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C I and O I non-LTE spectral line formation

Schloss Ringberg (Tegernsee, Germany) Tuesday, 29 November 2005 CRUMPS ESO/MPA Workshop. C I and O I non-LTE spectral line formation. D. Fabbian. Main collaborators. Asplund M. (Canberra, Australia) Carlsson M. (Oslo, Norway) Kiselman D. (Stockholm, Sweden)

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C I and O I non-LTE spectral line formation

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  1. Schloss Ringberg (Tegernsee, Germany) Tuesday, 29 November 2005 CRUMPS ESO/MPA Workshop C I and O Inon-LTE spectral line formation D. Fabbian

  2. Non-LTE carbon spectral line formation Main collaborators Asplund M. (Canberra, Australia) Carlsson M. (Oslo, Norway) Kiselman D. (Stockholm, Sweden) Nissen P. E. (Aarhus, Denmark)

  3. Non-LTE carbon spectral line formation Outline • What is non-LTE and why use it • Non-LTE calculations for C (completed) and O (under way) • C and O abundances in metal-poor halo stars

  4. Non-LTE carbon spectral line formation Stellar abundance analyses Abundances not observed but derived - Stellar parameters - Stellar atmosphere - Atomic data - Spectral line formation Potential for systematic errors  need accurate models

  5. Non-LTE carbon spectral line formation LTE LTE assumption still used for most stellar work

  6. Non-LTE carbon spectral line formation Non-LTE • Line formation in general not in LTE since radiative transfer intrinsically non-local process  Radiative rates > collisional onesoften in the line forming region of late-type stars

  7. Non-LTE carbon spectral line formation Non-LTE solution • Rate equations for N atomic levels in statistical equilibrium (dni/dt=0) • Populations for any level i • Transition rates Pij include all radiative and collisional transitions (b-b and b-f) • Radiative transfer equation

  8. Non-LTE carbon spectral line formation Increased complexity ! Radiation-matter coupling: “Everything depends on everything else, everywhere else” (non-LTE radiative transfer)

  9. Non-LTE carbon spectral line formation So, why bother ? • Accuracy achievable with modern instruments requires advances in modelling of stellar spectra in order to reach ≤ 0.1 dex abundance uncertainty  unravel chemical evolution history of our Galaxy • In general, low-excitation lines and/or minority ionization species more affected, but case-by-case calculations required  see extensive review by Asplund 2005, ARAA

  10. Non-LTE carbon spectral line formation Non-LTE radiative transfer Ingredients • MULTI (Carlsson 1986), MULTI3D (Botnen 1997) • 1D (MARCS) and 3D model atmospheres • Atomic models (TOPbase, NIST, VALD, literature data)

  11. Non-LTE carbon spectral line formation Main uncertainties Collisional excitations and ionizations • e- (forbidden: van Regemorter 1962; radiatively allowed: impact approximation) • neutral H atom (Drawin 1968)  still among main sources of uncertainty in current non-LTE studies

  12. Non-LTE carbon spectral line formation C and O stellar abundances • Abundant, crucial elements • Main C production site ? • [O/Fe] at low [Fe/H] (e.g. Israelian et al. 1998, Nissen et al. 2002, García Pérez et al. 2005) • [C/O] ratio in metal-poor halo stars (e.g. Akerman et al. 2004, Spite et al. 2005)

  13. Non-LTE carbon spectral line formation Carbon Main C production site in … massive stars ? (e.g. Akerman et al. 2004, Spite et al. 2005) or low and intermediate mass stars ? (e.g. Chiappini et al. 2003 , Romano & Matteucci 2003, Gavilán et al. 2005) or both important sources ? (e.g. Carigi et al. 2005, Bensby & Feltzing 2005)

  14. Non-LTE carbon spectral line formation C atomic model • 217-levels, 650 radiative transitions • High excitation (L>7.4 eV) C I features

  15. Non-LTE carbon spectral line formation H collisions “dilemma” • Drawin’s (1968) classical recipe may overestimate by 1-6 orders of magnitudes(Fleck et al. 1991, Belyaev et al. 1999, Belyaev & Barklem 2003) !  Scaling factor SH to Drawin formula used • C I non-LTE corrections relatively insensitive to H collisions efficiency  Still large when setting SH=1 (‘a la Drawin’)

  16. Non-LTE carbon spectral line formation Grid of stellar parameters • Range covering that of late-type stars • Non-LTE corrections by varying [C/Fe] (up to ±0.60 dex) • 3 different choices for SH (0, 10-3, 1) • >3000 different non-LTE runs in total

  17. Non-LTE carbon spectral line formation Non-LTE effects 910 nm lines • We find significant line strengthening in LTE • negative abundance corrections for all late-type stars Solar metallicity: line source function effect (S/B<1) Low metallicity: mainly opacity effect

  18. Non-LTE carbon spectral line formation Results for C I ~-0.35…-0.45 dex abundance corrections for halo turn-off stars at [Fe/H]~-3 910 nm lines H collisions neglected H collisions ‘a la Drawin’

  19. Non-LTE carbon spectral line formation Re-analysis of Akerman et al. 2004 Info on nucleosynthetic yields from massive stars ?  Non-LTE abundance corrections for sample of 34 metal-poor halo dwarfs (Akerman et al. 2004) No H coll. ‘a la Drawin’ vs. Teff

  20. Non-LTE carbon spectral line formation [C/Fe] trend • [C/Fe]~0 at low [Fe/H] Observational data: Akerman et al. (2004)+Bensby&Feltzing (2005) • Large spread(LTE and non-LTE) due to Teff errors ? H collisions neglected H collisions ‘a la Drawin’

  21. Non-LTE carbon spectral line formation [O/Fe] trend • O non-LTE corrections found to be larger at low [Fe/H] • [O/Fe] ~ flat below [Fe/H]<-1.5 at ~0.6 level • Observational data: Akerman et al. (2004)+Bensby&Feltzing (2005) LP815-43 Wrong temperature/EW ?

  22. Non-LTE carbon spectral line formation [C/O] in halo stars Implications for the chemical evolution of the (early) Galaxy from metal-poor unevolved stars  [O/H]non-LTE but [C/O]LTE, in the hope that C and O non-LTE corrections will compensate • Before: -3.2<[Fe/H]<-0.7 (Akerman et al. 2004, high-excitation C and O lines) Chemical evolution models with Chieffi&Limongi 2002 Pop. III yields (+normal/top-heavy IMF) “Standard”=No Pop. III

  23. Non-LTE carbon spectral line formation [C/O] in halo stars Halo: Akerman et al. (2004) + disk: Bensby&Feltzing (2005) • C/O upturn at [O/H]~-1? • LP815-43 has suspiciously low value of O ! • Spite et al. (2005) find near-solar values at very low metallicity, but do not correct CH (~-0.5 dex or more) ! • After: [C/O] down by 0.1-0.2 dex at low [O/H]  C and O non-LTE corrections do not compensate exactly • No need to invoke Pop. III stars contributions to C nucleosynthesis ? • Need to know more about oxygen !

  24. Non-LTE carbon spectral line formation Oxygen Halo Disk • [OI] 630 nm forbidden line: ~ plateau • OH UV lines: linear trend with slope ~ -0.3 but … 3D effects at low [Fe/H] (Asplund & Garcia Perez 2001) • OI 777nm permitted triplet: often in between but … non-LTE effects? • Other issues: new solar abundances (Asplund et al. 2005), stellar parameters (e.g. Melendez et al. 2005) 2

  25. Non-LTE carbon spectral line formation O I 777nm triplet • Formed in non-LTE: negative abundance corrections • IR O I triplet non-LTE effects should be well understood at solar metallicity (source function effect), not at low metallicity - Kiselman (1991): increasing corrections at low [Fe/H]  If so, agreement between different abundance indicators at [O/Fe]~+0.5 ? O overproduction in Type II SN (Arnett, 1978; Woosley, 1986) during early Galaxy chemical enrichment, then Type SN I kick in at [Fe/H]~-1 (disk formation age)

  26. Non-LTE carbon spectral line formation O atomic model 23-level atom 777 nm triplet

  27. Non-LTE carbon spectral line formation O abundance corrections • Driving non-LTE effect(s) ? Test different processes • Large abundance corrections at low metallicity • Large variations, from ~-0.2 to ~-0.7 dex at log O=6.50, [Fe/H]=-3.50, mainly depending on e- and H collisions  triplet-quintet coupling plays major role but rates uncertain, need best atomic data available (Barklem et al. calculations for these e- collisions)

  28. Non-LTE carbon spectral line formation Summary Non-LTE modelling necessary to reveal driving mechanisms of line formation

  29. Non-LTE carbon spectral line formation Our findings • Large negative abundance corrections for high excitation C lines (Fabbian et al. 2006 for full results) • Non-LTE corrections for oxygen triplet at 777 nm more uncertain (larger?) than stated in literature • (Slightly supra-) solar and ~ flat [C/Fe] at low [Fe/H] • Generally high O abundances at low metallicity. ~flat [O/Fe] trend below [Fe/H]≤-1.5 • No need to invoke yields from Pop. III stars to explain [C/O] trend down to [O/H]~-2.5 ?

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