排序方式： 共有10条查询结果，搜索用时 15 毫秒
1
1.
2.
3.
F. Spite 《Astrophysics and Space Science》1993,200(1):173-176
Specifications concerning names, designations, and nomenclature for astronomical radiation sources outside the solar system 相似文献
4.
P. Bonifacio E. Caffau P. Franois L. Sbordone H.‐G. Ludwig M. Spite P. Molaro F. Spite R. Cayrel F. Hammer V. Hill M. Nonino S. Randich B. Stelzer S. Zaggia 《Astronomische Nachrichten》2011,332(3):251-257
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
5.
E. Caffau L. Sbordone H.‐G. Ludwig P. Bonifacio M. Spite 《Astronomische Nachrichten》2010,331(7):725-730
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
6.
7.
The five known metal-poor dwarfs with an enhanced N/Fe ratio have been observed spectroscopically. Two of these dwarfs have
no lithium line; the absence of lithium is most probably accounted for by the usual convective destruction. The three other
dwarfs have the same lithium abundance as the normal metal-poor dwarfs (Spite, Maillard & Spite 1984). This excludes the deep
mixing process as the general source of nitrogen enhancement, since lithium is destroyed in deep (hot) layers. Deep mixing
had been previously found unlikely in metal-poor dwarfs (Da Costa & Demarque 1982). The discussion stresses the remarkable
uniformity of the lithium abundance in metal-poor dwarfs, and shows that the N-rich contaminating matter has a high N/H ratio.
Finally, the Al abundance is not greatly enhanced in these five stars.
Based on observations collected at ESO and CFHT. 相似文献
8.
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
9.
10.
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
1