Lithium abundances and isotope ratios,
and troublesome stellar atmospheres
Sean G. Ryan
School of Physics, Astronomy and Mathematics
University of Hertfordshire
Principal collaborators
Ana García Pérez(UH/Virginia), Adam Hosford(UH), Andy Gallagher(UH)
Wako Aoki (NAOJ), Keith Olive (Minnesota), John Norris (ANU)
Structure of this talk
Two halo-star lithium problems (7Li and 6Li)
Effective temperature scales
Line profiles in 1D LTE, 1D NLTE, and 3D
NGC 4414:
Hubble Heritage Team (AURA/STScI/NASA) + Hack/Ryan (OU)
[email protected]
1
Lithium problem #1: 7Li
• Measurements of CMBR by WMAP
give baryon density fraction
Bh2 = 0.0224±0.0009
(Spergel et al. 2003).
• BBN depends on B.
WMAP in excellent agreement
with B derived from 2H/1H.
• Uncomfortable discrepancy
for 7Li:
the “Lithium problem”
Coc & Vangioni (2005)
2
Lithium problem #1: 7Li
Several explanations offered to explain WMAP discrepancy
• intriguing particle physics possibilities
(failure of SBBN model):
– survival of metastable particles
for a few ×103 s, i.e. during BBN
Bird, Koopmans & Pospelov 2007,hep-ph/0703096:
X- + 7Be → 7BeX- ;
7BeX- (p,γ) 8BX- → 8BeX- + β+ + ν
e
Pospelov, M. 2007, hep-ph/0712.0647:
X- + 4He → 4HeX- ;
+4He → 8BeX- ;
8BeX- + n → 9BeX-* → 9Be + X-
– decay or annihilation of massive
supersymmetric particle,
modifying 7Li and 6Li production
Jedamzik 2004
3
Lithium problem #1: 7Li
Several explanations offered to explain WMAP discrepancy
• stellar destruction possibilities:
– Have some stars partially
destroyed 7Li?
• mundane possibilities:
Did we get the abundances wrong?
– Large uncertainty in low-Z
colour-effective-temperature scales
– E.g. comparison between
“cool” Ryan et al (2001) and
“hot” Melendez & Ramirez (2004)
Teff scales shows difference
of up to 400K for [Fe/H] < -3
– ΔTeff ≈ +400 K → ΔA(Li) ≈ +0.3 dex; close to discrepancy
4
Lithium problem #1: 7Li
• PhD: Adam Hosford
Effective temperature
scale for metal-poor stars:
• Use T-dependence of
Fe I LTE level populations:
Boltzmann factor exp-(χ/kT)
• Attention to error propagation
– Uncertainty in χ vs A(Fe) slope
being nulled
~ 60-80 K
– evolutionary state weakly
constrained
~ 12-24 K
– uncertainty in ξ ~ 30-90 K
(wrong physics anyway → 3D)
5
T(Ryan)
• Hosford: Fe I LTE
level populations
• Asplund et al. 2006:
Hα Balmer profile fits
• Melendez & Ramirez:
IRFM
• T,LTE similar to R01, A05
T(Asplund)
Lithium problem #1: 7Li
T(Hosford)
T(Hosford)
• Asplund’06 analysis:
– A05 in good agreement with b-y and
V-K IRFM of Nissen et al. (‘02,’04):
ΔTeff = -34 ± 95 K
– cooler than “hot” MR04 scale:
ΔTeff = 182 ± 72 K @ [Fe/H] < -2.6).
T(MR05)
Hosford, Ryan, Garcia Perez, Norris & Olive 2009, A&A, 493, 601
T(Hosford)
6
Lithium problem #1: 7Li
• Tχ,LTE assumes LTE Fe I level populations
• LTE holds at τcontinuum > 1, but lines form at τcontinuum < 1
• NLTE difficult to calculate reliably
– Collisional excitation very uncertain
• Collisions with hydrogen parametrized via SH (= 0.001? 1?)
– Model atom incomplete
• Ideally calculate populations and radiative & collisional transition
rates (need all oscillator strengths) for all levels (populations coupled
by radiative and collisional transitions), but ...
• ... our/Collett model atom contains just 524 levels for Fe I, II and III;
cf. NIST lists 493+578+567 levels for Fe I+II+III
•
–
–
Confucius say:
“Stay away from NLTE, and you can have a nice life.”
F. Thevenin, c.2000
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Lithium problem #1: 7Li
• Previous calculations at low Z point to overionisation as
major effect: underpopulates Fe I levels relative to LTE
e.g. Asplund et al. (1999, A&A, 346, L17; 2005 ARAA, 43, 481, §3.7)
– transparent layers with τcontinuum < 1 see photons from
deep/hot atmosphere, so photon intensity Jν > local Bν.
UV photons photoionise excited Fe I states.
• Additional factors: lack of collisions at τcontinuum < 1
– reduces collisional excitation of excited levels
compared to what local T suggests via Boltzmann
(i.e. populations not in thermal equilibrium with local temperature)
• Net result: excited level populations lower than in LTE;
Assess -dependence using MULTI calculations ...
8
Lithium problem #1: 7Li
b ≡ nNLTE/nLTE
(SH = 1)
• NLTE effects clearly depend on χ
• χLTE vs A(Fe) affected by NLTE, hence Tχ, LTE affected by NLTE
• Calculations vary from star to star, but (for six stars):
T,NLTE ~ 110-160 K hotter than R01, A05,
~ 190 K cooler than MR04
Hosford, García Pérez, Collet, Ryan, Norris, Olive, 2010, A&A, 511, 47
9
Lithium problem #2: 6Li
•
6Li
isotope shift = 0.15 Å; same as fine structure splitting
• Adds a little asymmetry to asymmetric line ... as does convection
– but hard to model in 3D
Cayrel ,et al. (incl. Ludwig), 2007, A&A, 473, 37
•
•
•
6Li
< 0.00001 ppb in standard bbn
Serpico et al. 2004
6Li not produced in stars: no stable A = 5 or 8 nuclei
6Li produced via galactic cosmic ray (GCR) spallation
– In Pop I alongside 9Be and 10,11B;
Steigman & Walker 1992, ApJ, 385, L13;
Boesgaard et al. 1999, AJ, 117, 1549 (BDKRVB)
•
6,7Li
at low Z via 4HeISM + αGCR
Yoshii et al. 1997, ApJ, 485, 605 (YKR)
Duncan et al. 1997, ApJ, 488, 338 (DPRBDHKR)
destroyed in stars in (p,α) reactions
– S-factor = 3140 keV barns for 6Li(p,3He)4He Elwyn et al. 79, PhysRevC, 20, 1984
– S-factor = 55 keV barns for 7Li(p,4He)4He Pizzone etal. 03, A&A, 398, 423
7Li(p,α)4He ~2.6×106 K
– 6Li(p,α)3He ~2.0×106 K
Survives (if at all) in warmest low-Z stars
Brown & Schramm 88, ApJ, 329, L103
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Lithium problem #2: 6Li
Aoki et al. 2004, A&A, 428, 579 (AIKRSST)
•
•
S/N = 1000
R = 90000
6Li/7Li
= 0.00, 0.04, 0.08
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Lithium problem #2: 6Li
Two major results from
Asplund et al. 2006:
• Abundance high
compared to models
that are consistent
with spallative 9Be,
10,11B, especially if
depletion allowed for.
• Trend with [Fe/H]
looks like plateau,
unlike strong [Fe/H]
dependence of
models.
12
Lithium problem #2: 6Li
Subaru/HRS data on 5 stars.
Isotope ratio VERY sensitive
to systematic uncertainties:
e.g. macroturbulent width,
wavelength shifts,
continuum errors,
flat field errors,
7Li abundance
fair choices → uncertainties
Δ(6Li/7Li) ~ 3-4%
García Pérez, Aoki, Inoue, Ryan, Suzuki, & Chiba,
2009, A&A, 504, 213
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Lithium problem #2: 6Li
VLT observations
Subaru/HRS data
very similar to
Asplund et al.
VLT/UVES data ...
4%
3%
2%
1%
Asplund et al. 2006
... but we are not
confident of our
“detections”
Working at margins of
significance due to
systematic limitations
Troublesome stellar atmospheres
• Barium isotope ratios
– Truran (1981) proposed that at low Z, r-process dominates over sprocess since s-process seeds have low abundance whereas rprocess seeds are made in the SN precursor (based partly on
Spite & Spite (1978) Eu/Ba)
Truran 1981, A&A, 97, 391
Spite & Spite 1978, A&A, 67, 23
– Travaglio et al (1999) numerical GCE simulations confirm
moderate-Z onset of s-process
Travaglio et al. 1999, ApJ, 521, 691
– But ... Magain (1995) found Ba 4554 isotope profile in HD 140283
more like s-process than r-process
Magain 1995, A&A, 297, 686
• Andy Gallagher (PhD thesis with SGR and AEGP):
Use 2 analysis techniques (ex-6Li) to attempt to study
135,137Ba isotopic splitting in low-Z stars
15
Troublesome stellar atmospheres
• Sensitivities: macroturbulent broadening key
(Lambert & Allende-Prieto, 2002, MNRAS, 335, 325)
– Fit via ~90 Fe I lines with WFe ~ WBa 4554
Gallagher, Ryan, Garcia Perez & Aoki, 2010, A&A, 523, A24
Gallagher, Ryan, Hosford, Garcia Perez, Aoki & Honda, 2012, A&A, 538, A118
• Co-add residuals for all Fe I lines to see if any asymmetry
– 4/4 dwarfs show asymmetric red wing ~ 130 mÅ from line core
– Not improved switching ATLAS to MULTI LTE, or LTE to NLTE
– 2/2 giants are symmetric (though still large residuals)
16
Troublesome stellar atmospheres
• Experimented with three formalisms for macroturbulence,
again fitting to ~90 Fe I lines
– Gaussian profile (+ Gaussian instrumental)
– Radial-tangential profile (+ Gaussian instrumental)
– vsini (+ Gaussian instrumental)
• Results:
– vsini :
rarely the best profile
(~ 5% of lines)
– Gaussian macroturbulence:
sometimes the best profile
(~20% of lines)
– Radial-tangential macroturbulence:
most often the best profile
(~80% of lines)
17
Concluding remarks
•
7Li:
temperature scales from colours, IRFM, T,LTE and
T,NLTE suggest 7Li not compatible with BBN/WMAP.
• 6Li: our Subaru data at best only marginally significant;
uncertainties ~3-4% of A(7Li); not significant detections.
• 6Li, Ba & Fe: asymmetries seen in Fe I line residuals
(and Ba II); could also be important for 6Li.
• Radial-tangential macroturbulence better than Gaussian
... but still artificial ... Need 3D atmospheres and radiative
transfer.
• Observation-based challenge for emerging 3D codes:
to reproduce observed shapes of Fe I lines in dwarfs and
giants.
18
Cautionary remark
• 3D modelling (in NLTE) motivated by:
• observed asymmetries in Fe I
• dissatisfaction with  (microturbulence)
• dissatisfaction with  and/or  (macroturbulence)
• realisation that 3D radiative transfer in dynamical
models may better explain line formation and hence
affect interpretation of spectra
• But it may not deliver!
M.Spite, 1997, IAUS, 189, 185
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NGC 4414:
Hubble Heritage Team (AURA/STScI/NASA) + Hack/Ryan (OU)
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