Download Chemical Evolution of the Galaxy and its satellites

Chemical Evolution of the
Galaxy and its Satellites
Francesca Matteucci
Cristina Chiappini
Francesco Calura
Antonio Pipino
Gabriele Cescutti
Universita’ di Trieste e OATS
How to model galactic chemical
evolution
• Initial conditions (open or closed-box;
chemical composition of the gas)
• Birthrate function (SFRxIMF)
• Stellar yields (how elements are produced
and restored into the ISM)
• Gas flows (infall, outflow, radial flow)
• Equations containing all of of this...
The Stellar Birthrate
• We define the stellar birthrate function as:
• B(m,t) =SFRxIMF
• The SFR is the star formation rate (how
many solar masses go into stars per unit
time)
• The IMF is the initial stellar mass function
describing the distribution of stars as a
function of stellar mass
The parametrization of the SFR
• The most common parametrization is the Schmidt
(1959) law where the SFR is proportional to some
power (k=2) of the gas density
• Kennicutt (1998) suggested k=1.4 from studying
star forming galaxies, but also a law depending on
the rotation angular speed of gas and the first
power of the gas density
The accretion history of the
Galaxy
The
gas infall rate can
simply be constant in
space and time
Or
described by an
exponential law:
Stellar Yields
• Low and intermediate mass stars (0.8-8 Msun):
produce He, N, C and heavy s-process elements.
They die as C-O white dwarfs, when single, and
can die as Type Ia SNe when binaries
• Massive stars (M>8-10 Msun, core-collapse SNe):
they produce mainly alpha-elements (O, Mg..),
some Fe, light s-process elements and r-process
elements and explode as core-collapse SNe
• Type Ia SNe produce mainly Fe (0.6-0.7M_sun
per SN)
Type Ia SN progenitors: SD
model
• Single-degenerate scenario (e.g. Whelan &
Iben 1974; Han & Podsiadlowsky 2004): a
binary system with a C-O white dwarf plus
a MS star. When the star becomes RG it
starts accreting mass onto the WD
• When the WD reaches 1.44 Msun
(Chandrasekhar mass), it explodes by Cdeflagration as Type Ia supernova
Type Ia SN progenitors: DD
model
• Double-Degenerate
scenario (Iben & Tutukov,
1984): two C-O WDs
merge after loosing
angular momentum due to
gravitational wave
radiation
• When the two WDs of 0.7
Msun merge, the
Chandrasekhar mass is
reached and Cdeflagration occurs
The clock for the explosion
• Single-Degenerate model: the clock to the
explosion is given by the lifetime of the secondary
star, m2. The minimum time for the appearence of
the first Type Ia SN is 30-35 Myr (the lifetime of
a 8 Msun star)
• Double-Degenerate model: the clock is given by
the lifetime of the secondary plus the gravitational
time-delay: 35 Myr + Delta_grav
• Minimum delay time 1 Myr (Greggio 2005),
maximum delay a Hubble time and more
The two-infall model of
Chiappini, FM & Gratton 1997
• The two-infall model of
Chiappini, FM & Gratton
(1997) predicts two main
episodes of gas accretion
• During the first one the
halo, bulge and part of
thick disk formed, the
second gave rise to the
disk
• Exponential infall law
with different timescales
The star formation rate in the
solar vicinity
• The predicted SFR
• The effect of a
threshold gas density
(7 Msun pc^(-2)) for
the SF is evident
• The IMF is that of
Scalo (1986)
• A gap in the SFR is
predicted
The SN rates in the solar vicinity
G-dwarf metallicity distribution
in the Solar Vicinity
[alpha/Fe] vs.[Fe/H] in the Solar
Vicinity: Francois et al. 04
Results of Francois, FM et al.
2004
Last data and models on C and N
How do the gradients form?
• If one assumes the disk to form inside-out,
namely that first collapses the gas which
forms the inner parts and then the gas which
forms the outer parts
• Namely, if one assumes a timescale for the
formation of the disk increasing with
galactocentric distance, the gradients are
well reproduced if
Predicted and Observed
Abundance Gradients
• Predicted and
observed abundance
gradients from
Chiappini et al. (2001)
• Data from HII regions,
PNe and B stars, red
dot is the Sun
• The gradients slightly
steepen with time
(from blue to red)
The Galactic Bulge
• Ballero, FM, Origlia & Rich (2007)
proposed a model where the Bulge forms
very quickly from gas shed by the halo
• The star formation efficiency was quite high
(starburst) and the IMF flatter than in the
disk
• In this situation a long plateau for
[alpha/Fe] ratios is predicted
Predictions and Observations for
the Bulge
[alpha/Fe] in galaxies with
different SFRs
Dwarf Speroidals vs. Milky Way
Do do the Dwarf Spheroidals
form?
• CDM models for galaxy formation predict dSph
systems (10^7 Msun) to be the first to form stars
(all stars should form < 1Gyr)
• Then heating and gas loss due to reionization
must have halted soon SF
• Observationally, all dSph satellites of the MW
contain old stars indistinguishable from those of
Galactic globular clusters and they have
experienced SF for long periods (>2 Gyr, Grebel
& Gallagher, 04)
Modelling the dSphs
• Lanfranchi & FM (03,04) computed the
evolution of 6 dSphs: Carina, Sextan,
Draco, Sculptor, Sagittarius and Ursa Minor
• They assumed the SF histories as measured
by the Color-Magnitude diagrams (Mateo,
1998;Dolphin 2002; Hernandez et al. 2000;
Rizzi et al. 2003)
Results: Carina galaxy
• Model Lanfranchi &
Matteucci (2004)
• SF history from Rizzi
et al. 03. Four bursts
of 2 Gyr, SF eff. =
0.15 Gyr^(-1)
wind=7xSFR
• Salpeter IMF
The metallicity distribution of
Carina
• Data from Koch et al.
(2005)
• Best model from
Lanfranchi & al.
(2006)
• This model well
reproduces also the
[alpha/Fe] ratios in
Carina
Chemical evolution of Sculptor
• Model and data for
Sculptor
• SF efficiency 0.05-0.5
Gyr^(-1), wind 7
XSFR
• One long SF episide
lasting 7 Gyr
• Salpeter IMF
Heavy elements in Sculptor
• Predicted and
observed s- and rprocess elements in
Draco
• The effect of the timedelay between Type II
and Ia SNe coupled
with slow SF produces
higher [s/Fe] than in
the Milky Way
Comparison Sculptor –Milky Way
• Blue line and blue data
refer to Sculptor
• Red line and red data
refer to the Milky Way
• The effect of the timedelay model is to shift
towards left the model
for Sculptor with a
lower SF efficiency
than in the MW
Conclusions on the Milky Way
• The Disk at the solar ring formed on a time scale
not shorter than 7 Gyr
• The whole Disk formed inside-out with timescales
of the order of 2 Gyr in the inner regions and 10
Gyr in the outer regions
• The inner halo formed on a timescale not longer
than 2 Gyr
• Time- delay model well explains abundance ratios.
Nature of N still unknown (probably primary N
from massive stars)
Conclusions on the Milky Way
• The best model for the Bulge suggests that
it formed by means of a strong starburst
• The efficiency of SF was 20 times higher
than in the rest of the Galaxy
• The IMF was very flat, as it is suggested for
starbursts
• The timescale for the Bulge formation was
0.1 Gyr and not longer than 0.5 Gyr
Conclusions on dSphs
• By comparing the [alpha/Fe] ratios in the
MW and dSphs one concludes that they had
different SF histories
• The [alpha/Fe] ratios in dSphs are always
lower than in the MW at the same [Fe/H], as
a consequence of the time delay model and
strong galactic wind
Conclusions on dSphs
• Good agreement both for [s/Fe] and [r/Fe]
ratios is obtained . These ratios are
generally higher, for a given [Fe/H], than
the corresponding ratios in S.N.
• This is again a consequence of the timedelay model
• It is unlikely that the dSphs are the building
blocks of the MW