About some traps in fundamental parameter determination of target stars

1. About some traps in fundamental parameter determination of target stars Friedrich Kupka Max-Planck-Institute for Astrophysics Hydrodynamics Group fk@mpa-garching.mpg.de

2. SOME POPULAR TRAPS • hidden use of model physics (circular argument) • neglection of systematic errors: ∆scatter ∆error • hidden systematic errors • silent break down of model physics • usage of calibrations outside their validity range • and for sure many more of them...

3. Hidden use of model physics • The determination of log(g) for Vega • Fundamental value unknown • inspite of this: primary calibration point for synthetic photometry • Moon & Dworetsky 1985:empirically corrected ATLAS6 grids for Strömgren colours • based on a mixture of true fundamental values andfurther calibrations (either log(g) or Teff unknown) • Vega's log(g): Balmer lines, Balmer jump fits from ATLAS6 models calibration point in MD ! • Castelli & Kurucz A&A 281, 817 (1994): values derived in this way depend on unknown He abundance • ATLAS9 based FeI / FeII as "supporting results”

4. Model atmosphere grids I • Current grids: why not trust them ? • ATLAS9 (  BaSeL, etc.), New MARCS, and PHOENIX: based on Kurucz atomic data • none fits Strömgren m0 metallicity index (A-G type stars) • none correctly predicts the Balmer jump for F stars (surface gravity, luminosity, ...) • outdated grids still widely used as black boxes (ATLAS9 C93 distribution)

5. Model atmosphere grids II • Current grids • are as poor in convection modelling as in 1970 • usually lack numerical resolution(computational costs, non-uniform over HRD) • incorrectly predict Balmer line profilesover the HRD for A-G stars (except for calibration star!) • are in disagreement with observational input from mid A stars (temperature gradients, microturbulence)

6. Neglection of systematic errors • Accuracy of Teff of fundamental stars • for many “fundamental stars”visual fluxes have not been measured(=L, M, R known without use of stellar models) • A0 – G2 MS: ~6: < 200 K, ~15: < 400 K  Why ? poor (post-Hipparcos!) parallaxes, spectrophotometry • between8000 K and 9500 K: no pairs with known M • even with these data:  some models excluded • Balmer line profiles for the Sun & other stars • confusing results [cf. Barklem et al. A&A 385, 951 (2002)] • resolved (perhaps ?) by improving convection physics

7. Hidden systematic errors • Spectral line ratios and absolute Teff • Line depth ratios of selected pairs correlate withphotometric temperature indicators (D. Gray) • Use colour index vs. Teff relation  line ratios f(Teff) • Problem: relative scale ! The Teff(line ratio) scaleinherits systematic errors from theTeff(colour) scale • IRFM also not free from systematics (binaries, IR fluxes) • Solar Teff: calibration errors, solar cycle, etc. ∆Teff~10 K. • A&A 411, 559 (2003): ∆Teff(sun)=0, ∆Teff(stars)~5-10 K ?!?

8. Breakdown of model physics • Abundance determinations • oscillator strengths: a few 1000 accurately measured • Wiese's law: the lowerthe accuracy,the more optimistic the error estimate(factor >2 in dex !) • successful “fits“ can be very deceiving (example Li) • 1D LTE/NLTE 3D LTE  3D NLTE: example: the Li determination of extreme Pop. II/Pop. III starsinternal, statisticalaccuracy estimates for abundances can be completely knocked over by (unexpected) systematic errors...

9. Calibration of parameters • Tuning convection in low mass stars • lack of alternatives  adjustments to fit the sun • but: an evolution model which fits the sundoesnot have to be good for anything else • one which does notis even more questionable ! • uncertainty of lower RGB (1Msolar):±100 K • uncertainty of PMS (D-burning phase): ±175 K • both due to convection alone... • different models/parameters in interior and atmosphere increase uncertainty

10. Conclusions I • General comments • a good direct measurement can never be substituted byclever, arbitrarily (?) accurate calibrations • systematic errors  intrinsic ones • Fundamental parameters • ASTRA project (S.J. Adelman, A. Gulliver, B. Smalley)new spectrophotometric fluxes (near UV – near IR), recalibration of stellar flux standards(50 cm robotic telescope, first light in spring 2004) • the long wait for the GAIA mission (too long for COROT)

11. Conclusions II • New model atmosphere grids • require adequate resolution in grid parameters • more cross-checking with fundamental stellar data • a better treatment of convection; diffusion, opacities, ... • taking black boxes from the shelf remains dangerous • Convection • “non-local models” and numerical simulations • solar calibration approach insufficientobservations (including particularly MOST & COROT...)

12. Extra slides and literature I • Figures shown • Smalley B., Kupka F., A&A 328, 349 (1997): Fig. 6 (m0-index)inability of models to match A-stars and the sun simultaneously • same paper: Fig. 5 (c1-index): systematics, “feature” for F-stars • Smalley et al., A&A 395, 601 (2002): Fig. 2 (H-profiles)small mixing length/flux overshooting • Stein R.F., Nordlund Å., ApJ 499, 914 (1998): Fig. 14, 15inhomogeneity of solar surface convection • Nordlund Å., Stein R.F., ASP Conf. Ser. 203, 362 (2000)photospheric levitation (1D / 3D, turbulent pressure) • Tables shown • Smalley et al., A&A 395, 601 (2002): Tables 2 and 5fundamental parameters: error sources; mid A-star problem • Asplund et al., A&A 399, L31 (2003): Table 1(electronic version) the Li problem (3D NLTE – 3D LTE – 1D LTE/NLTE)

13. Extra slides and literature II • Useful literature • Asplund M., Carlsson M., Botnen A.V., A&A 399, L31 (2003) • Barklem P.S. et al., A&A 385, 951 (2002) • Gray D.F., Johanson H.L., PASP 103, 439 (1991) • Moon T.T., Dworetsky M.M., MNRAS 217, 305 (1985) • Kurucz R.L., Astrophys. and Space Sci. Library, Vol. 274, Dordrecht: Kluwer Academic Publishers, ISBN 1-4020-0644-6, 2002, p. 3 – 14(http://kurucz.harvard.edu/papers.html PUERTOVALLARTA:2001) • Nordlund Å., Stein R.F., ASP Conf. Ser. 203, 362 (2000) • Smalley B., MNRAS 265, 1035 (1993) • Smalley B., Kupka F., A&A 328, 349 (1997) • Smalley B., Gardiner R.B., Kupka, F., Bessell M.S., A&A 395, 601 (2002) • Stein R.F., Nordlund Å., ApJ 499, 914 (1998)