Download James Binney: Chemical and dynamical evolution of the Milky Way

Chemical & dynamical evolution
of the Galaxy
James Binney
Oxford University
outline
• Secular heating
• Radial mixing
• Origin of the thin disc
Secular heating
• Fluctuations in © from spiral structure and molecular
clouds scatter stars from near-circular orbits
• Colour of MS stars related to mean age
• So random velocity of MS stars increases with B-V
Aumer & B 09
Modelling heating
• Assume
– SFR / e-t/t0
– ¾ / t¯
• Fit ¾(B-V)
and N(B-V)
Aumer & B 09
The solar neighbourhood is old
•
¿max > 10.5 Gyr,
best age 12.5 Gyr
• Peak cosmic SFR
z = 1-2 $
¿ = 7.8 – 10.5 Gyr
• Mean age
5.9 – 6.4 Gyr upper
end of disc ages at 10
kpc in
Scannapieco+09
simulations
Aumer & B 09
Radial mixing
• Dominant effect of spiral structure is
scattering @ corotation
• Does not heat disc but drives radial
migration
Sellwood & B 02
Impact on chemical evolution
• Conventional evolution models (van den
Bergh 62 ! Chiappini+97 ! Colavitti+08):
– Disc a series of accreting annuli
– Annuli evolve independently
• Schoenrich & B (09ab):
– Annuli accrete from IGM & next outwards
– Stars move radially on growing epicycles
(blurring) and epicenters “churned”
– ¾ / t¯
Schoenrich & B (09)
model
10
Flow over R
5
From IGM per area
Schoenrich & B (09)
ISM
• Leads to
– inhomogeneous
solar nhd
– steep Z gradient
d[Fe/H]/dR=-0.08
Local stars
Schoenrich & B (09)
• Parameters fitted to GenevaCopenhagen sample of
Hipparcos stars
• Selection effects crucial
GCS
cylinder
Chiappini 97
No churning
Origin of thick disc
• Reid & Gilmore (83)
identified thick disc
by vertical profile
• Later thick-disc stars
found to be old and
high-®
• Overlap of thin &
thick discs in Fe/H
(Bensby+03)
suggested hiatus in
SF between two
B 09
thin
Thick disc & SNIa
• In SB09 model infall rate
declines monotonically &
SFR determined by
Kennicutt law so total
continuity in SF
• But distribution of stars in
(Fe/H,®) bimodal
• Reason: high SF in ~1Gyr
before SNIa kick in
• Metal-poor stars in solar
nhd not historical relic but
reflect radial mixing
ISM tracks
Disc divided
Many predictions to test
Chemistry vs z
Rotation vs chemistry
Conclusions
• We are in the middle of an era of giant Galaxy surveys
(Hipparcos, UCAC2, DENIS, 2MASS, SDSS, RAVE,
HERMES, APOGEE, Gaia)
• From these data we should infer history of the Galaxy
• Models have a crucial role to play
• Dating stars is v hard; chemistry is a more accessible
proxy
• Chemistry & dynamics entangled
• Radial mixing has been too long neglected
• The simplest model of chemodynamical evolution with
radial mixing suggests the thick disc formed quiescently
and reflects dynamics of SNIa rather than lumpy
accretion