Detection of X-ray resonant scattering in active stellar coronae

1. Detection of X-ray resonant scattering in active stellar coronae Paola Testa1,2, J.J. Drake2, G. Peres1, E.E. DeLuca2 1University of Palermo 2 Harvard-Smithsonian CfA Cool Stars 13th Workshop - Hamburg, July 5th 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 Lya/Lyb in ACTIVE STARS: • detection of resonant scattering • ESTIMATE the SIZE of CORONAL STRUCTURES • IMPLICATIONS on CHARACTERISTICS of STRUCTURING

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

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. t ~ 1.16·10-14 · f l M1/2(nH/ne) AZ (nion/nel) ne l • 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 • Analysis of Stellar Emission: Ness et al.(2003) • Ness et al. (2003) analysis of large survey of stellar spectra • no clear evidence of resonant scattering from Fe lines • Structuring of stellar coronae • Optical depth as diagnostics for structuring:

7. 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:

8. Detection of X-ray • resonant scattering Optical Depth Analysis (Testa et al. 2004, ApJL, 609 L79) Analysis of Lya/Lyb in HETGS-Chandra spectra of active stars

9. Escape probability assumption of homogeneity: both emission and absorption occur over the whole l.o.s. through the corona Source Ion lt/LRTVa lt (cm) lt / R p(t) ~ 1 / (1 + 0.43 t) (Kastner & Kastner, 1990; Kaastra & Mewe, 1995) II Peg O VIII ~ 10 9.5 ·1010 0.04 IM Peg O VIII ~ 8 1.7 ·1010 0.02 Ne X [HEG] ~ 6 1.6 ·108 0.0002 Ne X [MEG] ~ 8 2.2 ·108 0.00018 aLoop length from RTV scaling lawsLRTV ~ T 3/[(1.4 ·103)3 p] lt R lt~ 10 LRTV Optical Depth Analysis Path Length Estimate

10. Conclusions first spectroscopic estimate of sizes of emitting structures in stellar coronae detection of resonant scattering implies non-uniformspatial distribution of the coronal plasma estimated characteristic lengths Rmost of all for hotter plasma, and ~10 LRTVboth 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. t ~ 1.16·10-14 · f l M1/2(nH/ne) AZ (nion/nel) ne l • Structuring of stellar coronae • Optical depth as diagnostics for structuring: t = s n l s = (pe2/mc) f l (M/2kT)1/2(1/p)1/2 n = (nH/ne) AZ (nion/nel) ne