Download observational requirements, feasability, expectations

Characterisation of stellar granulation
and 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, 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 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 /tbcz (Prot rotation period and
tbcz 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)
• Prot : hopefully from observations…
• tbcz : from models,but…
« variable » definition:
where exactly at the base of CZ
tbcz = Hp/w or aHp/w or aHp/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 sVis different from sCaHK…
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)
4s2t
1/2pt
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)
4s2t
1/2pt
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!
Conclusion: objectives
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? (a)
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
3
M
m
• Opacities binned over 4 color bands
m
M
6
Conclusion: requirements
Need for CZ!? ( M < 2MO)
Activity:
Even faint stars (even m=13, from exo channel)
Young stars, fast rotators
Granulation:
m<8
Massive ( > 1.5 MO) 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)