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

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

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

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

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 , 31 Com,  Cet,  Vel, Canopus Active Multiple Systems: ER Vul, 44 Boo, Algol, And, TZ CrB, TY Pyx, UX Ari,  UMa, II Peg, HR 1099, AR Lac, IM Peg

High Resolution Spectroscopy of Stellar Coronae Optical Depth - Ly  /Ly  (Ne, O) - Direct Path Length Estimate Density diagnostics - He-like triplets (Si, Mg, O) - Dependence on Stellar Parameters (L x, F x, gravity, rotation period, Rossby number) - Estimate of Coronal Filling Factors - Comparison with Loop Models Expectations

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

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

Structuring of stellar coronae Structuring of stellar coronae Optical depth as diagnostics for structuring :  =  n l  = (  e 2 /mc) f (M/2kT) 1/2 (1/  ) 1/2 n = (n H /n e ) A Z (n ion /n el ) n e  ~ 1.16· · f M 1/2 (n H /n e ) A Z (n ion /n el ) n e l 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) Analysis of Stellar Emission: Ness et al. (2003) analysis of large survey of stellar spectra no clear evidence for resonant scattering from Fe lines Ness et al. (2003)

Effectiveness of diagnostics - Patterns of Abundances in active stars: Audard (2003), Drake (2003), show that Fe is underabundant and Ne, O are overabundant in active stars Diagnostics from FeXVII lines: - 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 Optical Depth Analysis

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

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) p(t) ~ 1 / (  )  ~ 1.16· · f M 1/2 (n H /n e ) A Z (n ion /n el ) n e l (Kastner & Kastner, 1990; Kaastra & Mewe, 1995) Optical Depth

Spectroscopy of Stellar Coronae Path Length Estimate l  R  l ~ 10 L RTV

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

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

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  T  loop baseh ~ 1.7e10cm loop top (~3.5e10cm)

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 n e, 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

Structuring of stellar coronae Need for new Loop Models several observed EM(T)~ T  with  >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) (Sanz-Forcada et al.2002)

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

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

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.

Structuring of stellar coronae Hydrodynamic Loop Model  effective viscosity P(T) radiative losses function  Spitzer conductivity (Spitzer 1962)  fractional ionization  hydrogen ionization potential E H =E H (s,t)  ad hoc heating function

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) p(t) ~ 1 / (  )  =  n l  = (  e 2 /mc) f (M/2kT) 1/2 (1/  ) 1/2 n = (n H /n e ) A Z (n ion /n el ) n e  ~ 1.16· · f M 1/2 (n H /n e ) A Z (n ion /n el ) n e l (Kastner & Kastner, 1990; Kaastra & Mewe, 1995) Optical Depth

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