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\documentclass{mem}
\usepackage{natbib}\usepackage{txfonts}\usepackage{balance}
\usepackage{graphicx}
\usepackage[breaklinks]{hyperref}
\idline{75}{282}
\begin{document}
\def\teff{$T\rm_{eff }$}
\def\kms{$\mathrm {km s}^{-1}$}
\title{
Lithium abundances in Globular Clusters
}
\subtitle{}
\author{
P. \,Bonifacio\inst{1}
\and B. \, Rabbit\inst{1,2}
}
\institute{
Istituto Nazionale di Astrofisica --
Osservatorio Astronomico di Trieste, Via Tiepolo 11,
I-34131 Trieste, Italy
\and
Forest University, Department of Astronomy,
25 Long Street, 255255,
Somewhere, Elsewhere
\email{bonifaci@ts.astro.it}
}
\authorrunning{Bonifacio }
\titlerunning{Lithium in GCs}
\abstract{
Warm metal poor dwarfs display a constant Li abundance, regardelss of
their effective temperature or metallicity: the so-called ``Spite Plateau''.
If this constant value represents the primordial Li abundance, that is
the lithium synthesized in the Big Bang, the Universal baryonic
density may be derived comparing it
to nucleosynthesis calculations.
In the recent years there has been an active
debate on whether these stars have indeed
preserved their pristine Li or whether it
has been depleted by some stellar phenomenon.
Since the Globular Clusters are a homogeneous single-age population they are
an ideal testing ground for any theory which predicts Li depletion. As part
of the ESO Large Programme (165.L-0263, P.I. R. Gratton)
we observed turn-off stars in the
Globular Clusters NGC 6397([Fe/H]$\sim-2.0$ ) and
47 Tuc ([Fe/H]$\sim -0.7$) at high resolution and relatively high
signal to noise ratios, with the UVES spectrograph on the ESO Kueyen-VLT 8.2m
telescope. On behalf of the collaboration I report our results on
the Li abundances and abundance dispersion, or lack thereof, in these clusters
as well as a re-analysis of extant data of the metal-poor Globular Cluster M92.
\keywords{Stars: abundances --
Stars: atmospheres -- Stars: Population II -- Galaxy: globular clusters --
Galaxy: abundances -- Cosmology: observations }
}
\maketitle{}
\section{Introduction}
The original paper has been modified in order to be used as
demo file and display
some of the features of mem.cls.
Standard Big Bang cosmology predicts that protons and neutrons are
assembled into nuclei of D, $^4$He, $^3$He and $^7$Li during the first
three minutes \citep{wagoner}. The abundances of these nuclei are
a function of the baryon to photon ratio, which is simply related
to the baryonic density of the Universe $\Omega_b$.
The concordance, within errors, of the measured primordial abundances
for a unique value of $\Omega_b$
has been considered for the last twenty years one of the
observational pillars which support Big Bang cosmology.
The opportunity of measuring the primordial Li abundance arises
from the observation that the Li abundance in warm halo
dwarf stars is constant, regardless of the metallicity
and the effective temperature \citep{spite82}, the so-called
``Spite plateau''.
In order to intepret this constant Li value as the primordial
one two conditions must be fulfilled: a) the Li present in the
stellar atmosphere must have been preserved throughout the stellar
evolution ; b) the Li produced by Galactic sources must
be neglegible compared to that produced in the Big bang.
Li is a relatively fragile element and can be destroyed by
nuclear reactions at temperatures in excess of $2.5\times 10^6$ K.
Li destruction is believed to be responsible for the large scatter
in Li abundances observed in solar-metallicity stars and for the low
photospheric Li abundance in the Sun itself.
%
\begin{figure*}[t!]
\resizebox{\hsize}{!}{\includegraphics[clip=true]{pl_eta_6397.eps}}
\caption{\footnotesize
Comparison of the Li abundance measured in NGC 6397
(thick solid line)
with the predictions of BBN nucleosynthesis.
The middle curve is a Kawano-code computation performed
using a UNIX version of the code
(F. Villante private communication), the upper and lower
curves correspond to 1$\sigma$ errors as parametrized
by \citet{sarkar}.
The horizontal thin solid lines correspond to the
statistical error of 0.056 dex in the Li abundance,
while the dashed horizontal lines correspond
to the statistical error added linearly to
a systematic error of 0.05 dex, due to the uncertainty
on the temperature scale.
The vertical solid lines corresponds to
$\eta=6.137\times 10^{-10}$ as implied
by the WMAP measurement of $\Omega_b h^2 = 0.0224$
\citep{spergel}; the vertical dashed lines
correspond to the 1$\sigma$ error of
0.0009 on $\Omega_b h^2$, i.e. $0.246\times 10^{-10}$
on $\eta$.
}
\label{eta}
\end{figure*}
%
The convective structure of a metal-poor G dwarf should be, however,
different from that of the Sun, with a considerably thinner and more
superficial convective zone, which should allow Li to survive in the
star's atmosphere \citep{cayrel}.
However, models which are more sophisticated
than ``standard'' models and include additional
physical ingredients, such as rotational mixing \citep{pin01},
or diffusion
\citep{sal01}, or diffusion and turbulence \citep{richard},
or a combination of diffusion, rotation and
composition gradient
\citep{theado}, predict a mild Li depletion.
A common feature of such models is the prediction of
some star to star variation in Li abundances and, in some
cases the existence of a small number, of the order of a
few percent, of ``outliers'', i.e. stars which are consistently
Li depleted.
Globular Clusters (GCs) appear as the ideal testing ground for such
models, since the observations are not complicated by the
scatter in ages and metallicities which characterize
samples of field stars.
\section{Li observations}
\begin{table*}
\caption{Abundances for TO stars in M 92}
\label{abun}
\begin{center}
\begin{tabular}{lccccccc}
\hline
\\
Star \# & [Fe/H] & $\sigma$ & [Mg/Fe] & [Ca/Fe] & [Ti/Fe] & [Cr/Fe] & [Ba/Fe] \\
\hline
\\
18 &$ -2.63 $ & $0.22 $& $-0.02 $ &$+0.21$ & $+0.28$ & $-0.29 $ &$+0.11$ \\
21 &$ -2.57 $ & $0.27 $& $-0.55 $ &$+0.17$ & $+0.44$ & $-0.22 $ &$-0.18$ \\
34 &$ -2.58 $ & $0.24 $& $+0.06 $ &$+0.04$ & $+0.40$ & $ - $ &$-0.05$ \\
46 &$ -2.38 $ & $0.22 $& $-0.31 $ &$+0.22$ & $+0.17$ & $-0.27 $ &$-0.28$ \\
60 &$ -2.54 $ & $0.30 $& $+0.12 $ &$+0.43$ & $+0.44$ & $-0.06 $ &$+0.39$ \\
350 &$ -2.37 $ & $0.30 $& $-0.17 $ &$+0.37$ & $+0.34$ & $ - $ &$ - $
\\
\hline
\end{tabular}
\end{center}
\end{table*}
\subsection{NGC 6397}
This is the GC with the brightest TO in the sky
and, not surprisingly, was also the first
for which Li abundances in TO stars were measured
\citep{mp94,pm96}.
The advent of UVES at the VLT allowed the measurement
of Li in 12 TO stars with errors in the equivalent widths
of the order of 0.2 pm \citep{theve,B6397}, i.e. comparable
with what available for most field stars.
The homogeneous \balance analysis by \citet{B6397} of
all the high quality UVES data showed that there
is no evidence for scatter in the Li abundances above
what expected from the observational errors.
The Li abundance in NGC 6397 is A(Li)$=2.34\pm 0.056$ and is
very close to what derived from field stars analyzed in a
similar fashion, strongly suggesting that this is very close
to the primordial Li abundance.
A very tight limit on the maximum intrinsic
scatter in Li abundances compatible with the data
has been placed: 0.035 dex. This is a rather
robust constraint which must be fulfilled by
any theory which predicts Li depletion.
Considering all the data in the literature a total
of 15 TO stars have been observed in this cluster
and none is Li-depleted. However outliers are expected
to be of the order of a few percent, in order to
test their existence I used FLAMES at the VLT
in May 2003
to obtain medium quality high resolution spectra
for over 120 stars in this cluster.
The data is currently under analysis, however
a quick look at the data has so far
not revealed any outlier.
\begin{figure}[]
\resizebox{\hsize}{!}{\includegraphics[clip=true]{plot_li_vhel.eps}}
\caption{
\footnotesize
Li abundances
as a function of heliocentric
radial velocities in the Globular Cluster 47 Tuc.
The four stars observed in the course
of ESO LP 165.L-0263, are shown as filled circles,
while two stars observed by \citet{pm97}
are shown as open symbols. The vertical line denotes
the mean heliocentric radial velocity of the cluster.
}
\label{li_vhel}
\end{figure}
In Fig. \ref{eta} I compare this abundance with Big Bang
nucleosynthesis (BBN) predictions
computed with the Kawano code.
Two possible values for $\eta$
may be identified, however
concordance with the
fluctuations in the cosmic microwave background measured by
WMAP \citep{spergel} requires that the low $\eta$ root be rejected.
The plot shows that A(Li)=2.34 is compatible with the WMAP
measurement within 3$\sigma$. A large uncertainity is still linked
to the theoretical BBN prediction, which is largely due to the
uncertainty in the cross section for the
$\rm ^4 He(^3He,\gamma)^7Be$ reaction which is
of the order of 18\% and by itself results in an uncertainty
of about 20\% on the predicted Li abundance \citep{cuoco}.
The more recent computations of \citet{cuoco} with their new
BBN code arrive at a similar conclusion: the Li abundance
in NGC 6397, if taken as primordial, is consistent with BBN at less
than 3$\sigma$. I thus believe it premature to claim that concordance
with WMAP requires a Li depletion in metal-poor dwarfs.
\subsection{M 92}
For this metal-poor GC ([Fe/H]=-2.52 \citealt{king},
[Fe/H]=-2.26 \citealt{sneden})
a spread in Li abundances of the order of 0.5 dex
was claimed by \citet{boe98}.
A reanalysis of the equivalent widths
of \citet{boe98}by \citet{BM92} with a careful assessment
of the errors and the use of Monte Carlo simulations
concluded that there is no strong evidence for an
intrinsic dispersion of Li abundances in this cluster.
However, since the TO of this cluster is about 2 magnitudes
fainter than that of NGC 6397, the quality of the data is
considerably lower. Although an intrinsic dispersion
as large as 0.5 dex can be safely ruled out, a dispersion
of 0.18 dex could exist and be undetected.
Clearly higher quality data are needed to resolve the issue.
It is interesting to note that by using the
equivalent widths of \citet{boe98}, and the same
temperature scale used for NGC 6397,
\citet{BM92} derived for M 92 A(Li)$=2.36\pm 0.19$ in
remarkably good agreement with that found
in NGC 6397.
\subsection{47 Tuc}
This is among the most metal-rich GCs ([Fe/H]=-0.67,
Carretta et al. in preparation).
\citet{pm97} observed for Li three TO stars of
this cluster with the 3.5m ESO-NTT, in two the Li doublet
was detected, but not in the third one.
Although the detected Li was close to the value
of the ``Spite plateau'' of field stars, the lack of
detection of Li in the third star suggested that some
dispersion in Li abundances is present in this cluster.
Another four TO stars have been observed with UVES-VLT
in the course of the ESO LP 165.L-0263 led by R. Gratton.
Li was detected in all of them. However, the measured A(Li)
spans a range of almost 0.6 dex.
A proper Monte Carlo analysis of the data is underway. However,
the conclusion that there is real dispersion in Li content
among TO stars in 47 Tuc seems difficult to escape.
In Fig. \ref{li_vhel} I show the Li abundnaces for
all the 6 TO stars of 47 Tuc for which Li has been
detected versus the radial velocity.
\section{Conclusions}
The study of Li in GCs has just started
and I expect more exciting results in the next few
years from multi-fibre facilities such as FLAMES
and high efficiency spectrographs such as ESI and
X-shooter.
With the present data at hand it seems that Li in
metal-poor GCs tracks the ``Spite plateau''
while Li dispersion is present at higher metallicity.
This suggests that the mechanism(s) producing
such dispersion are not efficient at low metallicities.
\begin{acknowledgements}
I am grateful to R. Gratton and to all the researchers
of the Large Programme led by him, for the excellent
work done and for allowing me to present some
results in advance of publication.
It is a pleasure to acknowledge the
many helpful discussions on the topic
of Li abundances with M. Spite, F. Spite, P. Molaro and
R. Cayrel.
\end{acknowledgements}
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\end{document}