53 research outputs found
Overview of the lithium problem in metal-poor stars and new results on 6Li
Two problems are discussed here. The first one is the 0.4 dex discrepancy
between the 7Li abundance derived from the spectra of metal-poor halo stars on
the one hand, and from Big Bang nucleosynthesis, based on the cosmological
parameters constrained by the WMAP measurements, on the other hand. Lithium,
indeed, can be depleted in the convection zone of unevolved stars. The
understanding of the hydrodynamics of the crucial zone near the bottom of the
convective envelope in dwarfs or turn-off stars of solar metallicity has
recently made enormous progress with the inclusion of internal gravity waves.
However, similar work for metal-poor stars is still lacking. Therefore it is
not yet clear whether the depletion occurring in the metal-poor stars
themselves is adequate to produce a 7Li plateau. The second problem pertains to
the large amount of 6Li recently found in metal-poor halo stars. The
convection-related asymmetry of the 7Li line could mimic the signal attributed
so far to the weak blend of 6Li in the red wing of the 7Li line. Theoretical
computations show that the signal generated by the asymmetry of 7Li is 2.0,
2.1, and 3.7 per cent for [Fe/H]= -3.0, -2.0, -1.0, respectively (Teff =6250 K
and log g=4.0 [cgs]). In addition we re-investigate the statistical properties
of the 6Li plateau and show that previous analyses were biased. Our conclusion
is that the 6Li plateau can be reinterpreted in terms of intrinsic line
asymmetry, without the need to invoke a contribution of 6Li. (abridged)Comment: Invited talk at the 10th Symposium on Nuclei in the Cosmos - July 27
- August 1 2008 - Mackinac Island, Michigan, USA, Accepted version. Minor
changes following referee's suggestion
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
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
Solar twins in M67
The discovery of true solar analogues is fundamental for a better
understanding of the Sun and of the solar system. The open cluster M67 offers a
unique opportunity to search for solar analogues because its chemical
composition and age are very similar to those of the Sun. We analyze FLAMES
spectra of a large number of M67 main sequence stars to identify solar
analogues in this cluster.We first determine cluster members which are likely
not binaries, by combining proper motions and radial velocity measurements. We
concentrate our analysis on the determination of stellar effective temperature,
using analyses of line-depth ratios and H wings, making a direct
comparison with the solar spectrum obtained with the same instrument. We also
compute the lithium abundance for all the stars.Ten stars have both the
temperature derived by line-depth ratios and H wings within 100 K from
the Sun. From these stars we derive, assuming a cluster reddening
, the solar colour and a cluster
distance modulus of 9.63. Five stars are most similar (within 60 K) to the Sun
and candidates to be true solar twins. These stars have also a low Li content,
comparable to the photospheric abundance of the Sun, likely indicating a
similar mixing evolution. We find several candidates for the best solar
analogues ever. These stars are amenable to further spectroscopic
investigations and planet search. The solar colours are determined with rather
high accuracy with an independent method, as well as the cluster distance
modulus.Comment: 13pages and 8 figures, accepted for publication in A&
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
A T8.5 Brown Dwarf Member of the Xi Ursae Majoris System
The Wide-field Infrared Survey Explorer has revealed a T8.5 brown dwarf (WISE
J111838.70+312537.9) that exhibits common proper motion with a
solar-neighborhood (8 pc) quadruple star system - Xi Ursae Majoris. The angular
separation is 8.5 arc-min, and the projected physical separation is about 4000
AU. The sub-solar metallicity and low chromospheric activity of Xi UMa A argue
that the system has an age of at least 2 Gyr. The infrared luminosity and color
of the brown dwarf suggests the mass of this companion ranges between 14 and 38
Jupiter masses for system ages of 2 and 8 Gyr respectively.Comment: AJ in press, 12 pages LaTeX with 6 figures. More astrometric data and
a laser guide star adaptive optics image adde
Empirically Constrained Color-Temperature Relations. II. uvby
(Abriged) A new grid of theoretical color indices for the Stromgren uvby
photometric system has been derived from MARCS model atmospheres and SSG
synthetic spectra for cool dwarf and giant stars. At warmer temperatures this
grid has been supplemented with the synthetic uvby colors from recent Kurucz
atmospheric models without overshooting. Our transformations appear to
reproduce the observed colors of extremely metal-poor turnoff and giant stars
(i.e., [Fe/H]<-2). Due to a number of assumptions made in the synthetic color
calculations, however, our color-temperature relations for cool stars fail to
provide a suitable match to the uvby photometry of both cluster and field stars
having [Fe/H]>-2. To overcome this problem, the theoretical indices at
intermediate and high metallicities have been corrected using a set of color
calibrations based on field stars having accurate IRFM temperature estimates
and spectroscopic [Fe/H] values. Encouragingly, isochrones that employ the
transformations derived in this study are able to reproduce the observed CMDs
(involving u-v, v-b, and b-y colors) for a number of open and globular clusters
(including M92, M67, the Hyades, and 47Tuc) rather well. Moreover, our
interpretations of such data are very similar, if not identical, with those
given by VandenBerg & Clem (2003, AJ, 126, 778) from a consideration of BV(RI)c
observations for the same clusters. In the present investigation, we have also
analyzed the observed Stromgren photometry for the classic Population II
subdwarfs, compared our "final" (b-y)-Teff relationship with those derived
empirically in a number of recent studies, and examined in some detail the
dependence of the m1 index on [Fe/H].Comment: 70 pages, 26 figures. Accepted for publication in AJ (Feb 2004).
Postscript version with high resolution figures and complete Table 3
available at http://astrowww.phys.uvic.ca/~jclem/uvb
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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