Characterisation of stellar granulation and stellar activity (observational requirements, feasability, expectations) F. Baudin 1, R. Samadi 2, M-J Goupil 2, T. Appourchaux 1, K. Belkacem 2, P. Boumier 1, E. Michel 2 1 : Institut d'Astrophysique Spatiale, Orsay, France 2 : 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 d granul 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 = P rot /  bcz (P rot 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 in CaII H & K [Noyes et al, 1984, ApJ]

Activity (convection at the base of the CZ) P rot : hopefully from observations…  bcz : from models,but… « variable » definition: where exactly at the base of CZ  bcz = H p /w or  H p /w or  H p /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 and comparison to their rotation period

Which star to look at? (detection) 4242 1/2 

Which stars to look at? (granulation)

A sun at m=6 ? Strong optimism required

Which stars to look at? (granulation) M = 1.5 M O at m=6 ? OK until m=8

Which star to look at? (detection) 4242 1/2 

Which stars to look at? (activity)

Sun at m=11? ….yes?

Which stars to look at? (activity) Young M = 1.3 M O 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 scale  Clues 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

Need for CZ!? ( M < 2M O ) Activity: Even faint stars (even m=13, from exo channel) Young stars, fast rotators Granulation: m < 8 Massive ( > 1.5 M O ) stars Impossible in exo (photon noise + temporal sampling) Need for a precise correction of very low frequency instrumental noise!! Good to have ground observations to have Ca H & K measurements (Mt Wilson index) Conclusion: requirements