A view of the Galactic halo using beryllium as a time scale
Rodolfo Smiljanic$^1$ \thanks{Present address: ESO, Garching bei M\"unchen, Germany.}
L. Pasquini$^2$
P. Bonifacio$^{3,4,5}$
D. Galli$^6$
B. Barbuy$^1$
R. Gratton$^7$
S. Randich$^6$
Beryllium stellar abundances were suggested to be a good tracer of time in the early Galaxy. In an investigation of its use as a cosmochronometer, using a large sample of local halo and thick-disk dwarfs, evidence was found that in a log(Be/H) vs.\, [$\alpha$/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined. \keywords{stars: abundances � stars: late-type � Galaxy: halo} %% add here a maximum of 10 keywords, to be taken form the file <Keywords.txt>
rsmiljanic_etal.tex
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\title[The Galactic halo using beryllium as a time scale] %% give here short title %%
{A view of the Galactic halo using beryllium as a time scale}

\author[Smiljanic et al.]   %% give here short author list %%
{Rodolfo Smiljanic$^1$
 \thanks{Present address: ESO, Garching bei M\"unchen, Germany.},
  L. Pasquini$^2$,
  P. Bonifacio$^{3,4,5}$,
  D. Galli$^6$, \\
  B. Barbuy$^1$,
  R. Gratton$^7$,
 \and S. Randich$^6$
 }

\affiliation{$^1$IAG, Universidade de S\~ao Paulo, S\~ao Paulo, Brazil 
 \\ email: {\tt rodolfo@astro.iag.usp.br} \\[\affilskip]
$^2$ESO, Garching bei M\"unchen, Germany, 
$^3$GEPI Observatoire de Paris - Meudon, France, 
$^4$INAF, Osservatorio di Trieste, Trieste, Italy, 
$^5$CIFIST Marie Curie Excellence Team, 
$^6$INAF- Osservatorio di Arcetri, Firenze, Italy
$^7$INAF-Osservatorio di Padova, Padova, Italy, 
}

\pubyear{2009}
\volume{265}  %% insert here IAU Symposium No.
\pagerange{1--2}
% \date{?? and in revised form ??}
\setcounter{page}{1}
\jname{Chemical abundances in the Universe: connecting first stars to planets}
\editors{K. Cunha, M. Spite \& B. Barbuy, eds.}
\begin{document}

\maketitle

\begin{abstract}
Beryllium stellar abundances were suggested to be a good tracer of time 
in the early Galaxy. In an investigation of its use as a cosmochronometer, 
using a large sample of local halo and thick-disk dwarfs, evidence was found that in 
a log(Be/H) vs.\, [$\alpha$/Fe] diagram the halo stars 
separate into two components. One is consistent with predictions of evolutionary 
models while the other is chemically indistinguishable from the thick-disk stars. 
This is interpreted as a difference in the star formation history of the two components and 
suggests that the local halo is not a single uniform population where a clear 
age-metallicity relation can be defined.
\keywords{stars: abundances � stars: late-type � Galaxy: halo}
%% add here a maximum of 10 keywords, to be taken form the file <Keywords.txt>
\end{abstract}

\firstsection % if your document starts with a section,
              % remove some space above using this command.
\section{Introduction}

Be is a pure product of cosmic-ray spallation in the ISM involving mostly CNO nuclei. 
Abundances of Be in metal-poor stars show a linear relation with Fe (and O) with 
a slope close to one, implying that Be behaves as a primary element (\cite[Smiljanic et al. 2009, and references therein]{Sm09}). 
If cosmic rays are globally transported across the Galaxy, the production 
of Be is a widespread process and Be abundances should 
have a smaller scatter than the products of stellar nucleosynthesis at a given time in the early Galaxy 
(\cite[Suzuki \& Yoshii 2001]{SY01}). In other words Be should be a good tracer of time.

\cite[Pasquini et al. (2004, 2007)]{Pas04,07} calculated Be abundances in turn-off stars 
of two globular clusters, NGC 6397 and NGC 6752. The Be ages derived 
from a model of the evolution of Be with time (\cite[Valle et al. 2002]{Valle02}) were shown 
to agree with those derived from theoretical isochrones, supporting the use of Be as a 
cosmochronometer. 

\cite[Pasquini et al. (2005)]{Pas05} analyzed a sample of 20 
halo and thick-disk stars and found a possible separation between stars of the two components in a 
log(Be/H) vs.\, [$\alpha$/Fe] diagram.  This was interpreted as a difference in the time 
scales of star formation.

\cite[Smiljanic et al. (2009)]{Sm09} analyzed the largest sample of halo and 
thick-disk stars to date, extending the investigation of Be as cosmochronometer and its role 
as a discriminator of different stellar populations in the Galaxy. The thick disk was found to 
be a homogeneous population. The halo stars were found  to divide into 
two different components (Fig.\,\ref{fig1}).

\section{The Galactic halo}

\begin{figure}[t]
% \vspace*{-2.0 cm}
\begin{center}
 \includegraphics[width=4.6in]{figura_composta_iaus265.eps} 
% \vspace*{-1.0 cm}
 \caption{(a) log(Be/H) vs.\, [$\alpha$/Fe] for the halo stars of \cite[Smiljanic et al. (2009)]{Sm09}. 
The solid line is the model prediction of \cite[Valle et al. (2002)]{Valle02}. (b) log(Be/H) vs.\, [O/Fe] 
with new preliminary oxygen abundances for 
a subsample of the halo stars., corrected for NLTE (\cite[Fabbian et al. 2009]{Fabbian09})}
   \label{fig1}
\end{center}
\end{figure}

In a diagram of [O/Fe] vs. log(Be/H) the abscissa can be considered as increasing 
time and the ordinate as the star formation rate. In \cite[Smiljanic et al. (2009)]{Sm09} 
oxygen abundances were not available, so mean abundances of $\alpha$-elements 
were used instead. Here we present new preliminary oxygen abundances determined 
from the infrared triplet at 777nm. 
%The oxygen abundances were corrected from 

As shown in Fig.\,\ref{fig1}, using either $\alpha$ or oxygen abundances, the halo stars define 
two clear distinct sequences. One sequence is chemically similar to the thick disk, the other 
agrees with the models of \cite[Valle et al. (2002)]{Valle02}. The latter, however, with 
[$\alpha$/Fe] $\leq$ 0.25 and log(Be/H) $\leq$ $-$11.4, have similar kinematics. The stars have mostly  
velocity in the direction of the rotation of the Galaxy V $\sim$ 0 and the perigalactic 
distance Rmin $\leq$ 1 kpc (\cite[see Smiljanic et al. 2009 for details)]{Sm09}, as expected 
for accreted stars. 

The splitting into two components may be related to the accretion of external systems or to variations of star 
formation in different and initially independent regions of the early halo. The interpretetion is still open, 
it is however clear that the halo is not a single uniform population with a single age-metallicity relation. 
A similar division was found by \cite[Nissen \& Schuster (1997, 2009)]{NS97,NS09} but using Fe as 
a tracer of time. The division is clearer when Be is used as a time scale. In the same line, recent simulations of the formation of disk 
galaxies in a $\Lambda$CDM universe by \cite[Zolotov et al. (2009)]{Zolotov09} show that the inner halo has a 
dual nature, it is composed both by `in situ stars' formed in the potential well of the galaxy and by 
accreted stars, formed in subhalos later accreted by the galaxy.



\begin{acknowledgments}
R.S. acknowledges financial support from FAPESP (04/13667-4 and 08/55923-8).
\end{acknowledgments}


\begin{thebibliography}{}

\bibitem[Fabbian et al. (2009)]{Fabbian09}
{Fabbian, D., Asplund, M., Barklem, P. S., et al.} 2009,
A\&A, 500, 1221

\bibitem[Nissen \& Schuster (1997)]{NS97}
{Nissen, P. E. \& Schuster, W. J.} 1997, 
A\&A, 326, 751

\bibitem[Nissen \& Schuster (2009)]{NS09}
{Nissen, P. E. \& Schuster, W. J.} 2009,
in: Proc. IAU Symposium No. 254, p.\ 103
%J. Andersen, J. Bland-Hawthorn \& B. Nordstr\"om, (eds.),
%\textit{The Galaxy Disk in Cosmological Context},

\bibitem[Pasquini \etal (2004)]{Pasquini04}
{Pasquini, L., Bonifacio, P., Randich, S., Galli, D., \& Gratton, R. G.} 2004, 
A\&A, 426, 651

\bibitem[Pasquini \etal (2007)]{Pasquini07}
{Pasquini, L., Bonifacio, P., Randich, S., et al.
%Galli, D., Gratton, R. G., \& Wolff, B.
} 2007, 
A\&A, 464, 601

\bibitem[Pasquini \etal (2005)]{Pasquini05}
{Pasquini, L., Galli, D., Gratton, R. G., et al. 
%Bonifacio, P., Randich, S., \& Valle, G.
} 2005, 
A\&A, 436, L57

\bibitem[Smiljanic \etal (2009)]{Sm09}
{Smiljanic, R., Pasquini, L., Bonifacio, P., et al.} 2009,
A\&A, 499, 103

\bibitem[Suzuki \& Yoshii (2001)]{SY01}
{Suzuki, T. K. \& Yoshii, Y.} 2001, 
ApJ, 549, 303

%\bibitem[Suzuki \etal (1999)]{Suzuki99}
%{Suzuki, T. K., Yoshii, Y., \& Kajino, T.} 1999, 
%ApJ, 522, L125

\bibitem[Valle et al. (2002)]{Valle02}
{Valle, G., Ferrini, F., Galli, D., \& Shore, S. N.} 2002, 
ApJ, 566, 252

\bibitem[Zolotov et al. (2009)]{Zolotov09}
{Zolotov, A., Willman, B., Brooks, A. M., et al.} 2009, 
ApJ, 702, 1058

\end{thebibliography}

\end{document}

De ZOLOTOV: Most of the in situ halo
stars formed at high redshift out of smoothly accreted cold gas in the inner 1 kpc of the galaxies� potential
wells, possibly as part of their primordial disks. We find that the two galaxies with recently quiescent merger
histories have a higher fraction of in situ stars (?20\%�50\%) in their inner halos than the two galaxies with
many recent mergers (?5\%�10\% in situ fraction). Observational studies concentrating on stellar populations
in the inner halo of the Milky Way will be the most affected by the presence of in situ stars with halo
kinematics, as we find that their existence in the inner few tens of kpc is a generic feature of galaxy formation.

Their work has been highly successful
in explaining the discrepant chemical abundances of Milky
Way satellite galaxies and field halo stars (Font et al. 2006a,
2006b; Johnston et al. 2008).

In situ strs are bound to the potential well of the Galaxy

In Section 3.1, we show that while these
in situ stars reside in the halo at z = 0, they formed near the
center of the galaxies� potential wells, as part of their primordial
disks. The origin of the halo in situ stars in cold gas flows and their
formation in the innermost regions of MW1hr�s potential well
all highlight that this population of stars formed in a rapid inflow
and collapse of gas that also formed the disk ofMW1hr, but were
displaced through merger events into the halo.



\begin{discussion}

\discuss{Massey}{I�m wondering if you have considered the expected intrinsic dispersion in absolute
magnitude of WRs --� if you consider the (large) mass range that becomes an
early WN or late WC according to the evolutionary models, wouldn�t you expect a large
dispersion in M$_v$?}

\discuss{van der Hucht}{Indeed, we will be always left with some intrinsic scatter in M$_v$ due
to mass differences within the same spectral subtype. But in my opinion, the current
large dispersion is for a large fraction due to incertainties of the adopted distances of
open clusters and OB associations.}

\discuss{Walborn}{I think that the scatter in WNL absolute magnitudes is dominated by intrinsic
spread rather than errors. In the LMC, one finds a range of $�5$ to nearly $�8$. This
in turn likely reflects different formation channels: mass-transfer binaries, post-RSG,
and extremely massive stars in giant H {\scshape ii} regions.}

\discuss{van der Hucht}{As said above, there is likely to be intrinsic scatter. But, I wonder
whether a scatter of 3 magnitudes perhaps reflects undetected
multiplicity.} 

\discuss{Ma\'{\i}z-Apell\'{a}niz}{I could not agree more with your comment on the need for an updated
catalogue of O-type stars (as a follow up of that of Garmany
{\itshape et al.} 1982). We
are currently working on precisely that (see our poster, these Proceedings) and we will
soon make it available.}

\discuss{van der Hucht}{Wonderful.}

\discuss{Koenigsberger}{Is the ratio WR/O-stars in clusters similar or different from this ratio
for the field stars?}

\discuss{van der Hucht}{I think it is different because incompleteness among field stars is even
larger than that among cluster stars. But perhaps it should also be different because
WR stars are older and could have drifted away from clusters, more
than O-type stars.} 

\discuss{Gies}{How many of the WR stars in your catalogue might be low mass objects?}

\discuss{Walborn}{Comment: PN central stars in the WR sample would be only [WC].}

\discuss{van der Hucht}{Among the WR stars in our VIIth Catalogue we doubt only one:
WR109 (V617 Sgr), which is a peculiar object (not even a [WR] central star of a PN).
All other stars in our cataloge are true massive Population I WR stars, and properly
classified as such. We have not listed known Population II [WC] objects, as we did
separately in our VIth Catalogue (van der Hucht {\itshape et al.} 1981). [WN] objects are not
known to exist, see the comment by Nolan.}

\discuss{Zinnecker}{Are all Galactic WR stars in open clusters and OB association or are there
many WR stars in the field?}

\discuss{van der Hucht}{See the VIIth WR catalogue (van der Hucht 2001): of the listed 227
Galactic WR stars, only 53 are in open culsters and OB associations, or believed to be.
The other 184 are supposedly field stars.}
\end{discussion}

\end{document}

0909.2635.extracted_bib
\begin{thebibliography}{}

\bibitem[Fabbian et al. (2009)]{Fabbian09}
{Fabbian, D., Asplund, M., Barklem, P. S., et al.} 2009,
A\&A, 500, 1221

\bibitem[Nissen \& Schuster (1997)]{NS97}
{Nissen, P. E. \& Schuster, W. J.} 1997, 
A\&A, 326, 751

\bibitem[Nissen \& Schuster (2009)]{NS09}
{Nissen, P. E. \& Schuster, W. J.} 2009,
in: Proc. IAU Symposium No. 254, p.\ 103
%J. Andersen, J. Bland-Hawthorn \& B. Nordstr\"om, (eds.),
%\textit{The Galaxy Disk in Cosmological Context},

\bibitem[Pasquini \etal (2004)]{Pasquini04}
{Pasquini, L., Bonifacio, P., Randich, S., Galli, D., \& Gratton, R. G.} 2004, 
A\&A, 426, 651

\bibitem[Pasquini \etal (2007)]{Pasquini07}
{Pasquini, L., Bonifacio, P., Randich, S., et al.
%Galli, D., Gratton, R. G., \& Wolff, B.
} 2007, 
A\&A, 464, 601

\bibitem[Pasquini \etal (2005)]{Pasquini05}
{Pasquini, L., Galli, D., Gratton, R. G., et al. 
%Bonifacio, P., Randich, S., \& Valle, G.
} 2005, 
A\&A, 436, L57

\bibitem[Smiljanic \etal (2009)]{Sm09}
{Smiljanic, R., Pasquini, L., Bonifacio, P., et al.} 2009,
A\&A, 499, 103

\bibitem[Suzuki \& Yoshii (2001)]{SY01}
{Suzuki, T. K. \& Yoshii, Y.} 2001, 
ApJ, 549, 303

%\bibitem[Suzuki \etal (1999)]{Suzuki99}
%{Suzuki, T. K., Yoshii, Y., \& Kajino, T.} 1999, 
%ApJ, 522, L125

\bibitem[Valle et al. (2002)]{Valle02}
{Valle, G., Ferrini, F., Galli, D., \& Shore, S. N.} 2002, 
ApJ, 566, 252

\bibitem[Zolotov et al. (2009)]{Zolotov09}
{Zolotov, A., Willman, B., Brooks, A. M., et al.} 2009, 
ApJ, 702, 1058

\end{thebibliography}