348 research outputs found
The Hamburg/ESO R-process Enhanced Star survey (HERES) X. HE 2252-4225, one more r-process enhanced and actinide-boost halo star
We report on a detailed abundance analysis of the r-process enhanced giant
star, HE 2252-4225 ([Fe/H] = -2.63, [r/Fe] = 0.80). Determination of stellar
parameters and element abundances was based on analysis of high-quality
VLT/UVES spectra. The surface gravity was calculated from the NLTE ionisation
balance between Fe I and Fe II. Accurate abundances were determined for a total
of 38 elements, including 22 neutron-capture elements beyond Sr and up to Th.
This object is deficient in carbon, as expected for a giant star with Teff <
4800 K. The stellar Na-Zn abundances are well fitted by the yields of a single
supernova of 14.4 Msun. For the neutron-capture elements in the Sr-Ru, Ba-Yb,
and Os-Ir regions, the abundance pattern of HE 2252-4225 is in excellent
agreement with the average abundance pattern of the strongly r-process enhanced
stars CS 22892-052, CS 31082-001, HE 1219-0312, and HE 1523-091. This suggests
a common origin of the first, second, and third r-process peak elements in HE
2252-4225 in the classical r-process. We tested the solar r-process pattern
based on the most recent s-process calculations of Bisterzo et al. (2014) and
found that elements in the range from Ba to Ir match it very well. No firm
conclusion can be drawn about the relationship between the fisrt
neutron-capture peak elements, Sr to Ru, in HE 2252-4225 and the solar
r-process, due to the uncertainty in the solar r-process. The investigated star
has an anomalously high Th/Eu abundance ratio, so that radioactive age dating
results in a stellar age of tau = 1.5+-1.5 Gyr that is not expected for a very
metal-poor halo star.Comment: 20 pages, 6 tables, 9 figures, accepted for publication in A&
Fluorine Abundances of Galactic Low-Metallicity Giants
With abundances and 2{\sigma} upper limits of fluorine (F) in seven
metal-poor field giants, nucleosynthesis of stellar F at low metallicity is
discussed. The measurements are derived from the HF(1-0) R9 line at 23358{\AA}
using nearinfrared K-band high-resolution spectra obtained with CRIRES at the
Very Large Telescope. The sample reaches lower metallicities than previous
studies on F of field giants, ranging from [Fe/H] = -1.56 down to -2.13.
Effects of three-dimensional model atmospheres on the derived F and O
abundances are quantitatively estimated and shown to be insignificant for the
program stars. The observed F yield in the form of [F/O] is compared with two
sets of Galactic chemical evolution models, which quantitatively demonstrate
the contribution of Type II supernova (SN II) {\nu}-process and asymptotic
giant branch/Wolf-Rayet stars. It is found that at this low-metallicity region,
models cannot well predict the observed distribution of [F/O], while the
observations are better fit by models considering an SN II {\nu}-process with a
neutrino energy of E_{\nu} = 3 x 10^53 erg. Our sample contains HD 110281, a
retrograde orbiting low-{\alpha} halo star, showing a similar F evolution as
globular clusters. This supports the theory that such halo stars are possibly
accreted from dwarf galaxy progenitors of globular clusters in the halo.Comment: 8 pages, 8 figures, 2 tables, published in The Astrophysical Journa
An Update on the 0Z Project
We give an update on our 0Z Survey to find more extremely metal poor (EMP)
stars with [Fe/H] < -3 dex through mining the database of the Hamburg/ESO
Survey. We present the most extreme such stars we have found from ~1550
moderate resolution follow up spectra. One of these, HE1424-0241, has highly
anomalous abundance ratios not seen in any previously known halo giant, with
very deficient Si, moderately deficient Ca and Ti, highly enhanced Mn and Co,
and low C, all with respect to Fe. We suggest a SNII where the nucleosynthetic
yield for explosive alpha-burning nuclei was very low compared to that for the
hydrostatic alpha-burning element Mg, which is normal in this star relative to
Fe. A second, less extreme, outlier star with high [Sc/Fe] has also been found.
We examine the extremely metal-poor tail of the HES metallicity distribution
function (MDF). We suggest on the basis of comparison of our high resolution
detailed abundance analyses with [Fe/H](HES) for stars in our sample that the
MDF inferred from follow up spectra of the HES sample of candidate EMP stars is
heavily contaminated for [Fe/H](HES) < -3 dex; many of the supposed EMP stars
below that metallicity are of substantially higher Fe-metallicity, including
most of the very C-rich stars, or are spurious objects.Comment: to appear in conference proceedings "First Stars III", ed. B. O'Shea,
A. Heger & T.Abel, 4 pages, 2 figure
Efficient Searches for r-Process-Enhanced, Metal-Poor Stars
Neutron-capture-enhanced, metal-poor stars are of central importance to
developing an understanding of the operation of the r-process in the early
Galaxy, thought to be responsible for the formation of roughly half of all
elements beyond the iron peak. A handful of neutron-capture-rich, metal-poor
stars with [Fe/H] < -2.0 have already been identified, including the well known
r-process-enhanced stars CS 22892-052 and CS 31082-001. However, many questions
of fundamental interest can only be addressed with the assemblage of a much
larger sample of such stars, so that general properties can be distinguished.
We describe a new effort, HERES: The Hamburg/ESO R-Process-Enhanced Star
survey, nearing completion, which will identify on the order of 5-10 additional
highly r-process-enhanced, metal-poor stars, and in all likelihood, a similar
or greater number of mildly r-process-enhanced, metal-poor stars in the halo of
the Galaxy. HERES is based on rapid "snapshot" spectra of over 350 candidate
halo giants with [Fe/H] < -2.0, obtained at moderately high resolution, and
with moderate signal-to-noise ratios, using the UVES spectrograph on the
European VLT 8m telescope.Comment: Contributed paper to The Eighth Nuclei in the Cosmos conference, to
appear (in refereed form) in Nuclear Physics
The Frequency of Carbon Stars Among Extremely Metal-Poor Stars
We demonstrate that there are systematic scale errors in the [Fe/H] values
determined by the Hamburg/ESO Survey (and by inference by the HK Survey in the
past) for certain extremely metal poor highly C-enhanced giants. The
consequences of these scale errors are that a) the fraction of carbon stars at
extremely low metallicities has been overestimated in several papers in the
recent literature b) the number of extremely metal poor stars known is somewhat
lower than has been quoted in the recent literature c) the yield for extremely
metal poor stars by the HES Survey is somewhat lower than is stated in the
recent literature. A preliminary estimate for the frequency of Carbon stars
among the giants in the HES sample with -4 < [Fe/H] < -2.0 dex is 7.4 +-2.9%;
adding an estimate for the C-enhanced giants with [C/Fe] > 1.0 dex without
detectable C2 bands raises the fraction to 14 +-4$%.
We rely on the results of an extensive set of homogeneous detailed abundance
analyses of stars expected to have [Fe/H] < -3.0 dex selected from the HES to
establish these claims. We have found that the Fe-metallicity of the cooler
(Teff < 5200K) C-stars as derived from spectra taken with HIRES at Keck are a
factor of ~10 higher than those obtained via the algorithm used by the HES
project to analyze the moderate resolution follow-up spectra, which is
identical to that used until very recently by the HK Survey. This error in
Fe-abundance estimate for C-stars arises from a lowering of the emitted flux in
the continuum bandpasses of the KP (3933 A line of CaII) and particularly the
HP2 (Hdelta) indices used to estimate [Fe/H] due to absorption from strong
molecular bands.Comment: Accepted to the ApJL after a very lengthly duel with the 3
simultaneous referee
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