Structuring of stellar coronae
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Transcript of Structuring of stellar coronae
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Structuring of stellar coronae
Dip.Scienze Fisiche e Astronomiche - June 23rd 2004
Paola Testa
Supervisor: G. Peres1
Collaborations: J.J. Drake2, E.E. DeLuca2
1 University of Palermo, Italy
2 Harvard-Smithsonian CfA, USA
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Structuring of stellar coronaeStructuring 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
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Structuring of stellar coronaeStructuring of stellar coronae• Spatial structuring:
Hierarchy of Structures – Different Scales
Whole star -- Active regions -- Loops
smallest observed scale (~700Km)
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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
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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, 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
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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 (Lx, Fx, gravity, rotation period, Rossby number)
- Estimate of Coronal Filling Factors
- Comparison with Loop Models Expectations
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Spectroscopy of Stellar Coronae
Density diagnostics (Testa et al., ApJ 2004)
- correlation with Lx, Lx/Lbol
dwarfs
- 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
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Spectroscopy of Stellar Coronae
Surface Filling Factors:
- 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)
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Structuring of stellar coronaeStructuring of stellar coronae
• Optical depth as diagnostics for structuring:
= n l
= (e2/mc) f (M/2kT)1/2(1/)1/2
n = (nH/ne) AZ (nion/nel) ne
~ 1.16·10-14 · f 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: 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)
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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
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(Testa et al. 2004, ApJL)
- Detection of X-ray Resonant Scattering
Optical Depth Analysis
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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 + 0.43 )
~ 1.16·10-14 · f M1/2 (nH/ne) AZ (nion/nel) ne l
(Kastner & Kastner, 1990;
Kaastra & Mewe, 1995)
Optical Depth
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Spectroscopy of Stellar Coronae
Path Length Estimate
l R
l ~ 10 LRTV
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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
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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
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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 ~ 1.7e10cmloop top (~3.5e10cm)
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Models of Coronal Plasma StructuresModels 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
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Structuring of stellar coronaeStructuring of stellar coronaeNeed 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)
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Structuring of stellar coronaeStructuring 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
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Loop ModelsLoop ModelsHydrodynamic Loop Model
• heat pulses at the footpoints
• model: symmetric, with uniform cross-section
• solves equations for density, momentum, energy
constant heatingpulsed heating
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dynamic models of a loop impulsively heated at the footpoints (Testa, Peres & Reale, in prep.)
Loop ModelsLoop ModelsHydrodynamic 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.
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Structuring of stellar coronaeStructuring of stellar coronaeHydrodynamic Loop Model
effective viscosity
P(T) radiative losses function
Spitzer conductivity (Spitzer 1962)
fractional ionization
hydrogen ionization potential
EH=EH (s,t)ad hoc heating function
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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 + 0.43 )
= n l
= (e2/mc) f (M/2kT)1/2(1/)1/2
n = (nH/ne) AZ (nion/nel) ne
~ 1.16·10-14 · f M1/2 (nH/ne) AZ (nion/nel) ne l
(Kastner & Kastner, 1990;
Kaastra & Mewe, 1995)
Optical Depth
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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