From carbon to europium:
Chemical evolution models of
the Galaxy & its closest companions
Cescutti Gabriele
Francesca Matteucci , Cristina Chiappini
Pierre North, Andy McWilliam, Gustavo Lanfranchi
Sexten, 27/01/2012
Outline
Galactic chemical evolution & the analyzed stellar systems
Classical (homogenous) chemical evolution code:
The manganese in the different stellar systems:
Metal dependency in SNIa yields?
The phosphorus in the Milky Way:
Which is the main driver of its evolution?
The neutron capture elements
Stochastic (in the IMF) chemical evolution code:
results for the neutron capture elements
&
for CNO in the Galactic halo
Conclusions
Sexten, 27/01/2012
Galactic chemical evolution
Stars and interstellar gas in galaxies exhibit diverse chemical element abundance
patterns that are shaped by their environment and formation histories.
The aim of Galactic Chemical Evolution is to use the observed abundances in stars
and the interstellar medium to reconstruct the chemical history and unlock earlier
epochs in the Universe, probe the mechanisms of galaxy formation, and gain insight
into the stellar evolution, constraining the stellar yields.
Models for the chemical evolution of galaxies need to account for the collapse of gas
and metals into stars (star formation), the synthesis of new elements within these
stars, and the subsequent release of metal-enriched gas as stars lose mass and die. An
additional feature is the ongoing accretion of gas from outside the system.
Sexten, 27/01/2012
Galactic chemical evolution
An homogeneous model follows the time evolution of the gas fraction of
element A with this equation:
ĠA(R,t) =
1) Locked in stars
2) Infalling in the system
3) Produced by stars
Sexten, 27/01/2012
3)Flowing from the system
Galactic chemical evolution
An homogeneous model follows the time evolution of the gas fraction of
element A with this equation:
ĠA(R,t) =
-XA(R,t) Ψ (R, t) +
1) Locked in stars
2) Infalling in the system
3) Produced by stars
Sexten, 27/01/2012
3)Flowing from the system
Galactic chemical evolution
An homogeneous model follows the time evolution of the gas fraction of
element A with this equation:
ĠA(R,t) =
Ψ (R, t) = ν(R,t) G(R,t)k
-XA(R,t) Ψ (R, t) +
1) Locked in stars
2) Infalling in the system
3) Produced by stars
Sexten, 27/01/2012
3)Flowing from the system
Galactic chemical evolution
An homogeneous model follows the time evolution of the gas fraction of
element A with this equation:
ĠA(R,t) =
Ψ (R, t) = ν(R,t) G(R,t)k
-XA(R,t) Ψ (R, t) + ĠA,infall (R,t)
1) Locked in stars
2) Infalling in the system
3) Produced by stars
Sexten, 27/01/2012
3)Flowing from the system
Galactic chemical evolution
An homogeneous model follows the time evolution of the gas fraction of
element A with this equation:
ĠA(R,t) =
Ψ (R, t) = ν(R,t) G(R,t)k
-XA(R,t) Ψ (R, t) + ĠA,infall (R,t)
1) Locked in stars
2) Infalling in the system
3) Produced by stars
Sexten, 27/01/2012
-XA(R,t) ẆA (R, t) +
3)Flowing from the system
Galactic chemical evolution
An homogeneous model follows the time evolution of the gas fraction of
element A with this equation:
ĠA(R,t) =
Ψ (R, t) = ν(R,t) G(R,t)k
-XA(R,t) Ψ (R, t) + ĠA,infall (R,t)
1) Locked in stars
2) Infalling in the system
-XA(R,t) ẆA (R, t) +
3)Flowing from the system
+ ∫M Ψ (R, t - τ(ḿ)) φ (ḿ) Q(ḿ, z(t - τ(ḿ))A dḿ
3) Produced by stars
Sexten, 27/01/2012
Galactic chemical evolution
An homogeneous model follows the time evolution of the gas fraction of
element A with this equation:
ĠA(R,t) =
Ψ (R, t) = ν(R,t) G(R,t)k
-XA(R,t) Ψ (R, t) + ĠA,infall (R,t)
1) Locked in stars
2) Infalling in the system
-XA(R,t) ẆA (R, t) +
3)Flowing from the system
+ ∫M Ψ (R, t - τ(ḿ)) φ (ḿ) Q(ḿ, z(t - τ(ḿ))A dḿ
3) Produced by stars
Sexten, 27/01/2012
Galactic chemical evolution
An homogeneous model follows the time evolution of the gas fraction of
element A with this equation:
ĠA(R,t) =
Ψ (R, t) = ν(R,t) G(R,t)k
-XA(R,t) Ψ (R, t) + ĠA,infall (R,t)
1) Locked in stars
2) Infalling in the system
-XA(R,t) ẆA (R, t) +
3)Flowing from the system
+ ∫M Ψ (R, t - τ(ḿ)) φ (ḿ) Q(ḿ, z(t - τ(ḿ))A dḿ
3) Produced by stars
Sexten, 27/01/2012
Stellar
nucleosynthesis!!!
Studied stellar systems
Solar vicinity Located at about 8kpc from the centre of our Galaxy, belongs
to the galactic disc within the Orion spiral arm.
We can resolve individual stars at all evolutionary phases.
Galactic bulge In the center of the Milky Way, the distribution of stars
changes: a sign of a different component of the galaxy - the galactic bulge. It
is hard to study the galactic bulge due to dust. But the dust is patchy, and
there are holes which we can peek through, such as Baade's window.
Different teams were pursuing the detailed element abundances of bulge
giant stars following the pioneering effort by McWilliam & Rich (1994).
Draco
dSph
Dwarf spheroidal galaxies They are low luminosity, dwarf elliptical galaxies
with low surface brightness. The Milky Way has 12 identified dwarf
spheroidal galaxies (dSphs) companions.
It is possible the measure the chemical abundances in red giant stars with
high resolution spectroscopy.
Sexten, 27/01/2012
Studied stellar systems 2
The different stellar systems have had different evolutions.
To account for these, their models present some main differences:
Solar vicinity
Milkyway Bulge
Dwarf spheroidals
ν (SF efficiency)
1
20
<1
Galactic wind
No
Yes
Yes
Infall Law
Two infall model
(halo-disc)
One infall
(short timescale!)
Depends on the dwarf
spheroidal
More recent model
Cescutti et al
(2008)
Ballero et al.
(2007)
Lanfranchi et al.
(2008)
They share the same stellar nucleosynthesis!
Sexten, 27/01/2012
Manganese intro
The Mn in the ISM is enriched by two main sources
(as iron!):
Supernovae type II
Supernovae type Ia
Important ingredient of GCE, produce ~ half of iron at
the solar formation time.
Manganese is a gray-white metal,
resembling iron. It is a hard metal
and is very brittle, fusible with
difficulty, but easily oxidized.
Manganese metal and its common
ions are paramagnetic.
The scenario we take in
account in the models
for SNeIa is the single
degenerate: a white
dwarf accreting material
from a companion giant
star up to the
Chandrasekhar limit.
Caveat
recent paper on Nature
Schaefer & Pagnotta
They most important point is that the have a long timescale, in the
MW disc they start to be important at [Fe/H] = -1!
Sexten, 27/01/2012
Manganese 1
The Mn in the three systems with the yields used by François et al. (2004), to well
fit the data of the MW disc (without metallicity dependence)
Cescutti et al. (2008)
WM reddy et al
Bulge McWilliam 2003
Sagittarius McWilliam
2003
Sbordone 2007
Sexten, 27/01/2012
Manganese 2
To improve the results for Mn we use the yields for SNII computed by the whole
set of Woosley & Weaver (1995), with metallicity dependence
Cescutti et al. (2008)
Sexten, 27/01/2012
Manganese 3
We explore the role of the yields for SNIa, adopting yields again depending to
the metallicity : Z/Z⊙0.65 (cfr Badenes et al. 2008)
Cescutti et al. (2008)
This solution to the Mn problem in Sagittarius was suggested by McWilliam et al. (2003)
Sexten, 27/01/2012
Manganese 4
Applying this modification of this SNIa yields to other dwarf spheroidal systems
to check the goodness of this new assumption,
part of the new paper by North et al. (submitted to A&A)
SF histories assumed for Sculptor (model A and B)
Fornax (model C and D)
Carina (model E)
Sexten, 27/01/2012
The major points in this figures is
that without a metal dependence
(dashed line) it is not possible to
reproduce the data.
We obtain this results applying
very simple assumptions
for the chem. ev. models, just
assuming the SF histories from
the observational data.
A proper treatment of the gas
flows from these could improve
the fit of the results.
Sexten, 27/01/2012
The different timescale and
contribution to Mn by SNIa in
the models for Sculptor and
Fornax.
For Sculptor SNIa the
contribution is almost 50%
each, whereas in Fornax
SNeIa play the major role,
which is also the case of the
Milky Way.
Sexten, 27/01/2012
Phosphorus
Yields for phosphorus by
Woosley and Weaver 1995
Kobayashi et al. 2006
Iron by
Woosley and Weaver 1995
Data by Caffau et al 2011
Stars without planet
Stars with a planet
Cescutti et al. (2012) accepted by A&A
Sexten, 27/01/2012
Phosphorus
Yields for phosphorus and iron by
Kobayashi et al. 2006
with hypernovae
Kobayashi et al. 2006
without hypernovae
Data by Caffau et al 2011
Stars without planet
Stars with a planet
Sexten, 27/01/2012
Phosphorus
If we increase by a factor of 3 the
production of phosphorus in SNII
Kobayashi et al. 2006
with hypernovae
Kobayashi et al. 2006
without hypernovae
Data by Caffau et al 2011
Stars without planet
Stars with a planet
Sexten, 27/01/2012
Neutron capture elements: r-s process
The elements beyond the iron peak (A>60) are formed through
neutron capture on seed nuclei (iron and silicon).
Two cases:
s-process
τ
β
Different Timescale of the
neutron capture
Different process path
<< τc
P
n
Sexten, 27/01/2012
r-process
τ
β
>> τc
Yields for Neutron capture elements
s-process
r-process
low mass stars,
during third dredge up
?? SNII ??
?? less massive ones
see Quian & Wasserburg
Long time scale,
No enrichment of the halo ISM
Short time scale,
the polluter of the halo
(but see the caveat!)
Theoretical predictions OK:
see for example
Busso et al 2001
Cristallo et al 2011
Magrini talk
Still missing theoretical predictions
Caveat also in massive stars IF fast rotators
(Pignatari et al. 2008 - Frischknecht et al. 2011 & Meynet talk)
Sexten, 27/01/2012
Yields for Ba and Sr in massive stars
We set the yields of Ba to fit the observed data in the Galactic halo,
then we scaled these yields to the other neutron capture elements using
the r-process signature in the r-process rich stars.
Note that for the Sr-Y-Zr group the peak of production of these elements is
translate toward more massive stars.
Strontium
Barium
This procedure is not model independent, we use now the model for the halo
proposed by Chiappini et al (2006), refined in Cescutti & Chiappini (2011)
Sexten, 27/01/2012
Results for Barium
By construction of
the yields themself,
it fits the data...
BUT
This homogenous
model cannot be
used to have an
insight of the spread
observed in the halo
stars, only the trend
is recovered.
Data taken from the collection of Frebel 2010,
Astronomische Nachrichten, 331, 474
EXCLUDED STARS WITH [C/Fe]>1 Sexten, 27/01/2012
Inhomogeneous chemical evolution
model for the halo of the Milky Way
Problem to solve:
The neutron capture elements at low metallicities show spread
whereas α-elements do not
Main assumptions:
A random formation of new stars subjects to the condition that the cumulative
mass distribution follows a given initial mass function;
α-elements and neutron capture elements are produced in different mass ranges:
All the massive stars for α-elements

8-30 M⊙ for neutron capture elements (but with a peak at ~ 8-10Msun)

Sexten, 27/01/2012
Inhomogeneous
model
50 stars
105 stars
1000 stars
107 stars
Sexten, 27/01/2012
Inhomogeneous model
We divide the halo in boxes each one of the typical size of 200 pc and we treat
each box as isolate from the other boxes. The dimension of the box is the new
parameter that we add in this new approach.
200pc
Inside each box, we simulate for 1 Gyr the chemical enrichment.
The main parameters are the same as those of the homogeneous model but in each
box the masses of the formed stars are different and this fact produces different
enrichments.
Sexten, 27/01/2012
Inhomogeneous model: Barium
The spread in
[Ba/Fe] observed in
the halo stars can be
recovered assuming
a stochastic
formation of the
stars in this system.
Note that that the
r-process rich stars
are not reproduced
but... (see in 2 slides)
blue intensity area depending on the number at [Fe/H] vs [Ba/Fe]
Sexten, 27/01/2012
Inhomogeneous model
For [Sr/Fe] the
problem is that
assuming the exactly
the r-process pattern
we are not producing
enough Sr
???
Possible solution is
in s-process
production in
massive stars?
Sexten, 27/01/2012
Inhomogeneous model
Plotting the ratio of
[Sr/Ba] vs [Ba/H]
shows an IMPORTANT
result: with the assumed
yields we are able to
reproduce the trend of
[Sr/Ba] and also the
spread is well recovered.
(note still not enough Sr)
Problems in Fe yields?
(Limogi talk-->iron
uncertain)
Sexten, 27/01/2012
Inhomogeneous model for Ca
With this model
the prediction for the
[Ca/Fe] show a much
smaller spread, if we
exclude at extremely
low metallicity
(where actually also
the few observational
data seem to have
some spread)
Sexten, 27/01/2012
Strontium
s-process in
fast rotators massive stars
using the yields of
Frischknecht et al.
Sexten, 27/01/2012
P
M
I
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R
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Barium
s-process in
fast rotators massive stars
using the yields of
Frischknecht
Sexten, 27/01/2012
P
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R
IN
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R
A
S
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R
S
T
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s-process in
fast rotators massive stars
using the yields of Frischknecht
and
“r-process” from the empirical yields
Sexten, 27/01/2012
P
M
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R
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P
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Neutron Capture
elements in
What about the results
on dwarf spheroidal by
Lanfranchi et al. (20062008)?
Our predictions for Ba
are not much affected
since the integrated
yields are really similar,
and they improve the
agreement with the
EMP star measured by
Frebel et al (2010) in
Sculptor!
Sexten, 27/01/2012
dSphs (vs MW)
Neutron Capture
elements in
dSphs (vs MW)
Lanfranchi et al. (2008)
The galactic wind and
the different star
formation, produce
different chemical
evolution, in particular
in the case of ratio of
[Eu/Fe]
Bonifacio 2000 2004
Venn 2004 omogenized
Sculptor's data
MW's data
Sexten, 27/01/2012
Neutron Capture
elements in
dSphs (vs MW)
Lanfranchi et al. (2008)
Sculptor's data
MW's data
Sexten, 27/01/2012
Neutron Capture
elements in
dSphs (vs MW)
Lanfranchi et al. (2008)
In some theories of
galactic evolution, the
dSphs are believed to be
the building blocks of
the spiral galaxies as the
MW.
Sculptor's data
MW's data
Sexten, 27/01/2012
Neutron Capture
dSphs (vs MW)
elements in
Cescutti & Matteucci
The data by Bensby et
al. (2010) are the first
results for the barium in
Galactic bulge, they are
based on microlensing
effect in bulge stars.
but...
No data for Eu:
it is hard to understand
the trend.
Sexten, 27/01/2012
(2011)
Inhomogeneous model and CNO
Cescutti & Chiappini (2010)
We test 2 set of yields for fast
rotating massive and very
metal poor stars
(Hirschi et al. 2007)
1) Massive stars enrich during
both the stellar wind and
supernovae phases.
2) Stars with Z<10 −8 &
M>40M⊙
only enrich the
interstellar medium via stellar
winds (collapsing directly into
black holes).
Spite et al. (2006)
Lai et al. (2008)
Fabbian et al. (2009) for log(C/O)
Israelian et al. (2004) for log(N/O)
CEMP-no stars collected by Masseron et al. (2009)
CEMP-r star collected by Masseron et al. (2009)
Sexten, 27/01/2012
Inhomogeneous model and CNO
Cescutti & Chiappini (2010)
It is impossible to explain
the abundances observed
in CEMP-no stars. But the
model agrees (marginally)
with the abundance ratios
of the only CEMP-r star
with CNO and
HE 0107−5240 and
HE 0557−4840.
These results may suggest
that very metal-poor
massive stars would
collapse directly into black
holes as predicted by
Heger et al. (2003).
Spite et al. (2006)
Lai et al. (2008)
Fabbian et al. (2009) for log(C/O)
Israelian et al. (2004) for log(N/O)
CEMP-no stars collected by Masseron et al. (2009)
CEMP-r star collected by Masseron et al. (2009)
Sexten, 27/01/2012
Inhomogenous model and CNO
The same results shown in [X/Fe] vs [Fe/H]. Iron yields by Limongi for Z<10-8,
by Woosley and Weaver (1995) for other metallicity.
Sexten, 27/01/2012
Conclusions
Modeling different histories of star formation in the three systems (Solar neighborhood,
MW bulge and Sagittarius dwarf spheroidal) are insufficient to reproduce the different
behavior of the [Mn/Fe] ratio; rather, it is necessary to invoke metallicity-dependent type
Ia SN Mn yields.
By means of the comparison between of a new model for the halo and the new data at
low metallicity we conclude that the production of Ba (Sr, Y, Zr, La and Eu) is produced
by in massive stars (8-30M⊙), with a peak at 8Msun, compatible to the theoretical results
for the site of production of r-process.
Assuming a peak for slightly larger mass for Sr can explain the trend of the ratio of [Sr/
Ba] vs [Ba/H] at extremely low metallicity
When assumed in a inhomogeneous model the assumptions for the yields of Sr and Ba
in massive stars, we are able to well reproduce not only the trend but also the spread of this
2 elements seen in extremely metal poor stars in the halo (and a smaller spread for alpha
elements)
Some problems remain how to correctly scaling the r-process contribution to the
different neutron capture elements (and the iron yields) but it could be solved if we allow a
contribution by s-process in massive stars.
Sexten, 27/01/2012
Conclusions
For Ba and Eu the same nucleosynthesis prescriptions adopted to well fit the data in the
Milky Way produce a good fit to the data of dwarf spheroidal galaxies.
The results of the bulge model for Ba are still difficult to be compared to the data, the trend is
in fact still difficult to be understood.
With the same model we investigate the effect for CNO enrichment in the Galactic halo if
we take into account the stellar winds yield for extreme metal poor stars. It seems to be a
promising approach and it also suggest that very metal-poor massive stars would collapse
directly into black holes as predicted by Heger et al. (2003).
Sexten, 27/01/2012