neon and oxygen in stellar coronae - a unification with the sun

Neon and oxygen in stellar coronae

A unification with the Sun

Jan Robrade

Hamburger Sternwarte

From Atoms to Stars, July 2011, Oxford

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

Why care?

The chemical composition of the Sun is one of the most importantyardsticks in astronomy with implications for basically all fieldsfrom 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

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 modellinghigh quality atomic line data, includes non-LTE calculationsreduced abundances of C, N, O by 30 – 40%better agreement e.g. with ISM measurementsbut:significant disagreement with helioseismologymissing opacity

Way out needed!


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 linessolar wind/energetic particlesoxygen as reference element; determine ANe/AO (Ne/O)

AGS05: Ne/O = 0.15 quite low (same as AnGr89)


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 regionsNe/O = 0.18 (Schmelz et al. 2005)

bottom: transition region - quiet sun

Ne/O = 0.17 (Young 2005)


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!


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


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


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), ǫ 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


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. Ne ix (Acton, Catura, Joki, 1975)

Used lines:

Ovii r (21.6 A), Oviii Lyα (18.97 A), Ne ix r (13.45 A), Nex Lyα (12.13 A)

Oviii vs. Ne ix + 0.15 Nex – energy flux weighting (Drake & Testa, 2005)

0.67 Oviii - 0.17 Ovii vs. Ne ix + 0.02 Nex - photon f. w. (Liefke & Schmitt, 2006)


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


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 A) - ’cooler’ Nevii + viii

derive X-ray luminosities, coronal temperatures, Ne/O ratios

absolute abundances more uncertain - EM interdependence


Ne/O - X-ray CCD spectra

OVII

NeX

OVIII

NeIX

MOS CCD spectra of ǫ Eri (black) and Procyon (red) with line features labeled


Ne/O - X-ray grating spectra

High resolution X-ray spectrafrom XMM-Newton andChandra

good data quality obtainedNe ix line most crucial

Spectra of ǫ Eri (LETGS) and 61 Cyg (RGS, co-added)


X-ray data & coronal properties

Star Mission Obs.(No Exp.) log LX Tav. LX/Lbol

(ks) (erg/s) (MK) log

ǫEri (K2) XMM 13 (1) 28.2 3.8 -4.9HD81809 (G2+G9) XMM 72 (7) 28.7 4.0 -5.661Cyg (K5+K7) XMM 103 (11) 27.3 3.2 -5.6, -5.5β 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)αCen (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


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


Results - Altair

XMM-observation of Altair (Robrade & Schmitt, 2009)

A7 star, Teff ≈ 7800 K, M = 1.8M⊙, 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 plasmaminor activity, rotational modulation, long term stablesolar-like abundances and FIP effect

’classical interpretation’: thin outer convective layer


Results - Altair II

CHARA (Monnier et al. 2007)

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:

6900K (equator) up to 8500K (poles)

Ovii f /i-ratio high:tracer of density and UV-field

surface features at Teff . 7400 K

f /i = R0/(1 + φ/φc + ne/nc)

(e.g. Gabriel & Jordan 1969, Porquet+ 2001)

=⇒ Equatorial bulge corona


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 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 starsdiscrepancies for the coolest stars


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


Ne/O - results II

Ne/O ratios of individual stars

Ne/O: 0.35 – 0.40 for ǫEri (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

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)


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 methodtrend 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-curvedependence on sp. type, LX/Lbol vs. TX


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


Caveats & open Questions

Measured and predicted sound speed

(Asplund+ 2009)

Discrepancy to helioseismologyalleviated but still significant!

Solar problemssound speed profile wrongconvection zone depth too shallowinterior He abundance too low

Possible solutionsrevise abundance calculationsrevise opacity calculations for solar interiorrevise diffusion model - interior is more metal richinternal gravity waves - promising, but only qualitatively evaluated


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 parameterelements: charge, mass

stars: gravity, temperature gradients, radiation, electric & magnetic fields

details of abundance trends need to be refined

other elements need to be considered


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!


Ne/O - line fluxes

Measured line fluxes in 10−5 photons cm−2 s−1 from RGS or LETGS (C) and energyflux ratio of the Oviii(Lyα) to Ovii(r) line.

Star Ovii(r) Oviii Ne ix(r) Nex∗ O8/O7(r)

61 Cyg 6.6±0.4 9.1±0.5 2.0±0.3 1.6±0.2 1.57±0.13Altair 4.9±0.4 3.7±0.3 0.4±0.1 0.2±0.1 0.86±0.10αCen 33.5±1.3 27.0±1.2 4.1±0.5 1.6±0.2 0.92±0.05αCen 03/04 47.9±2.1 45.4±1.5 5.5±0.8 2.4±0.5 1.08±0.06αCen A (C) 9.2±1.0 3.2±0.5 0.5±0.3 . 0.2 0.40±0.07αCen B (C) 11.5±1.0 6.3±0.6 1.3±0.4 . 0.2 0.62±0.08β Com 3.8±0.6 5.9±0.6 0.9±0.3 0.6±0.3 1.77±0.33ǫEri 44.1±2.6 78.0±3.0 21.2±2.4 10.5±1.4 2.01±0.14ǫEri (C) 41.5±1.6 78.9±1.7 18.8±0.9 16.7±0.9 2.16±0.10HD 81809 1.0±0.3 2.3±0.3 0.5±0.2 0.5±0.2 2.62±0.86Procyon 35.6±1.1 22.9±0.9 1.8±0.3 0.5±0.2 0.73±0.04Procyon (C) 29.1±1.6 17.3±1.0 2.5±0.5 0.7±0.3 0.68±0.05Procyon (C) 30.3±1.7 18.9±1.1 1.4±0.3 . 0.2 0.71±0.06

∗ Blended with Fexvii.


neon and oxygen in stellar coronae - a unification with the sun

Education

neo x

neo ratiosabsolute abundances

robrade schmitt

dwarfs robrade

stellar coronae26

low activity stars robrade

neo spectral modeling

active stars hr