1,053 research outputs found
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
Sulphur abundances in halo stars from Multiplet 3 at 1045 nm
Sulphur is a volatile alpha-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 Mult.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 zeta line. We study the possibility of using Mult. 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.Comment: to be published in Astronomische Nachrichte
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
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
correcte
Cu I resonance lines in turn-off stars of NGC 6752 and NGC 6397. Effects of granulation from CO5BOLD models
Context. Copper is an element whose interesting evolution with metallicity is
not fully understood. Observations of copper abundances rely on a very limited
number of lines, the strongest are the Cu I lines of Mult. 1 at 324.7 nm and
327.3 nm which can be measured even at extremely low metallicities. Aims. We
investigate the quality of these lines as abundance indicators. Method. We
measure these lines in two turn-off (TO) stars in the Globular Cluster NGC 6752
and two TO stars in the Globular Cluster NGC 6397 and derive abundances with 3D
hydrodynamical model atmospheres computed with the CO5BOLD code. These
abundances are compared to the Cu abundances measured in giant stars of the
same clusters, using the lines of Mult. 2 at 510.5 nm and 578.2 nm. Results.
The abundances derived from the lines of Mult. 1 in TO stars differ from the
abundances of giants of the same clusters. This is true both using CO5BOLD
models and using traditional 1D model atmospheres. The LTE 3D corrections for
TO stars are large, while they are small for giant stars. Conclusions. The Cu I
resonance lines of Mult. 1 are not reliable abundance indicators. It is likely
that departures from LTE should be taken into account to properly describe
these lines, although it is not clear if these alone can account for the
observations. An investigation of these departures is indeed encouraged for
both dwarfs and giants. Our recommendation to those interested in the study of
the evolution of copper abundances is to rely on the measurements in giants,
based on the lines of Mult. 2. We caution, however, that NLTE studies may imply
a revision in all the Cu abundances, both in dwarfs and giants.Comment: to be published on A\&
Solar abundances and 3D model atmospheres
We present solar photospheric abundances for 12 elements from optical and
near-infrared spectroscopy. The abundance analysis was conducted employing 3D
hydrodynamical (CO5BOLD) as well as standard 1D hydrostatic model atmospheres.
We compare our results to others with emphasis on discrepancies and still
lingering problems, in particular exemplified by the pivotal abundance of
oxygen. We argue that the thermal structure of the lower solar photosphere is
very well represented by our 3D model. We obtain an excellent match of the
observed center-to-limb variation of the line-blanketed continuum intensity,
also at wavelengths shortward of the Balmer jump.Comment: Contributed paper, to be published in the proceedings of IAU
Symposium 265, eds. K. Cunha, M. Spite, and B. Barbuy, Cambridge University
Press (CUP). 2 figures, 4 page
The Galactic evolution of sulphur as traced by globular clusters
Sulphur is an important, volatile alpha element but its role in the Galactic
chemical evolution is still uncertain. We derive the S abundances in RGB stars
in three Galactic globular clusters (GC) that cover a wide metallicity range
(-2.3<[Fe/H]<-1.2), namely M4, M22, and M30. The halo field stars show a large
scatter in the [S/Fe] ratio in this metallicity span, which is inconsistent
with canonical chemical evolution models. To date, very few measurements of
[S/Fe] exist for stars in GCs, which are good tracers of the chemical
enrichment of their environment. However, some light and alpha elements show
star-to-star variations within individual GCs and it is yet unclear whether
sulphur also varies between GC stars. We used the the infrared spectrograph
CRIRES to obtain high-resolution (R~50000), high signal-to-noise (SNR~200 per
px) spectra in the region of the S I multiplet 3 at 1045 nm for 15 GC stars
selected from the literature (6 stars in M4, 6 stars in M22 and 3 stars in
M30). Multiplet 3 is better suited for S abundance derivation than the more
commonly used lines of multiplet 1 at 920 nm, since its lines are not blended
by telluric absorption or other stellar features at low metallicity. We used
spectral synthesis to derive the [S/Fe] ratio of the stars assuming local
thermodynamic equilibrium (LTE). We find mean [S/Fe] = 0.58 +/- 0.01 +/- 0.20
dex (statistical and systematic error) for M4, [S/Fe] = 0.57+/-0.01+/-0.19 dex
for M22, and [S/Fe] = 0.55+/-0.02+/-0.16 dex for M30. The negative NLTE
corrections are estimated to be in the order of the systematic uncertainties.
With the tentative exception of two stars with measured high S abundances, we
conclude that sulphur behaves like a typical alpha element in the studied
Galactic GCs, showing enhanced abundances with respect to the solar value at
metallicities below [Fe/H] = -1.0 dex without a considerable spread.Comment: 9 pages, 7 figures, accepted for publication in A&
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