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1.
The Small Magellanic Cloud is a close, irregular galaxy that has experienced a complex star formation history due to the strong interactions occurred both with the Large Magellanic Cloud and the Galaxy. Despite its importance, the chemical composition of its stellar populations older than ∼ 1–2 Gyr is still poorly investigated. I present the first results of a spectroscopic survey of ∼ 200 Small Magellanic Cloud giant stars performed with FLAMES@VLT. The derived metallicity distribution peaks at [Fe/H] ∼ –0.9/–1.0 dex, with a secondary peak at [Fe/H] ∼ –0.6 dex. All these stars show [α /Fe] abundance ratios that are solar or mildly enhanced (∼+0.1 dex). Also, three metal‐poor stars (with [Fe/H] ∼ –2.5 dex and enhanced [α /Fe] ratios compatible with those of the Galactic Halo) have been detected in the outskirts of the SMC: These giants are the most metal‐poor stars discovered so far in the Magellanic Clouds. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

2.
Data from our compiled catalog of spectroscopically determined magnesium abundances in stars with accurate parallaxes are used to select thin-disk dwarfs and subgiants according to kinematic criteria. We analyze the relations between the relative magnesium abundances in stars, [Mg/Fe], and their metallicities, Galactic orbital elements, and ages. The [Mg/Fe] ratios in the thin disk at any metallicity in the range ?1.0 dex <[Fe/H] < ?0.4 dex are shown to be smaller than those in the thick disk, implying that the thin-disk stars are, on average, younger than the thick-disk stars. The relative magnesium abundances in such metal-poor thin-disk stars have been found to systematically decrease with increasing stellar orbital radii in such a way that magnesium overabundances ([Mg/Fe] > 0.2 dex) are essentially observed only in the stars whose orbits lie almost entirely within the solar circle. At the same time, the range of metallicities in magnesium-poor stars is displaced from ?0.5 dex < [Fe/H] < +0.3 dex to ?0.7 dex < [Fe/H] < +0.2 dex as their orbital radii increase. This behavior suggests that, first, the star formation rate decreases with increasing Galactocentric distance and, second, there was no star formation for some time outside the solar circle, while this process was continuous within the solar circle. The decrease in the star formation rate with increasing Galactocentric distance is responsible for the existence of a negative radial metallicity gradient (grad R[Fe/H] = ?0.05 ± 0.01 kpc?1) in the disk, which shows a tendency to increase with decreasing age. At the same time, the relative magnesium abundance exhibits no radial gradient. We have confirmed the existence of a steep negative vertical metallicity gradient (grad Z[Fe/H] = ?0.29 ± 0.06 kpc?1) and detected a significant positive vertical gradient in relative magnesium abundance (grad Z[Mg/Fe] = 0.13 ± 0.02 kpc?1); both gradients increase appreciably in absolute value with decreasing age. We have found that there is not only an age-metallicity relation, but also an age-magnesium abundance relation, in the thin disk. We surmise that the thin disk has a multicomponent structure, but the existence of a negative trend in the star formation rate along the Galactocentric radius does not allow the stars of its various components to be identified in the immediate solar neighborhood.  相似文献   

3.
The space velocities and Galactic orbital elements of stars calculated from the currently available high-accuracy observations in our compiled catalog of spectroscopic magnesium abundances in dwarfs and subgiants in the solar neighborhood are used to identify thick-disk objects. We analyze the relations between chemical, spatial, and kinematic parameters of F–G stars in the identified subsystem. The relative magnesium abundances in thick-disk stars are shown to lie within the range 0.0 < [Mg/Fe] < 0.5 and to decrease with increasingmetallicity starting from [Fe/H] ≈ ?1.0. This is interpreted as evidence for a longer duration of the star formation process in the thick disk. We have found vertical gradients in metallicity (gradZ[Fe/H] = ?0.13 ± 0.04 kpc?1) and relative magnesium abundance (gradZ[Mg/Fe] = 0.06 ± 0.02 kpc?1), which can be present in the subsystem only in the case of its formation in a slowly collapsing protogalaxy. However, the gradients in the thick disk disappear if the stars whose orbits lie in the Galactic plane, but have high eccentricities and low azimuthal space velocities atypical of the thin-disk stars are excluded from the sample. The large spread in relative magnesium abundance (?0.3 < [Mg/Fe] < 0.5) in the stars of the metal-poor “tail” of the thick disk, which constitute ≈8% of the subsystem, can be explained in terms of their formation inside isolated interstellar clouds that interacted weakly with the matter of a single protogalactic cloud. We have found a statistically significant negative radial gradient in relative magnesium abundance in the thick disk (gradR[Mg/Fe] = ?0.03 ± 0.01 kpc? 1) instead of the expected positive gradient. The smaller perigalactic orbital radii and the higher eccentricities for magnesium-richer stars, which, among other stars, are currently located in a small volume of the Galactic space near the Sun, are assumed to be responsible for the gradient inversion. A similar, but statistically less significant inversion is also observed in the subsystem for the radial metallicity gradient.  相似文献   

4.
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5.
The analysis of the kinematics of solar neighbourhood stars shows that the low- and high-metallicity tails of the thin disc are populated by objects which orbital properties suggest an origin in the outer and inner Galactic disc, respectively. Signatures of radial migration are identified in various recent samples, and are shown to be responsible for the high-metallicity dispersion in the age–metallicity distribution. Most importantly, it is shown that the population of low-metallicity wanderers of the thin disc (−0.7 < [Fe/H] < −0.3 dex) is also responsible for the apparent hiatus in metallicity with the thick disc (which terminal metallicity is about −0.2 dex). It implies that the thin disc at the solar circle has started to form stars at about this same metallicity. This is also consistent with the fact that 'transition' objects, which have α-element abundance intermediate between that of the thick and thin discs, are found in the range [−0.4, −0.2] dex. Once the metal-poor thin disc stars are recognized for what they are – wanderers from the outer thin disc – the parenthood between the two discs can be identified on stars genuinely formed at the solar circle through an evolutionary sequence in [α/Fe] and [Fe/H]. Another consequence is that stars that can be considered as truly resulting of the chemical evolution at the solar circle have a metallicity restricted to about [−0.2, +0.2] dex, confirming an old idea that most chemical evolution in the Milky Way have preceded the thin disc formation.  相似文献   

6.
In this paper, the first statistically rigorous test is performed to determine whether there are dwarfs in the solar neighbourhood with [Fe/H] > +0.2 dex. The possibility that no such stars exist is rejected with a confidence limit exceeding 99.994 per cent. If the threshold value is raised to +0.38 dex, the corresponding hypothesis is rejected with a confidence limit of 98.5 per cent. It is tentatively found that this upper limit does not depend on the presence or absence of planets orbiting metal-rich stars. In a result that is unchanged from a previous paper, the upper limit for the known metallicities of giants is found to be about 0.2 dex lower than the limit for dwarfs. Stars that might be observed to improve the upper metallicity limit for dwarfs are listed.  相似文献   

7.
We present a revised metallicity distribution of dwarfs in the solar neighbourhood. This distribution is centred on solar metallicity. We show that previous metallicity distributions, selected on the basis of spectral type, are biased against stars with solar metallicity or higher. A selection of G-dwarf stars is inherently biased against metal-rich stars and is not representative of the solar neighbourhood metallicity distribution. Using a sample selected on colour, we obtain a distribution where approximately half the stars in the solar neighbourhood have metallicities higher than [Fe/H]=0 . The percentage of mid-metal-poor stars ([Fe/H]<−0.5) is approximately 4 per cent, in agreement with present estimates of the thick disc.
In order to have a metallicity distribution comparable to chemical evolution model predictions, we convert the star fraction to mass fraction, and show that another bias against metal-rich stars affects dwarf metallicity distributions, due to the colour (or spectral type) limits of the samples. Reconsidering the corrections resulting from the increasing thickness of the stellar disc with age, we show that the simple closed-box model with no instantaneous recycling approximation gives a reasonable fit to the observed distribution. Comparisons with the age–metallicity relation and abundance ratios suggest that the simple closed-box model may be a viable model of the chemical evolution of the Galaxy at solar radius.  相似文献   

8.
We collected a sample of 100 galaxies for which different observers have determined colour indices of globular cluster candidates. The sample includes representatives of galaxies of various morphological types and different luminosities. Colour indices (in most cases (V–I), but also (B–I) and (C–T1)) were transformed into metallicities [Fe/H] according to a relation by Kissler‐Patig (1998). These data were analysed with the KMM software in order to estimate similarity of the distribution with uni‐ or bimodal Gaussian distribution. We found that 45 of 100 systems have bimodal metallicity distributions. Mean metallicity of the metal‐poor component for these galaxies is 〈[Fe/H]〉 = –1.40 ± 0.02, of the metalrich component 〈[Fe/H]〉 = –0.69 ± 0.03. Dispersions of the distributions are 0.15 and 0.18, respectively. Distribution of unimodal metallicities is rather wide. These data will be analysed in a subsequent paper in order to find correlations with parameters of galaxies and galactic environment.  相似文献   

9.
A comparison of observed stellar distributions with a three-component model of the Galaxy is presented. The analysis is based on photometric and photoelectric data obtained along the main Galactic meridian and in two fields near the North Galactic pole (programme MEGA). The assumed model considers the Galaxy as composed of the disk (main sequence and disk red giants), the thick disk and spheroid populations. To model the observed colour distribution, we distinguish main sequence stars and disk red giants as the disk subsystem; white dwarfs, subdwarfs and intermediate giants as the thick disk subsystem; extreme subdwarfs, spheroid giants and horizontal branch stars as the spheroid subsystem. A statistical relation between the apparent and absolute magnitudes of stars which make the maximum contribution to the star counts for a given disk subsystem is derived. In order to achieve the best agreement between the model and observations, we fit the values of the ‘dip’ (aw) of the disk luminosity function, the correction to the absolute magnitude of disk red giants (ΔMVRG) and the expression for interstellar extinction. As the main result, we obtained aw = 0.6 (logarithmic scale) and ΔMVRG = 0.5 mag; the interstellar extinction has to be taken into account by the modified Sandage law.  相似文献   

10.
Abundances of O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Fe, Ni and Ba are determined for 30 nearby lower-main-sequence stars in the Northern sky using high-resolution, high signal-to-noise ratio spectra. Our results show an equilibrium of  [Fe/H]I  and  [Fe/H]II  and a much smaller star-to-star scatter of the abundance ratios as a function of metallicity compared with the results of Kotoneva et al. The non-local thermodynamic equilibrium (non-LTE) corrections for oxygen are considered and found to be small  (∼−0.04 dex)  . A flat trend of [O/Fe] exists over the whole metallicity range. The non-LTE effects for some important elements are discussed, and it is found that the abundance pattern for our programme stars is very similar to that of F and G dwarfs.  相似文献   

11.
The Edinburgh–Cape Blue Object Survey is an ongoing project to identify and analyse a large sample of hot stars selected initially on the basis of photographic colours (down to a magnitude limit     over the entire high-Galactic-latitude southern sky, and then studied with broad-band UBV photometry and medium-resolution spectroscopy. Due to unavoidable errors in the initial candidate selection, stars that are likely metal-deficient dwarfs and giants of the halo and thick-disc populations are inadvertently included, yet are of interest in their own right. In this paper we discuss a total of 206 candidate metal-deficient dwarfs, subgiants, giants, and horizontal-branch stars with photoelectric colours redder than     and with available spectroscopy. Radial velocities, accurate to ∼10–15 km s−1, are presented for all of these stars. Spectroscopic metallicity estimates for these stars are obtained using a recently recalibrated relation between Ca  ii K-line strength and     colour. The identification of metal-poor stars from this colour-selection technique is remarkably efficient, and competitive with previous survey methods. An additional sample of 186 EC stars with photoelectric colours in the range     composed primarily of field horizontal-branch stars and other, higher gravity, A- and B-type stars, is also analysed. Estimates of the physical parameters T eff, log  g , and [Fe/H] are obtained for cooler members of this subsample, and a number of candidate RR Lyrae variables are identified.  相似文献   

12.
We study the kinematics of the Galactic thin and thick disk populations using stars from the RAVE survey’s second data release together with distance estimates from Breddels et al. (2010). The velocity distribution exhibits the expected moving groups present in the solar neighborhood. We separate thick and thin disk stars by applying the X (stellar-population) criterion of Schuster et al. (1993), which takes into account both kinematic and metallicity information. For 1906 thin disk and 110 thick disk stars classified in this way, we find a vertical velocity dispersion, mean rotational velocity and mean orbital eccentricity of (σW, 〈VΦ〉, 〈e〉)thin = (18 ± 0.3 km s−1, 223 ± 0.4 km s−1, 0.07 ± 0.07) and (σW, 〈VΦ〉, 〈e〉)thick = (35 ± 2 km s−1, 163 ± 3 km s−1, 0.31 ± 0.16), respectively. From the radial Jeans equation, we derive a thick disk scale length in the range 1.5-2.2 kpc, whose greatest uncertainty lies in the adopted form of the underlying potential. The shape of the orbital eccentricity distribution indicates that the thick disk stars in our sample most likely formed in situ with minor gas-rich mergers and/or radial migration being the most likely cause for their orbits. We further obtain mean metal abundances of 〈[M/H]〉thin = +0.03 ± 0.17, and 〈[M/H]〉thick = −0.51 ± 0.23, in good agreement with previous estimates. We estimate a radial metallicity gradient in the thin disk of −0.07 dex kpc−1, which is larger than predicted by chemical evolution models where the disk grows inside-out from infalling gas. It is, however, consistent with models where significant migration of stars shapes the chemical signature of the disk, implying that radial migration might play at least part of a role in the thick disk’s formation.  相似文献   

13.
Several arguments that justify establishing a revised abundance calibration for DDO photometry of population I red giants are presented. The components of the blanketing vector in the DDOC(45–48) vsC(42–45) diagram are determined for late-type dwarfs and giants. We have redefined the DDO cyanogen anomaly and calibrated it against metallicity. The sample of field giants now available with abundances derived from high dispersion spectroscopy is substantially larger than previously available, leading to a more accurate abundance calibration. Iso-abundance lines in theC(41–42) vsC(42–45) diagram have been determined for population IG and K giants and an iterative method for deriving abundances of these stars is described. We show that the new DDO abundances are in very good agreement with those derived from high dispersion spectroscopy. The new method improves by about 0.1 dex the DDO abundances derived for early G and/or late K giants, with respect to the δCN method of Janes (1975).  相似文献   

14.
We present a method which separates field dwarfs and field giants by their 2MASS and V apparent magnitudes. This method is based on spectroscopically selected standards and is hence reliable.We applied it to stars in two fields, SA 54 and SA 82, and we estimated a full set of Galactic model parameters for giants including their total local space density. Our results are in agreement with the ones given in the recent literature. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
Sulphur is a volatile α ‐element which is not locked into dust grains in the interstellar medium (ISM). Hence, its abundance does not need to be corrected for dust depletion when comparing the ISM to the stellar atmospheres. The abundance of sulphur in the photosphere of metal‐poor stars is a matter of debate: according to some authors, [S/Fe] versus [Fe/H] forms a plateau at low metallicity, while, according to other studies, there is a large scatter or perhaps a bimodal distribution. In metal‐poor stars sulphur is detectable by its lines of multiplet 1 at 920 nm, but this range is heavily contaminated by telluric absorptions, and one line of the multiplet is blended by the hydrogen Paschen ζ line. We study the possibility of using multiplet 3 (at 1045 nm) for deriving the sulphur abundance because this range, now observable at the VLT with the infra‐red spectrograph CRIRES, is little contaminated by telluric absorption and not affected by blends at least in metal‐poor stars. We compare the abundances derived from multiplets 1 and 3, taking into account NLTE corrections and 3D effects. Here we present the results for a sample of four stars, although the scatter is less pronounced than in previous analysis, we cannot find a plateau in [S/Fe], and confirm the scatter of the sulphur abundance at low metallicity (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

16.
The abundances of heavy elements in EMP stars are not well explained by the simple view of an initial basic “rapid” process. In a careful and homogeneous analysis of the “First Stars” sample (eighty per cent of the stars have a metallicity [Fe/H] ≃ –3.1 ± 0.4), it has been shown that at this metallicity [Eu/Ba] is constant, and therefore the europium‐rich stars (generally called “r‐rich”) are also Ba‐rich. The very large variation of [Ba/Fe] (existence of “r‐poor” and “r‐rich” stars) induces that the early matter was not perfectly mixed. On the other hand, the distribution of the values of [Sr/Ba] vs. [Ba/Fe] appears with well defined upper and lower envelopes. No star was found with [Sr/Ba] < –0.5 and the scatter of [Sr/Ba] increases regularly when [Ba/Fe] decreases. To explain this behavior, we suggest that an early “additional” process forming mainly first peak elements would affect the initial composition of the matter. For a same quantity of accreted matter, this additional Sr production would barely affect the r‐rich matter (which already contains an important quantity of Sr) but would change significantly the composition of the r‐poor matter. The abundances found in the CEMP‐r+s stars reflect the transfer of heavy elements from a defunct AGB companion. But the abundances of the heavy elements in CEMP‐no stars present the same characteristics as the the abundances in the EMP stars. Direct stellar ages may be found from radioactive elements, the precision is limited by the precision in the measurements of abundances from faint lines in faint stars, and the uncertainty in the initial abundances of the radioactive elements. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

17.
In this paper we construct and analyze the uniform non-LTE distributions of the aluminium ([Al/Fe]-[Fe/H]) and sodium ([Na/Fe]-[Fe/H]) abundances in the sample of 160 stars of the disk and halo of our Galaxy with metallicities within ?4.07 ≤ [Fe/H] ≤ 0.28. The values of metallicity [Fe/H] and microturbulence velocity ξ turb indices are determined from the equivalent widths of the Fe II and Fe I lines. We estimated the sodium and aluminium abundances using a 21-level model of the Na I atom and a 39-level model of the Al I atom. The resulting LTE distributions of [Na/Fe]-[Fe/H] and [Al/Fe]-[Fe/H] do not correspond to the theoretical predictions of their evolution, suggesting that a non-LTE approach has to be applied to determine the abundances of these elements. The account of non-LTE corrections reduces by 0.05–0.15 dex the abundances of sodium, determined from the subordinate lines in the stars of the disk with [Fe/H] ≥ ?2.0, and by 0.05–0.70 dex (with a strong dependence on metallicity) the abundances of [Na/Fe], determined by the resonance lines in the stars of the halo with [Fe/H] ≤ ?2.0. The non-LTE corrections of the aluminium abundances are strictly positive and increase from 0.0–0.1 dex for the stars of the thin disk (?0.7 ≤ [Fe/H] ≤ 0.28) to 0.03–0.3 dex for the stars of the thick disk (?1.5 ≤ [Fe/H] ≤ ?0.7) and 0.06–1.2 dex for the stars of the halo ([Fe/H] ≤ ?2.0). The resulting non-LTE abundances of [Na/Fe] reveal a scatter of individual values up to Δ[Na/Fe] = 0.4 dex for the stars of close metallicities. The observed non-LTE distribution of [Na/Fe]-[Fe/H] within 0.15 dex coincides with the theoretical distributions of Samland and Kobayashi et al. The non-LTE aluminium abundances are characterized by a weak scatter of values (up to Δ[Al/Fe] = 0.2 dex) for the stars of all metallicities. The constructed non-LTE distribution of [Al/Fe]-[Fe/H] is in a satisfactory agreement to 0.2 dex with the theoretical data of Kobayashi et al., but strongly differs (up to 0.4 dex) from the predictions of Samland.  相似文献   

18.
Based on our compiled catalogue of positions, velocities, ages, and abundances of nine chemical elements for 221 classical Cepheids, we analyze the dependences of the relative abundances of α-elements as well as rapid and slow neutron capture elements on metallicity, space velocity components, and Galactocentric distance. We have found that the relative abundances of all elements in Cepheids do not depend on velocity but increase with Galactocentric distance and decrease with increasing metallicity, just as in thin-disk dwarfs and giants. In Cepheids, however, the [α/Fe]-[Fe/H] relation lies below, while the [r/Fe]-[Fe/H] and [s/Fe]-[Fe/H] relations lie above the analogous sequences for dwarfs and giants. We hypothesize that upon reaching a nearly solar metallicity in the interstellar medium of the thin disk, the most massive stars ceased to explode as type II supernovae, which mostly enriched the interstellar medium with α-elements. As a result, an underabundance of α-elements and a slight overabundance of r-process elements, which are ejected into the interstellar medium by less massive (8–10 M ⊙) type II supernovae, were formed in the next generations of stars. The overabundance of s-process elements in Cepheids can be explained by the fact that some of the s-elements were produced in the weak s-process in the interiors of massive stars, which may be able to eject the upper parts of their envelopes even without any explosion like asymptotic giant branch stars. And since such massive stars, exploding as type II supernovae, also enriched the interstellar medium with a considerable amount of iron atoms, the [s/Fe] ratios (along with [r/Fe]) in the next generations of stars must be higher in their absence.  相似文献   

19.
The metal abundance distribution (the metallicity function, MF) of stars and globular clusters is studied. It is found to have three gaps, near [Fe/H]=–1.0, –0.5 and –0.1. The gaps are shown to be statistically significant practically at the 100% confidence level. They divide the galactic population into four metallicity groups with the average [Fe/H] of about –1.5, –0.8, –0.25 and +0.10 (groups I, II, III and IV, respectively). The main contribution into the scatter of metallicity within the groups (apart from group I) comes from random errors in abundance measurements. So we infer that the actual MF must be essentially discrete. These results substantiate the ones obtained in our previous Papers I–IV; they support our idea on active phases in the evolution of the Galaxy, which imply the intermittent enrichment and star formation.We find that the kinematics and metallicity of groups III and IV of F and G dwarfs show a paradox: the metal-rich group (group IV) of G dwarfs turns out to be kinematically older than the group III of F dwarfs with half the metal abundance. The implication of this result for star formation is discussed. Also we show that the portion of metal poor disk population F dwarfs (group III) is the same or even larger than that of G dwarfs. This fact disagrees with the conventional idea that the young kinematics of F dwarfs owes to the absence of old F dwarfs, which are supposed to be evolved into red giants.  相似文献   

20.
We present and analyse the kinematics and orbits for a sample of 488 open clusters (OCs) in the Galaxy. The velocity ellipsoid for our present sample is derived as  (σ U , σ V , σ W ) = (28.7, 15.8, 11.0) km s−1  which represents a young thin-disc population. We also confirm that the velocity dispersions increase with the age of a cluster subsample. The orbits of OCs are calculated with three Galactic gravitational potential models. The errors of orbital parameters are also calculated considering the intrinsic variation of the orbital parameters and the effects of observational uncertainties. The observational uncertainties dominate the errors of derived orbital parameters. The vertical motions of clusters calculated using different Galactic disc models are rather different. The observed radial metallicity gradient of clusters is derived with a slope of   b =−0.070 ± 0.011   dex kpc−1. The radial metallicity gradient of clusters based on their apogalactic distances is also derived with a slope of   b =−0.082 ± 0.014   dex kpc−1. The distribution of derived orbital eccentricities for OCs is very similar to that derived for the field population of dwarfs and giants in the thin disc.  相似文献   

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