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  • 1
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Ongoing spectroscopy and photometry of stars selected in the HK objective-prism/interference-filter survey of Beers and colleagues has resulted in the identification of many hundreds of additional stars in the halo (and possibly the thick disk) of the Galaxy with abundances [Fe/H] ≤ -2.0. A new calibration of the technique for estimation of metal abundance based on a CaII K index as a function of broadband B - V color is applied to obtain metallicities for stars observed with the SSO 2.3m and INT 2.5m telescopes. This new data is combined with other samples of extremely metal-deficient stars (Ryan and Norris, 1991a; Beers et al., 1992; Carney et al., 1994) to form a large database of objects of low metallicity. The combined sample is examined and compared with expectations derived from a Simple Model of Galactic chemical evolution. There appears to be a statistically-significant deficit of stars more metal-weak than [Fe/H] = -3.0. An abundance of [Fe/H] ≈ -4.0 can be taken as the low-metallicity limit for presently-observable stars in the Galaxy.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present observations of the λ3130 Å Beii resonance doublet in stars of intermediate metal deficiency, −0.6≥[Fe/H]≥−1.1 obtained with the Intermediate Dispersion Spectrograph and IPCS detector at the 2.5 m Isaac Newton Telescope on La Palma. The derived beryllium abundances range from 5.6×10−12 (one hafl solar) to 2×10−12 (one sixth solar). These values, interpolated between the sets of previous beryllium measurements at higher and at lower metallicities, serve to confirm the monotonic variation of the Be abundance with Fe during the evolution of the galactic disc. We find that there was no extreme burst of Be production in the halo. To circumvent the effects of depletion, a statistical set of data is needed, for which the upper envelope can be used to trace galactic Be evolution. We note that no observations with sufficient sensitivity to detect Be in the extremely metal deficient stars of the halo have been reported to date.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Astrophysics and space science 157 (1989), S. 55-60 
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Intermediate resolution (λ/δλ ∼ 25 000) CCD spectra of the oxygen triplet at λ7770 Å have been analyzed to determine oxygen abundances in a sample of metal-deficient stars with metallicities covering the range −2.5≤[Fe/H]≤−0.2. Important oxygen overabundances ([O/Fe]≥1) are found in the more metal deficient stars of the sample. We briefly discuss the information that these observations provide about the early nucleosynthesis history and chemical evolution of the Galaxy.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Current observational constraints on anisotropies in the CMB (Cosmic Microwave Background) and on the clustering of galaxies have considerably narrowed the range of acceptable models for galaxy formation. Observations of these anisotropies on intermediate and large angular scales provide the best constraint on the amplitude of very long wavelength density fluctuations. We present results from the Tenerife CMB fluctuation experiment (a Jodrell Bank-IAC collaboration) at intermediate angular scales (6 deg) which taken together with recently reported upper limits to fluctuations on small (arc min) angular scales constrain severely the spectral index of initial fluctuations in baryon dominated model universes. Constraints on adiabatic fluctuations in ‘hot’ and ‘cold’ dark matter models are also briefly discussed.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Astrophysics and space science 157 (1989), S. 47-54 
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Measurements of lithium in stars of different galactic populations such as young open clusters (α Per, Pleiades, Praesepe, Coma, Hyades), very young stellar associations (Taurus-Auriga, Chamaeleon, Ophiuchus clouds), intermediate and old open clusters (NGC 752, M 67, NGC 188), old disc stars and halo stars give us the observational framework from which the galactic evolution of lithium has to be inferred. This element is produced mainly via three mechanisms: primordial nucleosynthesis, spallation reactions in the interstellar medium and thermonuclear reactions in some particular stellar evolutionary stages (novae, red giants). The complicated nucleosynthesis and the fact that ‘astration’ of lithium in stars is not well understood, makes a direct interpretation of the lithium evolutionary abundance curve difficult. The constraints set by recent lithium measurements in very old open clusters and metal-deficient stars on galactic lithium production mechanisms are discussed. Current problems in the determination of the primordial lithium abundance are briefly reviewed.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present an analysis of high-resolution (λ/δλ≈8×104) high signal-to-noise-ratio spectra of late-type chromospherically active dwarfs in Caii H (λ3968.47 Å), obtained with the Coudé Auxiliary Telescope plus the Coudé Echelle Spectrometer at ESO, La Silla. Observational parameters employed are the total emission flux attributable to the chromosphere, the asymmetry of the emission component and its bisector, the ratio of the intensities of the blue and red H2 emission peaks, and the absolute wavelengths of the H3 minimum and the mean emission feature. Given time-spaced sets of spectra for each object, which are as yet relatively sparse, we explore the potential use of these observational parameters to deepen our understanding of the chromospheric activity.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We discuss results on the oxygen abundance in a sample of 23 metal-poor (−3.0≤ [Fe/H] ≤ −0.3) unevolved stars and one giant. High resolutionspectroscopy of OH lines in the near UV allowed us to trace the early evolution of oxygenversus metallicity. Contrary to previous expectations, we find that oxygen abundances derived from these low excitation lines agree well withthose derived from the high excitation lines of the OI IR triplet and from the [OI] λ 6300 Å line. Our new oxygen abundances show a smooth extension of previouslyknown trends of [O/Fe] versus [Fe/H] in disk stars to much lower metallicities, with a slope of −0.31± 0.11. The [O/Fe] ratio increasesfrom 0.6 to 1 between [Fe/H] =−1.5 and −3.0. Comparison with oxygen abundances in giant stars of the same metallicity imply that the lattermay have suffered a process of oxygen depletion. We briefly discussthe impact of these results on the yields of Type II SNe in the early Galaxy and on the age of globular clusters.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We give the first results of a study in F-type MS stars using the Hei D 3 absorption feature at λ5876 Å as an indicator of stellar activity and structure. The observations were obtained with the QUB echelle spectrograph at the Cassegrain focus of the 4.2 m William Herschel telescope, and have spectral resolution λ/Δλ≈6×104, and signal-to-noise ratios ≈200. We chose as a first sample 17 F-stars of the Hyades Main Sequence. It was found that Hei D 3 and X-ray emission appear aroundB-V ∼0.3, indicating the onset of sub-surface convection zones. Below this value, in stars withB-V≤0.42–0.44 the behaviour of the Hei D 3 equivalent width against Rossby number is different from that observed in later type stars, indicating that the presence of nonradiative mechanisms other than dynamo must be important in heating the chromospheres of these hotter stars.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 377 (1995), S. 129-131 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Only about half a dozen extremely cool dwarfs with spectral types M9 or later have been identified5. At present they are the best candidates for brown dwarfs. Their mass-luminosity and mass-spectral-type relationships6'7 show that these objects are indeed very close to the substellar limit, but ...
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  • 10
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Extensive high sensitivity observations of the cosmic microwave background have been made on an angular scale of 8° covering a substantial fraction of the northern sky. An observed anisotropy in the sky emission at a level of δT/T = 3.7×l0–5 has ...
    Type of Medium: Electronic Resource
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