Neon and oxygen in stellar coronae - a unification with the Sun

808 views

Published on

Talk presented by Robrade, Jan

Published in: Education, Technology, Business
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
808
On SlideShare
0
From Embeds
0
Number of Embeds
414
Actions
Shares
0
Downloads
4
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Neon and oxygen in stellar coronae - a unification with the Sun

  1. 1. Neon and oxygen in stellar coronae A unification with the Sun Jan Robrade Hamburger SternwarteFrom Atoms to Stars, July 2011, Oxford
  2. 2. Overview1 Neon and the solar modeling problem2 Data and measurements3 X-ray properties of weakly active stars4 Coronal Ne/O ratios
  3. 3. Why care? The chemical composition of the Sun is one of the most important yardsticks in astronomy with implications for basically all fields from planetary science to the high-redshift Universe. (Asplund, Grevesse, Sauval 2005)Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 3 / 28
  4. 4. Our star - the Sun The ideal world Abundances from Grevesse & Sauval, 1998 Solar interior model Agreement with helioseismologic measurements Trouble in paradise? Revised abundances by Asplund+ 2005 sophisticated 3D hydrodynamic modelling high quality atomic line data, includes non-LTE calculations reduced abundances of C, N, O by 30 – 40% better agreement e.g. with ISM measurements but: significant disagreement with helioseismology missing opacity Way out needed!Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 4 / 28
  5. 5. Neon and oxygen Increase neon by a factor of 3 – 4 !! (e.g. Antia & Basu 2005, Bahcall 2005) Why neon? no photospheric lines in solar spectrum no useful meteorite data (noble gas, volatile) very common element strong source of opacity determined indirectly coronal and/or TR lines solar wind/energetic particles oxygen as reference element; determine ANe /AO (Ne/O) AGS05: Ne/O = 0.15 quite low (same as AnGr89)Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 5 / 28
  6. 6. The controversy ’The solar modelling problem solved by the abundance of neon in nearby stars’ top: solar corona - active regions Ne/O = 0.41 (Drake & Testa, 2005) Ne/O = 0.18 (Schmelz et al. 2005) mainly active stars bottom: transition region - quiet sun objections from solar observers Ne/O = 0.17 (Young 2005)Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 6 / 28
  7. 7. Ne/O in the Sun A short history of solar Ne/O ratios: 0.21 (0.16-0.31) solar corona (Acton et al. 1975) 0.17 (0.15-0.19) solar wind (von Steiger & Geiss 1989) 0.18 (0.15-0.22) Sun (Grevesse & Sauval 1998) transient deviations in individual flares observed, but in general: Ne/O ≈ 0.2 - independent of atmospheric layer and activity phase!Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 7 / 28
  8. 8. Ne/O in inactive stars Sun (von Steiger & Geiss 1989) α Centauri A (Raassen+ 2003) solar wind/energetic particles X-rays/corona TR/corona (Feldman 1992) Chemical fractionation Pt. I fractionation occurs in chromosphere weakly active stars show FIP-effect O and Ne are high FIP elements ⇒ Ne/O ratio unchangedJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 8 / 28
  9. 9. Ne/O in active stars HR 1099 (Brinkman+ 2001) active M dwarfs (Robrade+ 2005) X-rays/corona X-rays/corona Chemical fractionation Pt. II highly active stars show IFIP-effect strength depends on activity level Ne/O ratio changedJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 9 / 28
  10. 10. Neon and oxygen in weakly active stars Study coronal Ne/O of in a sample of stars similar to the Sun! Neon and oxygen in low activity stars (Robrade+ 2008; Robrade & Schmitt 2009) The sample: Altair (A7), Procyon (F5), β Com (G0), α Cen (G2+K1), HD 81809 (G2+G9), Eri (K2), 61 Cyg (K5+K7) broad range of effective temperatures low to moderately active stars; log LX /Lbol = −5... − 7 coronae dominated by cool plasma (T 5 MK) Ne/O from emission line ratios + global modelingJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 10 / 28
  11. 11. Ne/O - emission line ratios Method I - emission line ratios strong emission lines from oxygen and neon (fitted with CORA) covered by XMM-Newton (RGS) and Chandra (LETGS) virtually free of blends well determined atomic data Construct ’temperature-independent’ line ratios: 1975: O viii vs. Ne ix (Acton, Catura, Joki, 1975) Used lines: O vii r (21.6 ˚), O viii Lyα (18.97 A), Ne ix r (13.45 ˚), Ne x Lyα (12.13 ˚) A ˚ A A O viii vs. Ne ix + 0.15 Ne x – energy flux weighting (Drake & Testa, 2005) 0.67 O viii - 0.17 O vii vs. Ne ix + 0.02 Ne x - photon f. w. (Liefke & Schmitt, 2006)Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 11 / 28
  12. 12. Ne/O - emission line ratios Summed and scaled residuals of the emissivities ≡ flat EMD). Theoretical emissivity curves for Ne (CHIANTI 5, Landi+ 2006) and O and respective residuals. residuals smooth out only if EMD is very broad significant error for very cool stars possible L&S - too much neon, D&T - EM dependent trendJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 12 / 28
  13. 13. Ne/O - spectral modeling Method II -’global’ modeling fit spectra with multi-temperature VAPEC models in XSPEC free abundances of Ne, O, Fe (+ additional, if S/N sufficient) RGS/MOS or LETGS spectra check with Ne+O dominated spectral regions ˚ (11-14 + 18-23 A) include LETGS long-wavelength regime (85-100 ˚) - ’cooler’ Ne vii + viii A derive X-ray luminosities, coronal temperatures, Ne/O ratios absolute abundances more uncertain - EM interdependenceJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 13 / 28
  14. 14. Ne/O - X-ray CCD spectra NeX OVII OVIII NeIX MOS CCD spectra of Eri (black) and Procyon (red) with line features labeledJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 14 / 28
  15. 15. Ne/O - X-ray grating spectra High resolution X-ray spectra from XMM-Newton and Chandra good data quality obtained Ne ix line most crucial Spectra of Eri (LETGS) and 61 Cyg (RGS, co-added)Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 15 / 28
  16. 16. X-ray data & coronal properties Star Mission Obs.(No Exp.) log LX Tav. LX /Lbol (ks) (erg/s) (MK) log Eri (K2) XMM 13 (1) 28.2 3.8 -4.9 HD 81809 (G2+G9) XMM 72 (7) 28.7 4.0 -5.6 61 Cyg (K5+K7) XMM 103 (11) 27.3 3.2 -5.6, -5.5 β Com (G0) XMM 41 (1) 28.2 3.4 -5.6 Chandra 105 (1) Procyon (F5) XMM 138 (3) 27.9 1.9 -6.5 Chandra 139 (2) α Cen (G2+K1) XMM 73 (9) 27.1 2.2 -7.3, -6.2 Chandra 79 (1) Altair (A7) XMM 130 (1) 27.1 2.3 -7.4 data taken 1999-2007 , XMM/RGS binary data unresolved MOS/RGS spectral fit for basic parameter, LX in 0.2 – 3.0 keV bandJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 16 / 28
  17. 17. Results - weakly active stars (Hempelmann 2006, Robrade+, in prep.) Global X-ray properties Pt. I all coronae are cool, av. TX ≈ 2 − 4 MK weak to minor contribution of 5 – 10 MK plasma FIP effect in the lesser active stars (α Cen, β Com) weak/no FIP effect in in moderately active stars many weakly active G and K dwarfs show cyclic X-ray activityJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 17 / 28
  18. 18. Results - Altair XMM-observation of Altair (Robrade & Schmitt, 2009) A7 star, Teff ≈ 7800 K, M = 1.8 M , Vsini ≈ 220 km/s, i ≈ 60◦ , X-ray source X-ray properties similar to inactive sun LX = 1.4 × 1027 erg/s, log LX /Lbol = −7.4, 1 – 4 MK plasma minor activity, rotational modulation, long term stable solar-like abundances and FIP effect ’classical interpretation’: thin outer convective layerJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 18 / 28
  19. 19. Results - Altair II Altair - rotationally deformed Vrot 60% breakup-speed ⇒ X-ray saturation level very low oblate, axial ratio of a / b ≈ 1.1 – 1.2 gravity darkening ⇒ Teff range: 6900 K (equator) up to 8500 K (poles) O vii f /i-ratio high: tracer of density and UV-field surface features at Teff 7400 K CHARA (Monnier et al. 2007) f /i = R0 /(1 + φ/φc + ne /nc ) (e.g. Gabriel & Jordan 1969, Porquet+ 2001) =⇒ Equatorial bulge coronaJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 19 / 28
  20. 20. Neon and oxygen - results Global modeling - results Ne/O ratio robust Neon and oxygen line measurements virtually all lines detected in all stars few Ne x U.L. in LETGS spectra of coolest coronae account for Fe xvii blend in Ne x via emissivities (20 %) neglect Fe xix blends in Ne ix – low TX overall good agreement between multiple observations obs.-time average for cyclic stars Ne/O ratios - D&T vs. L&S similar for the hotter stars discrepancies for the coolest starsJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 20 / 28
  21. 21. Ne/O - results I Coronal stellar Ne/O ratios and the ’classical’ Sun (Robrade & Schmitt, 2009) (global fit: diamonds/solid line, D&T asterisks/dotted line, L&S squares/dashed line) Stellar X-ray data suggests Ne/O at upper bound of solar rangeJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 21 / 28
  22. 22. Ne/O - results II Ne/O ratios of individual stars Ne/O: 0.35 – 0.40 for Eri (0.37), 61 Cyg (0.36), HD 81809 (0.36) Ne/O: 0.25 – 0.35 for β Com (0.25), α Cen B (0.26) Ne/O: 0.20 – 0.25 for Procyon (0.22), α Cen A (0.21), Altair (0.20) overall good agreement with literature mod. active stars Eri (Wood & Linsky Ne/O = 0.36, Sanz-Forcada+ Ne/O = 0.4) weakly active stars Procyon (Raassen+ Ne/O = 0.22 , Sanz-Forcada+ Ne/O = 0.40) α Cen A/B (Raassen+ A: Ne/O = 0.18, B: Ne/O = 0.26, L&S Ne/O = 0.27)Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 22 / 28
  23. 23. Ne/O - results III Ne/O ratios of stellar coronae coronal Ne/O ratio increases with activity in weakly active stars trend independent of analysis method trend independent of spectral type Ne/O ≈ 0.2 at log LX /Lbol ≈ −6.5 Ne/O ≈ 0.4 at log LX /Lbol ≈ −4.5 solar values typical for low activity stars Ne/O apparently saturates at higher activity levels larger datasets required to reveal details of chemical fractionation shape of ratio-curve dependence on sp. type, LX /Lbol vs. TXJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 23 / 28
  24. 24. Solar modeling problem: further insights Other abundance determinations: low Ne/O ratio in photospheric study of early B stars (Przybilla et al. 2008) Ne/O= 0.21 (0.19-0.23) (absolute values intermediate to GS98 & AGS05) homogeneous distribution of elemental abundances in solar neighborhood Solar abundances revised (Asplund+ 2009) slightly higher solar metallicity (+ 10%) Ne/O = 0.17 (0.14-0.22) absolute abundances of Ne and O: agreement between Sun, B stars and H ii-regions within errors Lodders+ 2009, Ne/O = 0.21Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 24 / 28
  25. 25. Caveats & open Questions Measured and predicted sound speed (Asplund+ 2009) Discrepancy to helioseismology alleviated but still significant! Solar problems sound speed profile wrong convection zone depth too shallow interior He abundance too low Possible solutions revise abundance calculations revise opacity calculations for solar interior revise diffusion model - interior is more metal rich internal gravity waves - promising, but only qualitatively evaluatedJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 25 / 28
  26. 26. Caveats & open Questions II Chemical fractionation: cause of fractionation not fully explained transition: FIP- no-FIP - IFIP Laming-models promising (ponderomotive force) activity good tracer - importance of fundamental parameter elements: charge, mass stars: gravity, temperature gradients, radiation, electric & magnetic fields details of abundance trends need to be refined other elements need to be consideredJan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 26 / 28
  27. 27. Summary Neon and oxygen in stellar coronae Ne/O ratio depends on stellar activity Ne/O increases with activity in weakly to moderately active stars Ne/O ≈ 0.2 ± 0.05 in weakly active stars Neon is not the solution for the solar modelling problem The Sun is a typical star!Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 27 / 28
  28. 28. Ne/O - line fluxes Measured line fluxes in 10−5 photons cm−2 s−1 from RGS or LETGS (C) and energy flux ratio of the O viii(Lyα) to O vii(r) line. Star O vii(r) O viii Ne ix(r) Ne x∗ O8/O7(r) 61 Cyg 6.6±0.4 9.1±0.5 2.0±0.3 1.6±0.2 1.57±0.13 Altair 4.9±0.4 3.7±0.3 0.4±0.1 0.2±0.1 0.86±0.10 α Cen 33.5±1.3 27.0±1.2 4.1±0.5 1.6±0.2 0.92±0.05 α Cen 03/04 47.9±2.1 45.4±1.5 5.5±0.8 2.4±0.5 1.08±0.06 α Cen A (C) 9.2±1.0 3.2±0.5 0.5±0.3 0.2 0.40±0.07 α Cen B (C) 11.5±1.0 6.3±0.6 1.3±0.4 0.2 0.62±0.08 β Com 3.8±0.6 5.9±0.6 0.9±0.3 0.6±0.3 1.77±0.33 Eri 44.1±2.6 78.0±3.0 21.2±2.4 10.5±1.4 2.01±0.14 Eri (C) 41.5±1.6 78.9±1.7 18.8±0.9 16.7±0.9 2.16±0.10 HD 81809 1.0±0.3 2.3±0.3 0.5±0.2 0.5±0.2 2.62±0.86 Procyon 35.6±1.1 22.9±0.9 1.8±0.3 0.5±0.2 0.73±0.04 Procyon (C) 29.1±1.6 17.3±1.0 2.5±0.5 0.7±0.3 0.68±0.05 Procyon (C) 30.3±1.7 18.9±1.1 1.4±0.3 0.2 0.71±0.06 ∗ Blended with Fe xvii.Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 28 / 28

×