Transcript SRON - Personal profiles

HRXS - Utrecht
Neon and oxygen in low activity stars
Towards a coronal unification with the Sun
Jan Robrade and J.H.M.M. Schmitt
Hamburger Sternwarte, Universität Hamburg, e-mail: [email protected]
Abstract
We use XMM-Newton (RGS) and Chandra (LETGS) high-resolution X-ray spectra from a sample of weakly and moderately active stars to determine
their coronal Ne/O abundance ratio and to investigate an enhanced neon abundance as solution of the solar modeling problem;
a problem that arises from the disagreement between revised solar abundances and helioseismology.
We apply linear combinations of strong emission lines as well as a global-fitting method for each dataset and find a correlation between the Ne/O ratio
and stellar activity in the sense that stars with a higher activity level show a higher Ne/O ratio. The Ne/O abundance ratio decreases
from ~0.4 for more active stars down to ~0.2 for low activity stars, a ratio that is similar to 'classical' solar values.
We find no indications of a peculiar position of the Sun; the solar coronal Ne/O abundance ratio appears to be rather typical of low activity stars.
The sample: nearby, magnetically active stars, ~ 1 Ms data
A short history of Ne/O - ratios (Sun = corona + wind/photosphere)
61 Cyg (K5+K7), α Cen (G2+K1), β Com (G0), ε Eri (K2), Altair (A7),
HD 81809 (G2+G9), Procyon (F5)
0.21 (0.16-0.31)
0.18 (0.15-0.22)
0.18 (0.14-0.22)
0.17 (0.12-0.22)
0.21 (0.19-0.23)
0.17 (0.14-0.22)
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X-ray emission at log LX/Lbol = -5 ... -7
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broad range of eff. temperatures (spectral types A7 – K7)
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coronae dominated by rather cool plasma (< 5 MK)
We find:
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Ne/O ratio depends on stellar activity → chemical fractionation
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low coronal Ne/O ratio (Ne/O = 0.20 ± 0.05) in weakly active stars
●
Sun is not atypical !!!
–
–
–
–
–
–
solar corona (Acton et al. 1975)
Sun (Grevesse & Sauval 1998)
solar corona (Schmelz et al. 2005)
solar transition region (Young 2005)
OB stars (Przybilla et al. 2008)
Sun (Asplund et al. 2009)
Absolute oxygen abundance (H=1, linear scale, in 10-4):
8.51 (AnGr89), 6.74 (GrSa98), 4.57 (Asp05), 4.90 (Asp09)
Neon and oxygen in low activity stars
The solar abundance / modeling problem:
revised abundances lower by 30% (Asplund et al. 2005)
● opacity missing - wrong interior structure & helioseismology
● neon not directly measured (no photospheric lines)
● use suitable reference element (O) and relative abundance (Ne/O)
● coronal and transition region measurements of Ne/O ratio
● significantly higher Ne abundance (Ne/O=0.41) claimed (Drake & Testa, 2005)
● solves the problem, BUT: mainly active stars used (IFIP effect etc.)
● objections from solar observers (Ne/O is low!!!)
●
Slightly higher solar
metallicity in
Asplund et al. (2009):
discrepancy alleviated
but still significant.
Solar modeling problem
remains....
Ne/O abundance ratio of low to moderately active stars with spectral types
late-A to mid-K as well as the 'classical' Sun (red box). The coronal Ne/O
ratio decreases towards low activity stars. Data points/regression curves
from global fitting (diamonds/solid line) and two emission line ratios (Drake &
Testa: asterisks/dotted line, Liefke & Schmitt: squares/dashed line).
LETGS spectrum of Eps Eri with used X-ray lines labeled, the
H-like Lyα and He-like resonance line of oxygen and neon.
Abundance ratios from emission lines:
● construct temperature independent emission line ratios - direct measurement of relative abundances
● use strong lines, preferable without blends and with well determined atomic data
Jan Robrade
1975: O VIII vs. Ne IX (Acton, Catura, Joki)
2005: O VIII vs. Ne IX + 0.15 Ne X (Drake & Testa (2005), D&T) --- energy flux weighting
2006: 0.67 O VIII – 0.17 O VII vs Ne IX + 0.02 Ne X (Liefke & Schmitt (2006), L&S) --- photon flux weighting
:: residuals remain due to limited line/ion coverage
:: caveat: especially poor for low activity stars with cool coronae (e.g. quiescent Sun)
Publications: J. Robrade, J.H.M.M. Schmitt, F. Favata, A&A, 486, 995, 2008; J. Robrade, J.H.M.M. Schmitt, A&A, 497, 511, 2009