events.
Figure 3 compares the best-fit solutions to the mean abundance ratios of the sample stars using either Model A (continuous line) or Model B (dashed line). Good fits to the data are obtained for all chemical elements (except Ca, which is not shown in Figs. 2 and 3). The simulated curves rarely depart from the mean abundance ratios by more than 1σ. The best-fit curves using either Model A or Model B consistently reproduce the decreasing abundance ratios as a function of time of the Mg, Al, and Si α elements; and the Ti, Cu, and Zn iron-peak elements. Hence, they fit the increasing abundance ratios of these elements relative to iron as a function of age. The best-fit curves using either Model A or Model B also reproduce the increasing abundance ratios of the s-process elements as a function of time. They fit the decreasing abundance ratios relative to iron of these elements as a function of age. No acceptable fit to the [Ca/Fe] abundance ratio could be found, as measurements show a constant or slightly increasing trend with time and simulations show a decay.
The best-fit parameters derived from the fitting of the mean abundance ratios of several chemical elements are given in Table 1 for Model A and Model B, respectively. Although good fits to the measured abundance ratios are obtained for all elements, the derived ∑SFR values are different between elements for a given model. In particular, the ∑SFR values derived from the measured [Zr/Fe] and [Nd/Fe] abundance ratios appeared to be significantly larger than those derived from other elements. Measurement uncertainties were taken into account in deriving the mean abundance ratios of the sample stars, but systematic errors may be present. Delgado Mena et al. (2017) recommended the use of the [Zr/Fe] abundances derived from the weighted mean of four Zr II lines for stars with effective temperatures over 5300 K, but from only one line for stars cooler than 5300 K since the other three lines become blended at low effective temperatures. The authors calculated the Nd abundances using four Nd II lines that also become very blended for effective temperatures lower than 5000 K. In view of the difficulties in deriving abundances from the Zr and Nd spectral lines due to unknown blends and equivalent-width variations with photospheric temperatures, the best-fit model parameters derived from these elements may be uncertain and are not reported in Table 1. Similar difficulties in deriving the Zn and Cu abundances from neutral spectral lines are reported by Delgado Mena et al. (2017).
Another source of inconsistencies between the ∑SFR values derived from the abundances of different elements using a same model come from uncertainties on the nucleosynthesis prescription and in particular on production yields. Although major contributions are known to come from explosive nucleosynthesis in supernovae, Lach et al. (2020) argued that the origins of Cu and Zn are still uncertain. The authors analysed the nucleosynthesis yields of various scenarios of type Ia supernova explosions including pure detonations in sub-Chandrasekhar-mass white dwarfs; double detonations and pure-helium detonations of sub-Chandrasekhar-mass white dwarfs with an accreted helium envelope; a violent merger model of two white dwarfs; and deflagrations and delayed detonations in Chandrasekhar-mass white dwarfs. Their models involving helium shell detonations can produce Cu and Zn in supersolar ratios relative to Fe, thus suggesting that type Ia supernova yields in galactic chemical evolution models should be reconsidered.
The influence of stellar activity on stellar spectra and on the determination of stellar abundances (Borrero 2008) is also a candidate source of inconsistencies between the ∑SFR values derived from the fitting of the mean abundance ratios of different chemical elements using a same model. Magnetic fields in active stars are known to affect the profile of some spectral lines through the Zeeman effect and inhomogeneities in their atmospheres due to cool spots and active regions. Spina et al. (2016) studied the equivalent-width variations of 95 Fe lines and 146 lines of other elements (including Mg, Al, Si, Ca, Ti, Cu, Zn, Y, Zr, and Ba) in high-resolution HARPS spectra of 211 Sun-like stars observed at different phases of their activity cycles. The authors observed that the variation of micro-turbulence along the cycle of the most active stars is large enough to lower the abundances of Si, Ti, Fe, and Cu but does not change the abundance of other elements including Mg, Al, Ca, Y, Zr, and Ba. Yana-Galarza et al. (2019) also found that some of the stronger Fe I and Fe II lines in HARPS spectra of the young (~0.4 Gyr), active solar-type stars HD 59967 vary significantly in strength along its activity cycle of ~6 yr and that the derived atmospheric parameters change correspondingly. In contrast, magnetic activity did not significantly affect the abundance that Nissen et al. (2020) derived from weaker Fe lines in HARPS spectra of the older (1.8±0.8 Gyr) ζ1 Ret solar-type star. These studies indicate that magnetic activity can affect the accuracy of the abundance derivation of some elements in high-resolution spectra of young active dwarfs depending on the sensitivity to magnetic fields of the analysed spectral lines not excluding possible blends with weak temperature sensitive lines.
Although uncertainties on abundance measurements and nucleosynthesis yields alter the consistency of the integrated star formation estimates, they do not explain the opposite evolutionary trends of the abundance ratios of elements with low and high atomic numbers; that is, the systematic decay of the abundance ratio relative to iron with increasing index for light α and Fe peak elements and their systematic increase for heavier s-process elements. If one excludes the [Cu/Fe], [Zn/Fe], [Zr/Fe], and [Nd/Fe] abundance ratios, the integrated star formation rates derived from the other elements are consistent within a factor of ~2. The ∑SFR values derived from Model A are included between 15 and 36 M⊙ pc−2, while those derived from Model B are higher (between 160 and 350 M⊙ pc−2), as expected from the higher initial surface density assumed in this model. Furthermore, the age of maximum star formation τMax derived from the best-fit exercise to the measured abundance ratios of the sample stars is similar for all elements including Cu, Zn, Zr, and Nd (but not Ca) for a broad range of initial disc surface densities and star formation efficiencies. It is included between 5.5 and 6.0 Gyr according to Model A and between 5.7 and 6.2 Gyr according to Model B. Good fits to all abundance ratios are obtained for rms width σSFR = 0.6 Gyr of the star formation event. This value is comparable to the expected precision of the age derivation of the sample stars (Gondoin 2023).
Initial abundances and star formation parameters of models that best fit the abundance ratios of the sample stars as a function of age.
5 Discussion
In the present study, I addressed the chemical evolution of the MW disc using abundance measurements in nearby Sun-like stars and the emission in the core of their CaII H&K lines as an age indicator. The study sample consists of 157 nearby Sunlike stars with masses between 0.85 and 1.0 M⊙ and a near solar metallicity (−0.2 ≤ [Fe/H] ≤ +0.2). Their average indices and their masses were extracted from the catalogue of Gomes da Silva et al. (2021) that provides the mean chromospheric activity level of nearby F-, G-, and K-type stars obtained by averaging from a few to several thousand measurements of their chromospheric activity indices distributed on a time span from a few hours up to more than a decade. Their Mg, Al, Si, Ca, Ti, Cu, Zn, Sr, Y, Zr, Ba, Ce, and Nd abundances relative to iron have been derived from the observations of 1111 FGK stars with a high spectral resolution within the context of the HARPS-GTO program (Adibekyan et al. 2012; Delgado Mena et al. 2017). One major finding of the study is the existence of correlations between these abundances and the mean chromospheric activity indices of the sample stars. Although a significant dispersion is observed at any activity level, the abundance ratios of the α elements (with the exception of Ca) and of iron peak elements tend to decay with increasing activity index, while the abundance ratios of the heavier s-process elements increase with increasing activity index.
I used an empirical age–activity relationship (Gondoin 2023) to infer the age of the sample stars from their mean indices. This relationship combines rotation period measurements in intermediate-age open clusters with a rotation-activity relationship. Since late-type dwarfs experience a reduced braking efficiency and stalled spin-down between the ages of ~670 Myr and ~1 Gyr (Meibom et al. 2011, 2015; Rebull 2017; Agueros et al. 2018; Curtis et al. 2019; Douglas et al. 2019), the decay of their index inferred from rotation period measurements in intermediate-age (≥1 Gyr) open clusters is slightly steeper than the activity evolution previously derived from the best fits to measured data in young (<0.7 Gyr) open clusters and in M67 (~4 Gyr) or the Sun (e.g. Skumanich 1972; Lorenzo-Oliveira et al. 2018; Zhang et al. 2019).
The ISM composition in the region where the sample stars formed at the epoch of their formation was estimated by calculating the mean abundances of the various elements relative to iron in sub-samples of 20 stars with consecutive ages. The analysis revealed close relations between the mean abundance ratios of coeval stars and their ages. Two distinct evolutionary trends were observed. Light α elements including Mg, Al, and Si; and the Ti, Cu, and Zn iron-peak elements show increasing abundance ratios with age. In contrast, the abundance ratios of heavier s-process elements (Sr. Y, Zr, Ba, Ce, Nd) decay with age. Only, one α element, calcium, departs from this general behaviour, with a slight decay of its abundance relative to iron with age.
Several spectroscopic studies have derived accurate chemical abundances and atmospheric parameters from equivalent-width measurements in high-resolution spectra using line-by-line excitation and ionisation balance analysis relative to the solar spectrum. Nissen (2015) determined the atmospheric parameters and high-precision abundances of 14 elements from C to Y in a sample of 21 solar twins, spanning an age interval of 8 Gyr. Their analysis indicates linear correlations between the abundance ratios of several elements relative to iron and stellar ages. The correlations are positive for most of the elements from C to Zn with the exceptions of Ca and Cr that show a flat and a slightly negative correlation, respectively. In contrast, Y appears to decrease with increasing age, which is in agreement with previous studies of s-process elements. These evolutionary trends were confirmed by Spina et al. (2016) in a sample of 14 solar twins with, however, larger slopes for some elements. These results are consistent with the present study, showing an increase of the abundance ratios of light elements relatively to iron until [Zn/Fe] and a decay with age of the abundance ratio of elements heavier than Sr, including yttrium in particular. Bedell et al. (2018) also found two major [X/Fe] abundance trends among 79 nearby Sun-like stars. As observed in the present study, these ratios increase with age from C to Zn and decrease with age for s-process elements including Sr, Y, Zr, Ba, La, Ce, and Nd. In agreement with those trends, a variety of abundance ratios between s-process elements (from Sr to Ce) and lighter elements (from C to Zn) are sensitive chemical clocks (Jofre et al. 2020), including [Y/Mg] in particular (Nissen 2015; Tucci Maia et al. 2016). The relationship between [Y/Mg] and ages among solar twins is similar to that outlined by red clump stars in open clusters younger than 8 Gyr that are located at galactocentric distances between 6 and 11 kpc and have near solar metallicity (−0.2 ≤ [Fe/H] ≤ +0.2) (Casamiquela et al. 2021). However, several authors noted that such age-chemical clock relationships are not valid for the whole disc, but they depend on the galactocentric position (Casali et al. 2020; Viscasillas Vasquez et al. 2022). These results indicate that stellar age plays an important role in determining the abundance pattern of nearby stars. The correlations or anti-correlations between the chemical abundances and the age of the sample stars suggest a coherent evolution of the ISM composition on large scales since stars that are nearby today may have formed in distant regions.
During their life, stars can indeed migrate to different orbits due to interactions with non-axisymmetric disturbances within the disc such as bars, spiral arms, or mergers (e.g. Sellwood & Binney 2002; Röskar et al. 2008; Quillen et al. 2009; Schönrich & Binney 2009; Minchev & Famaey 2010; Grand et al. 2012; Frankel et al. 2018; Carr et al. 2022). These migration effects cause the oldest stars in the solar neighbourhood to be potentially issued from more distant regions. Although the information about the birthplace of nearby stars is lost, Minchev et al. (2018) attempted to retrieve their birth radii from spectroscopic data and age determinations using a few prior assumptions. Applying this approach to stars from the AMBRE:HARPS (De Pascale et al. 2014; Hayden et al. 2017) and HARPS-GTO (Adibekyan et al. 2012; Delgado Mena et al. 2017) samples with stellar ages computed using the STARHORSE code (Anders et al. 2018; Queiroz et al. 2018), the authors found that nearby stars beyond [Fe/H] > 0.2 have birth radii inside 5 kpc (see right column of Fig. 8 in Minchev et al. 2018). They also concluded that stars with the largest birth radii belong to the metal-poor end of the low-[Mg/Fe] sequence, as expected from their high rotational velocities (e.g. Haywood 2008). These results support the view that most metal-rich stars in the solar neighbourhood likely migrated from a region with earlier and more rapid star formation such as the inner Galaxy (Feuillet et al. 2018), while low-metallicity stars formed in outer regions of the disc (e.g. Luck & Lambert 2011; Lemasle et al. 2013; Anders et al. 2014; Hayden et al. 2014). This does not exclude that some migrators may be part of the present sample (Anders et al. 2018). The main effect of stellar migration on the present study of nearby stars with a near solar metallicity (−0.2 ≤ [Fe/H] ≤ +0.2) is thus that it averages the ISM abundance estimates in the disc on a radial scale that increases with age up to a few kpc around the present-day galactocentric radius of the Sun. Minchev et al. (2018) estimated the solar birth radius to 7.3±0.6 kpc compared with a recent measurement of the current Galactic centre distance at 8.178±0.035 kpc (GRAVITY Collaboration 2019).
Based on the hypothesis that the evolution of the metallicity of nearby Sun-like stars could be related to some gas infall (Gondoin 2023), I compared the mean abundance ratios of the sample stars as a function of age with chemical evolution models of metal-rich gaseous discs with different initial surface densities experiencing some gas infall after a quenching period of star formation. The magnitude of the event is described in the models by the integrated star formation density. It could be estimated from the abundance evolution in the disc if its initial surface density and the composition of the infalling material were known. The models use a number of simplifying assumptions including the hypothesis of a pristine gas infall. According to a recent spectroscopic study (Minelli et al. 2023), the metallicity distribution in a region that best approximates the conditions near the centre of the Sgr progenitor is characterised by a peak at [Fe/H] ~ −0.5 plus a wide and weak tail reaching [Fe/H] ≤ −2.0. Such a metallicity distribution in the progenitor of the Sgr system is reproduced by chemical evolution models (Muciarelli et al. 2017) assuming that the bulk of the stellar populations of Sgr formed from 14 to 7 Gyr ago and that the Sgr progenitor starts losing a large fraction of its gas due to the interaction with the MW, until the star formation definitively stops, about 6 Gyr ago. These studies and other metallicity distribution functions derived from large APOGEE samples (Hayes et al. 2020; Hasselquist et al. 2021) suggest that a fraction of the gas that fell onto the MW disc during its encounter with the Sgr dwarf galaxy was not pristine, but had a sub-solar metallicity.
The models also ignore starburst driven outflows from supernovae and winds from massive stars that regulate star formation and affect the metallicity of galaxy discs (Dalcanton 2007). They do not take into account stellar migration that plays an important role in the evolution of their metallicity gradient either (e.g. Johnson et al. 2021; Anders et al. 2023; Chien et al. 2023; Willet et al. 2023; Sun et al. 2024; Wang et al. 2024). These model simplifications affect the simulation results. In particular, Ratcliffe et al. (2024) illustrate how radial migration can change the [s-process/Mg] radial gradient with time explaining that there is no universal [s-process/Mg] relation across the Galaxy disc (Viscasillas Vasquez et al. 2022).
Ratcliffe et al. (2024) also suggest that the close relationships between the [s-process/Mg] abundance ratios and the ages of nearby solar-type stars is due to a tight correlation between their birth radii and their ages. The simulation results in the present study suggest that these chemical-age relationships among nearby Sun-like stars with a near solar metallicity could be also linked to a recent event in the Milky Way history. They show that gas infall models that encompass initial conditions compatible with a period of low star formation ~8 Gyr ago reproduce the opposite evolution trends to the abundance ratios of α and Fe-peak elements on one side and of s-process elements on the other. For a broad range of low initial surface densities of the disc, the models that best fit the mean abundance evolution of the sample stars as a function of age concur with a gas infall event and a concomitant burst of star formation that occurred between 6.2 and 5.5 Gyr ago. According to the models, the most massive stars (M > 8–10 M⊙) formed during this event would have soon exploded as core-collapse supernovae, thus rapidly enriching the ISM mainly with α elements (O, Ne, Mg, S, Si, Ca), but also with some Fe, light s-process elements, and perhaps r-process elements. In contrast, s-process elements such as Ba, Y, and Sr would have been released with some delays during the late AGB phases of intermediate-mass stars (1.3–8 M⊙) that die as C-O white dwarfs when single and can die as Type I supernovae when in binaries. A major burst of star formation 5.5–6.2 Gyr ago could thus explain the positive correlation between the abundance ratios of α and Fe-peak elements relative to iron in nearby Sun-like stars and their age. It could also explain the anti-correlation between their abundances in s-process elements relative to iron and their age.
The estimated age of the star formation burst is close to that of a prominent peak ~5.7 Gyr ago in the formation rate of stars within 2 kpc of the Sun derived from a colour-magnitude diagram fitting of Gaia data (Ruiz-Lara et al. 2020). The authors interpret this peak as being induced by the first pericentre passage of the Sgr dwarf galaxy discovered by Ibata et al. (1994). The first encounter between the Milky Way and the Sgr dwarf galaxy could thus have played a major role in determining the long-term evolution of the ISM composition in the region of the MW disc where the Sun and Sun-like stars formed.
The age of this event is consistent with an interpretation of the age-metallicity distribution among solar-type stars as evidence of two episodes of accretion of gas onto the Galactic disc with a quenching of star formation between them (Nissen et al. 2020). The Sgr galaxy would be a major actor in the evolution of the MW disc in this second episode. An important infall of pristine or low-metallicity gas from the Sgr galaxy onto the MW disc could explain that the mean iron abundance of nearby Sunlike stars first decays from a super-solar value ([Fe/H] ~ +0.05) ~ 6 Gyr ago to a sub-solar value ([Fe/H] ≤ −0.05) ~ 4 Gyr ago (Gondoin 2023). A burst of star formation triggered by this event would have then initiated a continuous metal re-enrichment of the disc over gigayears. The continuous mass return of evolved stars with different masses would have progressively compensated the decaying infall of low-metallicity gas leading to an increase in the mean stellar metallicity from ~4 Gyr ago until the recent past as observed. The present study suggests that successive explosions of carbon-oxygen white dwarfs that either accreted matter from companion red giant stars or that merged with a white dwarf companion would be major contributors of this increase of the mean iron abundance.
Such a scenario suggested by the correlations between the chemical abundances of nearby Sun-like stars and their age is supported by hydrodynamical simulations providing a very close pericentric passage (<20 kpc) of the Sgr precursor with a substantial mass loss (Annem & Khoperskov 2022). These simulations also indicate that the impact of the satellite perturbations, especially during its first encounters, is the largest in the outer disc, causing a dilution of the mean stellar metallicity. It thus affected the radial metallicity gradient of stellar populations formed soon after the passages. This effect could account for observed fluctuations in the disc metallicity gradient (Ratcliffe et al. 2023), which begins to steepen at the estimated time of the Sgr pericenter passages.
6 Summary
The present study addresses the chemical evolution of the MW disc during the last 6 Gyr using abundance measurements in nearby 0.85–1.0 M⊙ stars with −0.2 ≤ [Fe/H] ≤ +02. An empirical age–activity relationship derived from stellar-rotation-period measurements in intermediate-age open clusters was used to estimate their age from their mean chromospheric activity index. The analysis revealed close relations between the mean abundance ratios of coeval stars and their ages. Two distinct evolutionary trends were observed. Light α elements (Mg, Al, Si) and iron-peak elements (Ti, Cu, Zn) show increasing abundances relative to iron with age. In contrast, the abundance ratios of s-process elements (Sr. Y, Zr, Ba, Ce, Nd) decay with age. Only one α element, calcium, departs from this general behaviour.
I compared these measurement results with chemical evolution models of gaseous discs experiencing an infall of pristine gas. Models with initial super-solar metallicity and disc surface densities compatible with an absence of star formation ~8 Gyr ago account for the opposite evolution trends of the abundance ratios of α and Fe-peak elements on one side and of the s-process elements on the other. The models that best fit the mean chemical abundances of the sample stars as a function of age concur to a gas infall event and a concomitant burst of star formation that occurred between 6.2 and 5.5 Gyr ago. This age is similar to that of a peak ~5.7 Gyr ago in the star formation rate within about 2 kpc of the Sun derived from a colour-magnitude diagram fitting of Gaia data by Ruiz-Lara et al. (2020).
The present study supports a scenario (Gondoin 2023) where the significant infall of pristine or low-metallicity gas from the Sgr galaxy onto the MW disc first diluted its iron abundance, while a concomitant burst of star formation led to a progressive metal re-enrichment of the disc in the following gigayears. The most massive stars (M > 8 M⊙) formed in this event rapidly enriched the ISM with α elements via stellar winds and terminal explosions as type II supernovae. Successive explosions of carbon-oxygen white dwarfs then contributed to the increase of the mean iron abundance in the disc. Intermediate-mass stars formed during the encounter have been returning s-process elements to the ISM on a long timescale via stellar winds and envelope ejections. The observed correlation between the chemical composition and the age of the sample stars thus suggests that the first encounter between our Galaxy and the Sgr dwarf galaxy played a major role in determining the long-term evolution of the ISM composition in the region of the MW disc where the Sun and Sun-like stars formed.
Acknowledgements
This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France and of the NASA’s Astrophysics Data System Bibliographic Services. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. I am grateful to the anonymous referee for the helpful comments that allowed me to improve the paper.
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All Tables
Initial abundances and star formation parameters of models that best fit the abundance ratios of the sample stars as a function of age.
All Figures
Fig. 1 Evolutionary status and magnetic activity distribution of sample stars. Left: colour-magnitude diagram of sample stars compared with the PARSEC isochrones (Bressan et al. 2012) projected into the Gaia eDR3 photometric system (Evans et al. 2018). Right: mean ⟨⟩ index distribution of sample stars. The grey areas represent the ± 1σ envelopes of 1000 realisations of the index distribution assuming a Gaussian distribution of the average ⟨⟩ index of each star around the value provided in the AMBRE-HARPS catalogue of indices with the uncertainty given in the catalogue. |
|
In the text |
Fig. 2 Abundances of sample stars relative to iron as a function of their mean ⟨⟩ indices. The upper row provides the abundances of the Mg, Al, and Si α elements. The upper middle row provides the abundances of the Ti, Cu, and Zn iron-peak elements. The lower middle row and the lower row show the abundances of the Sr, Y, and Zr light; and the Ba, Ce, and Nd heavy s-process elements, respectively. |
|
In the text |
Fig. 3 Mean abundance ratios of sample stars in age bins of 20 stars as a function of the average age of the stars in those bins. The grey areas represent the ±1σ errors on the mean abundance values. The upper row and the upper middle row provide the mean abundances of the Mg, Al, and Si α elements; and the Ti, Cu, and Zn iron-peak elements, respectively. The lower middle row and the lower row show the mean abundances of the Sr, Y, and Zr light; and the Ba, Ce, and Nd heavy s-process elements, respectively. The circles indicate the corresponding element abundances in the Sun. The dashed lines represent best-fit simulations of the mean abundance ratios with a chemical evolution model of a metal-rich gaseous disc with low initial surface density experiencing an infall of pristine gas (see Model A in Table 1). The continuous lines represent the best-fit simulations of the mean abundance ratios with a chemical evolution model of a metal-rich gaseous disc with a moderate initial surface density experiencing an infall of pristine gas (see Model B in Table 1). |
|
In the text |
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