Neon and oxygen in stellar coronae

Transcription

1 Neon and oxygen in stellar coronae A unification with the Sun Jan Robrade Hamburger Sternwarte From Atoms to Stars, July 2011, Oxford

2 O v e r v i e w 1 Neon and the solar modeling problem 2 Data and measurements 3 X-ray properties of weakly active stars 4 Coronal Ne/O ratios

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 / 28

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 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 / 28

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 A Ne /A O (Ne/O) AGS05: Ne/O = 0.15 quite low (same as AnGr89) Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

6 The controversy The solar modelling problem solved by the abundance of neon in nearby stars Ne/O = 0.41 (Drake & Testa, 2005) mainly active stars objections from solar observers top: solar corona - active regions Ne/O = 0.18 (Schmelz et al. 2005) bottom: transition region - quiet sun Ne/O = 0.17 (Young 2005) Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

7 Ne/O in the Sun A short history of solar Ne/O ratios: 0.21 ( ) solar corona (Acton et al. 1975) 0.17 ( ) solar wind (von Steiger & Geiss 1989) 0.18 ( ) Sun (Grevesse & Sauval 1998) transient deviations in individual flares observed, but in general: Ne/O independent of atmospheric layer and activity phase! Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

8 Ne/O in inactive stars Sun (von Steiger & Geiss 1989) solar wind/energetic particles TR/corona (Feldman 1992) α Centauri A (Raassen+ 2003) X-rays/corona Chemical fractionation Pt. I fractionation occurs in chromosphere weakly active stars show FIP-effect O and Ne are high FIP elements Ne/O ratio unchanged Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

9 Ne/O in active stars HR 1099 (Brinkman+ 2001) X-rays/corona active M dwarfs (Robrade+ 2005) X-rays/corona Chemical fractionation Pt. II highly active stars show IFIP-effect strength depends on activity level Ne/O ratio changed Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

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 (G2+G9), ǫ Eri (K2), 61 Cyg (K5+K7) broad range of effective temperatures low to moderately active stars; logl X /L bol = coronae dominated by cool plasma (T 5 MK) Ne/O from emission line ratios + global modeling Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

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: Oviii vs. Neix (Acton, Catura, Joki, 1975) Used lines: Ovii r(21.6 Å), Oviii Lyα(18.97 Å), Neix r(13.45 Å), Nex Lyα(12.13 Å) Oviii vs. Neix Nex energy flux weighting (Drake & Testa, 2005) 0.67 Oviii Ovii vs. Neix Nex - photon f. w. (Liefke & Schmitt, 2006) Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

12 Ne/O - emission line ratios Theoretical emissivity curves for Ne and O and respective residuals. Summed and scaled residuals of the emissivities flat EMD). (CHIANTI 5, Landi+ 2006) 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 trend Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

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 ( Å) include LETGS long-wavelength regime ( Å) - cooler Ne vii + viii derive X-ray luminosities, coronal temperatures, Ne/O ratios absolute abundances more uncertain - EM interdependence Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

14 Ne/O - X-ray CCD spectra NeX OVII OVIII NeIX MOS CCD spectra of ǫ Eri (black) and Procyon (red) with line features labeled Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

15 Ne/O - X-ray grating spectra High resolution X-ray spectra from XMM-Newton and Chandra good data quality obtained Neix line most crucial Spectra of ǫ Eri (LETGS) and 61 Cyg (RGS, co-added) Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

16 X-ray data & coronal properties Star Mission Obs.(No Exp.) log L X T av. L X/L bol (ks) (erg/s) (MK) log ǫ Eri (K2) XMM 13 (1) HD (G2+G9) XMM 72 (7) Cyg (K5+K7) XMM 103 (11) , -5.5 β Com (G0) XMM 41 (1) Chandra 105 (1) Procyon (F5) XMM 138 (3) Chandra 139 (2) α Cen (G2+K1) XMM 73 (9) , -6.2 Chandra 79 (1) Altair (A7) XMM 130 (1) data taken , XMM/RGS binary data unresolved MOS/RGS spectral fit for basic parameter, L X in kev band Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

17 Results - weakly active stars (Hempelmann 2006, Robrade+, in prep.) Global X-ray properties Pt. I all coronae are cool, av. T X 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 activity Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

18 Results - Altair XMM-observation of Altair (Robrade & Schmitt, 2009) A7 star, T eff 7800 K, M = 1.8M, Vsini 220 km/s, i 60, X-ray source X-ray properties similar to inactive sun L X = erg/s, logl X /L bol = 7.4, 1 4 MK plasma minor activity, rotational modulation, long term stable solar-like abundances and FIP effect classical interpretation : thin outer convective layer Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

19 Results - Altair II Altair - rotationally deformed CHARA (Monnier et al. 2007) V rot 60% breakup-speed X-ray saturation level very low oblate, axial ratio of a/b gravity darkening T eff range: 6900K (equator) up to 8500K (poles) Ovii f/i-ratio high: tracer of density and UV-field surface features at T eff 7400 K f/i = R 0 /(1+φ/φ c +n e /n c ) (e.g. Gabriel & Jordan 1969, Porquet+ 2001) = Equatorial bulge corona Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

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 Nex U.L. in LETGS spectra of coolest coronae account for Fexvii blend in Nex via emissivities (20%) neglect Fexix blends in Neix low T X 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 stars Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

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 range Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

22 Ne/O - results II Ne/O ratios of individual stars Ne/O: for ǫeri (0.37), 61 Cyg (0.36), HD (0.36) Ne/O: for β Com (0.25), α Cen B (0.26) Ne/O: for Procyon (0.22), α Cen A (0.21), Altair (0.20) overall good agreement with literature mod. active stars ǫeri (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 / 28

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 logl X /L bol 6.5 Ne/O 0.4 at logl X /L bol 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, L X /L bol vs. T X Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

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 ( ) (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 ( ) absolute abundances of Ne and O: agreement between Sun, B stars and H ii-regions within errors Lodders+ 2009, Ne/O = 0.21 Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

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 evaluated Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

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 considered Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28

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 / 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 Oviii(Lyα) to Ovii(r) line. Star Ovii(r) Oviii Neix(r) Nex O8/O7(r) 61 Cyg 6.6± ± ± ± ±0.13 Altair 4.9± ± ± ± ±0.10 α Cen 33.5± ± ± ± ±0.05 α Cen 03/ ± ± ± ± ±0.06 αcen A (C) 9.2± ± ± ±0.07 αcen B (C) 11.5± ± ± ±0.08 β Com 3.8± ± ± ± ±0.33 ǫ Eri 44.1± ± ± ± ±0.14 ǫ Eri (C) 41.5± ± ± ± ±0.10 HD ± ± ± ± ±0.86 Procyon 35.6± ± ± ± ±0.04 Procyon (C) 29.1± ± ± ± ±0.05 Procyon (C) 30.3± ± ± ±0.06 Blended with Fexvii. Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae / 28