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

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

Structuring of stellar coronaeStructuring 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 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

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

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

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)

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)

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 + 0.43 )

~ 1.16·10-14 · f M1/2 (nH/ne) AZ (nion/nel) ne l

(Kastner & Kastner, 1990;

Kaastra & Mewe, 1995)

Optical Depth

Spectroscopy of Stellar Coronae

Path Length Estimate

l R

l ~ 10 LRTV

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

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

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)

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

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)

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

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

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.

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

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

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