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1.
The chemical composition of the Milky-Way halo is studied on the basis of a sample of metal-poor (logarithmic metallicity less than –1) globular clusters. The histogram obtained may be interpreted in the terms of the classical galaxy formation theory of Eggen, Lynden-Bell and Sandage. Interestingly enough, this is in a rough agreement with the data on metallicity of high-redshift damped Ly-alpha and Ly-alpha forest systems. This may serve the important purpose of constraining the nature and the formation timescale of MACHOs discovered through gravitational microlensing experiments. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

2.
We have determined the atmospheric abundances of selected Cepheids in order to study the large-scale chemical inhomogeneities across the galactic disk. The classical Cepheids were selected as probes to study the variation of metallicity in the galactic disk, because of their high intrinsic luminosity, small age and the existence of period-luminosity and period-age relationships. High dispersion spectra of programme stars WZ Sgr, X Sgr, ? Gem, T Mon and S V Mon were obtained using the 102-cm reflector of Kavalur Observatory. The atmospheric abundances were determined by theoretically synthesizing the selected portions of the stellar spectrum and comparing with the observed spectra. In order to compute the theoretical spectrum, the formal solution of the equation of radiative transfer was numerically evaluated with the simplifying assumptions of local thermodynamical equilibrium, plane-parallel geometry and hydrostatic equilibrium. These assumptions are reasonably good for the metallic lines of F-G supergiants and hence the observations were confined to the phases where Cepheids behave like nonvariable F-G supergiants. The atmospheric abundances of iron-peak elements, Fe, Cr, Ti, Ca and heavier s-process elements Y, Ba, La, Ce, Sm were obtained by synthesizing a selected spectral region in the range 4330 Å — 4650 Å. We derive a radial abundance gradient for iron \(\frac{{d(Fe/H)}}{{dr_{gc} }} = - 0.056 \pm 0.08\) for the region of galactic disk between 6.7 and 10.9 kpc from the galactic centre (assuming rgc = 8.5 kpc for the Sun). This value agrees with the one obtained from the general sample of Cepheids for which spectroscopic abundances are available, and also with the existing photometric determinations, but is shallower than the one derived by Luck (1982). Abundances of the elements derived in the present investigation do not show any significant correlation with atomic number. Also the abundance ratio of s-process elements does not show any correlation with Fe. This lack of correlation for disk population stars shows the inadequacy of simple models of galactic chemical evolution and favours the infall models. Alternately, the evolution of [s/Fe] may be determined by the ratio of intermediate-mass stars (which contribute s-process nuclei) to high-mass stars (which contribute Fe peak nuclei). Thus the different behaviour of halo and disk population may indicate a difference in the mass spectrum of star formation.  相似文献   

3.
On the basis of 30 observations the period of the new cepheid in M31,P=75.487 is obtained. Ths cepheid has an extremely long period and gives a possibility to determine the period-amplitude envelope toward the long-period region in M31 galaxy. A comparison of the period-amplitude diagrams in various galaxies is made.  相似文献   

4.
We present long-slit observations in the optical and near-infrared of 14 H  ii regions in the spiral galaxies NGC 628, 925, 1232 and 1637, all of them reported to have solar or oversolar abundances according to empirical calibrations. For seven of the observed regions, ion-weighted temperatures from optical forbidden auroral to nebular line ratios are obtained and, for six of them, the oxygen abundances derived by standard methods turn out to be significantly lower than solar. The other one, named CDT1 in NGC 1232, shows an oxygen abundance of     , and constitutes, to the best of our knowledge, the first high-metallicity H  ii region for which accurate line temperatures, and hence elemental abundances, have been derived.
For the rest of the regions no line temperature measurements could be made, and the metallicity has been determined by means of both detailed photoionization modelling and the sulphur abundance parameter S 23. Only one of these regions shows values of O 23 and S 23 implying a solar or oversolar metallicity.
According to our analysis, only two of the observed regions can therefore be considered as of high metallicity. These two fit the trends previously found in other high-metallicity H  ii regions, i.e., N/O and S/O abundance ratios seem to be higher and lower than solar respectively.  相似文献   

5.
Recent determinations of precise abundance ratios for nearby halo and thick disk stars in the metallicity range −1.3 < [Fe/H] < −0.5 have revealed a significant cosmic spread in the abundances of oxygen, magnesium, sodium, nickel, s-process and r-process elements relative to iron. Possible explanations of these variations are reviewed. In particular, it is discussed if the differences in abundance ratios are correlated with the kinematics of the stars, and hence can be used to identify stellar populations in the Galaxy. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

6.
We report the serendipitous discovery of a population of low-mass, pre-main-sequence (PMS) stars in the direction of the Wolf–Rayet/O-star binary system γ 2  Vel and the Vela OB2 association. We argue that γ 2  Vel and the low-mass stars are truly associated and approximately coeval, and that both are at distances between 360 and 490 pc, disagreeing at the 2 σ level with the recent Hipparcos parallax of γ 2  Vel, but consistent with older distance estimates. Our results clearly have implications for the physical parameters of the γ 2  Vel system, but also offer an exciting opportunity to investigate the influence of high-mass stars on the mass function and circumstellar disc lifetimes of their lower mass PMS siblings.  相似文献   

7.
We present line-strengths and kinematics from the central regions of 32 galaxies with Hubble types ranging from E to Sbc. Spectral indices, based on the Lick system, are measured in the optical and near-infrared (NIR). The 24 indices measured, in conjunction with models of the effects of varying abundance ratios, permit the breaking of age/metallicity degeneracy, and allow estimation of enhancements in specific light elements (particularly C and Mg). The large range of Hubble types observed allows direct comparison of line-strengths in the centres of early-type galaxies (E and S0) with those in spiral bulges, free from systematic differences that have plagued comparisons of results from different studies. Our sample includes field and Virgo cluster galaxies. For early-type galaxies our data are consistent with previously reported trends of Mg2 and Mgb with velocity dispersion. In spiral bulges we find trends in all indices with velocity dispersion. We estimate luminosity-weighted ages, metallicities and heavy-element abundance ratios (enhancements) from optical indices. These show that bulges are less enhanced in light ( α -capture) elements and have lower average age than early-type galaxies. Trends involving age and metallicity also differ sharply between early and late types. An anticorrelation exists between age and metallicity in early types, while, in bulges, metallicity is correlated with velocity dispersion. We consider the implications of these findings for models of the formation of these galaxies. We find that primordial collapse models of galaxy formation are ruled out by our observations, while several predictions of hierarchical clustering (merger) models are confirmed.  相似文献   

8.
《New Astronomy Reviews》2000,44(4-6):329-334
Heavy element abundances derived from high-quality ground-based and Hubble Space Telescope (HST) spectroscopic observations of low-metallicity blue compact galaxies (BCGs) with oxygen abundances 12+log O/H between 7.1 and 8.3 are discussed. None of the heavy element-to-oxygen abundance ratios studied here (C/O, N/O, Ne/O, Si/O, S/O, Ar/O, Fe/O) depend on oxygen abundance for BCGs with 12+log O/H≤7.6 (Z≤Z⊙/20). This constancy implies that all these heavy elements have a primary origin and are produced by the same massive (M≥10 M⊙) stars responsible for O production. The dispersion of the C/O and N/O ratios in these galaxies is found to be remarkably small, being only ±0.03 dex and ±0.02 dex respectively. This very small dispersion is strong evidence against any time-delayed production of C and primary N in the lowest-metallicity BCGs, and hence against production of these elements by intermediate-mass (3 M⊙≤M≤9 M⊙) stars at very low metallicities, as commonly thought.In higher metallicity BCGs (7.6<12+log O/H<8.2), the Ne/O, Si/O, S/O, Ar/O and Fe/O abundance ratios retain the same constant value they had at lower metallicities. By contrast, there is an increase of the C/O and N/O ratios along with their dispersions at a given O. We interpret this increase as due to the additional contribution of C and primary N production in intermediate-mass stars, on top of that by high-mass stars. BCGs show the same O/Fe overabundance with respect to the Sun (∼0.4 dex) as galactic halo stars, suggesting the same chemical enrichment history.  相似文献   

9.
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   

10.
In this paper we review the chemical evolution models for the Galactic bulge: in particular, we discuss the predictions of models as compared with the available abundance data and infer the mechanism as well as the time scale for the formation of the Galactic bulge. We show that good chemical evolution models reproducing the observed metallicity distribution of stars in the bulge predict that the [α/Fe] >0 over most of the metallicity range. This is a very important constraint indicating that the bulge of our Galaxy formed at the same time and even faster than the inner Galactic halo. We also discuss predictions for the evolution of light elements such as D and 7Li and conclude that the D astration should be maximum due to the high star formation rate required for the bulge whereas the evolution of the abundance of Li should be similar to that observed in the solar neighbourhood, but with an higher Li abundance in the interstellar medium at the present time. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
Detailed abundances are measured in cool (K type) supergiants in LMC young populous clusters (NGC 2004, NGC 2100 and NGC 1818), and compared to those, recently observed in samples of field supergiants in both Magellanic Clouds. This completes the work already done onthe SMC cluster NGC 330 (Hill et al., 1997b).The analysis of the field supergiants showed no evidence of any abundance dispersion among the stars, compatible with a well mixed gas in thesegalaxies. However, previous determinations of the metallicity ofNGC 330 and NGC 1818 gave values well below the respective field values. This difference is seeked, using a homogeneous method for the field andcluster samples, and the abundance ratios of various elements are examined. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

12.
In this work, we present a study of the strong optical collisional emission lines of Ne and Ar in an heterogeneous sample of ionized gaseous nebulae for which it is possible to derive directly the electron temperature and hence the chemical abundances of Ne and Ar. We calculate using a grid of photoionization models new ionization correction factors for these two elements and we study the behaviour of Ne/O and Ar/O abundance ratios with metallicity. We find a constant value for Ne/O, while there seems to be some evidence for the existence of negative radial gradients of Ar/O over the discs of some nearby spirals. We study the relation between the intensities of the emission lines of [Ne  iii ] at 3869 Å and [O  iii ] at 4959 and 5007 Å. This relation can be used in empirical calibrations and diagnostic ratios extending their applicability to bluer wavelengths and therefore to samples of objects at higher redshifts. Finally, we propose a new diagnostic using [O  ii ], [Ne  iii ] and Hδ emission lines to derive metallicities for galaxies at high z .  相似文献   

13.
Some insight on the first generation of stars can be obtained from the chemical composition of their direct descendants, extremely metal‐poor stars (EMP), with metallicity less than or equal to 1/1000 of the solar metallicity. Such stars are exceedingly rare, the most successful surveys, for this purpose, have so far provided only about 100 stars with 1/1000 the solar metallicity and 4 stars with about 1/10000 of the solar metallicity. The Sloan Digital Sky Survey has the potential to provide a large number of candidates of extremely low metallicity. X‐shooter has the unique capability of performing the necessary follow‐up spectroscopy providing accurate metallicities and abundance ratios for several elements (Mg, Al, Ca, Ti, Cr, Sr,...) for EMP candidates. We here report on the results for the first two stars observed in the course of our Franco‐Italian X‐shooter GTO. The two stars were targeted to be of metallicity around –3.0, the analysis of the X‐shooter spectra showed them to be of metallicity around –2.0, but with a low α to iron ratio, which explains the underestimate of the metallicity from the SDSS spectra. The efficiency of X‐shooter allows an in situ study of the outer halo, for the two stars studied here we estimate distances of 3.9 and 9.1 kpc, these are likely the most distant dwarf stars studied in detail to date (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

14.
Based on a sample of 72 Blue Compact Galaxies (BCGs) observed with the 2.16m telescope of the National Astronomical Observatories, Chinese Academy of Sciences (NAOC) and about 4000 strong emission line galaxies from the Sloan Digital Sky Survey, we analyzed their chemical evolution history using the revised chemical evolution model of Larsen et al. Our sample covers a much larger metallicity range (7.2 < 12 log(O/H) < 9.0). We found that, in order to reproduce the observed abundance pattern and gas fraction over the whole metallicity range, a relatively continuous star formation history is needed for high metallicity galaxies, while assuming a series of instantaneous bursts with long quiescent periods (some Gyrs) for low metallicity galaxies. Model calculations also show that only the closed-box model is capable of reproducing the observational data over the whole metallicity range. Models that consider the ordinary winds and/or inflow can only fit the observations in the low metallicity range, and a model with enriched wind cannot fit the data in the whole metallicity range. This implies that the current adopted simple wind and inflow models are not applicable to massive galaxies, where the underlying physics of galactic winds or inflow could be more complicated.  相似文献   

15.
We present theoretical evolutionary sequences of intermediate-mass stars  ( M = 3 − 6.5 M⊙)  with metallicity   Z = 0.004  . Our goal is to test whether the self-enrichment scenario by massive asymptotic giant branch stars may work for the high-metallicity globular clusters, after previous works by the same group showed that the theoretical yields by this class of objects can reproduce the observed trends among the abundances of some elements, namely the O–Al and O–Na anticorrelations, at intermediate metallicities, i.e.  [Fe/H]=−1.3  . We find that the increase in the metallicity favours only a modest decrease of the luminosity and the temperature at the bottom of the envelope for the same core mass, and also the efficiency of the third dredge-up is scarcely altered. On the contrary, differences are found in the yields, due to the different impact that processes with the same efficiency have on the overall abundance of envelopes with different metallicities. We expect the same qualitative patterns as in the intermediate-metallicity case, but the slopes of some of the relationships among the abundances of some elements are different. We compare the sodium–oxygen anticorrelation for clusters of intermediate metallicity ( Z ≈ 10−3) and clusters of metallicity large as in these new models. Although the observational data are still too scarce, the models are consistent with the observed trends, provided that only stars of   M ≳ 5 M⊙  contribute to self-enrichment.  相似文献   

16.
Nine new times of light maximum are given for SZ Lyn and the time base of observations is herewith extended to 26 years. 154 light maxima have been used to calculate the orbital elements and the secular change in the period of the pulsating component. The results provided here support the post-Main Sequence, high mass hypothesis for the evolutionary state of this dwarf cepheid.  相似文献   

17.
We present a simple metallicity estimator based on the logarithmic [N  ii ]   λ 6584/H α   ratio, hereafter N2, which we envisage will become very useful for ranking galaxies in a metallicity sequence from redshift survey-quality data even for moderately low spectral resolution.
We have calibrated the N2 estimator using a compilation of H  ii galaxies having accurate oxygen abundances, plus photoionization models covering a wide range of abundances. The comparison of models and observations indicates that both primary and secondary nitrogen are important for the relevant range of metallicities.
The N2 estimator follows a linear relation with log(O/H) that holds for the whole abundance range covered by the sample, from approximately  1/50th  to twice the Solar value  [7.2<12+log(O/H)<9.1]  . We suggest that the ([S  ii ]   λλ 6717,6731/H α )  ratio (hereafter S2) can also be used as a rough metallicity indicator. Because of its large scatter the S2 estimator will be useful only in systems with very low metallicity, where [N  ii ] λ 6584 is not detected or in low-resolution spectra where [N  ii ] λ 6584 is blended with H α .  相似文献   

18.
In extremely metal-poor stars ([Fe/H]≤ − 2.5) the neutron capture elementsare characterized by a 300-fold dispersion in M/Fe ratios which decreases with increasing metallicity, the median M/Fe ratio increases with increasing [Fe/H], but the averageM/Fe number ratio is approximately constant. These observations are consistent witha highly dispersed intrinsic yield of neutron-capture elements in supernova (SN) events,and a progression to increasing metallicity by stochastic chemical evolution.The abundance trends indicate that the synthesis of elements heavier thanbarium was dominated by the r-process. The Sr/Ba ratio shows a dispersionwhich suggests a stochastic source of Sr in excess of the r-process value;possibly due to the alpha-rich freeze out.The iron-peak elements Cr, Mn, and Co show non-solar abundance ratios forextreme metal-poor stars, and no measurableintrinsic dispersion relative to iron. We discuss chemical evolution models which explain these observations. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

19.
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   

20.
The abundances of two chemical elements of the iron group, viz., Cr and Ti, were determined by their high-resolution spectra for 22 A, F, and G supergiants in the solar neighborhood (within 700 pc). The titanium and chromium abundances were obtained using the Cr II and Ti II lines. The average chromium abundance of log?(Cr) = 5.70 ± 0.13 within the error limit corresponds to the solar abundance of log?⊙(Cr) = 5.64. The average titanium abundance of log?(Ti) = 4.89 ± 0.10 within the error limit is very close to the solar abundance of log?⊙(Ti) = 4.95. The average Cr and Ti abundances may be indicative of the fact that the average metallicity of young closely located stars is identical to that of the Sun.  相似文献   

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