Galactic Chemical Evolution and the abundances of lithium, beryllium and boron

2 Pages·0.12 MB·English

by  Andreu Alibes

GALACTIC CHEMICAL EVOLUTION AND THE
ABUNDANCES OF LITHIUM, BERYLLIUM AND BORON
ANDREUALIBE´S, JAVIER LABAYAND RAMONCANAL
Departament d’Astronomia i Meteorologia, Universitat de Barcelona, Mart´ı i
Franqu`es 1, 08028 Barcelona, Spain. E-mail: aalibes, javier, [email protected]
1
0 A LiBeB evolution model including Galactic Cosmic Ray nucleosynthesis, the ν-
0 process,novae, AGBandC-starsispresented.
2

n
We have included Galactic Cosmic Ray Nucleosynthesis (GCRN) in a
a
completeChemicalEvolutionModelthattakesintoaccount76stableisotopes
J
from hydrogen to zinc. Any successful LiBeB evolution model should also
1
3 be compatible with other observational constraints like the age-metallicity
relation, the G-dwarf distribution or the evolution of other elements. At the

1 same time, we have checked how different would be a model that took into
v
accountthelastobservationsbyWakkeretal. (1999)ofmetal-enrichedclouds
1
falling onto the disk, from a primordial infall model.
6
5 We have integrated the standard evolution equations, using a SFR pro-
1 portional to σmσn and a double exponential infall (Chiappini et al. 1997)
g
0 with τ = 1 Gyr, τ = 7 Gyr and t = 1 Gyr. We have used two kinds
h d max
1
of infall material: in the primordial model (PM), the galaxyacretes only pri-
0
mordial material during all its lifetime (13 Gyr), while the enriched model
/
h (EM)considersafirstGyrofprimordialinfall,followedby12Gyrofenriched
p
infall (0.1 Z⊙, indicated by recent observationsof clouds falling onto the disk
-
o by Wakker et al. (1999)).
r We use the Kroupa et al. (1993) IMF and different prescriptions for the
t
s nucleosynthesis in the various mass ranges: low and intermediate-mass stars
a
(van den Hoek and Groenewegen, 1997), type II SNe (Woosley and Weaver,
:
v 1995) and type Ia SNe (model W7 of Nomoto et al., 1997). Finally, novae
i
X have also been included, by means of the Jos´e and Hernanz (1998) yields.
r
a The main source of 6Li, 9Be and 10B is the GCRN. In the superbubble
(SB) scenario, we have calculated the production rate by this mechanism,
taking a GCR composition that comes from inside the SB, where newly syn-
thesized material ejected by a SN is accelerated by the shock wave of other
SNandmixedwiththe ISMatthatepoch. The energyspectrumofthe GCR
is:q(E)∝ p−2.2e−EE0,withE0 =10GeV/n. Forthe ν-processwehavefixed
β
thecontributionofWW95yields(about25%)bymeansof(11B/10B)⊙. AGB
alibes: submitted to World Scientific on February 1, 2008 1
4
3
Li − Molaro et al. 1996
Be − Molaro et al. 1998 Li − Romano et al. 1999
2 BBBBe −−− − GCP Bruaimonrcehaisaasg Leeaotta paardell.. z e21 et09 ta09 al09. l.1 1999998 3 LL66LLiippiill aa−−ttee aaHHuu oo−−bb MBbbosson l11aif99rao99c 99eiot &al .M 1o9l9a6ro 1997
SS − Grevesse & Sauval 1998 SS − Anders & Sauval 1998
PM PM
EM EM
[Be] or [B] 01 6[Li] or [Li] 2 Li
B
1
−1
6Li
0
Be
−2
−4 −3 −2 −1 0 −4 −3 −2 −1 0
[Fe/H] [Fe/H]
stars andC-starsarealsoproducersof7Li. Weusethetime-dependentpro-
duction rate suggested by Abia et al. (1993) and a current production rate
of 1.5·10−8 M⊙pc−2Gyr−1. Finally, we have takenanaverageyield per nova
outburstof1.03·10−10M⊙,consideringthat30%oftheoutburstscomefrom
ONe WD and 70% from CO ones.
Our two-infall model, both primordial and enriched, is able to reproduce
the main solar neighborhood data; in particular, the G-dwarf distribution,
which is the main constraint, and the evolution of the majority of chemical
elements, specially the CNO ones. Together with the superbubble scenario
for the acceleration of GCR is also able to reproduce the LiBeB evolution,
speciallythe linearrelationshipsofBevsFeandBvsFe,whentakingaGCR
composition as a mixture of ejecta of SNe (20%) and ISM material (80%).
All the 7Li sources considered are necessary to reproduce the Li evolution.
References
1. Abia, C., Isern, J., Canal, R. 1993 A&A 275, 96
2. Chiappini, C., Matteucci, F., & Gratton, R. 1997 ApJ 477, 765
3. Jos´e, J., & Hernanz, M. 1998 ApJ 494, 680
4. Kroupa, P., Tout, C., & Gilmore, G. 1993 MNRAS 262, 545
5. Nomoto, et al. 1997 Nucl. Phys. A 621, 467c
6. van den Hoek, L.B., & Groenewegen, M.A.T. 1997 A&ASS 123, 305
7. Wakker, B.P. et al. 1999 Nature 402, 338
8. Woosley, S.E., & Weaver, T.A. 1995 ApJS 101, 181
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