1. and chemical evolution
Nucleosynthesis,
stellar abundances,
and chemical evolution
Anna Frebel
P-329
Guest lecture
“Stars and planets” class
by Dimitar Sasselov, Fall 2010

2. Ulitmate question
How did the solar abundances come about?

3. Stellar spectra

4. What sort of stars are we looking for?
unevolved, low-mass
stars; <1 Msun
to ensure long
lifetimes
=>unmixed, too,
to avoid surface
abundances
contamination
with nuclear
burning products
© B.J. Mochejska (APOD)

5. Three Observational Steps to Find Metal-Poor Stars
Sample selection and visual
inspection:
Find appropriate candidates
(Ca scales with Fe!)
Follow-up spectroscopy
(medium resolution):
Derive estimate for [Fe/H]
from the Ca II K line
3. High-resolution
spectroscopy:
Detailed abundances analysis
Frebel et al. 2005b

6. spectroscopic comparison
“Look-back time”
Galactic chemical evolution
Abundances are derived from
integrated absorption line strengths
equals 1/250,000th
of the solar Fe
abundance
[Fe/H] = log(NFe/NH)*  log(NFe/NH)

7. important spectral absorption lines in stars
H lines 6562Å, 4860Å, 4340Å, 4101Å
CH 4313Å and others
6707Å
Mg b ~5170Å
Ca K 3933Å
Na D ~5880Å
4129Å
4077Å, 4215Å
4554Å
Fe lines are everywhere in the spectrum -- always easily accessible

8. Carbon & nitrogen Huge carbon abundance ([C/Fe]= +3.7): 5,000
(=> not so carbon-poor...)
5,000
and 12,000 times
more carbon and
nitrogen exist than
iron!
Synthetic spectrum: red lines
Carbon (CH) band
Huge nitrogen
abundance ([N/Fe]= +4.1):
(=> not so nitrogen-poor...)
Reminder:
Solar ratio [C,N/Fe] = 0
Synthetic spectrum: red lines
Nitrogen (NH) band
Frebel et al. 2008, ApJ subm.

9. Ca II K line HE 1327-2326 Calcium often used as
proxy for the Fe abundance!
(..and Fe for metallicity)
Interstellar Ca
(Ca scales with Fe!)
–5.4
Frebel et al. (2005), Nature

10. Mg b lines

11. Eu

12. Thorium II Line 4019Å Abundance:
Synthetic spectrum (based on atomic data and model atmosphere)
to match observed spectrum
Synthetic spectrum that
includes NO thorium
‘Best fit’ synthetic spectrum Th
Frebel et al. (2010), in prep. HE

13. Uranium in HE 1523-0901 ‘Best fit’ synthetic spectrum
Synthetic spectrum that includes NO uranium
Synthetic spectrum with U abundance if it had NOT decayed
Frebel et al. (2007)
‘Best fit’ synthetic spectrum

14. How do we interpret stellar spectra?
need to know:
model atmosphere analysis techniques
knowledge about nucleosynthesis,
stellar evolution,
chemical evolution,
cosmological understanding of galaxy formation

15. Model atmosphere analysis techniques
Stellar parameters fully characterize a star:
effective temperature Teff
surface gravity log g
metallicity [Fe/H]
(microturbulent velocity vmic)

16. ATomic data Stronger line <=> lower excit <=>
every absorption line is an atomic transition
determined by atomic physics parameters
Vienna Atomic Line Database (VALD)
National Institute for Standards and Technology (NIST)
From Mon Nov 1 10:03:
Date: Tue, 31 Aug :56:
From:
Subject: Re:
============= job =============
# begin request
# extract all
# default configuration
# short format #
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Damping parameters Lande
Elm Ion WL(A) Excit(eV) log(gf) Rad. Stark Waals factor References
'Ti 1', , , , 7.735,-5.924,-7.491, 0.230,' '
'Si 2', , , , 0.000, 0.000, 0.000,99.000,' '
'F 3', , , , 0.000, 0.000, 0.000,99.000,' '
'V 1', , , , 8.158,-5.083,-7.799, 1.000,' '
'Cr 1', , , , 8.330,-5.330,-7.720, 1.160,' '
'Co 1', , , , 4.653,-6.374,-7.867, 1.230,' '
Stronger line <=>
lower excit <=>
higher log gf

17. Definitions: log 
Stellar ‘abundances’ are number density calculations with respect to H and the solar value
On a scale where H is 12.0:
This quantity is the output of all model atmospheres!
i.e. MOOG code (of Chris Sneden, publicly available) + Kurucz models (=inhouse!)
for element X

18. definitions: [fe/h] for elements A and B
where NFe and NH is the no. of iron and hydrogen atoms per unit of volume respectively.
for elements A and B

19. Solar abundances Photospheric (=‘stellar’ abundance)
Anders, Grevesse & Sauval ‘89
Grevesse & Sauval ‘98
Asplund, Grevesse &Sauval ‘05
Grevesse, Asplund & Sauval ‘07
Asplund, Grevesse, Sauval & Scott ‘09
reference element: H
calculation
Meteoritic (=‘star dust’ grain analysis)
Lodders 03
Lodders, Palme & Gail 09
reference element: Si
measurement
Volatile elements depleted, incl. the most abundant elements: H, He, C, N, O, Ne cannot rely on meteorites to determine the primordial Solar System abundances for such elements
For each application, the most similarly obtained solar abundances should be use to minimize systematic uncertainties!

20. how to calculate chemical abundances
Need a spectrum => measure equivalent width of absorption lines (=integrated line strength)
Need atomic data (excit. potential+log gf values) => feed both into “model atmosphere”
Get: calculated abundance (number density) log  (X)
Calculate [Fe/H] with solar abundances
Example:
log  (Mg)star = 5.96; log  (Fe)star = 5.50
log  (Mg)sun = 7.60; log  (Fe)sun = 7.50
[Mg/H] = log  (Mg)star - log  (Mg)sun = -1.64
[Mg/Fe] = [Mg/H] - [Fe/H] = (-2.0) = 0.36

21. How metal-poor? classical example: early universe: primordial gas
how metal-poor is the next-generation star?
canonical SN Fe yield: 0.1 Msun
available gas mass: 106 Msun

22. classification scheme
Range Term Acronym #
[Fe/H] ≥ Super metal-rich SMR some
[Fe/H] = Solar — a lot!
[Fe/H] ≤ – Metal-poor MP very many
[Fe/H] ≤ – Very metal-poor VMP many
[Fe/H] ≤ – Extremely metal-poor EMP ~100
[Fe/H] ≤ – Ultra metal-poor UMP
[Fe/H] ≤ – Hyper metal-poor HMP
[Fe/H] ≤ – Mega metal-poor MMP
Extreme Pop II stars!
as suggested by Beers & Christlieb 2005

23. Halo Metallicity distribution function (MDF)
Previous ‘as observed’, raw MDF is not a realistic presentation!
(but shows that we have been doing a good job in finding these stars..)
Non-zero tail!!!
Schoerck et al. 2008
The most metal-poor stars are
extremely rare but extremely important!

24. “Surface Pollution” through accretion
Bondi-Hoyle-Lyttleton accretion:
dM/dt = 4 G2M2 / v3
stellar orbit V1
Galactic disk V2
Three-component potential:
disk, spheroid, halo
(Johnston 1998)
Accretion for 10 billions years?

25. blue, bluer, the bluest Lower metallicity leads to decreased opacity
stars are hotter than solar equivalents
look bluer (bluer colors)
needs to be taken into account!
for temperature
measurements,
abundance analyses,
stellar populations
studies

26. Nucleosynthesis
All elements heavier than Li, Be, B are made during stellar evolution and supernova explosions

27. Stellar nucleosynthesis

28. most important reactions in stellar nucleosynthesis:
* Hydrogen burning:
- The proton-proton chain
- The CNO cycle
* Helium burning:
- The triple-alpha process
- The alpha process
* Burning of heavier elements:
- Carbon burning process
- Neon burning process
- Oxygen burning process
- Silicon burning process
* Production of elements heavier than iron:
- Neutron capture:
- The R-process
- The S-process
- Proton capture:
- The Rp-process
- Photo-disintegration:
- The P-process
All textbooks, wikipedia
....
Timmes+ ~95
Woosely&Weaver 1995
Heger & Woosley 2008
Many details not
known, but good models
out there

29. neutron-capture processes
-decay: n => p + e- v_e
s-process: neutron-capture longer than beta-decay timescale
r-process: neutron-capture shorter than beta-decay timescale

30. slow n-cap process
in 1-8Msun AGB stars; AGB stars are major providers of C and s-process elements in the universe (through mass loss)
produce s-rich companions: CH stars, Ba stars, s-rich metal-poor stars
good knowledge of s-process theoretically;
important for calculating the solar r-process component

31. The two neutron sources
in AGB stars
13C(a,n)16O Ne(a,n)25Mg
Needs 13C !
Major neutron source
13C-pocket
Primary source!
T8 = 0.9-1
Interpulse phase
(1- 0.4) 105 yr
Radiative conditions
Nn = 107 cm-3
lower mass AGBs
Abundant 22Ne
Minor neutron source
Neutron burst
Secondary source
T8 = 3 (low 22Ne efficiency)
Thermal pulse
6 yr
Convective conditions
Nn (peak) = 1010 cm-3
higher mass AGBs

32. the AGB engine At the He, 12C, 22Ne, s-process elements: Zr, Ba, ...
stellar
surface:
C>O, s-
process
enhance
ments
He, 12C, 22Ne, s-process elements: Zr, Ba, ...

33. thermally pulsing AGB stars

35. r-process

36. r-Process Enhanced Stars (rapid neutron-capture process)
Responsible for the production of the heaviest elements
Most likely production site: SNe II => pre-enrichment
Chemical “fingerprint” of previous nucleosynthesis event
(only “visible” in the oldest stars because of low metallicity)
~5% of metal-poor stars with
[Fe/H] <  2.5 (Barklem et al. 05)
Only stars known so
far with [Eu/Fe] > 1.0
Nucleo-chronometry: obtain
stellar ages from decaying Th, U and
stable r-process elements (e.g. Eu, Os)
[Th and U can also be measured in the Sun, but the chemical evolution has progressed too far; required are old, metal-poor stars from times when only very few SNe had exploded in the universe]
SN star decay today

37. Our Cosmic Lab

38. The r-Process Pattern
Very good agreement with scaled solar r-process pattern
for Z>56
scaled solar
r-process pattern
decayed Th,U HE
According to
metal-poor star
abundances,
the r-process
is universal!
Frebel et al. (2007)

39. Precision at work! Scaled solar r-process element pattern!!CS HD
Scaled solar
r-process
element
pattern!! BD CS HD HE
Cowan 2007, priv. comm
They all have the same abundance pattern,
particularly among heavy neutron-capture elements!
r-process must be a universal process!

40. origin of the elements abundances trends
chemical evolution
origin of the elements
abundances trends

41. Zentrum fuer Astronomie und Astrophysik, TU Berlin
chemical evolution
All the atoms (except H, He & Li)
were created in stars!
Pop III: zero-metallicity stars
Pop II: old halo stars
Pop I: young disk stars
Zentrum fuer Astronomie und Astrophysik, TU Berlin
We are made of stardust!
Old stars contain fewer elements
(e.g. iron) than younger stars

42. How and when did these early stars form?
Big Bang
Primordial
gas cloud
First star
exploding
First chemical
enrichment
2nd generation stars
forming from
enriched material
e.g. HE
[X/Fe]
Tominaga et al. 2007
Heger & Woosley 2008
Why important?
Metal-poor stars provide the only available diagnosis for zero-metallicity Pop III nucleosynthesis and early chemical enrichment

43. Pre-enrichment by a “faint SN”
“Faint” SN with mixing and fallback: Post-explosion abundance distribution
Explains high C, N, O, Mg
(Smaller mass cut for HE to account for high [Mg/Fe])
Explain other metal-poor stars with [Fe/H]<3.5
Neutron-capture elements not included
Iwamoto et al Science
M=25M, Z=0, low E

44. … with some (new) upper limits
Iwamoto et al Science

45. Abundance trends Alpha-elements [Mg/Fe]
Alpha elements multiple of He: (C,O), Ne, Mg, Si, S, Ar, Ca, Ti (not pure)
Synthesis during stellar evolution and -capture in supernova explosion of massive stars (>8 M)
[Si/Fe]
[Ca/Fe]
Pagel & Tautvaišiene (1995))
Fe and -elements produced in the explosions of massive stars (SN type II)
Fe-rich ejecta from the SN of low-mass stars (SN type Ia)

46. What is so special About the most Fe-poor stars?
Aoki, Frebel et al. 2006, ApJ
hyper
Fe-poor
ultra Fe-poor
extremely Fe-poor
very
hyper
Fe-poor
ultra Fe-poor
extremely Fe-poor
very
A compilation of abundances of ~800 metal-poor stars with [Fe/H]~<-2.0 can be found at
(published in Frebel ‘10, review article on metal-poor stars)
The very different chemical signature of the hyper iron-poor stars
is crucial for understanding the formation of the elements!

47. plots with abundance trends

48. Frebel et al. 06, ApJ submitted
Lithium
Li in HE 1300 depleted in accordance with the star’s evolutionary status (subgiant)
Majority of depletion seems to be taking place in the range K
Li depletion does not significantly depend on metallicity
Frebel et al. 06, ApJ submitted

49. the bigger picture
using stars to study the hierarchical assembly of galaxy formation
“near-field cosmology”

50. A long time ago... today 2nd and later generations of stars (<1 M)
First stars
(100 M)
Big Bang
today
first galaxies
today’s galaxies
Larson & Bromm 2001
Cosmic time (not to scale)

51. metallicity-luminosity relation
Ultra-faint dwarfs
Stellar
archaeology with
the most metal-poor
stars in MW satellite
galaxies
Ultra-faint dwarfs
Classical dSphs
Martin et al. (2007)

52. Metallicity distribution function of dSph galaxies
More metal-poor stars in the ultra-faints than in halo!?!
Kirby et al. (2008)
(targets selected from
Simon & Geha 2007)
Ultra-faint dwarfs
MW halo stars
Classical dwarfs
“classical” dSph
have no extremely
metal-poor stars?!?
(Helmi et al. 2006)
=> yes, they do!

53. What can we learn from the existing dwarf galaxies?
Stellar archaeology: examine the chemical history in search for their oldest population to learn about
- early chemical evolution in small systems
chemical signatures that relate dwarf galaxies to MW
If surviving dwarfs are analogs of early MW building blocks then we should find chemical evidence of it!
Stellar metallicities & abundances of metal-poor stars in dwarf galaxies should agree with those found in the MW halo

54. Mg, Ca, Ti (-elements)
No discrepancy of ultra-faint dwarf galaxy stars with those of MW halo (at low metallicities)!
Stars in ultra-faint dwarfs studied by AF and colleagues (Ursa MajorII, Coma Berenices, LeoIV)
(Frebel+2010, Simon+2010)
Stars in ultra-faint dwarfs studied by others (Hercules, Bootes) (Koch+2008, Norris+2009)
Stars in classical dSphs (Sculptor, Carinae, Draco, Sextans, Ursa Minor, Fornax, Leo)
(Shetrone+2001,2003, Venn+2004, Sadakane+04, Aoki+2009)
Halo stars
(e.g. Cayrel+2004, Barklem+2005, Aoki+2005,
Lai+2008 plus many others!) See Frebel (2010) review for a complete list of abundances and references.
halo stars
dSphs stars
ultra-faints

55. ultra-faint dwarf galaxy abundances
Excellent
agreement with
the MW chemical
evolution
Comparison with Cayrel+ 04 halo data (black open circles)
Spread in some elements (C, n-capture elements)
red squares:
Ursa Major II
blue dots:
Coma Berenices
black diamond:
MW halo giant
Frebel et al. (2010a)

56. Summary spectroscopy abundance analyses stellar atmospheres
stellar evolution
nucleosynthesis
SN physics and explosions
nuclear+atomic physics
chemical evolution
(near-field) cosmology