Lithium in the cosmos 2012

February 2012, 27 / 29 - Institut d'Astrophysique de Paris



Adam Ritchey
Lithium isotope ratios near the supernova remnant IC 443oral
The rapid rise in the Li-7 abundance for stars of near solar metallicity requires the existence of one or more stellar sources of Li-7. Likely candidates include red giant and asymptotic giant branch stars, which produce Li-7 via the Cameron-Fowler mechanism, and Type II supernovae, in which Li-7 is synthesized by neutrino-induced spallation. Direct evidence of the neutrino-process remains elusive, yet could be provided by detailed studies of Li isotope ratios in interstellar gas surrounding supernova remnants. Here, we present the results of the first such investigation, based on high-resolution measurements of Li I along four lines of sight through the supernova remnant IC 443. While our observations probe material presumably contaminated by the ejecta of a core-collapse supernova, we find no evidence of Li-7 synthesis by neutrino-induced spallation. Rather, our results indicate that the abundance of Li-6 relative to Li-7 has been enhanced by interactions between shock-accelerated cosmic rays and the molecular cloud surrounding the remnant. Future observations will help to establish unequivocally the role that neutrino spallation plays in Li-7 production.
Alain Coc
Nuclear Reactions in Big Bang Nucleosynthesisoral
Primordial nucleosynthesis (or BBN) is one of the three observational evidences for the Big-Bang model. There is indeed a good overall agreement between primordial abundances of D, 3He, 4He and 7Li either deduced from observation or primordial nucleosynthesis calculations. However, there remains a tantalizing discrepancy of a factor of 3-5 between the primordial 7Li abundances either calculated or deduced from observations. Solutions to this problem have been proposed, involving non standard models of the Big-Bang or stellar physics, but first, possible nuclear physics solutions have to be investigated. Standard BBN is very special as it involves only a dozen nuclear reactions and because, contrary to stellar models, the thermodynamic conditions can be calculated exactly. Hence, it should be possible to accurately calculate the abundances of the produced 'light elements'. Nuclear reactions, candidates for the destruction of 7Be/7Li, have been proposed and nuclear physics experiments have been (or are being) conducted to determine their cross-sections.
Alessio Mucciarelli
Giant reveal what dwarf conceal: Li abundance in lower RGB starsoral
The lower RGB stars (after the completion of the ?rst dredge-up and before the RGB Bump) offer an alternative route to solve the discrepancy between the cosmological Li abundance inferred from Population II stars and that derived from the WMAP results. The Li abundance in lower RGB stars is significantly less sensitive to the efficiency of the atomic diffusion, compared with the dwarf stars, and basically unaffected by other parameters. Thus, this diagnostic provides additional and robust constraints to the pristine Li abundance. The chemical analysis of a sample of Halo and Globular Clusters lower RGB stars provides initial Li abundances 0.3-0.4 dex lower than the value from WMAP (confirming the discrepancy already observed among the stars of the Spite Plateau).
Ana Palacios
The puzzling weak G band starsoral
The weak G-band subclass includes about thirty cool and luminous stars that exhibit very weak or no G-band of the CH. In addition to their apparently very strong carbon deficiency a large number of these stars are also lithium-rich (A(Li) > 1.4 dex). In this presentation, we will review the evolutionary status and the possible explanation of the weak G-band phenomenon, and propose some clues to their lithium enrichment in the light of new stellar evolution models and abundance determinations.
Andreas Koch
A super-Li rich turnoff star in NGC 6397 - the puzzle persistsoral
This presentation focuses on the recently discovered super-Li rich turn-off star in the old, metal poor globular cluster NGC 6397. Its unusually high NLTE lithium abundance of A(Li^7) = 4.21, the highest Li enhancement found in a Galactic GC dwarf star to date, has defied any unambiguous explanation through canonical enrichment channels. The spectrum of the star shows a possible indication of binarity, but its line strengths and chemical element abundance ratios are fully compatible with other turn-off stars in this GC, seemingly ruling out mass transfer from an AGB companion as origin of the high A(Li). A possible cause is an interaction with a red giant that has undergone cool bottom processing.
Bharat Kumar Yerra
Study of Li-rich K giants: Evolution of Li, far-IR excess, and rotation in low mass giantsoral
A few low mass giants in the red giant branch are found to have anomalous high Li in their atmospheres. Adding to this anomaly, some of the Li-rich giants show far infrared excess and high rotation. Origin of overabundance of Li and associated peculiarities in Li-rich K giants are not well understood. To understand the underlying physical process for excess Li and to increase the sample size of Li-rich K giants, we have initiated a low resolution spectroscopic survey of 2000 K giants along the red giant branch covering all the way from base of the RGB to post-luminosity bump. This is the first major survey for Li-rich K giants which resulted in 15 new Li-rich K giants including 3 new super Li-rich ones. Survey resulted in increasing the total number of Li-rich K giants by a factor of two, and confirmed the rarity of the Li-rich K giants, just under 1%, in the solar neighbourhood. A significant result from the survey is finding of no Li-rich K giants before and after the luminosity bump. Importantly, all the Li-rich K giants occupy a narrow luminosity range in the HR diagram suggesting their connection to internal changes in the star. High resolution spectroscopic study suggests no correlation between Li enhancement, and C, N, O abundances or 12C/13C isotopic ratios. Abundances of C,N,O and values of 12C/13C suggest that the Li-rich K giants are normal and have undergone expected 1st dredge-up and extra-mixing. Infrared properties of Li-rich K giants may suggest a correlation between Li and mass-loss. Distribution of rotational velocities of stars from main sequence to giants indicate that a few stars among Li-rich K giants may have inherited high rotation from main sequence. In this presentation, we will describe the survey and the results which strongly favour internal nucleosynthesis to any other scenarios such as planet engulfment or preservation.
Charbonnel Corinne
Lithium processing in stars: Diagnostics for stellar structure and evolutionoral
Low-mass stars exhibit, at all stages of their evolution, the signatures of complex physical processes that require challenging modelling beyond standard stellar theory. In this talk I will focuss on the lithium abundances which are one of the best tracers of these mechanisms. After disecting the Li dip, I will discuss how large scale mixing due to rotation and internal gravity waves may interact to explain this feature. I will also brie?y discuss the impact that is expected on Population II stars, and in red giants.
Daniel Bemmerer
Study of the 2H(alpha,gamma)6Li nuclear reaction producing 6Li in standard big bang nucleosynthesisposter
D. Bemmerer and C. Gustavino for the LUNA collaboration The 2H(alpha,gamma)6Li reaction dominates the production of lithium-6 in standard big bang nucleosynthesis. Due to its exceedingly low cross section, this reaction has never been studied experimentally at the relevant energies, and consequently the adopted reaction rate depends on uncertain extrapolations. A direct study of the 2H(alpha,gamma)6Li cross section is currently underway at the LUNA 400 kV accelerator, located deep underground in the Gran Sasso laboratory, Italy. The expected data lie directly at big-bang energies. It is hoped that they help constrain non-standard lithium-6 production scenarios, by putting the standard Big Bang production on a solid experimental footing.
Evan Kirby
Discovery of Super-Lithium Rich Red Giants in Milky Way Satellite Galaxiesoral
The convective envelopes of evolved red giants reach temperatures of millions of K, hot enough to destroy lithium. Very few red giants more luminous than the luminosity function bump display any detectable lithium. Nonetheless, we found 13 lithium-rich red giants among a sample of 2961 red giants in Milky Way dwarf satellite galaxies. The abundances range from A(Li) = 2.15 to 4.27. Nine of the stars have lithium abundances higher than the primordial value. Therefore, the lithium in these stars must have been created rather than saved from destruction. The ages of most of these galaxies suggest that the lithium was produced in the red giants themselves rather than mass transfer from intermediate-mass asymptotic giant branch stars.
Evan  Skillman
The Primordial He abundance: No Problemoral
Izotov and Thuan (2010) have argued that their derived value of the primordial helium abundance (Yp) is inconsistent with the value derived from the microwave background radiation fluctuation measurements assuming standard big bang nucleosynthesis, in the sense that their determination is too high by two sigma. This would require 3.68 to 3.80 neutrino flavors to bring this high value of Yp into agreement with the deuterium and WMAP measurements. We have used Markov Chain Monte Carlo (MCMC) techniques to analyze the large dataset of Izotov, Thuan, and Stasinska (2007). We draw attention to the use of chi^2 as a clear measure of the quality of the physical solution for individual spectra and adopt this measure in order to cut the sample to reliable solutions. We also emphasize the importance of the He I 4026 emission line for its sensitivity to underlying absorption. The final dataset, after cuts, exhibits improved consistency. Regression to zero metallicity yields Yp = 0.2534 +/- 0.0083, in agreement with the WMAP result of 0.2487 +/- 0.0002.
Fields Brian
Cosmic-Ray Synthesis of Lithiumoral
Cosmic-ray interactions with interstellar gas provide a guaranteed non-primordial source of lithium, as well as beryllium and boron. Indeed, cosmic-ray nucleosynthesis provides the only conventional source of Li6, Be9, and B10. We will review the physics and astrophysics of lithium production by Galactic (and possibly pre-galactic) cosmic rays, highlighting the impact on the Lithium Problem(s). Namely, cosmic-ray-produced lithium "contaminates" halo star lithium measurements, and implies that the true primordial abundance is lower than the (undepleted) observed lithium content, worsening the Li7 problem. Moreover, the Li6 evolution from cosmic rays must be understood in order to assess the existence and severity of the Li6 problem. We will discuss how cosmic-ray lithium production can be constrained via observations of Be and B in halo stars, as well as gamma-ray data from Fermi, observables which thus also probe the cosmic-ray history of the universe.
Francesca Primas
Lithium in dwarf spheroidals of the Local Grouporal
Very little is known about lithium in dwarf spheroidal, despite the large amount of data so far collected on different galaxies of the Local Group. This is probably due to the fact that the stars targeted for medium/high-resolution spectroscopy are cool, evolved giants, hence lithium is expected to be significantly depleted, if at all detectable. Here, we present preliminary results of our new investigation of the Li region in several UVES and GIRAFFE spectra belonging to the DART survey of Local Group dwarf spheroidal galaxies. So far, we found one giant star in Fornax with a rather strong Li line that will be compared to the Li-rich giants detected so far in different (extra-)galactic environments. Authors: Primas, Shetrone, Venn, Hill, Jablonka, Tolstoy
Francesco Villante
The cosmological lithium-7 problem from a nuclear physics perspectiveoral
We discuss the cosmological Lithium-7 problem from the nuclear physics perspective. In order to do this, we introduce an efficient method to calculate the response of the 7Li primordial abundance to an arbitrary modification of the nuclear reaction rates. We, then, apply our approach to the various possible Beryllium destruction channels. We improve with respect to previous analysis by introducing a better description of the nuclear processes. As a results of this, the parameter space for a nuclear physics solution of the Lithium-7 puzzle is considerably reduced.
Georges Meynet
Impact of rotation, disc lifetime and magnetic fields on lithium depletion of solar-type starsoral
The influence of rotation and disc lifetime on lithium depletion of pre-main sequence (PMS) solar-type stars is discussed in the context of models including shellular rotation. The surface lithium abundance at the end of the PMS is shown to decrease when rotational effects are included. The impact of rotation on PMS lithium depletion is also found to be sensitive to the duration of the disc-locking phase. When the disc lifetime increases, the PMS lithium content decreases due to the higher efficiency of rotational mixing in the radiative zone. The effects of rotation and magnetic fields on the surface abundances of solar-type stars with and without detected planets are also studied. We show that the larger efficiency of rotational mixing predicted in exoplanet-host stars can result in lower lithium abundances for these stars during the main sequence.
Giancarlo Pace
Lithium in M67: from the main sequence to the red giant branch.poster
Lithium abundances in open clusters have been widely used as probe of mixing processes in the sun. The open cluster M67 is especially interesting in that its age and metallicity make it a perfect tool to help understand the large depletion of lithium in the sun. Despite M67 have been studied several times, a homogeneous global analysis of lithium in stars from subsolar up to the most massive members, was never accomplished for a large sample based on high-quality spectra. We collected literature data to follow, for the first time in a homogeneous way, NLTE lithium abundances of all observed single stars in M67 more massive than about 0.9 solar masses, and we then used these data to test our non-standard models. Our grid of evolutionary models were computed with non-standard mixing at metallicity [Fe/H] = 0.01 dex, using the Toulouse-Geneva evolution code.
Gregory Ruchti
Li-Production as Probed by Metal-Poor Li-Rich Giants in RAVEoral
The abundance of Li in stellar atmospheres is a very sensitive probe of the structure of the stellar interior and the physical processes taking place there. Understanding the processes whereby lithium is created and destroyed is critical to clarifying tension with standard big bang nucleosynthesis. Li is expected to be readily destroyed in giant stars, however, Li-rich giants have been found in both the field and globular clusters. In a recent paper, we reported on the discovery of eight lithium-rich field giants found in a high-resolution spectroscopic sample of metal-poor stars ([Fe/H] < -0.5) selected from the Radial Velocity Experiment survey. The majority of the Li-rich giants in our sample are very metal-poor ([Fe/H] <~ -1.9), and have a Li abundance between 2.30 and 3.63, well above the typical upper RGB limit. We further discovered a new Li-rich giant in the globular cluster, M68. We performed a detailed abundance analysis of all stars, finding that the majority of our sample stars have elemental abundances indistinguishable from those of Li-normal halo giants. I will discuss these abundance properties, as well as other observational properties, of the Li-rich giants in our sample and compare to predictions made by present theories of Li-production in giant stars.
Herbert Lau
Lithium production in SAGB starsoral
We have been modelling the evolution of a grid of super-AGB stars, from Z=0.02 down to Z=10^-4, through the TPAGB phases. The lithium yields are calculated through the nuclear network. Typically, SAGB stars with higher initial masses and lower metallcity gives higher lithium enhancement. Most of models adopted Vassililadis & Wood (1993) mass loss rate, and Li enhancements for around 1 order of magnitude around 1 order of magnitude and less than 2 order of magnitude for all models. Higher mass-loss rate, particularly in the early AGB phase, will result in stronger Li enhancement. How mass rate and also treatment of convection affect the Li yield will be discussed.
J. Christopher Howk
Lithium in the Interstellar Medium: an Independent Probe of Lithium Evolution oral
Most of the observational constraints we have on the cosmic evolution of Li comes by way of the Li abundance in stellar atmospheres. Of course the concern is that the stars may have burned some of their original Li, complicating the interpretation of the derived abundances. Another approach to understanding the cosmic evolution of Li is to study its abundance and properties in the gas of galaxies. This has its own difficulties associated with the uncertain corrections for the ionization of the gas and the incorporation of Li into dust grains. However, it also provides a significantly easier route to measuring the isotopic ratio of Li in low metallicity environments than through stellar atmospheric studies. I will summarize the techniques involved in measuring interstellar Li abundances and the state-of-the art for measurements in low metallicity gas. I will also comment on future prospects, notably for measuring the isotopic abundance of Li and using the results as constraints on BBN and nucleosynthesis.
Jennifer Schober
Implications of Primordial Lithium for the Origin of Magnetic Fieldsposter
The primordial element abundances regulate the ability of the gas to cool, as well as its overall ionization degree. Detailed calculations show that at densities above 10^9 cm^{-3}, Li^+ becomes the main charge carrier and thus determines the magnetic properties of the gas. We use a detailed chemical network to determine the evolution of the abundances during the collapse of a primordial halo. We study the amplification of the magnetic field by the small-scale dynamo, which depends on the turbulent properties as well as the Ohmic resistivity (Schober et al. 2012). Finally, we calculate the evolution of the small-scale magnetic field during the collapse for the two extreme cases of turbulence (incompressible and highly compressible turbulence). We find that in both cases, equipartition is reached very quickly. Thus, the presence of primordial lithium is particularly important to maintain this magnetic field at high densities.
Jorge Melendez
Depletion signatures in the Spite plateau: new insights from UVES dataoral
The standard model of big bang nucleosynthesis for the WMAP-based baryon density, predicts a primordial lithium abundance about 2-6 times higher than the Li abundance inferred from metal-poor stars in the Spite plateau. This cosmological discrepancy could be explained either by new exciting physics or by stellar depletion. We will present new precise Li abundances based on recently acquired UVES spectra of metal-poor stars and employing precise effective temperatures based on the infrared flux method. Adding the new UVES stars to our previous high precision sample, we cover a broad range of stellar masses in the metallicity range -3.5 < [Fe/H] < -1.0, providing firm constraints on stellar depletion in the Spite plateau.
Karin Lind
Li depletion in halo starsoral
A popular solution to the cosmological 7Li problem is offered by atomic diffusion, slowly draining the surface layers of low-mass stars of Li and other elements. Metal-poor globular clusters provide the necessary means, i.e. the statistics and the precise information of evolutionary status, to constrain the efficiency of diffusion and non-standard mixing processes. Thereby we can determine how much Li these stars were born with. Previous work have mapped in detail the evolutionary dependence of 7Li abundance at the metallicity of NGC6397 ([Fe/H]=-2.1). I will present new results from VLT/FLAMES observations of one of the most metal-poor globular clusters in the Galaxy (M30, [Fe/H]=-2.4), shedding light on the metallicity dependence of the depletion mechanisms. While it may successfully account for the missing amount of cosmological 7Li, the stellar solution is faced with a formidable problem in explaining the detections of 6Li in the atmospheres of metal-poor stars. How can this fragile isotope have been produced in such amounts as to survive significant surface depletion? In a second part of the talk, I will present our combined 3D, NLTE re-investigation of the 6Li abundances of well-observed halo field stars at similar metallicities.
Korn A
Shedding light on lithium evolution: the globular-cluster perspectiveoral
I shall review what has been learnt during 20 years of lithium observations in stars belonging to metal-poor globular clusters. The focus will be on little evolved main-sequence, turnoff-point and sub-giant stars expected to display Spite-plateau lithium abundances like those found in the majority of field stars of similar metallicities. But is the Spite plateau of globular clusters the same as those of field stars? What effect does, e.g., cluster-internal pollution have on lithium abundances in the now dominant second generation of stars? It will be shown that it is primarily our incomplete knowledge of the temperature scale of Population II stars which currently limits the diagnostic power of globular clusters as regards the stellar-surface evolution of lithium.
Lorenzo Monaco
The Lithium content in the globular clusters M4 and Omega Centauri oral
Globular clusters (GCs) are among the oldest objects in the universe. By studying their Li content, insights can be gained about both the cosmological Lithium problem ---namely the discrepancy between the Li abundance predicted by standard big bang nucleosynthesis coupled with the baryonic density measured by WMAP and the constant value observed in warm metal poor halo stars, i.e. the Spite plateau--- and about the ubiquitous multi-population identified in GCs and, hence, about their chemical enrichment history. I will present the results of two spectroscopic campaigns aimed at studying the Li content of the GCs WCen and M4: (i) WCen is commonly considered as the remnant of a dwarf galaxy accretion event in the Milky Way. The Li content of un-evolved main sequence stars is compatible with the Spite Plateau. Solutions to the "cosmological lithium problem" that propose a special evolution for the Milky Way halo are, hence, disfavored by our results. (ii) The GC M4 has an almost bi-modal distribution of Na abundances, yet it does present very shallow Na-Li anti-correlation among un-evolved main sequence stars, if any. This suggests that lithium is produced in parallel to sodium. Additionally, one star in the sample, # 37934, shows the remarkably high lithium abundance of A(Li)=2.87, in good agreement with current estimates of the primordial lithium abundance, and much higher than the Spite plateau. This can be interpreted as preservation of the primordial lithium abundance, or as lithium produced by a previous generation of stars. The fact that the star is also sodium-rich favors the latter hypothesis, yet the former cannot be easily dismissed.
Monika Adamow
Lithium Abundances for 1000 Evolved Starsoral
Lithium is a very intriguing element. Its primordial abundance is still uncertain. While built into the stars it may be easily destroyed at temperatures of only 2.5 mln K, what makes it very sensitive indicator of processes in stellar interiors. Hence, investigation of lithium abundances requires understanding of various mechanisms occurring during stellar evolution, particularly those responsible for lithium production (overabundant giants) and destruction (lithium - poor dwarfs). Moreover, lithium over- abundance is sometimes connected with planet engulfment. We therefore pay much attention to Lithium within the PennState-Torun Planet Search (PTPS). PTPS is an ongoing project of the Pennsylvania State University and Torun Centre for Astronomy dedicated to search for planets around evolved stars with the radial velocity technique using the Hobby-Eberly Telescope. Based on PTPS data we have determined 7Li 6708A° abundance for ~1000 giants, sub-giants and dwarfs from the upper envelope of the Main Sequence using SME package by Valenti & Piskunov (1998). In particular we have tested possible relations between Li abundance and binarity, planetary occurrence, stellar age, atmospheric and integrated stellar parameters as well as with rotation velocity, which have been determined within PTPS. The richness of the PTPS sample in stars at various stages of stellar evolution allows for lithium abundance evolution discussion in relation to evolutionary status of stars. As the sample comes from a precise radial velocity survey we may discuss lithium abundances also in the context of stellar binarity or low-mass companions as well. Here we present preliminary results of the investigation of the complete PTPS sample and report a discovery of 5 lithium - rich giants with A(Li) >3. For these very intriguing stars we present complete data on age, mass, radius, Fe/H and kinematics which allows us to speculate about the origin of the observed Li overabundance.
Moshe Gai
The Destruction of 7Be by Neutronsoral
The Destruction of 7Be by neutrons via the 7Be(n,ga) and 7Be(n,a) was first and last studied in 1962 and compiled by Wagoneer in 1967. The Wagoneer cross section is used today in all BBN codes and hence the 7Be(n,a) reaction is ignored and not included among the twelve canonical BBN reactions. But the extrapolated cross section (from thermal to ~100 keV) could be off by a large factor. We commenced a study of the 7Be(n,a) reaction with a planned experiment at the Soreq Nuclear Center in Israel and the 7Be target produced at the PSI and ISOLDE in Switzerland. We will discuss several possibility that may led to the small cross section extrapolated from thermal neutrons to BBN energies (~100 keV) and the proposed experiment in Israel.
Nina Polosukhina
Lithium in roAp stars with strong magnetic fields.poster
Last time we study the problem of Lithium and the Li6/Li7 ratio in the atmospheres of some sharplined roAp stars with magn.fields Our synthetiic spectrum calculations take into account magnetic splitting and other line-brouding effects. Lithium everabundances were found in the atmospheres of the some roApstars based on our analysis of both lines 6103 and 6708A lithium lines, with high values of the Li6/Li7 ratio (0.2-0.5) This can be explaned if Lithium produced in spallation reactions and Li 7 and Li6 is preserved by strong magnetic fields in upper layers atm. around magnetic poles!
Olivier Richard
Atomic diffusion and lithium processing in old metal poor starsoral
Due to nuclear reactions within their interior, stars are the generators of the chemical evolution of galaxies. In this context, it is important to recognize that the chemical abundances observed in stellar surfaces are most often not the original ones. In fact, due to atomic diffusion, the atomic species in stars move either downwards or upwards. For solar metallicity stars, it is well accepted within the international community that stellar modeling should include atomic diffusion. However it is not yet considered as a standard process in lower metallicity stars. In this talk I will briefly present the physical processes leading to atomic diffusion that are included in the self-consistent stellar evolutionary models. I will present few effects of atomic diffusion on evolution and structure in old metal poor stars. Finally the effect of atomic diffusion on surface abundances, and on lithium, will be presented and discussed.
Cosmic ray nucleosynthesis, Origin, formation, and abundances of the elementsoral
Cosmic structure formation leads to large-scale shocked baryonic flows which are expected to produce a cosmological population of structure-formation cosmic rays (SFCRs). Interactions between SFCRs and ambient baryons will produce lithium isotopes via a+a?Li6,7. This pre-galactic (but nonprimordial) lithium should contribute to the primordial Li7 measured in halo stars and must be subtracted in order to arrive to the true observed primordial lithium abundance. In this paper we point out that the recent halo star Li6 measurements can be used to place a strong constraint to the level of such contamination, because the exclusive astrophysical production of Li6 is from cosmic-ray interactions. We find that the putative Li6 plateau, if due to pre-galactic cosmic-ray interactions, implies that SFCR-produced lithium represents LiSFCR/Liplateau˜15% of the observed elemental Li plateau. Taking the remaining plateau Li to be cosmological Li7, we find a revised (and slightly worsened) discrepancy between the Li observations and big bang nucleosynthesis predictions by a factor of LiBBN7/Liplateau7˜3.7. Moreover, SFCRs would also contribute to the extragalactic gamma-ray background (EGRB) through neutral pion production. This gamma-ray production is tightly related to the amount of lithium produced by the same cosmic rays; the Li6 plateau limits the pre-galactic (high-redshift) SFCR contribution to be at the level of I?pSFCR/IEGRB?5% of the currently observed EGRB.
Poul Erik Nissen
Lithium abundances in high- and low-alpha halo stars.oral
A previous study of 94 F and G dwarf stars in the solar neighborhood has revealed the existence of two distinct halo populations with a clear separation in [alpha/Fe] for the metallicity range -1.6 < [Fe/H] < -0.7. Galactic formation models and the kinematics of the stars suggest that the "high-alpha" stars formed in situ, whereas the "low-alpha" ones have been accreted from satellite galaxies. In order to see if there is any systematic difference in the lithium abundances of "high-" and "low-alpha" halo stars, equivalent widths of the LiI 6707.8 line have been measured from VLT/UVES and NOT/FIES spectra and used to derive Li abundances. Furthermore, stellar masses are determined from evolutionary tracks in the log(T_eff) - log(g) diagram. For the large majority of stars, the Li abundance is well fitted by a relation A(Li) = const + a M + b Z + c M Z, where M is the stellar mass and Z the heavy element fraction. Extrapolating this relation to Z = 0 leads to A(Li)= 2.61 +-0.08 close to the primordial Li abundance predicted from Big Bang nucleosynthesis calculations and the WMAP baryon density. There is no systematic difference in A(Li) between "high-" and "low-alpha" halo stars. We suggest that all these old stars originally had a primordial value of the Li abundance, and that lithium has been depleted in time with a linear dependence on Z and stellar mass, except for a small fraction of stars, which stand out by having very low Li abundances.
Ryan S.G.
The reliance of lithium abundances and isotopes on troublesome stellar atmospheresoral
No abstract
Sbordone Luca
Lithium abundances in extremely metal poor turn-off starsoral
We present Lithium abundances and upper limits for a sample of ~30 extremely metal poor dwarf / turn-off stars spanning the range -5<[Fe/H]<-2.8, derived from a number of observing programs, in particular the ongoing search of EMP-UMP TO stars in the SDSS survey. While in metal-poor stars above [Fe/H]~-2.5 Li abundances fall mostly into a thin plateau (the "Spite plateau") for stars above Teff~5800 K, this is no longer the case when metallicity descends below [Fe/H]=-3. Here, it appears that the Spite plateau only constitutes the upper limit of the Li abundance envelope, with a growing number of stars displaying lower abundances. Moreover, all the known dwarf stars below [Fe/H]=-4 show heavily depleted Lithium, allowing only the determination of upper limits. The exact explanation of this observational fact is controversial, both due to the still scarce observational sample and to the limitations of present modeling capabilities
Scilla Degl'Innocenti
Lithium seven abundances in pre-MS open cluster starsoral
The disagreement between theoretical predictions and observations for surface lithium abundance in stars is a long-standing problem not settled yet. Indeed, despite the large amount of lithium observations in isolated stars, binaries, and clusters, and the strong effort done from the theoretical point of view, the unsatisfactory agreement with the data is a challenging issue for stellar modellers. Thus, it is meaningful to examine the old lithium problem under the light of the new improvements both on models and observations. In the talk we perform a consistent and quantitative analysis of the theoretical uncertainties affecting the models and compare our predictions on lithium abundance against the observations available for five young open clusters, of different ages and chemical composition, namely, Ic 2602, $\alpha$ Per, Blanco1, Pleiades, and Ngc 2516. We present also the comparison for lithium observations available in four detached double-lined eclipsing binary systems. Finally we investigated and discuss the possibility of adopting a a different convection efficiency during the pre-MS and MS phase.
Silvio Bonometto
Lithium & Cosmologyoral
BBN predictions on light element (including 7Li) abundances find a quasi-agreement for small \Omega_b h^2 values, i.e. for a baryon density significantly smaller than the value LCDM requires to agree with CMB and related data. The best fit value of \Omega_b h^2 slightly decreases if a DE state equation w \neq -1 is allowed. By using standard MCMC procedures, we show that a general agreement with the whole dataset, including 7Li, is reapproached when w is not left free, but required to take suitable values <<-1. The resulting picture also softens upper limits of neutrino mass.
Spite Monique
The cosmic lithium problem. One observer's perspective.oral
The light elements D, 3He, 4He, and Li are formed in the first minutes of the expansion of the Universe, and the abundances reached at this time are linked with the baryons to photons ratio and therefore to the baryonic density of the Universe. This baryonic density is now determined with a striking precision by recent cosmic microwave background measurements and this allows unprecedented precision on the prediction of the yields of Standard Big Bang Nucleosynthesis and, in particular the ratio Li/H. On the other hand the abundance of lithium in the matter just after the BB is generally deduced from the lithium abundance in the atmosphere of the old galactic stars. But we know that lithium is fragile and its abundance may have been depleted in the atmosphere of the stars. It is thus important to make these measurements in stars where stellar physics predicts that lithium can survive. When we compare the quantity of the lithium created by the primordial nucleosynthesis and the abundance of lithium in the atmosphere of the old galactic stars, there is a very significant discrepancy. This is the main reason advanced for holding this workshop, and we will try to explore the different possibilities to explain this discrepancy.
Stefan Uttenthaler
Lithium destruction and production observed in red giant starsoral
According to standard stellar evolution, lithium is destroyed throughout most of the evolution of low- to intermediate-mass stars. However, a number of evolved stars on the RGB and AGB are known to contain a considerable amount of Li, whose origin is not understood well. Here we present the latest development on the observational side to obtain a better understanding of Li-rich K giants (RGB), moderately Li-rich low-mass stars on the AGB, as well as very Li-rich intermediate-mass AGB stars probably undergoing the standard hot bottom burning phase. These last ones possibly also enrich the interstellar medium with freshly produced Li.
Steffen Matthias
Li detection in metal-poor stars: can 3D model atmospheres solve the second lithium problem?oral
The presence of $^6$Li in the atmospheres of metal-poor halo stars can only be detected in spectra of the highest quality, where it causes a subtle extra depression in the red wing of the $^7$Li doublet line at 670.8 nm. A systematic analysis of high-resolution, high signal-to-noise UVES and Keck spectra of about 30 bright metal-poor stars resulted in the detection of $^6$Li (at the 2 sigma level) in about one third of these objects (Asplund et al.\ 2006, Asplund and Melendez 2008). The average $^6$Li/$^7$Li isotopic ratio in the stars in which $^6$Li has been detected is about 4\% and is very similar in each of these stars, defining a $^6$Li plateau. A convincing theoretical explanation of this new $^6$Li plateau turned out to be problematic: the high abundance of $^6$Li at the lowest metallicities cannot be explained by current models of galactic cosmic-ray production. A possible solution of this so-called 'second Lithium problem' was proposed by Cayrel et al. (2007), who pointed out that the intrinsic line asymmetry caused by convection in the photospheres of cool stars is almost indistinguishable from the asymmetry produced by a weak $^6$Li blend on a (presumed) symmetric $^7$Li profile. Based on 3D hydrodynamical model atmospheres computed with the CO$^5$BOLD code and 3D non-LTE line formation calculations, we investigate the consequences of this convective line asymmetry on the derived $^6$Li abundance as a function of effective temperature, gravity, and metallicity. We find that the asymmetry effect reduces the resulting $^6$Li/$^7$Li ratios significantly. A careful reanalysis of individual objects is under way to see whether the 'second Lithium problem' actually disappears when accounting properly for convection and non-LTE line formation in 3D stellar atmospheres.
Sylvie Theado (and Sylvie Vauclair)
Metal-rich Accretion and Thermohaline Instabilities in Exoplanet-host Starsoral
The early evolution of planetary systems is expected to depend on various periods of disk matter accretion onto the central star, which may include the accretion of metal-rich matter after the star settles on the main sequence. When this happens, the accreted material is rapidly mixed within the surface convective zone and induces an inverse mean-molecular-weight gradient, unstable for thermohaline convection. The induced mixing, which dilutes the metal excess, may also have important consequences on the light elements abundances. We model and analyze this process, and present the results according to various possible accretion scenarios. We give a detailed discussion of the different ways of treating thermohaline mixing, as proposed by previous authors, and converge on a consistent view, including the most recent numerical simulations. We show how the observations of light elements in stars can be used as tracers of such events.
Tamara Mishenina
Li abundance in the stars with the solar-like type of activity poster
Li abundances, atmospheric parameters and rotational velocities for 150 dwarfs have been determined from the high resolution echelle spectra, obtained with the ELODIE spectrograph at the OHP (France). Among them, there are 100 stars with a determined level of activity. The level of chromospheric and coronal activity of the target has been evaluated through the log R'HK index and X-ray flux. The behavior of the lithium abundance with Mbol, Teff, [FeH], vsini and the level of activity was considered. The Li is detected in 62% stars with the high level of solar-type activity whereas it is only detected in 31% of stars with the weak level of solar-type activity. Some correlations between the Li abundances, and of the level of chromospheric activity and rotational velocities vsini have been found. The correlation of the Li abundances and the index of chromospheric activity log R'HK was not established. The obtained results confirm that the principal causes of the formation of high activity and detectable Li abundance are determined mainly by fast axial rotation and younger age, respectively.
Thomas Bensby
Lithium in the Galactic thin and thick disksposter
I will present lithium abundances for 703 F and G dwarf stars in the Galactic thin and thick disk.
Tijana Prodanovic
Lithium Production in Galactic Flybysposter
Unlike Li-7 plateau which is expected but observed with a level that is difficult to explain, Li-6 observations in low-metallicity halo stars also reveal a plateau which is completely unexpected within the standard picture of Li-6 origin in galactic cosmic-ray interactions. This indicates the existence of a new pre-galactic Li source, however all considered sources (e.g. supernovae, shocks due to structure-formation etc.) either completely fail to explain the observed abundance or require some fine-tuning. Here we present another possible source of lithium – cosmic rays that arise due to galactic flybys. Gravitational interaction between two galaxies flying by each other could result in weak, but large-scale, short-lived shocks that would give rise to a new, low-energy cosmic-ray population. Such cosmic-ray population would result in a homogeneous increase of Li-6 (and Li-7) abundance, but would not be accompanied with increased metallicity or with increased gas content. The details, of course, depend on interacting galaxies. In our preliminary results we consider close flybys between Milky Way and Small Magellanic Cloud (SMC), and analyze impact on resulting lithium abundances. Our results are discussed in the light of the new lithium abundance measurements and gamma-ray observations of the SMC.
Wako Aoki
Li abundances in very metal-poor, main-sequence turn-off starsoral
Li abundances in very metal-poor stars show a decreasing trend with decreasing metallicity or some scatter in the lowest metallicity range. In order to make such trend or scatter clear, we are conducting Li abundance measurements for extremely metal-poor (EMP: [Fe/H}<-3) main-sequence turn-off stars found by SDSS/SEGUE. Our preliminary result for EMP stars, including [Fe/H]<-3.5 objects, supports previous suggestions for a small Li depletion in some EMP stars (A(Li)~-2.0), while others show the Spite plateau values. The correlation between Li and metallicity, as well as abundances of other elements, will be discussed. Our new measurement of Li abundances for a very metal-poor, double-lined spectroscopic binary will be also reported in the context of the mass dependence of the Li depletion.
Walter Maciel
Lithium abundances and metallicities: trends from metal-poor and AGB/RGB starsposter
Authors: W. J. Maciel, R. D. D. Costa Recent determinations of Li abundances in the Galaxy resulted in two important conclusions: first, there is a significant discrepancy between the predicted values by Standard Big Bang Nucleosynthesis (SBBN) and the observed values, reaching about 0.6 dex for the oldest, most metal-poor stars; second, for these stars there seems to exist an almost linear relationship between the Li abundances relative to H and the metallicity as given by the Fe abundances relative to the Sun. Concerning higher metallicity objects, present results show that a larger dispersion of Li abundances is observed for a given metallicity up to about [Fe/H] = 0.5. In the case of RGB and AGB stars, most of the objects analyzed have probably destroyed most of the pristine lithium. However, there is a number of galactic AGB and RGB stars which show clear signs of Li enhancements, in particular those for which the Li abundance relative to H is higher than 1.5 dex, which are generally known as Li-rich giants. Part of the Li observed in these objects may have been produced by Hot Bottom Burning (HBB), or by other mechanisms. Since the metallicities of the Li-rich giants are generally known, it is interesting to investigate whether or not the trend presented by the metal-poor stars can also be observed in their metal-rich counterparts. This will provide a better understanding of the chemical evolution of lithium during the galactic lifetime, as well as place constraints on the Li production in AGB stars. In this work, we consider a large sample of AGB and RGB stars for which accurate Li abundances are available, and investigate the existence of any trends between the Li abundances and the metallicities. It is concluded that, in average, Li-rich AGB stars maintain the same average increase of Li abundances at higher metallicities, so that a similar slope is obtained both for metal-poor and metal-rich stars. This fact possibly has some consequences on the mechanisms of Li production in AGB stars. (FAPESP/CNPq)
Xiaoting Fu
The discovery of 13 lithium-rich red giants in Milky Way dwarf satellite galaxiesposter
The convective envelopes of evolved red giants reach temperatures of millions of K, hot enough to destroy lithium. Very few red giants more luminous than the luminosity function bump display any detectable lithium. Nonetheless, we found 13 lithium-rich red giants among a sample of 2961 red giants in Milky Way dwarf satellite galaxies. The lithium stars are present in Sculptor, Fornax, Leo I, Leo II, and Canes Venatici I. The abundances range from A(Li) = 2.15 to 4.27. Nine of the stars have lithium abundances higher than the primordial value. Therefore, the lithium in these stars must have been created rather than saved from destruction. The ages of most of these galaxies suggest that the lithium was produced in the red giants themselves rather than mass transfer from intermediate-mass asymptotic giant branch stars. This work is the first lithium giants survey in the MW dSphs that quantifies the demographics of lithium giants in our dwarf galaxies for the very first time. It is also the largest lithium giants survey ever performed.
Xiaoting Fu
The discovery of 13 lithium-rich red giants in Milky Way dwarf satellite galaxiesoral
The convective envelopes of evolved red giants reach temperatures of millions of K, hot enough to destroy lithium. Very few red giants more luminous than the luminosity function bump display any detectable lithium. Nonetheless, we found 13 lithium-rich red giants among a sample of 2961 red giants in Milky Way dwarf satellite galaxies. The lithium stars are present in Sculptor, Fornax, Leo I, Leo II, and Canes Venatici I. The abundances range from A(Li) = 2.15 to 4.27. Nine of the stars have lithium abundances higher than the primordial value. Therefore, the lithium in these stars must have been created rather than saved from destruction. The ages of most of these galaxies suggest that the lithium was produced in the red giants themselves rather than mass transfer from intermediate-mass asymptotic giant branch stars. This work is the first lithium giants survey in the MW dSphs that quantifies the demographics of lithium giants in our dwarf galaxies for the very first time. It is also the largest lithium giants survey ever performed.