Galactocentric Distance Research Articles

Characterizing dynamical states of Galactic open clusters with Gaia DR2

ABSTRACT In this work, we investigate the dynamical properties of 38 Galactic open clusters: 34 of them are located at low Galactic latitudes (|b| < 10°) and are projected against dense stellar fields; the other four comparison objects present clearer contrasts with the field population. We determine structural and time-related parameters that are associated with the clusters’ dynamical evolution: core (rc), tidal (rt), and half-mass (rhm) radii, ages (t), and crossing times (tcr). We have also incorporated results for 27 previously studied clusters, creating a sample of 65, spanning the age and Galactocentric distance (RG) ranges $7.0 \lt \textrm {log}\, t \lt 9.7$ and 6 < RG (kpc) < 13. We employ a uniform analysis method which incorporates photometric and astrometric data from the Gaia DR2 catalogue. Member stars are identified by employing a decontamination algorithm which operates on the 3D astrometric space of parallax and proper motion and attributes membership likelihoods for stars in the cluster region. Our results show that internal relaxation causes rc to correlate negatively with the dynamical ratio τdyn = t/tcr. This implies that dynamically older systems tend to be more centrally concentrated. The more concentrated ones tend to present smaller rhm/rt ratios, which means that they are less subject to tidal disruption. The analysis of coeval groups at compatible RG suggests that the inner structure of clusters is reasonably insensitive to variations in the external tidal field. Additionally, our results confirm, on average, an increase in rt for regions with less-intense Galactic gravitational fields.

Kinematic Properties of Young Intermediate- and Low-Mass Stars from the Gaia DR2 Catalogue

We have studied the kinematic properties of young pre-main-sequence stars. We have selected these stars based on data from the Gaia DR2 catalogue by invoking a number of photometric infrared surveys. Using 4564 stars with parallax errors less than 20\%, we have found the following parameters of the angular velocity of Galactic rotation: $\Omega_0 =28.84\pm0.10$ km s$^{-1}$ kpc$^{-1}$, $\Omega^{'}_0=-4.063\pm0.029$ km s$^{-1}$ kpc$^{-2}$ and $\Omega^{''}_0=0.766\pm0.020$ km s$^{-1}$ kpc$^{-3}$, where the Oort constants are $A=16.25\pm0.33$ km s$^{-1}$ kpc$^{-1}$ and $B=-12.58\pm0.34$ km s$^{-1}$ kpc$^{-1}$. The circular rotation velocity of the solar neighborhood around the Galactic center is $V_0=230.7\pm4.4$ km s$^{-1}$ for the adopted Galactocentric distance of the Sun $R_0=8.0\pm0.15$ kpc. The residual velocity dispersion for the stars considered is shown to be low, suggesting that they are extremely young. The residual velocity dispersion averaged over three coordinates is $\sim$11 km s$^{-1}$ for Herbig Ae/Be stars and $\sim$7 km s$^{-1}$ for T Tauri stars.

Discovery of Magellanic Stellar Debris in the H3 Survey

Abstract We report the discovery of 15 stars in the H3 survey that lie, in projection, near the tip of the trailing gaseous Magellanic Stream (MS). The stars have Galactocentric velocities <−155 km s−1, Galactocentric distances of ≈40 to 80 kpc (increasing along the MS), and [Fe/H] consistent with that of stars in the Small Magellanic Cloud. These 15 stars comprise 94% (15 of 16) of the H3 observed stars to date that have R GAL > 37.5 kpc, −350 km s−1 < V GSR < −155 km s−1, and are not associated with the Sagittarius Stream. They represent a unique portion of the Milky Way’s outer halo phase space distribution function and confirm that unrelaxed structure is detectable even at radii where H3 includes only a few hundred stars. Due to their statistical excess, their close association with the MS and HI compact clouds in the same region, both in position and velocity space, and their plausible correspondence with tidal debris in a published simulation, we identify these stars as debris of past Magellanic Cloud encounters. These stars are evidence for a stellar component of the tidal debris field far from the Clouds themselves and provide unique constraints on the interaction.

Mass–radius relation of intermediate-age disc super star clusters of M82

ABSTRACT We present a complete set of structural parameters for a sample of 99 intermediate-age super star cluster (SSCs) in the disc of M82, and carry out a survival analysis using the semi-analytical cluster evolution code emacss. The parameters are based on the profile-fitting analysis carried out in previous work, with the mass-related quantities derived using a mass-to-light ratio for a constant age of 100 Myr. The SSCs follow a power-law mass function with an index α = 1.5, and a lognormal size function with a typical half-light radius, Rh = 4.3 pc, which is both comparable with the values for clusters in the Magellanic Clouds, rather than in giant spirals. The majority of the SSCs follow a power-law mass−radius relation with an index of b = 0.29 ± 0.05. A dynamical analysis of M82 SSCs using emacss suggests that 23 per cent of the clusters are tidally limited, with the rest undergoing expansion at present. Forward evolution of these clusters suggests that the majority would dissolve in ∼2 Gyr. However, a group of four massive compact clusters, and another group of five SSCs at relatively large galactocentric distances, are found to survive for a Hubble time. The model-predicted mass, Rh, μV, and core radius of these surviving SSCs at 12 Gyr are comparable with the corresponding values for the sample of Galactic globular clusters.

Young stellar population gradients in central cluster galaxies from NUV and optical spectroscopy

ABSTRACT Central cluster galaxies are the largest and most massive galaxies in the Universe. Although they host very old stellar populations, several studies found the existence of blue cores in some BCGs indicating ongoing star formation. We analyse VLT/X-Shooter stacked spectra of 6 nearby massive central galaxies with high central velocity dispersions (σ > 300 km s−1) at different galactocentric distances. We quantify the young stellar population out to 4 kpc by fitting near-UV and optical absorption line indices with predictions of composite stellar populations modelled by an old and a young stellar component. We also use IMF-sensitive indices since these galaxies have been found to host a bottom-heavy IMF in their central regions. We derive negative young stellar populations gradients, with mass fractions of stars younger than 1 Gyr decreasing with galactocentric distance, from 0.70 per cent within 0.8 kpc to zero beyond 2 kpc. We also measure the mass fraction in young stars for individual galaxies in the highest S/N central regions. All the galaxies have young components of less than one percent. Our results clearly suggest that the star formation in massive central cluster galaxies takes place in their galaxy cores (<2 kpc), which, with deeper gravitational potential wells, are capable of retaining more gas. Among the possible sources for the gas required to form these young stars, our results are consistent with an in situ origin via stellar evolution, which is sufficient to produce the observed young stellar populations.

Open Cluster Chemical Homogeneity throughout the Milky Way

Abstract The chemical homogeneity of surviving stellar clusters contains important clues about interstellar medium (ISM) mixing efficiency, star formation, and the enrichment history of the Galaxy. Existing measurements in a handful of open clusters suggest homogeneity in several elements at the 0.03 dex level. Here we present (i) a new cluster member catalog based only on APOGEE radial velocities and Gaia-DR2 proper motions, (ii) improved abundance uncertainties for APOGEE cluster members, and (iii) the dependence of cluster homogeneity on Galactic and cluster properties, using abundances of eight elements from the APOGEE survey for 10 high-quality clusters. We find that cluster homogeneity is uncorrelated with Galactocentric distance, , age, and metallicity. However, velocity dispersion, which is a proxy for cluster mass, is positively correlated with intrinsic scatter at relatively high levels of significance for [Ca/Fe] and [Mg/Fe]. We also see a possible positive correlation at a low level of significance for [Ni/Fe], [Si/Fe], [Al/Fe], and [Fe/H], while [Cr/Fe] and [Mn/Fe] are uncorrelated. The elements that show a correlation with velocity dispersion are those that are predominantly produced by core-collapse supernovae (CCSNe). However, the small sample size and relatively low correlation significance highlight the need for follow-up studies. If borne out by future studies, these findings would suggest a quantitative difference between the correlation lengths of elements produced predominantly by Type Ia SNe versus CCSNe, which would have implications for Galactic chemical evolution models and the feasibility of chemical tagging.

Extending the view of ArH+chemistry in diffuse clouds

Context.One of the surprises of theHerschelmission was the detection of ArH+towards the Crab Nebula in emission and in absorption towards strong Galactic background sources. Although these detections were limited to the first quadrant of the Galaxy, the existing data suggest that ArH+ubiquitously and exclusively probes the diffuse atomic regions of the interstellar medium.Aims.In this study, we extend the coverage of ArH+to other parts of the Galaxy with new observations of itsJ= 1−0 transition along seven Galactic sight lines towards bright sub-millimetre continuum sources. We aim to benchmark its efficiency as a tracer of purely atomic gas by evaluating its correlation (or lack of correlation as suggested by chemical models) with other well-known atomic gas tracers such as OH+and H2O+and the molecular gas tracer CH.Methods.The observations of theJ= 1−0 line of ArH+near 617.5 GHz were made feasible with the new, sensitive SEPIA660 receiver on the APEX 12 m telescope. Furthermore, the two sidebands of this receiver allowed us to observe theNKaKc= 11,0−10,1transitions of para-H2O+at 607.227 GHz simultaneously with the ArH+line.Results.We modelled the optically thin absorption spectra of the different species and subsequently derived their column densities. By analysing the steady state chemistry of OH+and o-H2O+, we derive on average a cosmic-ray ionisation rate,ζp(H), of (2.3 ± 0.3) × 10−16s−1towards the sight lines studied in this work. Using the derived values ofζp(H) and the observed ArH+abundances we constrain the molecular fraction of the gas traced by ArH+to lie below 2 × 10−2with a median value of 8.8 × 10−4. Combined, our observations of ArH+, OH+, H2O+, and CH probe different regimes of the interstellar medium, from diffuse atomic to diffuse and translucent molecular clouds. Over Galactic scales, we see that the distribution ofN(ArH+) is associated with that ofN(H), particularly in the inner Galaxy (within 7 kpc of the Galactic centre) with potentially even contributions from the warm neutral medium phase of atomic gas at larger galactocentric distances. We derive an average ortho-to-para ratio for H2O+of 2.1 ± 1.0, which corresponds to a nuclear spin temperature of 41 K, consistent with the typical gas temperatures of diffuse clouds.

Galactopause Formation and Gas Precipitation during Strong Galactic Outflows

Abstract Using X-ray constrained β-models for the radial distribution of gas in the outskirts of galaxies, we analyze the termination of galactic winds and the formation and evolution of halo clouds by thermal instability. At low mass-loss rates ( ) galactic winds are trapped within the halo, but they burst into the intergalactic medium during intermittent strong outflows with . We develop analytic models of halo clouds as they cool radiatively over condensation timescales for hydrogen number densities , gas temperatures T ≈ (106 K)T 6, and metallicities (Z/Z ⊙). Halo gas can form kiloparsec-scale clouds out to galactocentric distances r ≈ 30–65 kpc, where efficient radiative cooling from 106 K down to T ≈ 104–5 K occurs at Z ≥ 0.3 Z ⊙ on timescales less than 1 Gyr. After condensing to column densities N H ≥ 4 × 1016 cm−2, these clouds lose hydrostatic pressure support and fall inward on dynamical timescales of ∼200 Myr. Our baseline analysis will be followed by numerical calculations to understand the governing principles of halo cloud formation and transport of gas to the galactic disk.

MUSE view of Arp220: Kpc-scale multi-phase outflow and evidence for positive feedback

Context. Arp220 is the nearest and prototypical ultra-luminous infrared galaxy; it shows evidence of pc-scale molecular outflows in its nuclear regions and strongly perturbed ionised gas kinematics on kpc scales. It is therefore an ideal system for investigating outflow mechanisms and feedback phenomena in detail. Aims. We investigate the feedback effects on the Arp220 interstellar medium (ISM), deriving a detailed picture of the atomic gas in terms of physical and kinematic properties, with a spatial resolution that had never before been obtained (0.56″, i.e. ∼210 pc). Methods. We use optical integral-field spectroscopic observations from VLT/MUSE-AO to obtain spatially resolved stellar and gas kinematics, for both ionised ([N II]λ6583) and neutral (Na IDλλ5891, 96) components; we also derive dust attenuation, electron density, ionisation conditions, and hydrogen column density maps to characterise the ISM properties. Results. Arp220 kinematics reveal the presence of a disturbed kpc-scale disc in the innermost nuclear regions as well as highly perturbed multi-phase (neutral and ionised) gas along the minor axis of the disc, which we interpret as a galactic-scale outflow emerging from the Arp220 eastern nucleus. This outflow involves velocities up to ∼1000 km s−1 at galactocentric distances of ≈5 kpc; it has a mass rate of ∼50 M⊙ yr−1 and kinetic and momentum power of ∼1043 erg s−1 and ∼1035 dyne, respectively. The inferred energetics do not allow us to distinguish the origin of the outflows, namely whether they are active galactic nucleus- or starburst-driven. We also present evidence for enhanced star formation at the edges of – and within – the outflow, with a star-formation rate SFR ∼ 5 M⊙ yr−1 (i.e. ∼2% of the total SFR). Conclusions. Our findings suggest the presence of powerful winds in Arp220: They might be capable of heating or removing large amounts of gas from the host (“negative feedback”) but could also be responsible for triggering star formation (“positive feedback”).

Chasing Accreted Structures within Gaia DR2 Using Deep Learning

Abstract In previous work, we developed a deep neural network classifier that only relies on phase-space information to obtain a catalog of accreted stars based on the second data release of Gaia (DR2). In this paper, we apply two clustering algorithms to identify velocity substructure within this catalog. We focus on the subset of stars with line-of-sight velocity measurements that fall in the range of Galactocentric radii and vertical distances . Known structures such as Gaia Enceladus and the Helmi stream are identified. The largest previously unknown structure, Nyx, is a vast stream consisting of at least 200 stars in the region of interest. This study displays the power of the machine-learning approach by not only successfully identifying known features but also discovering new kinematic structures that may shed light on the merger history of the Milky Way.

Stellar streams in chameleon gravity

Theories of gravity that incorporate new scalar degrees of freedom typically require screening mechanisms to ensure consistency with Solar System tests. One widely-studied mechanism -- the chameleon -- can lead to violations of the equivalence principle (EP), as screened and unscreened objects fall differently. If the stars are screened but the surrounding dark matter is not, this leads to asymmetry between leading and trailing streams. We provide analytic estimates of the magnitude of this effect for realistic Galactic mass distributions. Using a restricted N-body code, we simulate 4 satellites with a range of masses and orbits, together with a variety of strengths of the fifth force and screening levels of the Milky Way and satellite. The ratio of the cumulative number function of stars in the leading and trailing stream as a function of longitude from the satellite is computable from simulations, measurable from the stellar data and can provide a direct test. We forecast constraints for streams at large Galactocentric distances, using the specific example case of Hu-Sawicki gravity. Streams with apocentres between 100 and 200 kpc provide attainable constraints at the level of $|f_{R0}| = 10^{-7}$. Still more stringent constraints at the level of $10^{-7.5}$ or even $10^{-8}$ are plausible provided the environmental screening of the satellite is accounted for. These would be among the tightest astrophysical constraints to date. We note further signatures of chameleon gravity: (i) the trailing stellar stream may become detached from the dark matter progenitor if all the stars are lost, (ii) in the extreme fifth force regime, striations in the stellar trailing tail may develop, (iii) if the satellite is fully screened, its orbital frequency is lower than that of the associated dark matter, which is preferentially liberated into the leading tidal tail.

A Catalog of 406 AGNs in MaNGA: A Connection between Radio-mode AGNs and Star Formation Quenching

Abstract Accurate active galactic nucleus (AGN) identifications and spatially resolved host galaxy properties are a powerful combination for studies of the role of AGNs and AGN feedback in the coevolution of galaxies and their central supermassive black holes. Here, we present robust identifications of 406 AGNs in the first 6261 galaxies observed by the integral field spectroscopy survey Mapping Nearby Galaxies at Apache Point Observatory. Instead of using optical line flux ratios, which can be difficult to interpret in light of the effects of shocks and metallicity, we identify the AGNs via mid-infrared Wide-field Infrared Survey Explorer colors, Swift/BAT ultrahard X-ray detections, NVSS and FIRST radio observations, and broad emission lines in SDSS spectra. We subdivide the AGNs into radio-quiet and radio-mode AGNs, and examine the correlations of the AGN classes with host galaxy star formation rates and stellar populations. When compared to the radio-quiet AGN host galaxies, we find that the radio-mode AGN host galaxies are preferentially elliptical, lie further beneath the star-forming main sequence (with lower star formation rates at fixed galaxy mass), have older stellar populations, and have more negative stellar age gradients with galactocentric distance (indicating inside-out quenching of star formation). These results establish a connection between radio-mode AGNs and the suppression of star formation.

Diagonal Ridge Pattern of Different Age Populations Found in Gaia-DR2 with LAMOST Main-sequence Turnoff and OB-type Stars

Abstract We revisit the diagonal ridge feature (diagonal distributions in the R, v ϕ plane) found in Gaia and present a timing analysis for it between Galactocentric distances of R = 7.5 and 12 kpc, using main-sequence turnoff and OB stars selected from the LAMOST Galactic spectroscopic surveys. We recover the ridge pattern in the R–v ϕ plane color coded by mean radial velocity and find that this feature is presented from very young (OB stars, few hundred megayears) to very old populations (τ = 9–14 Gyr). Meanwhile, some ridge features are also revealed in the metallicity [Fe/H], [α/Fe], and v z distributions. In the L Z , v ϕ plane, one of the ridge patterns, with constant angular momentum per unit mass, shows variations with different age populations compared. However, the remaining two are relatively stable, implying there might be two kinds of ridge patterns with different dynamical origins and evolutions.

Sulphur and carbon isotopes towards Galactic centre clouds

Context. Measuring isotopic ratios is a sensitive technique used to obtain information on stellar nucleosynthesis and chemical evolution. Aims. We present measurements of the carbon and sulphur abundances in the interstellar medium of the central region of our Galaxy. The selected targets are the +50 km s−1 Cloud and several line-of-sight clouds towards Sgr B2(N). Methods. Towards the +50 km s−1 Cloud, we observed the J = 2–1 rotational transitions of 12C32S, 12C34S, 13C32S, 12C33S, and 13C34S, and the J = 3–2 transitions of 12C32S and 12C34S with the IRAM-30 m telescope, as well as the J = 6–5 transitions of 12C34S and 13C32S with the APEX 12 m telescope, all in emission. The J = 2–1 rotational transitions of 12C32S, 12C34S, 13C32S, and 13C34S were observed with ALMA in the envelope of Sgr B2(N), with those of 12C32S and 12C34S also observed in the line-of-sight clouds towards Sgr B2(N), all in absorption. Results. In the +50 km s−1 Cloud we derive a 12C/13C isotopic ratio of 22.1−2.4+3.3, that leads, with the measured 13C32S/12C34S line intensity ratio, to a 32S/34S ratio of 16.3−2.4+3.0. We also derive the 32S/34S isotopic ratio more directly from the two isotopologues 13C32S and 13C34S, which leads to an independent 32S/34S estimation of 16.3−1.7+2.1 and 17.9 ± 5.0 for the +50 km s−1 Cloud and Sgr B2(N), respectively. We also obtain a 34S/33S ratio of 4.3 ± 0.2 in the +50 km s−1 Cloud. Conclusions. Previous studies observed a decreasing trend in the 32S/34S isotopic ratios when approaching the Galactic centre. Our result indicates a termination of this tendency at least at a galactocentric distance of 130−30+60 pc. This is at variance with findings based on 12C/13C, 14N/15N, and 18O/17O isotope ratios, where the above-mentioned trend is observed to continue right to the central molecular zone. This can indicate a drop in the production of massive stars at the Galactic centre, in the same line as recent metallicity gradient ([Fe/H]) studies, and opens the work towards a comparison with Galactic and stellar evolution models.

On the Origin of a Rotating Metal-poor Stellar Population in the Milky Way Nuclear Cluster

Abstract We explore the origin of a population of stars recently detected in the inner parsec of the Milky Way (MW) nuclear cluster, which exhibits subsolar metallicity and a higher rotation compared to the dominant population. Using state-of-the-art N-body simulations, we model the infall of massive stellar systems into the Galactic center, both of Galactic and extragalactic origin. We show that the newly discovered population can either be the remnant of a massive star cluster formed a few kiloparsecs away from the Galactic center (Galactic scenario) or be accreted from a dwarf galaxy originally located at 10–100 kpc (extragalactic scenario) and that reached the Galactic center 3–5 Gyr ago. A comparison between our models and characteristic Galactocentric distance and metallicity distributions of MW satellites and globular clusters favors the Galactic scenario. A comparison with clusters associated with the Enceladus-Sausage, Sequoia, Sagittarius, and Canis Major structures suggests that the progenitor of the observed metal-poor substructure formed in situ rather than being accreted.

The Keck Baryonic Structure Survey: using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at z ∼ 2

ABSTRACT We present new measurements of the spatial distribution and kinematics of neutral hydrogen in the circumgalactic and intergalactic medium surrounding star-forming galaxies at z ∼ 2. Using the spectra of ≃3000 galaxies with redshifts 〈z〉 = 2.3 ± 0.4 from the Keck Baryonic Structure Survey, we assemble a sample of more than 200 000 distinct foreground-background pairs with projected angular separations of 3–500 arcsec and spectroscopic redshifts, with 〈zfg〉 = 2.23 and 〈zbg〉 = 2.57 (foreground, background redshifts, respectively.) The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess $\rm{H\,{\small I}}$$\rm Ly\,\alpha$ optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 ≲ Dtran/pkpc ≲ 4000) and line-of-sight velocity. We obtain accurate galaxy systemic redshifts, providing significant information on the line-of-sight kinematics of $\rm{H\,{\small I}}$ gas as a function of projected distance Dtran. We compare the map with cosmological zoom-in simulation, finding qualitative agreement between them. A simple two-component (accretion, outflow) analytical model generally reproduces the observed line-of-sight kinematics and projected spatial distribution of $\rm{H\,{\small I}}$. The best-fitting model suggests that galaxy-scale outflows with initial velocity vout ≃ 600 km s$^{-1}\,$ dominate the kinematics of circumgalactic $\rm{H\,{\small I}}$ out to Dtran ≃ 50 kpc, while $\rm{H\,{\small I}}$ at Dtran ≳ 100 kpc is dominated by infall with characteristic vin ≲ circular velocity. Over the impact parameter range 80 ≲ Dtran/pkpc ≲ 200, the $\rm{H\,{\small I}}$ line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame $\rm Ly\,\alpha$ equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond Dtran ≃ 300 pkpc (∼1 cMpc), the line-of-sight kinematics are dominated by Hubble expansion.

Calibrating the BHB star distance scale and the halo kinematic distance to the Galactic Centre

ABSTRACT We report the first determination of the distance to the Galactic Centre based on the kinematics of halo objects. We apply the statistical-parallax technique to the sample of ∼2500 blue horizontal branch (BHB) stars compiled by Xue et al. to simultaneously constrain the correction factor to the photometric distances of BHB stars as reported by those authors and the distance to the Galactic Centre to find R = 8.2 ± 0.6 kpc. We also find that the average velocity of our BHB star sample in the direction of Galactic rotation, V0 = −240 ± 4 km s−1, is greater by about 20 km s−1 in absolute value than the corresponding velocity for halo RR Lyrae type stars (V0 = −222 ± 4 km s−1) in the Galactocentric distance interval from 6 to 18 kpc, whereas the total (σV) and radial (σr) velocity dispersion of the BHB sample are smaller by about 40–45 km s−1 than the corresponding parameters of the velocity dispersion ellipsoid of halo RR Lyrae type variables. The velocity dispersion tensor of halo BHB stars proved to be markedly less anisotropic than the corresponding tensor for RR Lyrae type variables: the corresponding anisotropy parameter values are equal to βBHB = 0.51 ± 0.02 and βRR = 0.71 ± 0.03, respectively.

Chromospheric activity of nearby Sun-like stars

Context.The chromospheric emission in the cores of the Ca IIH & K lines of late-type dwarfs is a well known indicator of magnetic activity that decreases with increasing stellar age.Aims.I use this indicator to investigate the formation history of nearby G- and early K-type stars with origins at galactocentric distances similar to that of the region where the Sun was born.Methods.A parent sample of single main-sequence stars with near-solar metallicity and known magnetic activity levels is built from catalogues of stellar atmospheric parameters and chromospheric activity indices. A kinematical approach usesGaiaastrometric data to differentiate thin disc stars from thick disc stars. Measured distributions ofR′HKchromospheric activity indices are compared with Monte Carlo simulations based on an empirical model of chromospheric activity evolution.Results.The thin disc includes a significant fraction of Sun-like stars with intermediate activity levels (2 × 10−5≤R′HK≤ 6 × 10−5), while most early K- and G-type stars from the thick disc are inactive (R′HK< 2 × 10−5). The chromospheric activity distribution among nearby Sun-like dwarfs from the thin disc can be explained by a combination of an old (>6–7 Gyr) star formation event (or events) and a more recent (<3 Gyr) burst of star formation. Such an event is not required to account for theR′HKindex distributions of nearby thick disc stars.Conclusions.The distribution of magnetic activity among local G- and early K-type stars with a near-solar metallicity bears the imprint of an important star formation event that occurred ~1.9–2.6 Gyr ago in the thin disc of the Milky Way.

Bar effect on gas-phase abundance gradients. I. Data sample and chemical abundances

ABSTRACT Studies of gas-phase radial metallicity profiles in spirals published in the last decade have diminished the importance of galactic bars as agents that mix and flatten the profiles, contradicting results obtained in the 1990s. We have collected a large sample of 2831 published H ii region emission-line fluxes in 51 nearby galaxies, including objects both with and without the presence of a bar, with the aim of revisiting the issue of whether bars affect the radial metal distribution in spirals. In this first paper of a series of two, we present the galaxy and the H ii region samples. The methodology is homogeneous for the whole data sample and includes the derivation of H ii region chemical abundances, structural parameters of bars and discs, galactocentric distances, and radial abundance profiles. We have obtained O/H and N/O abundance ratios from the Te-based (direct) method for a subsample of 610 regions, and from a variety of strong-line methods for the whole H ii region sample. The strong-line methods have been evaluated in relation to the Te-based one from both a comparison of the derived O/H and N/O abundances for individual H ii regions and a comparison of the abundance gradients derived from both methodologies. The median value and the standard deviation of the gradient distributions depend on the abundance method, and those based on the O3N2 indicator tend to flatten the steepest profiles, reducing the range of observed gradients. A detailed analysis and discussion of the derived O/H and N/O radial abundance gradients and y-intercepts for barred and unbarred galaxies is presented in the companion Paper II. The whole H ii region catalogue including emission-line fluxes, positions, and derived abundances is made publicly available on the CDS VizieR facility, together with the radial abundance gradients for all galaxies.

Chemical evolution of the Milky Way: constraints on the formation of the thick and thin discs

ABSTRACT We study the evolution of Milky Way thick and thin discs in the light of the most recent observational data. In particular, we analyse abundance gradients of O, N, Fe, and Mg along the thin disc as well as the [Mg/Fe] versus [Fe/H] relations and the metallicity distribution functions at different Galactocentric distances. We run several models starting from the two-infall paradigm, assuming that the thick and thin discs formed by means of two different infall episodes, and we explore several physical parameters, such as radial gas flows, variable efficiency of star formation, different times for the maximum infall on to the disc, different distributions of the total surface mass density of the thick disc, and enriched gas infall. Our best model suggests that radial gas flows and variable efficiency of star formation should be acting together with the inside-out mechanism for the thin disc formation. The time-scale for maximum infall on to the thin disc, which determines the gap between the formation of the two discs, should be tmax ≃ 3.25 Gyr. The thick disc should have an exponential, small-scale length density profile and gas infall on the inner thin disc should be enriched. We also compute the evolution of Gaia–Enceladus system and study the effects of possible interactions with the thick and thin discs. We conclude that the gas lost by Enceladus or even part of it could have been responsible for the formation of the thick disc but not the thin disc.