Characterisation of stellar granulation

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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
2

• 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

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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)

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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
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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

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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)
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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

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Which star to look at? (detection)
4s2t
1/2pt
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Which stars to look at? (granulation)
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Which stars to look at? (granulation)
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Which stars to look at? (granulation)
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Which stars to look at? (granulation)
A sun at m6 ?
Strong optimism required
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Which stars to look at? (granulation)
M 1.5 MO at m6 ?
OK until m8
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Which star to look at? (detection)
4s2t
1/2pt
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Which stars to look at? (activity)
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Which stars to look at? (activity)
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Which stars to look at? (activity)
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Which stars to look at? (activity)
Sun at m11?
.yes?
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Which stars to look at? (activity)
Young M 1.3 MO star at m13 ?
Yes!
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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)

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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

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Conclusion requirements
Need for CZ!? ( M lt 2MO) Activity Even faint
stars (even m13, from exo channel) Young stars,
fast rotators Granulation m lt 8 Massive ( gt 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)