Structuring of stellar coronae

1. Structuring of stellar coronae Paola Testa Supervisor: G. Peres1 Collaborations: J.J. Drake2, E.E. DeLuca2 1University of Palermo, Italy 2 Harvard-Smithsonian CfA, USA Dip.Scienze Fisiche e Astronomiche - June 23rd 2004

2. Structuring of stellar coronae • Spatial structuring • Temperature, Density, EM(T) structuring Comparison with physical models • insights into: • - astrophysical plasma physics • - plasma heating mechanisms • - characteristics of magnetic field • - dynamo processes • - atomic physics

3. Structuring of stellar coronae • Spatial structuring: • Hierarchy of Structures – Different Scales • Whole star -- Active regions -- Loops • smallest observed scale (~700Km)

4. Physics of Coronal Plasma • AIM:UNIFIED SCENARIO of CORONAL PHENOMENA • Coronal Observations (X-ray, EUV) • STELLAR CORONAE : spectral diagnostics • SOLAR CORONA : spatial + spectral information • Comparison with Loop Models • Development of Existing Loop Models • Hydrostatic • Hydrodynamic

5. High Resolution Spectroscopy of Stellar Coronae HETG spectra of a sample of 22 active stars at different activity level, different evolutionary stages • Single Dwarfs: AU Mic, Prox Cen, EV Lac, AB Dor, TW Hya • Single Giants: HD 223460, 31 Com, b Cet, m Vel, Canopus • Active Multiple Systems: ER Vul, 44 Boo, Algol, l And, • TZ CrB, TY Pyx, UX Ari, x UMa, II Peg, HR 1099, • AR Lac, IM Peg

6. High Resolution Spectroscopy of Stellar Coronae • Density diagnostics - He-like triplets (Si, Mg, O) • Dependence on Stellar Parameters (Lx, Fx, gravity, rotation period, Rossby number) • Estimate of Coronal Filling Factors • Comparison with Loop Models Expectations • Optical Depth - Lya/Lyb (Ne, O) • Direct Path Length Estimate

7. - electron density: < 1013 cm-3 from Si XIII (T~10 MK) ~ 1012 cm-3 from Mg XI (T~6-7 MK) ~ 1010 cm-3 from O VII (T~2-3 MK) higher p for higher T - correlation with Lx, Lx/Lbol dwarfs Spectroscopy of Stellar Coronae Density diagnostics(Testa et al., ApJ 2004)

8. -remarkably COMPACT CORONAL STRUCTURES especially for the hotter plasma Mg XI f ~ 10-4 – 10-1 O VII f ~ 10-3 – 1 X-ray surface flux observed in solar AR (Withbroe & Noyes, ARAA, 1977) Spectroscopy of Stellar Coronae Surface Filling Factors:

9. Analysis of Stellar Emission: Ness et al.(2003) • Ness et al. (2003) analysis of large survey of stellar spectra • no clear evidence for resonant scattering from Fe lines • Structuring of stellar coronae • Optical depth as diagnostics for structuring: t ~ 1.16·10-14 · f l M1/2(nH/ne) AZ (nion/nel) ne l t = s n l s = (pe2/mc) f l (M/2kT)1/2(1/p)1/2 n = (nH/ne) AZ (nion/nel) ne • Study of SOLAR STRUCTURES: • Controversial results from the analysis of FeXVII resonance line at ~15.03Å: Phillips et al. (1996), Schmelz et al. (1997), Saba et al. (1999)

10. Patterns of Abundances in active stars: • Audard (2003), Drake (2003), show that Fe is underabundant and Ne, O are overabundant in active stars Effectiveness of diagnostics Optical Depth Analysis • Atomic physics: • Doron & Behar (2002), Gu(2003) show the relevance of radiative recombination, dielectronic recombination and resonance excitation for interpreting the relative strength of FeXVII-FeXX lines • Diagnostics from FeXVII lines:

11. Optical Depth Analysis (Testa et al. 2004, ApJL) - Detection of X-ray Resonant Scattering

12. Spectroscopy of Stellar Coronae Path Length Escape probability (assumption of homogeneity: both emission and absorption occur over the whole l.o.s. through the corona) (Kastner & Kastner, 1990; Kaastra & Mewe, 1995) p(t) ~ 1 / (1 + 0.43 t) Optical Depth t ~ 1.16·10-14 · f l M1/2(nH/ne) AZ (nion/nel) ne l

13. Spectroscopy of Stellar Coronae Path Length Estimate l  R l ~ 10 LRTV

14. Spectroscopy of Stellar Coronae • Summary • - Coexisting Classes of Coronal Structures with different • density, temperature, filling factors • - data suggest dependence of ne and filling factors on parameters of stellar activity • - higher Fx values correspond to higher surface filling factors • - characteristic lengths  R most of all for hotter plasma

15. Solar Coronal Loops Data time series of observations with - TRACE -EUV narrow band imager (171Å, 195Å) high spatial resolution and temporal cadence - CDS/SoHO -EUV spectra detailed information on thermal structure

16. loop base h ~ 1.7e10cm loop top (~3.5e10cm) • Solar Coronal Loops • Main Results • - spatial distribution of plasma very different at different T • -EM(T) along the l.o.s. points to thermal structuring of the plasma along the l.o.s. filamentary structure • - EM(T): • similar at different heights • with ascending portion  T5

17. Models of Coronal Plasma Structures • Loop Models • Hydrostatic • Hydrodynamic • can be used as diagnostic tools for interpreting both solar and stellar data • Direct comparison of ne, T structure inside a single loop for spatially resolved solar observations (e.g. Reale ApJ 2002, Testa et al. ApJ 2002) • Analysis of EM(T) as distribution of loops composing the corona

18. (Sanz-Forcada et al.2002) • Structuring of stellar coronae • Need for new Loop Models • several observed EM(T)~ Ta with a>3/2 typical of hydrostatic loop models (e.g., Rosner, Tucker & Vaiana 1978)with uniform heating and constant cross-section: • e.g. Capella (Dupree et al. 1993, Mewe et al. 2001, Argiroffi et al. 2003); • several RS CVns (e.g. Sanz-Forcada et al. 2001,2002); • giants (e.g. Ayres et al. 1998)

19. Structuring of stellar coronae • ?loop models with EM(T) with slope steeper than 3/2 ? We are exploring • hydrodynamic loops with heating concentrated at the footpoints • hydrostatic models allowing loop expansion in the lower layers

20. pulsed heating constant heating • Loop Models • Hydrodynamic Loop Model • heat pulses at the footpoints • model: symmetric, with uniform cross-section • solves equations for density, momentum, energy

21. dynamic models of a loop impulsively heated at the footpoints (Testa, Peres & Reale, in prep.) • Loop Models • Hydrodynamic Loop Model • heat pulses at the footpoints • model: symmetric, with uniform cross-section • solves equations for density, momentum, energy EM(T) of the Sun (Brosius et al. 1996) and of Capella (Dupree et al. 1996), scaled arbitrarily for clarity.

22. Structuring of stellar coronae Hydrodynamic Loop Model m effective viscosity P(T) radiative losses function kSpitzer conductivity (Spitzer 1962) b fractional ionization chydrogen ionization potential EH=EH (s,t)ad hoc heating function

23. Spectroscopy of Stellar Coronae Path Length Escape probability (assumption of homogeneity: both emission and absorption occur over the whole l.o.s. through the corona) (Kastner & Kastner, 1990; Kaastra & Mewe, 1995) p(t) ~ 1 / (1 + 0.43 t) Optical Depth t = s n l s = (pe2/mc) f l (M/2kT)1/2(1/p)1/2 n = (nH/ne) AZ (nion/nel) ne t ~ 1.16·10-14 · f l M1/2(nH/ne) AZ (nion/nel) ne l

24. Future Work - development of more realistic plasma models, e.g., multi-species models including allowance for species-dependent heating - detailed comparison with observations - modeling of X-ray emitting astrophysical sources other than stellar coronae