Looking for nano-flares and stellar micro-variability as

mechanisms for coronal heating

Isabella Pagano1

I. Busà1, G. Cutispoto1, A.F. Lanza1, A. Lanzafame2, G. Leto1, S.Messina1, I. Ribas3, M. Rodonò2, and R. Ventura1

1)INAF, Catania Astrophysical Observatory, Italy2)INAF & Department of Physics and Astrophysics of Catania University, Italy3)University of Barcelona

Berlin, 12 Dec 2003 COROT Week #5 - I.Pagano

What heat stellar coronae?

q Does the energy input involve microflaring, and is therefore sporadic, or does it involve MHD waves,electric current dissipation, and/or other effects which could be quasisteady?

q That magnetic energy release occurring in discretemuch smaller events than observed, i.e. involving events of 1024 to 1025 ergs (nanoflares), does sustainthe coronal heating has been proposed and widely accepted by several authors (cf. Haisch, Strong & Rodonò1991, ARAA 29, 275; Parker et al. 1988, ApJ 330, 474, andreferences therein).

Berlin, 12 Dec 2003 COROT Week #5 - I.Pagano

Flares and Coronal Heating for the Sun

Here is α>2, Lf is dominated by the large events and flares cannot account for coronal heating.

However if the statistics of micro-nano flares is such that α>2, than Lf is dominated by small events.

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Berlin, 12 Dec 2003 COROT Week #5 - I.Pagano

What we know today

qO n t h e S u n α i s 1 . 8 f r o m f l a r e e n e r g y d i s t r i b u t i o n d o w n t o t h e t h r e s h o l d l i m i t o f 1 0 2 7 e r g s .

qH o w e v e r , t h e r e a r e s o m e s t u d i e s s u g g e s t i n g t h a t α i s g r e a t e r t h a n 2 f o r v e r y l o w e n e r g y f l a r e s b o t h f o r t h e S u n a n d o t h e r a c t i v e s t a r s ( P o r t e r e t a l . 1 9 9 5 , A p J 4 3 8 , 4 7 2 ) .

qT h i s m e a n s t h a t m i c r o /n a n o - f l a r e s a r e s t i l l a v i a b l e h e a t i n g m e c h a n i s m c a n d i d a t e .

Berlin, 12 Dec 2003 COROT Week #5 - I.Pagano

Other stars: micro/nano flare statistics

We actual ly do not know the f lare distr ibution for low energy events because of threshold l imits.

I f one assumes that the same power law distr ibut ion known for f lares holds for micro and nano flares, the coronal heat ing cannot be susta ined by these phenomena.

EV Lac f lare cumulat ive distribution (Leto et al. 1997, A&A 327, 1114)

Berlin, 12 Dec 2003 COROT Week #5 - I.Pagano

Hints f rom UV l ight curve

from Robinson et al 2001 ApJ 554, 368

For AU Mic (M1 V), observed by HST wi th HST/STIS, low energy f lares have been detected and the power law of the distr ibution is actual ly steeper than for “normal” f lares.

α =2.2 and 2.8 for cont inuum and C IV l ine, respect ively!!

Berlin, 12 Dec 2003 COROT Week #5 - I.Pagano

Time scales and amplitudes of microvariabil ity

No enough stat ist ics on such phenomena . High temporal resolut ionand continuous observat ions can help to constrain the flare physics and the mechanisms of coronal heating.

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Are pre-flare dipsphotospheric clues for coronal flare first

phases?

Flare stars show periodic variabil ity(few minutes t ime-scales) with amplitude of the order of 10 -2-10 - 3 mag lasting typically 15-60 min.

Are these f ingerprints of oscil lations in coronal loops?

Berlin, 12 Dec 2003 COROT Week #5 - I.Pagano

Required Observations

q T h e m o n i t o r i n g o f a f e w d K e / d M e s t a r s i n t h e s e i s m o l o g y c h a n n e l i s n e e d e d t o p u r s u e o u r o b j e c t i v e s .

q I n f a c t t h e s t e l l a r n a n o f l a r e s a n d m i c r o - v a r i a b i l i t y c a n b e i n v e s t i g a t e d o n l y w i t h t h e m i n i m u m i n t e g r a t i o n t i m e ( 1 s e c ) o f f e r e d b y C O R O T .

q O u r p r o g r a m c a n b e c a r r i e d o u t d u r i n g s h o r t r u n s ( S R ) .

q T h e f r e q u e n c y o f f l a r e w i t h e n e r g y h i g h e r t h a n 1 0 2 9

e r g s i s t y p i c a l l y o f 0 . 2 f l a r e s / h o u r s . W e d o n o t k n o w a n y t h i n g a b o u t f l a r e f r e q u e n c y f o r n a n o f l a r e s.

q W e e s t i m a t e t h a t a n o b s e r v a t i o n r u n o f 1 w e e k- t e n d a y s d u r a t i o n i s a p p r o p r i a t e f o r o u r i n v e s t i g a t i o n .

Berlin, 12 Dec 2003 COROT Week #5 - I.Pagano

Flare stars in the COROT field

qA m o n g t h e k n o w n f l a r e s t a r s w e h a v e s e l e c t e d t w o o f t h e m w h i c h a r e b r i g h t e r t h a n V = 9 . 5 :

qG J 7 5 2 A ( d M 3 . 5 e , V = 9 . 1 , R A = 1 9 1 6 5 5 . 2 6 , D E C = + 0 5 1 0 0 8 . 1 i n F K 5 )

qP Z M o n ( d K 2 e , V = 9 . 0 , R A = 0 6 4 8 2 1 . 0 7 , D E C = + 0 1 1 3 0 8 . 3 i n F K 5 ) .

qT h e l a t t e r i s v e r y c l o s e t o t h e f i e l d o f t h e p r i m a r y t a r g e t s : H D 4 9 9 3 3 , H D 4 9 9 3 4 .

qW e a r e o p e n t o e v a l u a t e o t h e r p o s s i b l e t a r g e t s .

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Berlin, 12 Dec 2003 COROT Week #5 - I.Pagano

Relation between f lare-sitesand spots

COROT data on f lare stars can al low us to c leary understand the spat ia l/temporal re lat ion betweenphotospher ic spots and f lares.

It is diff iculty to col lect good monitor ing f lare data and photometry contemporary in a t ime comparable to the act ive region l ifetime.

In fact , apart f rom the ground based study reported by Leto et al. (1997 A&A 327, 1114) for EV Lac, only HST FES data could be used to th is end (Benedict et al. 1998 AJ 116, 429).