Galactic Thin Disk Research Articles

Radial Metallicity Gradients for the Chemically Selected Galactic Thin Disc Main‐Sequence Stars

ABSTRACTWe present the radial metallicity gradients within the Galactic thin disc population through main‐sequence stars selected on the chemical plane using GALAH DR3 accompanied with Gaia DR3 astrometric data. The [Fe/H], [/Fe] and [Mg/H] radial gradients are estimated for guiding radius as , , dex kpc and for the traceback early orbital radius as , , dex kpc for 66,545 thin‐disc stars, respectively. Alteration of the chemical structure within the Galactic disc caused by the radial orbital variations complicates results for the radial metallicity gradient. The effect of radial orbital variations on the metallicity gradients as a function on time indicates the following results: (i) The presence of a gradient along the disc throughout the time for which the model provides similar prediction, (ii) the radial orbital variations becomes more pronounced with the age of the stellar population and (iii) the effect of radial orbital variations on the metallicity gradients is minimal. The effect of radial orbital variations is found to be at most 6% which does not statistically affect the radial gradient results. These findings contribute to a better understanding of the chemical evolution within the Galactic disc and provide an important basis for further research.

Investigating the vertical distribution of the disk as a function of radial action: Results from simulations

Previous research has established a relationship between radial action and scale height in Galactic disks, unveiling a correlation between radial and vertical heating. This finding poses a challenge to our existing comprehension of heating theories and consequently encodes crucial insights into the formation and heating history of Galactic disks. In this study, we perform N-body simulations with the aim of verifying the existence of this correlation between radial action and scale height, thereby enhancing our comprehension of the heating history of Galactic disks. We conducted a simulation featuring a disk embedded within a static dark matter halo potential, and systematically analyzed the correlation between radial action and scale height across every snapshot. Furthermore, we augmented this simulation by incorporating massive, long-lasting particles to examine their impact on the aforementioned relationship. We find that the relationship between radial action and scale height in our simulations can be described by the same functional form observed in previous work. Furthermore, the relationships derived from our simulations align well with those of the Galactic thin disk. However, they do not coincide with the inner thick disk but exhibit a rough correspondence with the outer thick disk, suggesting the possibility that additional heating mechanisms may be required to explain the inner thick disk. We also find that the mean radial action and scale height undergo rapid increases during the initial stages of the simulation, yet remain relatively unchanged as the disk evolves further. By tracing example particles, we uncover a correlation between radial and vertical heating in our simulation: as a particle in the disk gains or loses radial action, its vertical motion tends to oscillate on a more or less extended orbit, accompanied by a tendency to migrate outward or inward, respectively. The massive, long-lasting particles in our simulation contribute to disk heating by solely enhancing the rate of increase in scale height with radial action, while maintaining the functional form that describes the relationship between these two variables. We have successfully replicated the functional form previously reported in research, thereby confirming a correlation between radial and vertical heating. This achievement enhances our understanding of heating theories in galactic disks.

Gaia/GSP-spec spectroscopic properties of γ Doradus pulsators

Context. The third Data Release of the ESA Gaia mission has provided a large sample of new gravity-mode pulsators, among which more than 11 600 are γ Dor stars. Aims. The goal of the present work is to present the spectroscopic parameters of these γ Dor pulsators estimated by the GSP-Spec module that analysed millions of Gaia spectra. Such a parametrisation could help confirm their γ Dor nature and provide their chemo-physical properties. Methods. The Galactic positions, kinematics, and orbital properties of these new Gaia pulsators were examined in order to define a sub-sample belonging to the Milky Way thin disc, in which these young stars should preferentially be found. The stellar luminosities, radii, and astrometric surface gravities were estimated without adopting any priors from uncertain stellar evolution models. These parameters, combined with the GSP-Spec effective temperatures, spectroscopic gravities, and metallicities were then validated by comparison with recent literature studies. Results. Most stars are found to belong to the Galactic thin disc, as expected. It is also found that the derived luminosities, radii, and astrometric surface gravities are high quality and have values typical of genuine γ Dor pulsators. Moreover, we show that Teff and [M/H] of pulsators with high enough S/N spectra or slow to moderate rotation rates are robust. This allowed to define a sub-sample of genuine slow-rotating Gaiaγ Dor pulsators. Their Teff were found to be between ∼6500 and ∼7800 K, log(g) is around 4.2, and the luminosities and stellar radii peak at ∼5 L⊙ and ∼1.7 R⊙, The median metallicity is close to the Solar value, although γ Dor with higher and lower metallicities by about ±0.5 dex were also identified. The [α/Fe] content is fully consistent with the chemical properties of the Galactic disc. Conclusions.Gaia/DR3 spectroscopic properties of γ Dor stars therefore confirm the nature of these pulsators and allow to chemo-physically parametrise a new large sample of such stars. Moreover, future Gaia data releases should drastically increase the number of γ Dor stars with parameters spectroscopically derived with good precision.

The Brown Dwarf Kinematics Project (BDKP). VI. Ultracool Dwarf Radial and Rotational Velocities from SDSS/APOGEE High-resolution Spectroscopy

We present precise measurements of radial (RV) and projected rotational ( vsini ) velocities of a sample of 258 M6 to L2 dwarfs with multiepoch, high-resolution (λ/Δλ = 22,500), near-infrared (1.514–1.696 μm) spectroscopic observations reported in the Apache Point Observatory Galactic Evolution Experiment Data Release 17. The spectra were modeled using a Markov Chain Monte Carlo forward-modeling method, which achieved median precisions of σ RV = 0.4 km s−1 and σvsini = 1.1 km s−1. One-half of our sample (138 sources) are previously known members of nearby young clusters and moving groups, and we identified three new kinematic members of the Argus or Carina Near moving groups, 2MASS J05402570+2448090, 2MASS J14093200+4138080, and 2MASS J21272531+5553150. Excluding these sources, we find that the majority of our sample have kinematics consistent with the Galactic thin disk, and 11 sources are associated with the intermediate thin/thick disk. The field sample has a velocity dispersion of 38.2 ± 0.3 km s−1, equivalent to an age of 3.30 ± 0.19 Gyr based on empirical age–velocity dispersion relations, and a median vsini of 17 km s−1. For 172 sources with multiepoch observations, we identified 37 as having significant RV variations, and determined preliminary orbit parameters for 26 sources with four or more epochs, nine of which are short-period binary candidates. For 40 sources with photometric variability periods from the literature less than 5 days and vsini > 20 km s−1, we find a decline in projected radii ( Rsini ) with age congruent with evolutionary models. Finally, we also present multiepoch RV and vsini measurements for additional 444 candidate ultracool dwarfs.

Chemical compositions of semiregular variable red giants

ABSTRACT A sample of warm, low-level semiregular variables chosen from the General Catalogue of Variable Stars is studied for their chemical compositions by analysing high-resolution optical spectra. The abundance ratios from Na/Fe to Eu/Fe displayed by these and previously analysed semiregular variables are quite similar to ratios displayed by normal red giants across the Galactic thin and thick discs and halo populations in the solar neighbourhood, suggesting from this perspective that the variables may be among the more photometrically active red giants.

Populating the Milky Way

Context.Stellar populations and their distribution differ widely across the Galaxy, which is likely to affect planet demographics. Our local neighbourhood is dominated by young, metal-rich stars in the galactic thin disc, while the stellar halo and galactic bulge host a large fraction of older, metal-poor stars.Aims.We study the impact of these variations on planet populations in different regions of the Galaxy by combining a high-resolution galaxy formation simulation with state-of-the-art planet population synthesis models.Methods.We constructed a population model to estimate occurrence rates of different planet types, based on the New Generation Planet Population Synthesis (NGPPS). We applied this model to a simulated Milky Way (MW) analogue in the HESTIA galaxy formation simulation. We studied the planet occurrence rate in the metal-rich regions of the inner Galaxy, namely, in the galactic bulge and thin disc. We compared these result with the frequencies in the more distant, metal-poor region such as the thick disc and stellar halo.Results.We find that the planet demographics in the central, metal-rich regions of the MW analogue differ strongly from the planet populations in the more distant, metal-poor regions. The occurrence rate of giant planets (>300M⊕) is 10–20 times larger in the thin disc compared to the thick disc, driven by the low amounts of solid material available for planet formation around metal-poor stars. Similarly, low-mass Earth-like planets around Sun-like stars are most abundant in the thick disc, being 1.5 times more frequent than in the thin disc. Moreover, low-mass planets are expected to be abundant throughout the galaxy, from the central regions to the outer halo, due to their formation processes being less dependent on stellar metallicity. The planet populations differ more strongly around Sun-like stars compared to dwarfs with masses 0.3–0.5M⊙, caused by a weaker correlation between [Fe/H] metallicity and planet mass. However, it is important to note that the occurrence rates of low-mass planets are still uncertain, making our findings strongly model-dependent. Massive planets are more comprehensively understood and our findings are more robust. Nonetheless, other systematic effects have the potential to alter the giant planet population that we have not addressed in this study. We discuss some of these limitations and offer further directions for future research.

The Prince and the Pauper: Evidence for the early high-redshift formation of the Galactic α-poor disc population

Context. The presence of [α/Fe]–[Fe/H] bi-modality in the Milky Way disc has intrigued the Galactic archaeology community over more than two decades. Aims. Our goal is to investigate the chemical, temporal, and kinematical structure of the Galactic discs using abundances, kinematics, and ages derived self-consistently with the new Bayesian framework SAPP. Methods. We employed the public Gaia-ESO spectra, as well as Gaia EDR3 astrometry and photometry. Stellar parameters and chemical abundances are determined for 13 426 stars using NLTE models of synthetic spectra. Ages were derived for a sub-sample of 2898 stars, including subgiants and main-sequence stars. The sample probes a large range of Galactocentric radii, ∼3 to 12 kpc, and extends out of the disc plane to ±2 kpc. Results. Our new data confirm the known bi-modality in the [Fe/H]–[α/Fe] space, which is often viewed as the manifestation of the chemical thin and thick discs. The over-densities significantly overlap in metallicity, age, and kinematics and none of them offer a sufficient criterion for distinguishing between the two disc populations. In contrast to previous studies, we find that the α-poor disc population has a very extended [Fe/H] distribution and contains ∼20% old stars with ages of up to ∼11 Gyr. Conclusions. Our results suggest that the Galactic thin disc was in place early, at lookback times corresponding to redshifts of z ∼ 2 or more. At ages of ∼9 to 11 Gyr, the two disc structures shared a period of co-evolution. Our data can be understood within the clumpy disc formation scenario that does not require a pre-existing thick disc to initiate the formation of the thin disc. We anticipate that a similar evolution can be realised in cosmological simulations of galaxy formation.

High-resolution spectroscopy of the young open cluster M 39 (NGC 7092)

M 39 is a nearby young open cluster hardly studied in the last few decades. No giant is known among its members and its chemical composition has never been studied. In order to investigate it, we performed high-resolution spectroscopy of 20 expected cluster members with the HARPS and FIES spectrographs. By combining our observations with archival photometry andGaia-DR3 data we searched for evolved members and studied cluster properties such as the radial velocity, extinction, and age. For the first time, we provide stellar parameters and chemical abundances for 21 species with atomic numbers up to 56. We have not found any new giant as a likely member and we notice a negligible reddening along the cluster field, which we place at 300 pc. We obtain a mean radial velocity for M 39 of −5.5 ± 0.5 km s−1and an isochrone-fitting age of 430 ± 110 Ma, which corresponds to a MSTO mass of around 2.8M⊙. This value is consistent with the Li content and chromospheric activity shown by its members. Based on main-sequence stars, the cluster exhibits a solar composition, [Fe/H] = +0.04 ± 0.08 dex, compatible with its Galactocentric location. However, it has a slightly subsolar abundance of Na and an enriched content of neutron-capture elements, specially Ba. In any case, the chemical composition of M 39 is fully compatible with that shown by other open clusters that populate the Galactic thin disc.

Maser Investigation toward Off-Plane Stars (MIOPS): Detection of SiO Masers in the Galactic Thick Disk and Halo

Studying stars that are located off the Galactic plane is important for understanding the formation history of the Milky Way. We searched for SiO masers toward off-plane O-rich asymptotic giant branch (AGB) stars from the catalog presented by Mauron et al. in order to shed light on the origins of these objects. A total of 102 stars were observed in the SiO J = 1−0, v = 1 and 2 transitions with the Effelsberg 100 m and Tianma 65 m telescopes. SiO masers were discovered in eight stars, all first detections. The measured maser velocities allow for initial estimates of the host AGB stars’ radial velocities. We find that the radial velocities of three stars (namely G068.881−24.615, G070.384−24.886, and G084.453−21.863) significantly deviate from the values expected from Galactic circular motion. The updated distances and 3D motions indicate that G068.881−24.615 is likely located in the Galactic halo, while G160.648−08.846 is probably located in the Galactic thin disk, and the other six stars are probably part of the Galactic thick disk.

North–South asymmetries in the Galactic thin disc associated with the vertical phase spiral as seen using LAMOST-Gaia stars

ABSTRACT We select 1052 469 (754 635) thin disc stars from Gaia eDR3 and LAMOST DR7 in the range of Galactocentric radius R (guiding centre radius Rg) from 8 to 11 kpc to investigate the asymmetries between the North and South of the disc mid-plane. More specifically, we analyse the vertical velocity dispersion profiles ($\sigma _{v_{z}}(z$)) in different bins of R (Rg) and [Fe/H]. We find troughs in the profiles of $\sigma _{v_{z}}(z)$ located in both the North (z ∼ 0.7 kpc) and South (z ∼ −0.5 kpc) of the disc at all radial and chemical bins studied. The difference between the Northern and Southern vertical velocity dispersion profiles ($\Delta \sigma _{v_{z}}(|z|)$) shows a shift between curves of different R and Rg. A similar shift exists in these North–South (NS) asymmetry profiles further divided into different [Fe/H] ranges. The sample binned with Rg more clearly displays the features in the velocity dispersion profiles. The shift in the peaks of the $\Delta \sigma _{v_{z}}$ profiles and the variation in the phase spiral shape binned by metallicity indicate the variation of the vertical potential profiles and the radial metallicity gradient. The wave-like signal in NS asymmetry of $\sigma _{v_{z}}(z)$ largely originates from phase spiral; while the NS asymmetry profiles of [Fe/H] only display a weak wave-like feature near solar radius. We perform a test particle simulation to qualitatively reproduce the observed results. A quantitative explanation of the NS asymmetry in the metallicity profile needs careful consideration of the spiral shape and the perturbation model, and we leave this for future work.

An Inward-moving and Asymmetric Velocity Wave Detected in LAMOST-Gaia

The phase space, as coded by kinematic parameters and chemical abundances, is crucial for understanding the formation of the Galactic disk. Using red giant stars from the Galactic thin disk with [Fe/H] > − 0.8 and low-α ratios identified in LAMOST-Gaia, we detect numerous ridges and undulations in the R–V ϕ diagram coded by median V R . Strikingly, the slope of these features changes from −22 km s−1 kpc−1 to −8 km s−1 kpc−1 at R ∼ 11.5 kpc. Accordingly, the R g–V ϕ plane, also coded by median V R , reveals wave-like structures that propagate outwards in the inner disk but reverse direction and move inwards beyond R g = 11.5 kpc. The most prominent feature is the G1 group, distinguished by its wider spread and negative median V R at R g ∼ 15 kpc, contrasting with the narrower G0 group that exhibits positive median V R at R g < 11.5 kpc. Furthermore, the [C/N] versus [Fe/H] relationship for the G1 group mirrors the opposite trend compared to the G0 group. Since [C/N] serves as a proxy for age, this contrasting behavior suggests an inverse age–metallicity relation for the G1 group. Comparison with the Sagittarius dwarf galaxy reveals that the G1 group possesses distinct [Mg/Fe] and [Al/Fe] ratios, yet its [C/N] versus [Fe/H] pattern is similar to that of the Sgr dwarf galaxy. Based on these observations, we proposed that the inward-moving and asymmetric velocity wave G1 might be linked to the minor merge of the Sagittarius galaxy.

Stellar rotations and the disentanglement of the Galactic thin and thick discs

ABSTRACT The kinematics, mean α-elements, and ages of the stars reveal two distinct disc populations observed in the Solar neighbourhood. Although several studies have been carried out to characterize abundances of chemical elements and kinematic identifications in these populations, there are few studies that dealt with the analysis of stellar rotation and characteristics of populations focused on low-mass stars of the F and G types. In this work, we propose a new approach to classify stellar populations from the thin and thick disc for F and G dwarf stars by relating chemical abundance and rotation (v sin i) of these stars in the rotational–chemical–plane (v sin i–[α/Fe]–[Fe/H] plot). Our results show that the two rotational-chemical sequences disentangled, high- and low-[α/Fe] components, present properties (α-enhanced, ages, kinematics, space velocity gradients, local density ratio) closely linked to the stars of the chemically and kinematically defined as thin and thick disc components. This study has an impact on calibrations that relate ages to rotation (gyro-chronology) as well as age to chemical abundances (chemochronology), as well as sheds light on the understanding of the processes of Galactic disc formation.

The evolution of hot Jupiters revealed by the age distribution of their host stars.

The unexpected discovery of hot Jupiters challenged the classical theory of planet formation inspired by our solar system. Until now, the origin and evolution of hot Jupiters are still uncertain. Determining their age distribution and temporal evolution can provide more clues into the mechanism of their formation and subsequent evolution. Using a sample of 383 giant planets around Sun-like stars collected from the kinematic catalogs of the Planets Across Space and Time project, we find that hot Jupiters are preferentially hosted by relatively younger stars in the Galactic thin disk. We subsequently find that the frequency of hot Jupiters declines with age as [Formula: see text]. In contrast, the frequency of warm/cold Jupiters shows no significant dependence on age. Such a trend is expected from the tidal evolution of hot Jupiters' orbits, and our result offers supporting evidence using a large sample. We also perform a joint analysis on the planet frequencies in the stellar age-metallicity plane. The result suggests that the frequencies of hot Jupiters and warm/cold Jupiters, after removing the age dependence are both correlated with stellar metallicities as [Formula: see text] and [Formula: see text], respectively. Moreover, we show that the above correlations can explain the bulk of the discrepancy in hot Jupiter frequencies inferred from the transit and radial velocity (RV) surveys, given that RV targets tend to be more metal-rich and younger than transits.

Spectral study of the planetary nebula NGC 2452

We provide a detailed study for the Galactic planetary nebula (PN) NGC 2452, which has long been suspected to be a member of the Galactic cluster NGC2453. The present study uses integral field unit (IFU) spectroscopic observations of the PN to explore its physical conditions, chemical abundances, kinematical characteristics, and morphological features. The strength of the singly ionized helium (HeII λ4686) line reveals that the PN is of high excitation class. The presence of the neutral lines of [OI] and [NI] a long with the value of the flux ratio F([Nii]λ6583)/F(Hα) indicates that the object is slightly optically thick. We compute the nebular density and temperature from a large number of diagnostic line ratios. The ionic and total abundances of helium (He) and carbon (C) are derived from their optical recombination lines (ORLs), while the abundances of nitrogen (N), oxygen (O), neon (Ne), argon (Ar), sulphur (S), and chlorine (Cl) are derived from their collisionlly-excitation lines (CELs). Faint lines like Krypton (Kr), Manganese (Mn), and Chromium (Cr), which are known to be products of the slow-process (s-process), are also seen in the PN spectrum. To illustrate the morphology and excitation structures of the PN, we generate a gallery of emission-line maps using the nebular data cubes. We find that the PN has an overall elliptical shape with a few interior clumps. In terms of physical and chemical properties, we find no significant differences between the entire PN and its accompanying clumps, considering the uncertainties associated with these parameters. Both the astronomical data from Gaia and the inferred radial velocity suggest that the PN belongs to the Galactic thin-disk.

Oxygen, sulfur, and iron radial abundance gradients of classical Cepheids across the Galactic thin disk

Context. Classical Cepheids (CCs) are solid distance indicators and tracers of young stellar populations. Dating back to the beginning of the 20th century, they have been safely adopted to trace the rotation, kinematics, and chemical enrichment history of the Galactic thin disk. Aims. The main aim of this investigation is to provide iron, oxygen, and sulfur abundances for the largest and most homogeneous sample of Galactic CCs analyzed so far (1118 spectra of 356 objects). The current sample, containing 70 CCs for which spectroscopic metal abundances are provided for the first time, covers a wide range in galactocentric distances, pulsation modes, and pulsation periods. Methods. Optical high-resolution spectra with a high signal-to-noise ratio that were collected with different spectrographs were adopted to provide homogeneous estimates of the atmospheric parameters (effective temperature, surface gravity, and microturbulent velocity) that are required to determine the abundance. Individual distances were based either on trigonometric parallaxes by the Gaia Data Release 3 (Gaia DR3) or on distances based on near-infrared period-luminosity relations. Results. We found that iron and α-element radial gradients based on CCs display a well-defined change in the slope for galactocentric distances larger than ~12 kpc. We also found that logarithmic regressions account for the variation in [X/H] abundances from the inner to the outer disk. Radial gradients for the same elements, but based on open clusters covering a wide range in cluster ages, display similar trends. This means that the flattening in the outer disk is an intrinsic feature of the radial gradients because it is independent of age. Empirical evidence indicates that the S radial gradient is steeper than the Fe radial gradient. The difference in the slope is a factor of two in the linear fit (−0.081 vs. −0.041 dex kpc−1) and changes from −1.62 to −0.91 in the logarithmic distance. Moreover, we found that S (explosive nucleosynthesis) is underabundant on average when compared with O (hydrostatic nucleosynthesis). The difference becomes clearer in the metal-poor regime and for the [O/Fe] and [S/Fe] abundance ratios. We performed a detailed comparison with Galactic chemical evolution models and found that a constant star formation efficiency for galactocentric distances larger than 12 kpc accounts for the flattening observed in both iron and α-elements. To further constrain the impact of the predicted S yields for massive stars on radial gradients, we adopted a toy model and found that the flattening in the outermost regions requires a decrease of a factor of four in the current S predictions. Conclusions. CCs are solid beacons for tracing the recent chemical enrichment of young stellar populations. Sulfur photospheric abundances, when compared with other α-elements, have the key advantage of being a volatile element. Therefore, stellar S abundances can be directly compared with nebular sulfur abundances in external galaxies.

Eccentricity dynamics of wide binaries – I. The effect of Galactic tides

ABSTRACT A major puzzle concerning the wide stellar binaries (semimajor axes a ≳ 103 au) in the Solar neighbourhood is the origin of their observed superthermal eccentricity distribution function (DF), which is well approximated by P(e) ∝ eα with α ≈ 1.3. This DF evolves under the combined influence of (i) tidal torques from the Galactic disc and (ii) scattering by passing stars, molecular clouds, and substructure. Recently, it was demonstrated that Galactic tides alone cannot produce a superthermal eccentricity DF from an initially isotropic, non-superthermal one, under the restrictive assumptions that the eccentricity DF was initially of power-law form and then was rapidly phase-mixed toward a steady state by the tidal perturbation. In this paper, we first prove analytically that this conclusion is valid at all times, regardless of these assumptions. We then adopt a thin Galactic disc model and numerically integrate the equations of motion for several ensembles of tidally perturbed wide binaries to study the time evolution in detail. We find that even non-power-law DFs can be described by an effective power-law index αeff which accurately characterizes both their initial and final states, and that a DF with initial (effective or exact) power-law index αi is transformed by Galactic tides into another power law with index αf ≈ (1 + αi)/2 on a time-scale $\sim 4\, \mathrm{Gyr}\, (a/10^4\mathrm{AU})^{-3/2}$. In a companion paper, we investigate separately the effect of stellar scattering. As the GAIA data continues to improve, these results will place strong constraints on wide binary formation channels.

Interstellar Bow Shocks around Fast Stars Passing through the Local Interstellar Medium

Bow shocks are produced in the local interstellar medium by the passage of fast stars from the Galactic thin-disk and thick-disk populations with velocities V * = 40–80 km s−1. Stellar transits of local H i clouds occur every 3500–7000 yr on average and last between 104 and 105 yr. There could be 10–20 active bow shocks around low-mass stars inside clouds within 15 pc of the Sun. At local cloud distances of 3–10 pc, their turbulent wakes have transverse radial extents R wake ≈ 100–300 au, angular sizes 10″–100″, and Lyα surface brightnesses of 2–8 R in gas with total hydrogen density n H ≈ 0.1 cm−3 and V * = 40–80 km s−1. These transit wakes may cover an area fraction f A ≈ (R wake/R cl) ≈ 10−3 of local H i clouds and be detectable in IR (dust), UV (Lyα, two-photon), or nonthermal radio emission. Turbulent heating in these wakes could produce the observed elevated rotational populations of H2 (J ≥ 2) and influence the endothermic formation of CH+ in diffuse interstellar gas at T > 103 K.

Origin of neutron-capture elements with the Gaia-ESO survey: the evolution of s- and r-process elements across the Milky Way

ABSTRACT We investigate the origin of neutron-capture elements by analysing their abundance patterns and radial gradients in the Galactic thin disc. We adopt a detailed two-infall chemical evolution model for the Milky Way, including state-of-the-art nucleosynthesis prescriptions for neutron-capture elements. We consider r-process nucleosynthesis from merging neutron stars (MNS) and magneto-rotational supernovae (MR-SNe), and s-process synthesis from low- and intermediate-mass stars (LIMS) and rotating massive stars. The predictions of our model are compared with data from the sixth data release of the Gaia-ESO survey, from which we consider 62 open clusters with age ≳ 0.1 Gyr and ∼1300 Milky Way disc field stars. We conclude that: (i) the [Eu/Fe] versus [Fe/H] diagram is reproduced by both prompt and delayed sources, with the prompt source dominating Eu production; (ii) rotation in massive stars significantly contributes to the first peak s-process elements, but MNS and MR-SNe are necessary to match the observations; and (iii) our model slightly underpredicts Mo and Nd, while accurately reproducing the [Pr/Fe] versus [Fe/H] trend. Regarding the radial gradients, we find that: (i) our predicted [Fe/H] gradient slope agrees with observations from Gaia-ESO and other high-resolution spectroscopic surveys; (ii) the predicted [Eu/H] radial gradient slope is steeper than the observed one, regardless of how quick the production of Eu is, prompting discussion on different Galaxy-formation scenarios and stellar radial migration effects; and (iii) elements in the second s-process peak as well as Nd and Pr exhibit a plateau at low-Galactocentric distances, likely due to enhanced enrichment from LIMS in the inner regions.

The fragility of thin discs in galaxies – II. Thin discs as tracers of the assembly history of galaxies

ABSTRACT Thin galactic discs and nuclear stellar discs (NSDs) are fragile structures that can be easily disturbed by merger events. By studying the age of the stellar populations in present-day discs, we can learn about the assembly history of galaxies and place constraints on their past merger events. Following on the steps of our initial work, we explore the fragility of such disc structures in intermediate-mass-ratio dry encounters using the previously constructed N-body model of the Fornax galaxy NGC 1381 (FCC 170), which hosts both a thin galactic disc and an NSD. We dismiss major and minor encounters, as the former were previously shown to easily destroy thin-disc structures, whereas the latter take several Hubble times to complete in the specific case of FCC 170. The kinematics and structure of the thin galactic disc are dramatically altered by the mergers, whereas the NSD shows a remarkable resilience, exhibiting only a smooth increase of its size when compared to the model evolved in isolation. Our results suggest that thin galactic discs are better tracers for intermediate-mass-ratio mergers, while NSDs may be more useful for major encounters. Based on our simulations and previous analysis of the stellar populations, we concluded that FCC 170 has not experienced any intermediate-mass-ratio dry encounters for at least ∼10 Gyr, as indicated by the age of its thin-disc stellar populations.

On the origin of the Galactic thin and thick discs, their abundance gradients and the diagnostic potential of their abundance ratios

ABSTRACT Using a semi-analytical model of the evolution of the Milky Way, we show how secular evolution can create distinct overdensities in the phase space of various properties (e.g. age versus metallicity or abundance ratios versus age) corresponding to the thin and thick discs. In particular, we show how key properties of the Solar vicinity can be obtained by secular evolution, with no need for external or special events, like galaxy mergers or paucity in star formation. This concerns the long established double-branch behaviour of [alpha/Fe] versus metallicity and the recently found non-monotonic evolution of the stellar abundance gradient, evaluated at the birth radii of stars. We extend the discussion to other abundance ratios and we suggest a classification scheme, based on the nature of the corresponding yields (primary versus secondary or odd elements) and on the lifetimes of their sources (short-lived versus long-lived ones). The latter property is critical in determining the single- or double- branch behaviour of an elementary abundance ratio in the Solar neighbourhood. We underline the high diagnostic potential of this finding, which can help to separate clearly elements with sources evolving on different time-scales and help determining the site of e.g. the r-process(es). We define the ‘abundance distance’ between the thin and thick disc sequences as an important element for such a separation. We also show how the inside-out evolution of the Milky Way disc leads rather to a single-branch behaviour in other disc regions.