621 research outputs found
Lithium abundances in extremely metal-poor turn-off stars
We discuss the current status of the sample of Lithium abundances in
extremely metal poor (EMP) turn-off (TO) stars collected by our group, and
compare it with the available literature results. In the last years, evidences
have accumulated of a progressive disruption of the Spite plateau in stars of
extremely low metallicity. What appears to be a flat, thin plateau above
[Fe/H]\sim-2.8 turns, at lower metallicities, into a broader distribution for
which the plateau level constitutes the upper limit, but more and more stars
show lower Li abundances. The sample we have collected currently counts
abundances or upper limits for 44 EMP TO stars between [Fe/H]=-2.5 and -3.5,
plus the ultra-metal poor star SDSS J102915+172927 at [Fe/H]=-4.9. The
"meltdown" of the Spite plateau is quite evident and, at the current status of
the sample, does not appear to be restricted to the cool end of the effective
temperature distribution. SDSS J102915+172927 displays an extreme Li depletion
that contrasts with its otherwise quite ordinary set of [X/Fe] ratios.Comment: 6 pages, 4 figures, proceedings of the "Lithium in the Cosmos"
conference, Paris, 27-29 February 201
Chemical evolution of the Milky Way: the origin of phosphorus
Context. Recently, for the first time the abundance of P has been measured in
disk stars. This provides the opportunity of comparing the observed abundances
with predictions from theoretical models. Aims. We aim at predicting the
chemical evolution of P in the Milky Way and compare our results with the
observed P abundances in disk stars in order to put constraints on the P
nucleosynthesis. Methods. To do that we adopt the two-infall model of galactic
chemical evolution, which is a good model for the Milky Way, and compute the
evolution of the abundances of P and Fe. We adopt stellar yields for these
elements from different sources. The element P should have been formed mainly
in Type II supernovae. Finally, Fe is mainly produced by Type Ia supernovae.
Results. Our results confirm that to reproduce the observed trend of [P/Fe] vs.
[Fe/H] in disk stars, P is formed mainly in massive stars. However, none of the
available yields for P can reproduce the solar abundance of this element. In
other words, to reproduce the data one should assume that massive stars produce
more P than predicted by a factor of ~ 3. Conclusions. We conclude that all the
available yields of P from massive stars are largely underestimated and that
nucleosynthesis calculations should be revised. We also predict the [P/Fe]
expected in halo stars.Comment: Accepted for publication in A&A (minor changes with respect to the
submitted version
MyGIsFOS: an automated code for parameter determination and detailed abundance analysis in cool stars
The current and planned high-resolution, high-multiplexity stellar
spectroscopic surveys, as well as the swelling amount of under-utilized data
present in public archives have led to an increasing number of efforts to
automate the crucial but slow process to retrieve stellar parameters and
chemical abundances from spectra. We present MyGIsFOS, a code designed to
derive atmospheric parameters and detailed stellar abundances from medium -
high resolution spectra of cool (FGK) stars. We describe the general structure
and workings of the code, present analyses of a number of well studied stars
representative of the parameter space MyGIsFOS is designed to cover, and
examples of the exploitation of MyGIsFOS very fast analysis to assess
uncertainties through Montecarlo tests. MyGIsFOS aims to reproduce a
``traditional'' manual analysis by fitting spectral features for different
elements against a precomputed grid of synthetic spectra. Fe I and Fe II lines
can be employed to determine temperature, gravity, microturbulence, and
metallicity by iteratively minimizing the dependence of Fe I abundance from
line lower energy and equivalent width, and imposing Fe I - Fe II ionization
equilibrium. Once parameters are retrieved, detailed chemical abundances are
measured from lines of other elements. MyGIsFOS replicates closely the results
obtained in similar analyses on a set of well known stars. It is also quite
fast, performing a full parameter determination and detailed abundance analysis
in about two minutes per star on a mainstream desktop computer. Currently, its
preferred field of application are high-resolution and/or large spectral
coverage data (e.g UVES, X-Shooter, HARPS, Sophie).Comment: 15 pages, 14 figures, accepted for publication by A&
Granulation properties of giants, dwarfs, and white dwarfs from the CIFIST 3D model atmosphere grid
3D model atmospheres for giants, dwarfs, and white dwarfs, computed with the
CO5BOLD code and part of the CIFIST grid, have been used for spectroscopic and
asteroseismic studies. Unlike existing plane-parallel 1D structures, these
simulations predict the spatially and temporally resolved emergent intensity so
that granulation can be analysed, which provides insights on how convective
energy transfer operates in stars. The wide range of atmospheric parameters of
the CIFIST 3D simulations (3600 < Teff (K) < 13,000 and 1 < log g < 9) allows
the comparison of convective processes in significantly different environments.
We show that the relative intensity contrast is correlated with both the Mach
and Peclet numbers in the photosphere. The horizontal size of granules varies
between 3 and 10 times the local pressure scale height, with a tight
correlation between the factor and the Mach number of the flow. Given that
convective giants, dwarfs, and white dwarfs cover the same range of Mach and
Peclet numbers, we conclude that photospheric convection operates in a very
similar way in those objects.Comment: 16 pages, 17 figures, 37 pages online appendix, accepted for
publication in Astronomy and Astrophysic
An upper limit on the sulphur abundance in HE 1327-2326
Context: Star HE 1327-2326 is a unique object, with the lowest measured iron
abundance ([Fe/H] ~ -6) and a peculiar chemical composition that includes large
overabundances of C, N, and O with respect to iron. One important question is
whether the chemical abundances in this star reflect the chemical composition
of the gas cloud from which it was formed or if they have been severely
affected by other processes, such as dust-gas winnowing. Aims: We measure or
provide an upper limit to the abundance of the volatile element sulphur, which
can help to discriminate between the two scenarios. Methods: We observed HE
1327-2326 with the high resolution infra-red spectrograph CRIRES at the VLT to
observe the S I lines of Multiplet 3 at 1045 nm. Results: We do not detect the
S I line. A 3sigma$upper limit on the equivalent width (EW) of any line in our
spectrum is EW<0.66 pm. Using either one-dimensional static or
three-dimensional hydrodynamical model-atmospheres, this translates into a
robust upper limit of [S/H]<-2.6. Conclusions: This upper limit does not
provide conclusive evidence for or against dust-gas winnowing, and the evidence
coming from other elements (e.g., Na and Ti) is also inconclusive or
contradictory. The formation of dust in the atmosphere versus an origin of the
metals in a metal-poor supernova with extensive "fall-back" are not mutually
exclusive. It is possible that dust formation distorts the peculiar abundance
pattern created by a supernova with fall-back, thus the abundance ratios in HE
1327-2326 may be used to constrain the properties of the supernova(e) that
produced its metals, but with some caution.Comment: Accepted by Astronomy & Astrophysics Minor typos in the abstract
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Lithium abundance and 6Li/7Li ratio in the active giant HD123351 I. A comparative analysis of 3D and 1D NLTE line-profile fits
Current three-dimensional (3D) hydrodynamical model atmospheres together with
NLTE spectrum synthesis, permit to derive reliable atomic and isotopic chemical
abundances from high-resolution stellar spectra. Not much is known about the
presence of the fragile 6Li isotope in evolved solar-metallicity RGB stars, not
to mention its production in magnetically active targets like HD123351. From
fits of the observed CFHT spectrum with synthetic line profiles based on 1D and
3D model atmospheres, we seek to estimate the abundance of the 6Li isotope and
to place constraints on its origin. We derive A(Li) and the 6Li/7Li isotopic
ratio by fitting different synthetic spectra to the Li-line region of a
high-resolution CFHT spectrum (R=120 000, S/R=400). The synthetic spectra are
computed with four different line lists, using in parallel 3D hydrodynamical
CO5BOLD and 1D LHD model atmospheres and treating the line formation of the
lithium components in non-LTE (NLTE). We find A(Li)=1.69+/-0.11 dex and
6Li/7Li=8.0+/-4.4 % in 3D-NLTE, using the line list of Mel\'endez et al.
(2012), updated with new atomic data for V I, which results in the best fit of
the lithium line profile of HD123351. Two other line lists lead to similar
results but with inferior fit qualities. Our 2-sigma detection of the 6Li
isotope is the result of a careful statistical analysis and the visual
inspection of each achieved fit. Since the presence of a significant amount of
6Li in the atmosphere of a cool evolved star is not expected in the framework
of standard stellar evolution theory, non-standard, external lithium production
mechanisms, possibly related to stellar activity or a recent accretion of rocky
material, need to be invoked to explain the detection of 6Li in HD123351.Comment: 16 pages, 11 figures. Accepted for publication in A&
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