37 research outputs found
Non-LTE effects on the lead and thorium abundance determinations for cool stars
Knowing accurate Pb abundances of metal-poor stars provides constraints on
the Pb production mechanisms in the early Galaxy. Accurately deriving Th
abundances permits a nucleo-chronometric age determination of the star. We
improve the calculation of the Pb I and Th II lines in stellar atmospheres
based on non-LTE line formation and evaluate the influence of departures from
LTE on Pb and Th abundance determinations through a range of stellar
parameters. Comprehensive model atoms for Pb I and Th II are presented. The
departures from LTE lead to systematically depleted total absorption in the Pb
I lines and positive abundance corrections. Non-LTE removes the discrepancy
between the solar and the meteoritic Pb abundance. With the Holweger & Mueller
(1974) solar model atmosphere, log eps(Pb, non-LTE) = 2.09. We revise the Pb
and Eu abundances of the strongly r-process enhanced (r-II) stars CS 31082-001
and HE 1523-0901 and the Roederer et al. (2010) stellar sample. Our results
provide strong evidence for universal Pb/Eu relative r-process yields during
course of the Galaxy evolution. The stars with -2.3<[Fe/H]< -1.4 have, on
average, 0.51 dex higher Pb/Eu ratios compared with that of the r-II stars
suggesting that the s-process synthesis of Pb started as early as the time when
Galactic metallicity had grown to [Fe/H] = -2.3. The average Pb/Eu ratio of the
-1.4<[Fe/H]< -0.59 stars is close to the solar value, in line with the
predictions of Travaglio et al. (2001) that AGB stars with [Fe/H] ~ -1 provided
the largest contribution to the solar s-nuclei of Pb. Non-LTE leads to weakened
Th II lines. Overall, the abundance correction does not exceed +0.2 dex when
collisions with H I atoms are taken into account in non-LTE calculations.Comment: A&A accepted: 12 pages, 7 figures, 3 table
Non-LTE line formation for Pr II and Pr III in A and Ap stars
Non-LTE line formation for Pr II and Pr III is considered through a range of
effective temperatures between 7250 K and 9500 K. A comprehensive model atom
for Pr II/III is based on the measured and the predicted energy levels, in
total, 6708 levels of Pr II and Pr III. We describe calculations of the Pr II
energy levels and oscillator strengths for the transitions in Pr II and Pr III.
The influence of departures from LTE on Pr abundance determinations is
evaluated. At Teff >= 8000 K departures from LTE lead to overionization of Pr
II and to systematically depleted total absorption in the line and positive
abundance corrections. At the lower temperatures, different lines of Pr II may
be either weakened or amplified depending on the line strength. The non-LTE
effects strengthen the Pr III lines and lead to negative abundance corrections.
Non-LTE corrections grow with effective temperature for the Pr II lines, and,
in contrast, they decline for the Pr III lines. The Pr II/III model atom is
applied to determine the Pr abundance in the atmosphere of the roAp star HD
24712 from the lines of two ionization stages. In the chemically uniform
atmosphere with [Pr/H] = 3, the departures from LTE may explain only small part
(0.3 dex) of the difference between the LTE abundances derived from the Pr II
and Pr III lines (2 dex). We find that the lines of both ionization stages are
described for the vertical distribution of the praseodymium where the Pr
enriched layer with [Pr/H] > 4 exists in the outer atmosphere at log tau_5000 <
-4. The departures from LTE for Pr II/III are strong in the stratified
atmosphere and have the opposite sign for the Pr II and Pr III lines. Using the
revised partition function of Pr II and experimental transition probabilities,
we determine the solar non-LTE abundance of Pr as log (Pr/H) = -11.15\pm0.08.Comment: 17 pages, 4 tables, 11 figures, accepted for publication in A&
Chemical abundances of distant extremely metal-poor unevolved stars
Aims: The purpose of our study is to determine the chemical composition of a
sample of 16 candidate Extremely Metal-Poor (EMP) dwarf stars, extracted from
the Sloan Digital Sky Survey (SDSS). There are two main purposes: in the first
place to verify the reliability of the metallicity estimates derived from the
SDSS spectra; in the second place to see if the abundance trends found for the
brighter nearer stars studied previously also hold for this sample of fainter,
more distant stars. Methods: We used the UVES at the VLT to obtain
high-resolution spectra of the programme stars. The abundances were determined
by an automatic analysis with the MyGIsFOS code, with the exception of lithium,
for which the abundances were determined from the measured equivalent widths of
the Li I resonance doublet. Results: All candidates are confirmed to be EMP
stars, with [Fe/H]<= -3.0. The chemical composition of the sample of stars is
similar to that of brighter and nearer samples. We measured the lithium
abundance for 12 stars and provide stringent upper limits for three other
stars, for a fourth star the upper limit is not significant, owing to the low
signal-to noise ratio of the spectrum. The "meltdown" of the Spite plateau is
confirmed, but some of the lowest metallicity stars of the sample lie on the
plateau. Conclusions: The concordance of the metallicities derived from
high-resolution spectra and those estimated from the SDSS spectra suggests that
the latter may be used to study the metallicity distribution of the halo. The
abundance pattern suggests that the halo was well mixed for all probed
metallicities and distances. The fact that at the lowest metallicities we find
stars on the Spite plateau suggests that the meltdown depends on at least
another parameter, besides metallicity. (abridged)Comment: A&A in pres
Rare-earth elements in the atmosphere of the magnetic chemically peculiar star HD 144897. New classification of the Nd III spectrum
We have obtained a UVES spectrum of a slowly rotating strongly magnetic Ap
star, HD 144897, that exhibits very large overabundances of rare-earth
elements. Here we present a detailed spectral analysis of this object, also
taking into account effects of non-uniform vertical distribution
(stratification) of chemical elements. We have determined the photospheric
abundances of 40 ions. For seven elements (Mg, Si, Ca, Ti, Cr, Mn, Fe), we have
obtained a stratification model that allow us to produce a satisfactory fit to
the observed profiles of spectral lines of various strength. REEs abundances,
that for the first time in the literature have been determined from the lines
of the first and second ions, have been found typically four dex larger than
solar abundances. Our analysis of REE spectral lines provide a strong support
to the laboratory line classification and determination of the atomic
parameters. The only remarkable exception is Nd III, for which spectral
synthesis was found to be inconsistent with the observations. We have therefore
performed a revision of the Nd III classification. We have confirmed the
energies for 11 out of 24 odd energy levels classified previously, and we have
derived the energies for additional 24 levels of Nd III, thereby increasing
substantially the number of classified Nd III lines with corrected wavelengths
and atomic parameters.Comment: 22 pages; accepted by A&
New Rare Earth Element Abundance Distributions for the Sun and Five r-Process-Rich Very Metal-Poor Stars
We have derived new abundances of the rare-earth elements Pr, Dy, Tm, Yb, and
Lu for the solar photosphere and for five very metal-poor, neutron-capture
r-process-rich giant stars. The photospheric values for all five elements are
in good agreement with meteoritic abundances. For the low metallicity sample,
these abundances have been combined with new Ce abundances from a companion
paper, and reconsideration of a few other elements in individual stars, to
produce internally-consistent Ba, rare-earth, and Hf (56<= Z <= 72) element
distributions. These have been used in a critical comparison between stellar
and solar r-process abundance mixes.Comment: 48 pages, 11 figures, 12 tables: To appear in the Astrophysical
Journal Supplemen
Lithium on the surface of cool magnetic CP stars. II. Spectrum analysis of HD 83368 and HD 60435 with lithium spots.
As a further step in our Lithium project we present results of abundance
determination of some elements in the roAp star HD 60435. Possible NLTE
effects were considered.
Equivalent widths of spectral lines vary with rotational phase which suggests
nonuniform distribution of chemical elements over the stars' surface. Large
abundance differences derived from various ions of some rare earth elements
indicate their possible vertical stratification.
Two spots of enhanced lithium abundance were distinguished at the
magnetic poles on HD 60435 and their parameters derived.
The rotational axis inclination or and surface
magnetic field kG were determined for the first time
for HD 60435. The results are compared with HD 83368, the other star with
lines of lithium remarkably variable in intensity and wavelength
The formation of the milky way halo and its dwarf satellites: a NLTE-1D abundance analysis. IV. Segue 1, Triangulum ii
The Pristine survey – XII. Gemini-GRACES chemo-dynamical study of newly discovered extremely metal-poor stars in the Galaxy
High-resolution optical spectra of 30 metal-poor stars selected from the Pristine survey are presented, based on observations taken with the Gemini Observatory GRACES spectrograph. Stellar parameters T-eff and logg are determined using a Gaia DR2 colour-temperature calibration and surface gravity from the Stefan-Boltzmann equation. GRACES spectra are used to determine chemical abundances (or upper limits) for 20 elements (Li, O, Na, Mg, K, Ca, Ti, Sc, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Ba, La, Nd, Eu). These stars are confirmed to be metal-poor ([Fe/H]<-2.5), with higher precision than from earlier medium-resolution analyses. The chemistry for most targets is similar to other extremely metal-poor stars in the Galactic halo. Three stars near [Fe/H] = -3.0 have unusually low Ca and high Mg, suggestive of contributions from few SN II where alpha-element formation through hydrostatic nucleosynthesis was more efficient. Three new carbon-enhanced metal-poor (CEMP) stars are also identified (two CEMP-s and one potential CEMP-no star) when our chemical abundances are combined with carbon from previous medium-resolution analyses. The GRACES spectra also provide precision radial velocities (sigma(RV) <= 0.2kms(-1)) for dynamical orbit calculations with the Gaia DR2 proper motions. Most of our targets are dynamically associated with the Galactic halo; however, five stars with [Fe/H]<-3 have planar-like orbits, including one retrograde star. Another five stars are dynamically consistent with the Gaia-Sequoia accretion event; three have typical halo [alpha/Fe] ratios for their metallicities, whereas two are [Mg/Fe]-deficient, and one is a new CEMP-s candidate. These results are discussed in terms of the formation and early chemical evolution of the Galaxy