1. Detection of X-ray resonant scattering in active stellar coronae Paola Testa 1,2, J.J. Drake 2, G. Peres 1, E.E. DeLuca 2 1 University of Palermo 2 Harvard-Smithsonian CfA Cool Stars 13 th Workshop - Hamburg, July 5 th 2004

2. RATIONALE GENERAL PROBLEM : which is the structure of stellar coronae? DIAGNOSTIC TOOLS to understand STRUCTURING : eclipse and rotational modulation, flares analysis, density measurements, optical depth, etc ANALYSIS of Ly  /Ly  in ACTIVE STARS : detection of resonant scattering ESTIMATE the SIZE of CORONAL STRUCTURES IMPLICATIONS on CHARACTERISTICS of STRUCTURING

3. smallest observed scale (~700Km) Structuring of stellar coronae Solar Corona : Hierarchy of structures Whole star Active regions Loops

4. Structuring of stellar coronae Stellar Coronae : eclipse and rotational modulation, evolution during flares, density measurements, optical depth, etc, all provide information on scale height and location of active regions e.g. Brickhouse et al. (2001) : X-ray Doppler imaging of 44Boo Schmitt & Favata (1999) : flare analysis + eclipse mapping for Algol emitting plasma localized at high latitude, and rather compact (H  0.5 R  ) but also alternative interpretations of rotational broadening in terms of extended loops; e.g. Chung et al. (2004), Redfield et al. (2003)

5. Structuring of stellar coronae Stellar Coronae : how are very active stars structured? needs for larger volumes and/or higher densities can simple hydrostatic loop models ( e.g. RTV - Rosner, Tucker & Vaiana, 1978 ) explain the emission from active stars?

6.  Analysis of Stellar Emission: Ness et al. (2003) analysis of large survey of stellar spectra no clear evidence of resonant scattering from Fe lines Ness et al. (2003)  Study of SOLAR STRUCTURES: Controversial results from the analysis of FeXVII resonance line at ~15.03Å: e.g. Phillips et al. (1996), Schmelz et al. (1997), Saba et al. (1999) discrepancy in the derived direction and magnitude of the center-to-limb trend Structuring of stellar coronae Optical depth as diagnostics for structuring :  ~ 1.16·10 -14 · f M 1/2 (n H /n e ) A Z (n ion /n el ) n e l

7. 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

8. (Testa et al. 2004, ApJL, 609 L79) Analysis of Ly  /Ly  in HETGS- Chandra spectra of active stars Optical Depth Analysis D etection of X-ray resonant scattering

9. Escape probability assumption of homogeneity: both emission and absorption occur over the whole l.o.s. through the corona p(  ) ~ 1 / (1 + 0.43  ) (Kastner & Kastner, 1990; Kaastra & Mewe, 1995) Optical Depth Analysis Path Length Estimate l   R  l  ~ 10 L RTV 0.000182.2 ·10 8 ~ 8Ne X [MEG] 0.00021.6 ·10 8 ~ 6Ne X [HEG] 0.021.7 ·10 10 ~ 8O VIIIIM Peg 0.049.5 ·10 10 ~ 10O VIIIII Peg l  / R  l  (cm) l  / L RTV a IonSource a Loop length from RTV scaling laws L RTV ~ T 3 / [( 1.4 ·10 3 ) 3 p ]

10. Conclusions first spectroscopic estimate of sizes of emitting structures in stellar coronae detection of resonant scattering implies non-uniform spatial distribution of the coronal plasma estimated characteristic lengths  R  most of all for hotter plasma, and ~10 L RTV both for hot and cool plasma results consistent with other findings of compact structures as inferred from several flares and eclipse analyses general scenario of coexisting classes of coronal structures: remarkably compact structures especially at higher temperatures

11. 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  ~ 1.16·10 -14 · f M 1/2 (n H /n e ) A Z (n ion /n el ) n e l