Stellar Yields and Chemical Evolution — I. Abundance Ratios and Delayed Mixing in the Solar Neighbourhood

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.     

Stellar archaeology: Exploring the Universe with metal‐poor stars

The abundance patterns of the most metal‐poor stars in the Galactic halo and small dwarf galaxies provide us with a wealth of information about the early Universe. In particular, these old survivors allow us to study the nature of the first stars and supernovae, the relevant nucleosynthesis processes responsible for the formation and evolution of the elements, early star‐ and galaxy formation processes, as well as the assembly process of the stellar halo from dwarf galaxies a long time ago. This review presents the current state of the field of “stellar archaeology” – the diverse use of metal‐poor stars to explore the high‐redshift Universe and its constituents. In particular, the conditions for early star formation are discussed, how these ultimately led to a chemical evolution, and what the role of the most iron‐poor stars is for learning about Population III supernovae yields. Rapid neutron‐capture signatures found in metal‐poor stars can be used to obtain stellar ages, but also to constrain this complex nucleosynthesis process with observational measurements. Moreover, chemical abundances of extremely metal‐poor stars in different types of dwarf galaxies can be used to infer details on the formation scenario of the halo and the role of dwarf galaxies as Galactic building blocks. I conclude with an outlook as to where this field may be heading within the next decade. A table of ~ 1000 metal‐poor stars and their abundances as collected from the literature is provided in electronic format (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The structure of the Galactic halo: SDSS versus SuperCOSMOS

The halo structure at high Galactic latitudes near both the north and south poles is studied using Sloan Digital Sky Survey (SDSS) and SuperCOSMOS data. For the south cap halo, the archive of the SuperCOSMOS photographic photometry sky survey is used. The coincident source rate between SuperCOSMOS data in B _J band from 16.5 to 20.5 mag and SDSS data is about 92 per cent, in a common sky area in the south. While that in the R _F band is about 85 per cent from 16.5 to 19.5 mag. Transformed to the SuperCOSMOS system and downgraded to the limiting magnitudes of SuperCOSMOS, the star counts in the North Galactic Cap from SDSS show up to an  16.9 ± 6.3  per cent  asymmetric ratio (defined as relative fluctuations over the rotational symmetry structure) in the B _J band, and up to  13.5 ± 6.7  per cent  asymmetric ratio in the R _F band. From SuperCOSMOS B _J and R _F bands, the structure of the Southern Galactic hemisphere does not show the same obvious asymmetric structures as the northern sky does in both the original and downgraded SDSS star counts. An axisymmetric halo model with n = 2.8 and q = 0.7 can fit the projected number density from SuperCOSMOS fairly well, with an average error of about 9.17 per cent. By careful analysis of the difference of star counts between the downgraded SDSS northern halo data and SuperCOSMOS southern halo data, it is shown that no asymmetry can be detected in the South Galactic Cap at the accuracy of SuperCOSMOS, and the Virgo overdensity is likely a foreign component in the Galactic halo.     

The main sequence from F to K stars of the solar neighbourhood in SDSS colours

For an understanding of Galactic stellar populations in the SDSS filter system well defined stellar samples are needed. The nearby stars provide a complete stellar sample representative for the thin disc population. We compare the filter transformations of different authors applied to the main sequence stars from F to K dwarfs to SDSS filter system and discuss the properties of the main sequence. The location of the mean main sequence in colour‐magnitude diagrams is very sensitive to systematic differences in the filter transformation. A comparison with fiducial sequences of star clusters observed in g ′, r ′, and i ′ show good agreement. Theoretical isochrones from Padua and from Dartmouth have still some problems, especially in the (r – i) colours. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A revision of the solar neighbourhood metallicity distribution

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.     

On the Galactic disc age–metallicity relation

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.     

Radioactive Ages of Metal-Poor Halo Stars

The abundances of long-lived radioactive elements Th and U observed in metal-poor halo stars can be used as chronometers to determine the age of individual stars, and hence set a lower limit on the age of the Galaxy and hence of the universe. This radioactive dating requires the zero-decay productions of Th and U, which involves complicated r-process nucleosynthesis calculations. Several parametric r-process models have been used to calculate the initial abundance ratios of Th/Eu and U/Th, but, due to the sharp sensitivity of these models to nuclear physics inputs, the calculations have relatively large uncertainties which lead to large uncertainties in the age determinations. In order to reduce these uncertainties, we present a simple method to estimate the initial productions of Th and U, which only depends on the solar system abundances and the stellar abundances of stable r-process elements. From our calculations of the initial abundance ratios of Th/Eu and U/Th, we re-estimate the ages of those ver     

A search for previously unrecognized metal-poor subdwarfs in the Hipparcos astrometric catalogue

We have identified 317 stars included in the Hipparcos astrometric catalogue that have parallaxes measured to a precision of better than 15 per cent, and the location of which in the ( M _V ,( B − V ) _T ) diagram implies a metallicity comparable to or less than that of the intermediate-abundance globular cluster M5. We have undertaken an extensive literature search to locate Strömgren, Johnson/Cousins and Walraven photometry for over 120 stars. In addition, we present new UBV ( RI )_C photometry of 201 of these candidate halo stars, together with similar data for a further 14 known metal-poor subdwarfs. These observations provide the first extensive data set of R _C I _C photometry of metal-poor, main-sequence stars with well-determined trigonometric parallaxes. Finally, we have obtained intermediate-resolution optical spectroscopy of 175 stars.
47 stars still lack sufficient supplementary observations for population classification; however, we are able to estimate abundances for 270 stars, or over 80 per cent of the sample. The overwhelming majority have near-solar abundance, with their inclusion in the present sample stemming from errors in the colours listed in the Hipparcos catalogue. Only 44 stars show consistent evidence of abundances below [Fe/H]=−1.0 . Nine are additions to the small sample of metal-poor subdwarfs with accurate photometry. We consider briefly the implication of these results for cluster main-sequence fitting.     

On the metallicity of the Milky Way thin disc and photometric abundance scales

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.     

Dark haloes of spiral galaxies: ISO photometry

We exclude hydrogen-burning stars, of any mass above the hydrogen-burning limit and any metallicity, as significant contributors to the massive haloes deduced from rotation curves to dominate the outer parts of spiral galaxies. We present and analyse images of four nearly edge-on bulgeless spiral galaxies (UGC 711, NGC 2915, UGC 12426, UGC 1459) obtained with ISOCAM (The CAMera instrument on board the Infrared Space Observatory ) at 14.5 and 6.75 μm. Our sensitivity limit for detection of any diffuse infrared emission associated with the dark haloes in these galaxies is a few tens of μJy per 6 × 6 arcsec² pixel, with this limit currently set by remaining difficulties in modelling the non-linear behaviour of the detectors. All four galaxies show zero detected signal from extended non-disc emission, consistent with zero halo-like luminosity density distribution. The 95 per cent upper limit on any emission, for NGC 2915 in particular, allows us to exclude very low mass main-sequence stars ( M  > 0.08 M⊙) and young brown dwarfs (≲1 Gyr) as significant contributors to dark matter in galactic haloes. Combining our results with those of the Galactic microlensing surveys, which exclude objects with M  < 0.01 M⊙, excludes almost the entire possible mass range of compact baryonic objects from contributing to Galactic dark matter.     

Kinematics and metallicity analysis for nearby F,G and K stars

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)