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Detection of X-ray resonant scattering in active stellar coronae

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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. RATIONALE GENERAL PROBLEM :

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slide1
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

slide2
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
slide3
Structuring of stellar coronae

Solar Corona: Hierarchy of structures

Whole star Active regions Loops

  • smallest observed scale (~700Km)
slide4
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)
slide5
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?
slide6
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:
slide7
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:
slide8
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

slide9
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

slide10
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

slide11
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

ad