Chemodynamics of the Milky Way. I. The first year of APOGEE data
F. AndersC. ChiappiniB. X. SantiagoH. J. Rocha–PintoL. GirardiL. N. da CostaM. A. G. MaiaMatthias SteinmetzIvan MinchevM. SchultheisC. BoecheA. MiglioJ. MontalbánDonald P. SchneiderTimothy C. BeersKátia CunhaCarlos Allende PrietoE. BalbinotDmitry BizyaevD. E. BrauerJ. BrinkmannPeter M. FrinchaboyA. E. García PérezMichael R. HaydenFrederick R. HeartyJon A. HoltzmanJennifer A. JohnsonKaren KinemuchiSteven R. MajewskiElena MalanushenkoViktor MalanushenkoDavid L. NideverR. W. O’ConnellKaike PanA. C. RobinRicardo P. SchiavonMatthew ShetroneMichael F. SkrutskieVerne V. SmithKeivan G. StassunGail Zasowski
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We investigate the chemo-kinematic properties of the Milky Way disc by exploring the first year of data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), and compare our results to smaller optical high-resolution samples in the literature, as well as results from lower resolution surveys such as GCS, SEGUE and RAVE. We start by selecting a high-quality sample in terms of chemistry ($\sim$ 20.000 stars) and, after computing distances and orbital parameters for this sample, we employ a number of useful subsets to formulate constraints on Galactic chemical and chemodynamical evolution processes in the Solar neighbourhood and beyond (e.g., metallicity distributions -- MDFs, [$\alpha$/Fe] vs. [Fe/H] diagrams, and abundance gradients). Our red giant sample spans distances as large as 10 kpc from the Sun. We find remarkable agreement between the recently published local (d $<$ 100 pc) high-resolution high-S/N HARPS sample and our local HQ sample (d $<$ 1 kpc). The local MDF peaks slightly below solar metallicity, and exhibits an extended tail towards [Fe/H] $= -$1, whereas a sharper cut-off is seen at larger metallicities. The APOGEE data also confirm the existence of a gap in the [$\alpha$/Fe] vs. [Fe/H] abundance diagram. When expanding our sample to cover three different Galactocentric distance bins, we find the high-[$\alpha$/Fe] stars to be rare towards the outer zones, as previously suggested in the literature. For the gradients in [Fe/H] and [$\alpha$/Fe], measured over a range of 6 $ < $ R $ <$ 11 kpc in Galactocentric distance, we find a good agreement with the gradients traced by the GCS and RAVE dwarf samples. For stars with 1.5 $<$ z $<$ 3 kpc, we find a positive metallicity gradient and a negative gradient in [$\alpha$/Fe].Keywords:
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Abstract We present Non-Local Thermodynamic Equilibrium (Non-LTE) abundance corrections for Mg, Ca, and Fe in 12 ultra metal-poor (UMP) stars ([Fe/H] < −4.00). We show that they increase in absolute value toward the lower metallicity up to 0.45 dex for Mg, 0.30 dex for Ca, and 1.00 dex for Fe. This represents a first step toward a full Non-LTE analysis of chemical species in the UMP stars that will enable us to put useful constraints on the properties of the “First” stars.
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Aims. The early evolution of protostellar disks with metallicities in the Z = 1.0 − 0.01 Z ⊙ range was studied with a particular emphasis on the strength of gravitational instability and the nature of protostellar accretion in low-metallicity systems. Methods. Numerical hydrodynamics simulations in the thin-disk limit were employed that feature separate gas and dust temperatures, and disk mass-loading from the infalling parent cloud cores. Models with cloud cores of similar initial mass and rotation pattern but distinct metallicity were considered to distinguish the effect of metallicity from that of the initial conditions. Results. The early stages of disk evolution in low-metallicity models are characterized by vigorous gravitational instability and fragmentation. Disk instability is sustained by continual mass-loading from the collapsing core. The time period that is covered by this unstable stage is much shorter in the Z = 0.01 Z ⊙ models than in their higher metallicity counterparts thanks to the higher rates of mass infall caused by higher gas temperatures (which decouple from lower dust temperatures) in the inner parts of collapsing cores. Protostellar accretion rates are highly variable in the low-metallicity models reflecting the highly dynamic nature of the corresponding protostellar disks. The low-metallicity systems feature short but energetic episodes of mass accretion caused by infall of inward-migrating gaseous clumps that form via gravitational fragmentation of protostellar disks. These bursts seem to be more numerous and last longer in the Z = 0.1 Z ⊙ models than in the Z = 0.01 Z ⊙ case. Conclusions. Variable protostellar accretion with episodic bursts is not a particular feature of solar metallicity disks. It is also inherent to gravitationally unstable disks with metallicities up to 100 times lower than solar.
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We report new spectroscopic observations obtained with the Michigan/Magellan Fiber System of 308 red giants (RGs) located in two fields near the photometric center of the bar of the Large Magellanic Cloud. This sample consists of 131 stars observed in previous studies (in one field) and 177 newly-observed stars (in the second field) selected specifically to more reliably establish the metallicity and age distributions of the bar. For each star, we measure its heliocentric line-of-sight velocity, surface gravity and metallicity from its high-resolution spectrum (effective temperatures come from photometric colors). The spectroscopic Hertzsprung-Russell diagrams---modulo small offsets in surface gravities---reveal good agreement with model isochrones. The mean metallicity of the 177-RG sample is $\rm [Fe/H]=-0.76\pm0.02$ with a metallicity dispersion $\sigma=0.28\pm0.03$. The corresponding metallicity distribution---corrected for selection effects---is well fitted by two Gaussian components: one metal-rich with a mean $-0.66\pm0.02$ and a standard deviation $0.17\pm0.01$, and the other metal-poor with $-1.20\pm0.24$ and $0.41\pm0.06$. The metal-rich and metal-poor populations contain approximately 85% and 15% of stars, respectively. We also confirm the velocity dispersion in the bar center decreases significantly from $31.2\pm4.3$ to $18.7\pm1.9$ km s$^{-1}$ with increasing metallicity over the range $-2.09$ to $-0.38$. Individual stellar masses are estimated using the spectroscopic surface gravities and the known luminosities. We find that lower mass hence older RGs have larger metallicity dispersion and lower mean metallicity than the higher-mass, younger RGs. The estimated masses, however, extend to implausibly low values ($\rm \sim 0.1~M_{\odot}$) making it impossible to obtain an absolute age-metallicity or age distribution of the bar.
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Abstract We present a low-metallicity map of the Milky Way consisting of ∼110,000 metal-poor giants with −3.5 < [Fe/H] < −0.75, based on public photometry from the second data release of the SkyMapper survey. These stars extend out to ∼7 kpc from the solar neighborhood and cover the main Galactic stellar populations, including the thick disk and the inner halo. Notably, this map can reliably differentiate metallicities down to [Fe/H] ∼ −3.0, and thus provides an unprecedented view into the ancient, metal-poor Milky Way. Among the more metal-rich stars in our sample ([Fe/H] > −2.0), we recover a clear spatial dependence of decreasing mean metallicity as a function of scale height that maps onto the thick disk component of the Milky Way. When only considering the very metal-poor stars in our sample ([Fe/H] < −2), we recover no such spatial dependence in their mean metallicity out to a scale height of ∣ Z ∣ ∼ 7 kpc. We find that the metallicity distribution function (MDF) of the most metal-poor stars in our sample (−3.0 < [Fe/H] < −2.3) is well fit with an exponential profile with a slope of and [Fe/H] = 1.52 ± 0.05, and slightly shifts to after accounting for target selection effects. For [Fe/H] < −2.3, the MDF is largely insensitive to scale height ∣ Z ∣ out to ∼5 kpc, showing that very and extremely metal-poor stars are in every galactic component.
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We present the largest, publicly available, sample of Damped Lyman-$\alpha$ systems (DLAs) along Gamma-ray Bursts (GRB) line of sights in order to investigate the environmental properties of long GRBs in the $z=1.8-6$ redshift range. Compared with the most recent quasar DLAs sample (QSO-DLA), our analysis shows that GRB-DLAs probe a more metal enriched environment at $z\gtrsim3$, up to $[X/H]\sim-0.5$. In the $z=2-3$ redshift range, despite the large number of lower limits, there are hints that the two populations may be more similar (only at 90\% significance level). Also at \hiz, the GRB-DLA average metallicity seems to decline at a shallower rate than the QSO-DLAs: GRB-DLA hosts may be polluted with metals at least as far as $\sim 2$kpc from the GRB explosion site, probably due to previous star-formation episodes and/or supernovae explosions. This shallow metallicity trend, extended now up to $z\sim5$, confirms previous results that GRB hosts are star-forming and have, on average, higher metallicity than the general QSO-DLA population. Finally, our metallicity measurements are broadly consistent with the hypothesis of two channels of GRB progenitors, one of which is mildly affected by a metallicity bias. The metallicity evolution of modeled GRB hosts agrees reasonably well with our data up to intermediate redshift, while more data are needed to constrain the models at $z\gtrsim 4$.
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We obtain high-resolution and high signal-to-noise ratio spectra of 39 red clump giants selected from the Hipparcos Catalogue. We determine their atmospheric parameters, iron abundances, α-element enhancements, and masses. We find that the sample can be divided into a metal-poor group and a metal-rich group. The majority of the stars are metal-rich (Z > 0.3 Z☉) with mass around 2 M☉, while the metal-poor group has lower surface gravity and lower mass. The variation of α-element abundances with [Fe/H] agrees with that of local G and K disk dwarfs. We also show that the metallicity is weakly correlated with the I-band absolute magnitude and the V-I color, in agreement with Udalski's recent findings. We make the high-resolution spectra available over the internet for interested readers.
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Carbon-enhanced metal poor stars (CEMP) form a significant proportion of the metal-poor stars, their origin is not well understood. Three very metal-poor C-rich turnoff stars were selected from the SDSS survey, observed with the ESO VLT (UVES) to precisely determine the element abundances. In turnoff stars (unlike giants) the carbon abundance has not been affected by mixing with deep layers and is therefore easier to interpret. The analysis was performed with 1D LTE static model atmospheres. When available, non-LTE corrections were applied to the classical LTE abundances. The 3D effects on the CH and CN molecular bands were computed using hydrodynamical simulations of the stellar atmosphere (CO5BOLD) and are found to be very important. To facilitate a comparison with previous results, only 1D abundances are used in the discussion. The abundances (or upper limits) of the elements enable us to place these stars in different CEMP classes. The carbon abundances confirm the existence of a plateau at A(C)= 8.25 for [Fe/H] \geq -3.4. The most metal-poor stars ([Fe/H] < -3.4) have significantly lower carbon abundances, suggesting a lower plateau at A(C) \approx 6.5. Detailed analyses of a larger sample of very low metallicity carbon-rich stars are required to confirm (or refute) this possible second plateau and specify the behavior of the CEMP stars at very low metallicity.
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Abstract I discuss three different topics concerning the chemical evolution of the Milky Way (MW). 1) The metallicity distribution of the MW halo; it is shown that this distribution can be analytically derived in the framework of the hierarchical merging scenario for galaxy formation, assuming that the component sub-haloes had chemical properties similar to those of the progenitors of satellite galaxies of the MW. 2) The age-metallicity relationship (AMR) in the solar neighborhood; I argue for caution in deriving from data with important uncertainties (such as the age uncertainties in the Geneva-Copenhagen Survey) a relationship between average metallicity and age: derived relationships are shown to be systematically flatter than the true ones and should not be directly compared to models. 3) The radial mixing of stars in the disk, which may have important effects on various observables (scatter in AMR, extension of the tails of the metallicity distribution, flatenning of disk abundance profiles). Recent SPH + N-body simulations find considerable radial mixing, but only comparison to observations will ultimately determine the extent of that mixing.
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Abstract We present [O/Fe] and other $\alpha$-elements/Fe ratios in a sample of 24 mildly metal-poor stars. The sample stars are thought to be brighter than 9.0 magnitude and have available $\mathit{uvby}$ photometric data. Also, based on the typical LTE abundance analysis, we find that [Si/Fe] and [Ca/Fe] are correlated with each other. Combining the kinematic data and the metallicity, we can classify the sample stars into three groups. An abundance analysis shows some evidence that these groups are chemically discrete from each other. Further, the general trend of a decreasing overabundance of the $\alpha$ elements with increasing metallicity has been confirmed.
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