1

Stellar Populations in the Milky Way

Recall the major components of the Milky Way:

Dark matter halo

Stellar/globular cluster halo

Bulge

Thick Disk

Disk

Nucleus – to be discussed in the next lecture

2

Star Formation History and Evolution of Galactic Disk

• primary diagnostics are ages, abundances, kinematics

• several age indicators

– isochrone ages ---->

– chromospheric ages

– white dwarf cooling ages

– open cluster CMD ages

3

Noh, Scalo 1990, ApJ, 352, 605

Disk Star Formation History

• average star formation rate (SFR) roughly constant, +- factor of two – IMF, metallicities of stars roughly

consistent with constant SFR – ~constant SFR difficult to

reconcile with small gas fraction today-- suggests disk did not evolve as closed system

• some hints of sporadic star formation bursts

• way too few low-metallicity stars for simple model “G-dwarf problem”

4

Rocha-Pinto et al. 2000, A&A, 358, 869

This study used “chromospheric”ages based on Ca H and K lines.

These authors conclude that the SFR in the disk has not been constant.

5

Wyse, Gilmore 1995, AJ, 110, 2771 Edvardsson et al. 1993, A&A, 275, 101

Solar Neighborhood

Outer bulge

6

Nucleosynthesis and Sites for Heavy Element Production

• He: Most He was produced in the Big Bang (0.235 by mass)Big Bang production confirmed by comparing He to

heavier elements in stars and metal-poor galaxies and extrapolating back to zero metallicity

Note that lower He at a fixed Z results in a brighter zero age main sequence.• Heavier elements: Almost all come from stars one way or another. Li was produced in the Big Bang and can be destroyed instars so it requires special treatment.• Review CNO cycle for production of those elements• Some elements get produced in massive stars as the result of nuclear reactions involving burning of heavier elements:

These elements may also be called “primary nuclides” as they are produced in stars with no dependence on initial metallicity. Only elements up to Fe56 can be produced in this fashion.

20 24 28 32 36 40Ne, Mg, Si, S, Ar, Ca nuclides= α

7

r, s Process ElementsAnother route to produce elements is via neutron capture followed by β- decay. To enrich the ISM, these elements will have to be blown out of a star by winds, PN formation or SN explosions

– s = slow neutron capture– r = rapid neutron capture– Slow versus rapid determined by whether the nuclide decays too rapidly

to a stable nuclide to capture another neutron or not.• r-process nuclides include 80Se, 81Br, 84Kr, 128,130Te, 127I, 192Os,193Ir, 196,198Pt• s-process nuclides include 88Sr, 89Y, 90Zr, 138Ba, 139La, 140Ce, 141Pr,

208Pb, 209Bi– s-process nuclides are produced from heavy nuclides synthesized in a

previous generation of stars– r-process nuclides are produced from heavy nuclides that may or may

not have come from an earlier generation of stars

8

Supernova Yields

Arnett 1991 ASPC 20 389.

• Type II Sn are the result of massive star evolution and can produce heavy elements without regard to initial metallicity. Yields through Fe can be computed and SN1987a yielded a large amount of data that helped to normalize the calcs.

• Abundances in the Sun show more Fe that just summed SNII would produce – the other major source of Fe-peak elements is Type Ia SN where a C/O white dwarf explodes.

• Note that because Ia SNs come from stars of a few solar masses, approximately a Gyr has to elapse before this route produces any Fe

9

Wheeler, Sneden, Truran 1989, ARAA, 27, 279

early core-collapseSNe (Type II, Ic)rich in O, α ejecta

after ~1 Gyr whitedwarf binary SNe(Type Ia) begin to enrich more in Fe

10

Models for Chemical EnrichmentZ

yeN(Z) y=yield y

yield= Z for closed box, gas exhausted

−

=

From Zocalli et al. 2001, bulge model

• Closed box model: 1) region is initially a metal-free gas; 2) no gas enters or leaves; 3) turbulence keeps the gas well-mixed; 4) eventually all the gas will be consumed. Another assumption often use is that of instantaneous recycling meaning the high mass stars evelove and return material to the ISM instantly.

Metallicity as a function of time:

Stars at time t with lower metallicity:

Closed box predicts too many low Z stars in the solar neighborhood

gas

gas

M (t)Z(t) y ln

M (0)⎡ ⎤

= − ⎢ ⎥⎢ ⎥⎣ ⎦

Z(t )y

stars gasM [ Z(t)] M (0) 1 e−⎡ ⎤< = −⎢ ⎥⎣ ⎦

11

More Complex Models• Leaky-box Model: some gas released by stars is driven out the region by SN

– mass loss rate will be proportional to the high mass SFR. Assume that stars have velocity dispersion σ*

( )Stars Stars 2

Z

*

2

*

totalstars

y yZ = Z1+c 1

0.6 200km / seccf

M (0) (1 c)ZM ( Z) 1 exp1 c y

=σ+ σ

⎛ ⎞= ⎜ ⎟σ⎝ ⎠

⎧ ⎫⎡ ⎤+< = − −⎨ ⎬⎢ ⎥+ ⎣ ⎦⎩ ⎭

There is substantial evidence in support of this model: metal-poor dwarf galaxies, very metal-rich giant ellipticals.

• Accreting Box Model: Assume that initially the mass in interstellar material is small. After some period, most, say 90%, will be converted to stars. Metal-free gas is then added at a rate equal to the SFR. This will have the long term consequence of building up high Z stars relative to the low Z numbers.

totalgas stars gas

gas

M ZZ y 1 exp 1 M constant M ( Z) M ln 1M y

⎡ ⎤⎛ ⎞ ⎛ ⎞= − − = < = − −⎢ ⎥⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎢ ⎥⎝ ⎠⎣ ⎦

Comes close to explaining the G-dwarf problem.

12

Extremely Low Metallicity Stars

From Beers & Christlieb ARAA Vol43:

Nomenclature for stars of different metallicity

[Fe/H] Term Acronym> +0.5 Super metal-rich SMR

0.0 Solar —

< −1.0 Metal-poor MP

< −2.0 Very metal-poor VMP

< −3.0 Extremely metal-poor EMP < −4.0 Ultra metal-poor UMP

< −5.0 Hyper metal-poor HMP

< −6.0 Mega metal-poor MMP

• Lowest metallicity stars found so far have [Fe/H] = -5.4

[Fe/H]=-5.4

Cannot tell yet whether these abundance patterns result from a Pop III SN precursor or from Pop III stars which have accreted gas containing some metals.

13

Edvardsson et al. 1993, A&A, 275, 101

Chemical Clues

14

Velocity Ellipsoid• Data on stellar motions are measured with respect to the Sun in a reference frame called the Local Standard of Rest (LSR). • The LSR is a point instantaneously centered on the Sun and moving on a circular orbit around the Galactic Center at the Sun’s present distance (Ro) from the Galactic Center.• A star’s velocity w.r.t the LSR is its peculiar velocity denoted as (u,v,w)• The Sun’s motion w.r.t. the is the solar motion and is in the sense of -R (towards the G.C.) and +z (away from the midplane).

LSR LSR 0 LSR

0

R z

LSR

LSR 0

LSR

dR d dz =R Z=dt dt dt

0 Z 0220km / sec

Peculiar velocity=(V ,V ,V ) (u, v, w)u v= - w=Z-Z Z

θ

θΠ = Θ

Π = Θ = Θ =Θ =

== Π −Π = ΠΘ Θ = Θ−Θ

=

15Edvardsson et al. 1993

Kinematic Clues ---> evidence for a “thick disk”

Nissen, Schuster 1991, A&A, 251, 457

16

Disk Chemical History

• MW’s disk has not evolved in a smooth, homogenous fashion

• Age and metallicity are loosely correlated but any one volume or kinematic selection appears to have a range of properties

• Extended periods (~1 Gyr long) of elevated star formation appear to have occurred

• Thin and thick disk metallicity distributions overlap so great care is needed in isolating samples

• Exact formation mechanism of the thick disk still unclear – large overlap in metallicities hard to reconcile with merger models

17

The Galactic Bulge and Halo

18

The Bulge: How Metal Rich?Early work (using a prime focus grism) revealed dramatic differences between the Milky Way Bulge and the Magellenic Clouds.

Blanco et al. 1978 IAU Symp.

19

Detailed Study of M Giants in the

Bulge• The prevalence of M

giants as opposed to carbon stars implies a metal-rich population.

• By observing bulge fields ranging from 4° to 12°, Frogel et al. demonstrated a metallicity gradient in the bulge with highest metallicity closest to the center.

• This result should be re-visited in the light of what we now know about the MW’s bar.

Frogel et al. 1990 353 494

20

Zoccali et al. 2003, A&A, 399, 931

Bulge

• Distinct population from disk – no stars with age <10

Gyr– bar appears to be old– distinct metal

abundances,

kinematics

21

Zoccali et al. 2003, A&A, 399, 931

22

Photometric Z agrees well with smaller spectroscopic samples.

23

Bulge SED from Zoccoli et al.

• M/LV = 3.67• M/LI = 3.25• M/LJ = 1.28• M/LH = 1.00• M/LK = 0.87

Curious that these authors did not comment on the apparent disagreement between an “Sc” SED and the lack of young stars in the bulge.

24Eggen, Lynden-Bell, Sandage 1962, ApJ, 136, 748 (“ELS”)

Galactic Halo

• First clues came from in situ studies of halo stars in the solarneighborhood

UV excessmetallicity index(lower metallicity = bigger excess)

25

Sample ELS Stars

26

No metal-rich, low angular momentum stars

27

ELS: Results and Conclusions

• stellar metal abundances are strongly correlated with orbital properties (eccentricity, vertical velocity, apicenter radius, angular momentum, anisotropy)

• interpretation– first generation metal-poor stars formed at large radial

distances from Galactic center, in spherical protogalaxy– radial collapse of protogalaxy produced eccentric orbits in

metal-poor stars– subsequent generations of more metal-rich stars formed at

smaller radii, on more circular orbits– strong abundance-kinematics trends require spheroid to

form on order of dynamical timescale (~100 Myr)• differential abundances of halo stars (e.g., O vs Fe) consistent

with rapid formation/enrichment time

28Zinn 1985, ApJ, 293, 424

The Accretion (“Searle-Zinn”) Scenario• Globular clusters do not fit neatly into ELS

picture– abundance - kinematics correlations due

to disk/halo bimodality, not continuous trends

– halo clusters alone show no ELS patterns • further studies of field stars show some ELS

patterns due to proper motion selection effects -- when stars selected via metallicity, trends weaken

• presence of thick disk confuses distinction between halo and disk

• Searle & Zinn propose that halo stars formed in protogalactic fragments and dwarf satellites, which were accreted one by one later (Searle, Zinn 1978, ApJ, 225, 357)

29Carney et al. 1996, AJ, 112, 668

Low metallicity stars are found with nearly disk-like orbits: Not predicted in the ELS model. Sample also includes a few stars at large distances from the plane which have retrograde orbits and no evidence of a metallicity gradient: Relics from an accretion event?

Evidence from Stars

30Ibata et al. 1997, AJ, 113, 634

Direct Evidence for AccretionSagittarius dwarf galaxy

– currently merging with Milky Way

– diameter >20 degrees, with larger tidal streamer

– L = 1-2 x 107 L– M ~ 108 M– includes 5 globular

clusters

31

Tidal Streamers

Newberg et al. 2002, ApJ, 569, 245 simulation by P. Harding, CWRU