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16-Galactic Chemical evolution (Stellar evolution end products…
16-Galactic Chemical evolution
Models
5 inputs
Initial conditions
Initial mass function to predict the birth rate of stars of different masses
star formation rate as a function of time, gas mass, gas density and other parameters
Understanding of how stellar evolution ends (how much material is ejected, what type of material is ejected and at what time)
other relevant processes excluding star birth and death
Initial conditions
Simple
Star with pure gas with primordial abundances predicted by BBNS
More complicated
Includes prior enrichment of the local system by products of some close-by evolved system
e.g. effect on local non-evolved system in the galactic disk from evolved systems in galactic bulge or halo
Observations
Analysis of carbonaceous chondrite meteorites shows excess of Mg26 correlated with abundance of aluminium
this is caused by the decay of Al26
inferred abundance of 26Al relative to Al27 is 5e-5
as half life for Al26 is only 0.73 Myr the early solar nebular must have been enriched by a relatively recent event
either nearby SNII or wind from wolf rayet star
Initial Mass function
describes the relative birth rates of stars with mass m in given interval dm in a given region
maths
Related to present day mass function
Determining it
Local IMF requires knowledge of
scale heights
mass luminosity relation
Luminosity function for some local volume
Corrections for
Evolved stars
unresolved binary or multiple systems
Assumptions about
age of galaxy
past history of SFR
Important properties
Mass fraction of stars with a lifetime less than the age of the system is typically 0.45
Return fraction R is the mass fraction of a generation of stars that is returned to the interstellar medium. R increases with t as progressively smaller stars complete their evolution
lock up fraction alpha defined (math) is the mass fraction that remains locked up in long lived stars 0.7-0.8
Imf also predicts yield of primary elements from massive stars. Yield is the mass of elements freshly produced and ejected by a generation of stars in units of mass that remains locked into long-lived stars and compact remnants
Star formation rates
no easy equation
In disk galaxies star formation is sporadic
gas-rich dwarf galaxies star formation occurs in short bursts
in large starburst and luminous IRAS galaxies violent bursts of star formation can be triggered by merger events
In our galaxy highest star formation occurs in a ring approximately 4kpc from galactic centre
Many physical processes can play a role in sfr
total surface density, gas density, gas pressure, temperature
chemical composition, gravitational potential, Galactic rotation effects
spiral shocks, magnetic fields, collisions between clouds
Stellar evolution end products
In principle can be predicted from theory
In practice many process make these calculations complicated
mixing,mass loss,stellar explosion mechanisms, nuclear reaction rates, evolution of close binaries, initial chemical composition (metallicity)
In general a stars contribution to nucleosynthesis depends primarily of four factors
Initial mass
Chemical composition
mass loss history during its life
effects of close binaries
Low metallicty stars have small mass loss
Distribution of primary elements in ejecta is mainly due to hydrostatic evolution (although can be modified late in star's evolution during periods of explosive nucleosynthesis
Production of C,He,O is insensitive to initial chemical composition although affected by the fact that mass loss increases with metallicity (especially for large mass stars)
Production of secondary elements (e.g. N14 Ne22) depends of initial abundance of progenitors
Production of odd-numbered elements (Na, Al) may depend on neutron excess and hence chemical composition
Stars of higher metallicity (Z>0.02) and initial mass >35 Msun undergo significant mass loss during their evolution
Ejection of significant amounts of He and C via stellar winds prior to core collapse
Hence final masses of He, CO cores are reduced so fewer heavy elements finally ejected
Nature of final remnant also important (neutron star vs black hole) as later will have far less ejecta
Intermediate mass starts (Minit >3.5 Msun) undergo so called dredge up episodes during their evolution that change the chem composition of outer layers
Outer convective zone reaches into the region where 12C was partially converted to C13 and N14, enhances C13,14N at the surface at the expense of 12C. Happens during ascent of RGB
Following ignition of He-burning shell convective envelope penetrates the He core dredging up He,N surface abundances of N14 and 4He enhances and C,O reduced. AGB
In later AGB evolution there is alternation of H shell burning (most of the time) and He shell burning (in brief flashes or thermal pulses)
Thermal pulses cause He, C and s-process elements to be brought to the stars surface. Also hydrogen shell burning (CNO) will convert some 12C into 14N. AGB
Meanwhile star is continually losing mass via stellar wind which increses as star expands and cools. AGB star can lose >50% of its mass
This wind and the material ejected (excluding the remnant ) at the end of the stars life affects the galactic chemical evolution
Type 1a SNe and interacting binary systems are important contributors to GCE
it is estimated that 2/3 of the iron in the solar system comes from SNe1a