Talk presented by Robrade, Jan

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

2. Overview
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 26.07.2011 3 / 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+ 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. 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. 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. 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. 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 unchanged
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 8 / 28

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 changed
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 9 / 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 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 modeling
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 10 / 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: 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. 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 trend
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 12 / 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 ˚
(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 interdependence
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 13 / 28

14. Ne/O - X-ray CCD spectra
NeX
OVII
OVIII
NeIX
MOS CCD spectra of Eri (black) and Procyon (red) with line features labeled
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 14 / 28

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. 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 band
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 16 / 28

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 activity
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 17 / 28

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 layer
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 18 / 28

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 corona
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 19 / 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 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 stars
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 20 / 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 26.07.2011 21 / 28

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. 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. TX
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 23 / 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 (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 = 0.21
Jan Robrade (Hamburger Sternwarte) Ne/O in stellar coronae 26.07.2011 24 / 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 26.07.2011 25 / 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 26.07.2011 26 / 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 26.07.2011 27 / 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