65 research outputs found
Lithium abundance in a turnoff halo star on an extreme orbit
The lithium abundance in turnoff stars of the old population of our Galaxy is
remarkably constant in the metallicity interval -2.8\textless{}[Fe/H]
\textless{}-2.0, defining a plateau. The Li abundance of these turnoff stars is
clearly lower than the abundance predicted by the primordial nucleosynthesis in
the frame of the standard Big Bang nucleosynthesis. Different scenarios have
been proposed for explaining this discrepancy, along with the very low scatter
of the lithium abundance around the plateau. The recently identified very high
velocity star, WISE J072543.88-235119.7 appears to belong to the old Galactic
population, and appears to be an extreme halo star on a bound, retrograde
Galactic orbit. In this paper, we study the abundance ratios and, in particular
the lithium abundance, in this star. The available spectra (ESO-Very Large
Telescope) are analyzed and the abundances of Li, C, Na, Mg, Al, Si, Ca, Sc,
Ti, Cr, Mn, Fe, Co, Ni, Sr and Ba are determined.The abundance ratios in WISE
J072543.88-235119.7 are those typical of old turnoff stars. The lithium
abundance in this star ~is in close agreement with the lithium abundance found
in the metal-poor turnoff stars located at moderate distance from the Sun. This
high velocity star confirms, in an extreme case, that the very small scatter of
the lithium plateau persists independent of the dynamic and kinematic
properties of the stars
High resolution study of the abundance pattern of the heavy elements in very metal-poor field stars.
9 pagesThe 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 Eu-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-rs 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
Lithium-6 : Evolution from Big Bang to Present
The primordial abundances of Deuterium, he4, and li7 are crucial to
determination of the baryon density of the Universe in the framework of
standard Big Bang nucleosynthesis (BBN). li6 which is only produced in tiny
quantities and it is generally not considered to be a cosmological probe.
However, recent major observational advances have produced an estimate of the
li6/li7 ratio in a few very old stars in the galactic halo which impacts the
question whether or not the lithium isotopes are depleted in the outer layers
of halo stars, through proton induced reactions at the base of (or below) the
convective zone. li6 is a pure product of spallation through the major
production reactions, fast oxygen and alphas interacting on interstellar H, He
(especially in the early Galaxy). The rapid nuclei are both synthesized and
accelerated by SN II. In this context, the \li6 evolution should go in step
with that of beryllium and boron, recently observed by the Keck and HST
telescopes. Li6 adds a new constraint on the early spallation in the Galaxy. In
particular, if confirmed, the Li6/Be9 ratio observed in two halo stars (HD
84937, BD +263578) gives strong boundary conditions on the composition and the
spectrum of the rapid particles involved. We show that Li6 is essentially
intact in halo stars, and a fortiori \li7. We can define a range of the Li6
abundance in the very early Galaxy consistent with Big Bang nucleosynthesis
(5.6 10(-14) to 3. 10(-13) . Following the evolution at increasing metallicity,
we explain the abundance in the solar system within a factor of about 2.Comment: 16 pages, 4 figure
Li isotopes in metal-poor halo dwarfs, a more and more complicated story
The nuclei of the lithium isotopes are fragile, easily destroyed, so that, at
variance with most of the other elements, they cannot be formed in stars
through steady hydrostatic nucleosynthesis. The 7Li isotope is synthesized
during primordial nucleosynthesis in the first minutes after the Big Bang and
later by cosmic rays, by novae and in pulsations of AGB stars (possibly also by
the "nu" process). 6Li is mainly formed by cosmic rays. The oldest (most
metal-deficient) warm galactic stars should retain the signature of these
processes if, (as it had been often expected) lithium is not depleted in these
stars. The existence of a "plateau" of the abundance of 7Li (and of its slope)
in the warm metal-poor stars is discussed. At very low metallicity ([Fe/H]<-2.7
dex the star to star scatter increases significantly towards low Li abundances.
The highest value of the lithium abundance in the early stellar matter of the
Galaxy (A(7Li) = 2.2 dex) is much lower than the the value A(7Li) = 2.72
predicted by the standard Big Bang nucleosynthesis, according to the
specifications found by the satellite WMAP. After gathering a homogeneous
stellar sample, and analysing its behaviour, possible explanations of the
disagreement between Big Bang and stellar abundances are discussed (including
early astration and diffusion). On the other hand, possibilities of lower
productions of 7Li in the standard and/or non-standard Big Bang nucleosyntheses
are briefly evoked. A surprisingly high value (A(6Li)=0.8 dex) of the abundance
of the 6Li isotope has been found in a few warm metal-poor stars. Such a high
abundance of 6Li independent of the mean metallicity in the early Galaxy cannot
be easily explained. But are we really observing 6Li
Carbon-enhanced metal-poor stars: the most pristine objects?
Carbon-enhanced metal poor stars (CEMP) form a significant proportion of the
metal-poor stars, their origin is not well understood. Three very metal-poor
C-rich turnoff stars were selected from the SDSS survey, observed with the ESO
VLT (UVES) to precisely determine the element abundances. In turnoff stars
(unlike giants) the carbon abundance has not been affected by mixing with deep
layers and is therefore easier to interpret. The analysis was performed with 1D
LTE static model atmospheres. When available, non-LTE corrections were applied
to the classical LTE abundances. The 3D effects on the CH and CN molecular
bands were computed using hydrodynamical simulations of the stellar atmosphere
(CO5BOLD) and are found to be very important. To facilitate a comparison with
previous results, only 1D abundances are used in the discussion. The abundances
(or upper limits) of the elements enable us to place these stars in different
CEMP classes. The carbon abundances confirm the existence of a plateau at A(C)=
8.25 for [Fe/H] \geq -3.4. The most metal-poor stars ([Fe/H] < -3.4) have
significantly lower carbon abundances, suggesting a lower plateau at A(C)
\approx 6.5. Detailed analyses of a larger sample of very low metallicity
carbon-rich stars are required to confirm (or refute) this possible second
plateau and specify the behavior of the CEMP stars at very low metallicity
Non-LTE abundances of Mg and K in extremely metal-poor stars and the evolution of [O/Mg], [Na/Mg], [Al/Mg] and [K/Mg] in the Milky Way
LTE abundances of light elements in extremely metal-poor (EMP) stars have
been previously derived from high quality spectra. New derivations, free from
the NLTE effects, will better constrain the models of the Galactic chemical
evolution and the yields of the very first supernovae. The NLTE profiles of the
magnesium and potassium lines have been computed in a sample of 53 extremely
metal-poor stars with a modified version of the program MULTI and adjusted to
the observed lines in order to derive the abundances of these elements. The
NLTE corrections for magnesium and potassium are in good agreement with the
works found in the literature. The abundances are slightly changed, reaching a
better precision: the scatter around the mean of the abundance ratios has
decreased. Magnesium may be used with confidence as reference element. Together
with previously determined NLTE abundances of sodium and aluminum, the new
ratios are displayed, for comparison, along the theoretical trends proposed by
some models of the chemical evolution of the Galaxy, using different models of
supernovae
Line shift, line asymmetry, and the 6Li/7Li isotopic ratio determination
Accepted for publication on A&A LettersContext: Line asymmetries are generated by convective Doppler shifts in stellar atmospheres, especially in metal-poor stars, where convective motions penetrate to higher atmospheric levels. Such asymmetries are usually neglected in abundance analyses. The determination of the 6Li/7Li isotopic ratio is prone to suffering from such asymmetries, as the contribution of 6Li is a slight blending reinforcement of the red wing of each component of the corresponding 7Li line, with respect to its blue wing. Aims: The present paper studies the halo star HD 74000 and estimates the impact of convection-related asymmetries on the Li isotopic ratio determination. Method: Two methods are used to meet this aim. The first, which is purely empirical, consists in deriving a template profile from another element that can be assumed to originate in the same stellar atmospheric layers as Li I, producing absorption lines of approximately the same equivalent width as individual components of the 7Li I resonance line. The second method consists in conducting the abundance analysis based on NLTE line formation in a 3D hydrodynamical model atmosphere, taking into account the effects of photospheric convection. Results: The results of the first method show that the convective asymmetry generates an excess absorption in the red wing of the 7Li absorption feature that mimics the presence of 6Li at a level comparable to the hitherto published values. This opens the possibility that only an upper limit on 6Li/7Li has thus far been derived. The second method confirms these findings. Conclusions: From this work, it appears that a systematic reappraisal of former determinations of 6Li abundances in halo stars is warranted
NLTE determination of the calcium abundance and 3D corrections in extremely metal-poor stars
Accepted in Astronomy and Astrophysics(Abridged) Extremely metal-poor stars contain the fossil records of the chemical composition of the early Galaxy. The NLTE profiles of the calcium lines were computed in a sample of 53 extremely metal-poor stars with a modified version of the program MULTI. With our new model atom we are able to reconcile the abundance of Ca deduced from the Ca I and Ca II lines in Procyon. -We find that [Ca/Fe] = 0.50 ± 0.09 in the early Galaxy, a value slightly higher than the previous LTE estimations. -The scatter of the ratios [X/Ca] is generally smaller than the scatter of the ratio [X/Mg] where X is a "light metal" (O, Na, Mg, Al, S, and K) with the exception of Al. These scatters cannot be explained by error of measurements, except for oxygen. Surprisingly, the scatter of [X/Fe] is always equal to, or even smaller than, the scatter around the mean value of [X/Ca]. -We note that at low metallicity, the wavelength of the Ca I resonance line is shifted relative to the (weaker) subordinate lines, a signature of the effect of convection. -The Ca abundance deduced from the Ca I resonance line (422.7 nm) is found to be systematically smaller at very low metallicity, than the abundance deduced from the subordinate lines
Oxygen Abundance in the Template Halo Giant HD 122563
HD 122563 is a well-known bright (V ¼ 6:2) halo giant of low metallicity ((Fe/H ��� 2:7). We have observed HD 122563 for infrared OH lines at 1.5-1.7 lm in the H band with the NIRSPEC high-resolution spectrograph at the 10 m Keck Telescope. Optical spectra were obtained with the UVES spectrograph at the 8 m VLT UT2 telescope at ESO (Paranal) and the FEROS spectrograph at ESO (La Silla). Based on the opti- cal high-resolution data, a detailed analysis has been carried out, and data on the forbidden (O i) 6300 Aline, unblended by telluric or sky lines, was obtained with the FEROS spectrograph. Signal-to-noise ratios of 200- 400 were obtained at resolutions of 37,000 in the H band and 45,000 in the optical. For the analysis we have adopted a photometric effective temperature Teff ¼ 4600 K. Two values for the gravity were adopted: a value deduced from ionization equilibrium, log g ¼ 1:1, with corresponding metallicity (Fe/H �¼� 2:8 and micro- turbulence velocity vt ¼ 2: 0k m s � 1 ; and log g ¼ 1:5, derived from the Hipparcos parallax, implying (Fe/H �¼� 2:71 and vt ¼ 2: 0k m s � 1 . The forbidden (O i) 6300 Aand the permitted O i 7771 Alines give O/Fe ratios essentially insensitive to model parameter variations, whereas the oxygen abundances from OH lines are sensitive to gravity, giving (O/Fe �¼þ 0:9 and +0.7, respectively, for log g ¼ 1:1 and 1.5. We derive the following oxygen abundances: for model 1, (O/Fe �¼þ 0:6, +1.1, and +0.9; and for model 2, (O/Fe �¼þ 0:6, +1.1, and +0.7, based on the (O i) 6300 A ˚ ,O i 7771 A ˚ , and IR OH 1.6 lm lines, respectively. The different oxygen abundance indicators give different oxygen abundances, illustrating the problem of oxy- gen abundance derivation in metal-poor giants. This is important because the age of globular clusters and the production of Li, Be, and B from spallation of C, N, and O atoms in the early Galaxy depend on the oxygen abundance adopted for the metal-poor stars. Subject headings: stars: abundances — stars: individual (HD 122563) — stars: Population II On-line material: machine-readable tabl
An extremely primitive halo star
The early Universe had a chemical composition consisting of hydrogen, helium
and traces of lithium1, almost all other elements were created in stars and
supernovae. The mass fraction, Z, of elements more massive than helium, is
called "metallicity". A number of very metal poor stars have been found some of
which, while having a low iron abundance, are rich in carbon, nitrogen and
oxygen. For theoretical reasons and because of an observed absence of stars
with metallicities lower than Z=1.5E-5, it has been suggested that low mass
stars (M<0.8M\odot, the ones that survive to the present day) cannot form until
the interstellar medium has been enriched above a critical value, estimated to
lie in the range 1.5E-8\leqZ\leq1.5E-6, although competing theories claiming
the contrary do exist. Here we report the chemical composition of a star with a
very low Z\leq6.9E-7 (4.5E-5 of that of the Sun) and a chemical pattern typical
of classical extremely metal poor stars, meaning without the enrichment of
carbon, nitrogen and oxygen. This shows that low mass stars can be formed at
very low metallicity. Lithium is not detected, suggesting a low metallicity
extension of the previously observed trend in lithium depletion. Lithium
depletion implies that the stellar material must have experienced temperatures
above two million K in its history, which points to rather particular formation
condition or internal mixing process, for low Z stars.Comment: Published on Nature, 2011 Volume 477, Issue 7362, pp. 67-6
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