Characterisation of stellar granulationand stellar activity

(observational requirements, feasability, expectations)  F. Baudin1, R. Samadi2, M-J Goupil2, T. Appourchaux1,K. Belkacem2, P. Boumier1, E. Michel2

  1 : Institut d'Astrophysique Spatiale, Orsay, France2 : LESIA, Observatoire de Paris, Meudon, France

Expectations:

• constraints on models (of convection)• refinement of empirical laws relating activity to star characteristics… and better understanding of dynamo in stars?

Convection and dynamo are still among the most crucial open questions in stellar (and even solar) physics

Granulation (convection at the surface)

Granulation spectrum = function of:• dL/L (border/center of the granule) (= temperature)• eddie size at the surface dgranul

• overturn time of the eddies at the surface• (+ star radius)

Activity (convection at the base of the CZ)

Activity spectrum = function of:• Rossby number Ro = Prot /bcz (Prot rotation period and bcz overturn time of eddies where dynamo occurs (base of CZ)• activity (variability) time scale

Empirical law relating Ro to the observed flux inCaII H & K [Noyes et al, 1984, ApJ]

Activity (convection at the base of the CZ)

• Prot : hopefully from observations…

• bcz : from models,but…« variable » definition:where exactly at the base of CZ bcz = Hp/w or Hp/w or Hp/2w[see the poster of L. Mendez et al]

Activity (convection at the base of the CZ)

Remark: variability observed in visible light = spots variability in CaII H & K = faculaes

If Vis different from CaHK… Information on the magnetic field manifestation(ratio spots/faculae)

Activity (time scale)

Activity time scale with COROT (visible light) :spots lifetime combined with rotation period(solar case not so simple; instrumental low frequency noise)

No real law, even empirical, to estimate the activity time scale

exploratory approach based on many stars andcomparison to their rotation period

Which star to look at? (detection)

42

1/2

Which stars to look at? (granulation)

Which stars to look at? (granulation)

Which stars to look at? (granulation)

Which stars to look at? (granulation)

A sun at m=6 ?

Strong optimism required

Which stars to look at? (granulation)

M = 1.5 MO at m=6 ?

OK until m=8

Which star to look at? (detection)

42

1/2

Which stars to look at? (activity)

Which stars to look at? (activity)

Which stars to look at? (activity)

Which stars to look at? (activity)

Sun at m=11?

….yes?

Which stars to look at? (activity)

Young M = 1.3 MO star at m=13 ?

Yes!

Constraints on surface convection time scale Refined models of convection

Amplitude of variability versus Rossby number (empirical)+ exploratory approach of variability time scaleClues to understand better stellar dynamo+ constraints on models? ()

Conclusion: objectives

Modelling the granulation characteristics (continue)

Future work :

doing the same with 3D simulations

of Stein & Nordlund :

• Cartesian geometry

• Navier Stockes Eq.

• Realistic LTE radiatif transfer

• Opacities binned over 4 color bands

3 M

m

Need for CZ!? ( M < 2MO)

Activity:Even faint stars (even m=13, from exo channel)Young stars, fast rotators

Granulation:m < 8Massive ( > 1.5 MO) starsImpossible in exo (photon noise + temporal sampling)

Need for a precise correction of very lowfrequency instrumental noise!!

Good to have ground observations to haveCa H & K measurements (Mt Wilson index)

Conclusion: requirements