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1.
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   

2.
High signal-to-noise ratio spectra were obtained of 10 high-proper-motion stars having  −1 ≲[Fe/H] < 0  , and a comparable number of disc stars. All but two of the high-proper-motion stars were confirmed to have  [Fe/H] > −1.0  , some approaching solar metallicity, but, even so, earlier measurements overestimated the metallicities and velocities of some of these stars. Models of stellar populations were used to assign membership probabilities to the Galactic components to which the high-velocity stars might belong. Many were found to be more probably thick-disc than halo objects, despite their large space motions, and two might be associated with the inner Galaxy. It may be necessary to reassess contamination of previous halo samples, such as those used to define the metallicity distribution, to account for contamination by high-velocity thick-disc stars, and to consider possible subcomponents of the halo.
The change in [α/Fe] ratios at  [Fe/H]≃−1.0  is often used to constrain the degree and timing of Type Ia supernova nucleosynthesis in Galactic chemical-evolution models. [Ti/Fe] values were measured for eight of the high-velocity stars. Both high- and low-[Ti/Fe] halo stars exist; likewise high- and low-[Ti/Fe] thick-disc stars exist. We conclude that the [Ti/Fe]'break' is not well defined for a given population; nor is there a simple, continuous evolutionary sequence through the break. Implications for the interpretation of the [α/Fe] break in terms of SN Ia time-scales and progenitors are discussed. The range of [Ti/Fe] found for high -velocity (low rotation) thick-disc stars contrasts with that for the low -velocity (high rotation) thick-disc sample studied by Prochaska et al.  相似文献   

3.
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   

4.
We demonstrate that low-resolution Ca  ii triplet (CaT) spectroscopic estimates of the overall metallicity ([Fe/H]) of individual red giant branch (RGB) stars in two nearby dwarf spheroidal galaxies (dSphs) agree to ±0.1–0.2 dex with detailed high-resolution spectroscopic determinations for the same stars over the range  −2.5 < [Fe/H] < −0.5  . For this study, we used a sample of 129 stars observed in low- and high-resolution modes with VLT/FLAMES in the Sculptor and Fornax dSphs. We also present the data-reduction steps we used in our low-resolution analysis and show that the typical accuracy of our velocity and CaT [Fe/H] measurement is ∼2 km s−1 and 0.1 dex, respectively. We conclude that CaT–[Fe/H] relations calibrated on globular clusters can be applied with confidence to RGB stars in composite stellar populations over the range  −2.5 < [Fe/H] < −0.5  .  相似文献   

5.
The mean metallicity of the Milky Way thin disc in the solar neighbourhood is still a matter of debate, and we recently proposed an upward revision. Our star sample was drawn from a set of solar neighbourhood dwarfs with photometric metallicities. In a very recent study, it has been suggested that our metallicity calibration, based on Geneva photometry, is biased. We show here that the effect detected is not a consequence of our adopted metallicity scale, and we confirm that our findings are robust. On the contrary, the application to Strömgren photometry of the Schuster & Nissen metallicity scale is problematic. Systematic discrepancies of  ∼0.1–0.3 dex  affect the photometric metallicity determination of metal-rich stars, on the colour interval  0.22 < b − y < 0.59  , i.e. including F and G stars. For F stars, it is shown that this is a consequence of a mismatch between the standard sequence   m 1( b − y )  of the Hyades used by Schuster & Nissen to calibrate their metallicity scale, and the system of Olsen. It means that although the calibration of Schuster & Nissen and Olsen's photometry are intrinsically correct, they are mutually incompatible for metal-rich F-type stars. For G stars, the discrepancy is most probably the continuation of the same problem, albeit worsened by the lack of spectroscopic calibrating stars. A corrected calibration is proposed that renders the calibration of Schuster & Nissen applicable to the catalogues of Olsen. We also give a simpler calibration referenced to the Hyades sequence, valid over the same colour and metallicity ranges.  相似文献   

6.
A comparison is made between the age–metallicity relations obtained from four different types of studies: F and G stars in the solar neighbourhood, analysis of open clusters, galactic structure studies with the stellar population synthesis technique and chemical evolution models. Metallicities of open clusters are corrected for the effects of the radial gradient, which we find to be −0.09 dex kpc−1 and most likely constant in time. We do not correct for the vertical gradient, because its existence and value are not firmly established.
Stars and clusters trace a similar age–metallicity relation, showing an excess of rather metal-rich objects in the age range 5–9 Gyr. Galactic structure studies tend to give a more metal-poor relation than chemical evolution models. Neither relation explains the presence of old, relatively metal-rich stars and clusters. This might be caused by uncertainties in the ages of the local stars, or pre-enrichment of the disc with material from the bulge, possibly as a result of a merger event in the early phases of the formation of our Galaxy.  相似文献   

7.
We present high-resolution Utrecht Echelle Spectrograph spectra of the quasar PHL 957, obtained in order to study the foreground damped Lyα (DLA) galaxy at z =2.309. Measurements of absorption lines lead to accurate abundance determinations of Fe, S and N which complement measurements of Zn, Cr and Ni already available for this system. We find [Fe/H]=−2.0±0.1, [S/H]=−1.54±0.06 and [N/H]=−2.76±0.07. The ratio [Fe/Zn]=−0.44 provides evidence that ≈74 per cent of iron and ≈28 per cent of zinc are locked into dust grains with a dust-to-gas ratio of ≈3 per cent of the Galactic one. The total iron content in both gas and dust in the DLA system is [Fe/H]=−1.4. This confirms a rather low metallicity in the galaxy, which is in the early stages of its chemical evolution. The detection of S ii allows us to measure the S ii /Zn ii ratio, which is a unique diagnostic tool for tracing back its chemical history, since it is not affected by the presence of dust. Surprisingly, the resulting relative abundance is [S/Zn]=0.0±0.1, at variance with the overabundance found in the Galactic halo stars with similar metallicity. We emphasize that the [S/Zn] ratio is solar in all the three DLA absorbers with extant data. Upper limits are also found for Mn, Mg, O and P and, once the dust depletion is accounted for, we obtain [Mg/Fe]c<+0.2, [O/Fe]c<+0.4, [Mn/Fe]c<+0.0 and [P/Fe]c<−0.7. The [α/Fe] values do not support Galactic halo-like abundances, implying that the chemical evolution of this young galaxy is not reproducing the evolution of our own Galaxy.  相似文献   

8.
Abundances of europium for 112 FGK dwarf stars of thick and thin disks have been determined in the metallicity range of ?1.0 < [Fe/H] < +0.3. Spectra of the studied stars have been obtained using the 1.93-m telescope of Haute-Provence Observatory (France) with spectral resolution R = 42000 and signal-to-noise ratio S/N = 100?300. Eu content has been calculated with assumption of LTE using the synthetic spectrum approach with detailed consideration of superfine structure. Analysis of europium abundances as a function of metallicity in kinematically selected stars of the Galactic thick and thin disks revealed different values in the disks. Comparison of europium abundances with magnesium abundances makes it possible to assume that at [Fe/H] < ?0.2 dex the origins of these elements are similar and at [Fe/H] > ?0.2 dex they are, probably, different.  相似文献   

9.
We present the first 3D observations of a diffuse elliptical galaxy (dE). The good quality data (S/N up to 40) reveal the kinematical signature of an embedded stellar disc, reminiscent of what is commonly observed in elliptical galaxies, though similarity of their origins is questionable. Colour map built from Hubble Space Telescope Advanced Camera for Surveys (ACS) images confirms the presence of this disc. Its characteristic scale (about 3 arcsec =250 pc) is about a half of galaxy's effective radius, and its metallicity is 0.1–0.2 dex larger than the underlying population. Fitting the spectra with synthetic single stellar populations (SSP), we found an SSP-equivalent age of 5 Gyr and nearly solar metallicity [Fe/H]  =−0.06  dex. We checked that these determinations are consistent with those based on Lick indices, but have smaller error bars. The kinematical discovery of a stellar disc in dE gives additional support to an evolutionary link from dwarf irregular galaxies due to stripping of the gas against the intracluster medium.  相似文献   

10.
Models of the chemical evolution of our Galaxy are extended to include radial migration of stars and flow of gas through the disc. The models track the production of both iron and α-elements. A model is chosen that provides an excellent fit to the metallicity distribution of stars in the Geneva–Copenhagen survey (GCS) of the solar neighbourhood and a good fit to the local Hess diagram. The model provides a good fit to the distribution of GCS stars in the age–metallicity plane, although this plane was not used in the fitting process. Although this model's star formation rate is monotonically declining, its disc naturally splits into an α-enhanced thick disc and a normal thin disc. In particular, the model's distribution of stars in the ([O/Fe], [Fe/H]) plane resembles that of Galactic stars in displaying a ridge line for each disc. The thin-disc's ridge line is entirely due to stellar migration, and there is the characteristic variation of stellar angular momentum along it that has been noted by Haywood in survey data. Radial mixing of stellar populations with high  σ z   from inner regions of the disc to the solar neighbourhood provides a natural explanation of why measurements yield a steeper increase of  σ z   with age than predicted by theory. The metallicity gradient in the interstellar medium is predicted to be steeper than in earlier models, but appears to be in good agreement with data for both our Galaxy and external galaxies. The models are inconsistent with a cut-off in the star formation rate at low gas surface densities. The absolute magnitude of the disc is given as a function of time in several photometric bands, and radial colour profiles are plotted for representative times.  相似文献   

11.
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   

12.
A sample containing 1 026 stars of spectral types F, G, and K, mainly dwarfs, from the solar neighbourhood with available space velocities and metallicities is treated. The treatment comprises a statistical analysis of the metallicity and velocity data and calculation of galactocentric orbits. Sample stars identified as members of the galactic halo are detached from the rest of the sample based on the values of their metallicities, velocity components and galactocentric orbits. In identifying halo stars a new, kinematical, criterion is proposed. Except one, these halo stars are the metal‐poorest ones in the sample. Besides, they have very high velocities with respect to LSR. On the other hand, the separation between the thin disc and thick one is done statistically based on LSR space velocities, membership probability (Schwarzschild distribution with assumed parameters) and galactocentric orbits. In the metallicity these two groups are not much different. For each of the three subsamples the mean motion and velocity ellipsoid are calculated. The elements of the velocity ellipsoids agree well with the values found in the literature, especially for the thin disc. The fractions of the subsystems found for the present sample are: thin disc 93%, thick disc 6%, halo 1%. The sample stars established to be members of the thin disc are examined for existence of star streams. Traces of both, known and unknown, star streams are not found (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

13.
High-resolution spectra of five candidate metal-weak thick-disc stars suggested by Beers & Sommer-Larsen are analysed to determine their chemical abundances. The low abundance of all the objects has been confirmed, with metallicity reaching [Fe/H]=−2.9. However, for three objects the astrometric data from the Hipparcos catalogue suggest they are true halo members. The remaining two, for which proper-motion data are not available, may have disc-like kinematics. It is therefore clear that it is useful to address properties of putative metal-weak thick-disc stars only if they possess full kinematic data. For CS 22894−19 an abundance pattern similar to those of typical halo stars is found, suggesting that chemical composition is not a useful discriminant between thick-disc and halo stars. CS 29529−12 is found to be C-enhanced with [C/Fe]=+1.0; other chemical peculiarities involve the s-process elements: [Sr/Fe]=−0.65 and [Ba/Fe]=+0.62, leading to a high [Ba/Sr], considerably larger than that found in more metal-rich carbon-rich stars, but similar to those in LP 706-7 and LP 625-44, discussed by Norris et al. Hipparcos data have been used to calculate the space velocities of 25 candidate metal-weak thick-disc stars, thus allowing us to identify three bona fide members, which support the existence of a metal-poor tail of the thick disc, at variance with a claim to the contrary by Ryan & Lambert.  相似文献   

14.
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   

15.
We have calculated the orbital parameters for 90 stars in Chen et al. and updated the kinematic data for stars in Edvardsson et al. by using the accurate Hipparcos parallaxes and proper motions, and recalculated the \\\\\\\\\\\\-element abundances in Edvardsson et al. in a way consistent with Chen et al. The two sets of data are combined in a study of stellar populations and characteristics of F & G stars in the solar neighborhood. We confirm the result of Chen et al. that a distinguishable group of stars may belong to the thick disk rather than the thin disk. The ages for the stars are determined using the theoretical isochrones of VandenBerg et al. The age-metallicity relation is investigated for different subgroups according to distance from the sun and galactic orbital parameters. It is found that a mixing of stars with different orbital parameters significantly affect the age-metallicity relation for the disk. Stars with orbits confined to the solar circle all have metallicities [Fe/H] > -0.3 irresp  相似文献   

16.
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   

17.
We interpret the de‐reddened UBV data for the field SA 133 to deduce the stellar density and metallicity distribution functions. The logarithmic local space density for giants, D*(0) = 6.40, and the agreement of the luminosity function for dwarfs and sub‐giants with the one of Hipparcos confirms the empirical method used for their separation. The metallicity distribution for dwarfs gives a narrow peak at [Fe/H] = +0.13 dex, due to apparently bright limiting magnitude, Vo = 16.5, whereas late‐type giants extending up to z ∼ 4.5 kpc from the galactic plane have a multimodal distribution. The metallicity distribution for giants gives a steep gradient d[Fe/H]/dz = –0.75 dex kpc–1 for thin disk and thick disk whereas a smaller value for the halo, i.e. d[Fe/H]/dz = –0.45 dex kpc–1. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

18.
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   

19.
邱红梅  赵刚  仲佳勇 《天文学报》2002,43(3):257-263
在第1篇论文的基础上，确定了样本星的恒星大气参数，得到这些星中9种元素的丰度。讨论了各种元素丰度随[Fe/H]的变化。平均的[Na/Fe]～－0．01dex，接近于太阳丰度。α元素Si和Ca具有几乎相同的丰度模式，而[Ti/Fe]弥散较大，但三者均有随[Fe/H]的减小而增加的趋势。铁峰元素V、Cr、Ni在不同丰度处有较大的弥散，[Cr/Fe]在所有样本星中均表现超丰；而[Mn/Fe]却明显过贫，且随金属丰度的增加而增加。  相似文献   

20.
We analyse two recent computations of Type II supernova nucleosynthesis by Woosley & Weaver (hereafter WW95) and Thielemann, Nomoto & Hashimoto (hereafter TNH96), focusing on the ability to reproduce the observed [Mg/Fe] ratios in various galaxy types. We show that the yields of oxygen and total metallicity are in good agreement. However, TNH96 models produce more magnesium in the intermediate and less iron in the upper mass range of Type II supernovae than WW95 models. To investigate the significance of these discrepancies for chemical evolution, we calculate simple stellar population yields for both sets of models and different initial mass function slopes. We conclude that the Mg yields of WW95 do not suffice to explain the [Mg/Fe] overabundance either in giant elliptical galaxies and bulges or in metal-poor stars in the solar neighbourhood and the Galactic halo. Calculating the chemical evolution in the solar neighbourhood according to the standard infall model, we find that, using WW95 and TNH96 nucleosynthesis, the solar magnesium abundance is underestimated by 29 and 7 per cent, respectively.   We include the relaxation of the instantaneous mixing approximation in chemical evolution models by splitting the gas component into two different phases. In additional simulations of the chemical evolution in the solar neighbourhood, we discuss various time-scales for the mixing of the stellar ejecta with the interstellar medium. We find that a delay of the order of 108 yr leads to a better fit of the observational data in the [Mg/Fe]–[Fe/H] diagram without destroying the agreement with solar element abundances and the age–metallicity relation.  相似文献   

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