Rota1on!and!convec1on!in!cool!MS!stars! ! On!the!MS:! ! - PowerPoint PPT Presentation

! What!do!numerical!simula1ons!tell!us! about!solar/stellar!dynamos?! ! ! Laurène!Jouve! IRAP>Toulouse>France! ! Cool$Stars$2016.Uppsala$

Rota1on!and!convec1on!in!cool!MS!stars! ! On!the!MS:! ! Fully!convec1ve!stars!below!M " !<!0.35! ! Convec1ve!envelope!for!0.35!<!!M " !<!1.4! ZAMS! Reiners!(2008),!Siess!et!al.!(2000)! ! Rela1vely!fast!rota1on! !!!!!!but!large!spread! McQuillan!et!al.!(2014)!

Magne1c!fields!in!cool!MS!stars! Morin,!Dona1!et!al.!(2008>2010)! Dona1!(2011)! ! Mostly!mul1polar!for!M " !>!0.35! ! Mostly!dipolar!for!M " !<!0.35! ! Bistability!for!M " !<!0.2! ! Field!strength!increases!with!rota1on! ! More!and!more!toroidal!with!rota1on! Pe1t!et!al.!2008,!S.!Marsden’s!talk!

Observa1ons!of!magne1c!cycles?! Chromospheric!ac1vity!(Mount! Wilson!data,!Ca!II!HK!lines): Noyes!et!al.!1984! P cyc =(1/Ro) 1.28+/>0.48! ! where!the!Rossby!number! Ro=P rot / τ ! " Dona1!et!al!2008,!Fares!et!al!2009,!Mengel!et!al!2016:! τ !boo:!2!years! Pe1t!et!al!2009,!Morgenthaler!et!al!2011:!HD!190771!(complex!variability)! Garcia!et!al!2010:!HD!49933:!120!days?! Boro>Saika!et!al!2016:!HD!201091:!14!years!

Main!physical!ingredients!to!the ! genera1on!and!transport!of!magne1c!fields ! Dynamo mechanism: process through which motions of a conducting fluid can permanently regenerate and maintain a magnetic field against its ohmic dissipation It consists of the regeneration of both poloidal and toroidal fields Sources!of!magne1c!field! Transport!of!magne1c!field! Poloidal Toroidal Toroidal Poloidal # !Large>scale!flows!! (meridional!! circula1on)!! # α effect # !Downward!pumping!by!! penetra1ve!convec1on!! # Ω effect # !Transport!from!the!base!of!the!! convec1on!zone!to!the!surface! (magne1c!buoyancy)!! # Babcock-Leighton effect

Schema1c!theore1cal!view!! of!the!magne1c!cycle!in!solar>like!stars 4:!Parker!instability! 1:!magne1c!field!genera1on,!self>induc1on! 5:!emergence+rota1on! 2:!pumping!of!mag.!field!! 6:!recycling!through! α - effect!or! or!! 7:!emergence!of!twisted!bipolar! 2’:!transport!by!meridional!flow! structures!at!the!surface! 3:!stretching!of!field!lines!through! Ω >effect!! !

Solar>like!stars:!role!of!the!the!tachocline?! Browning!et!al.!(2006)! ! Organize!field!at!large!scales! Ghizaru!et!al.!(2010)! ! From these concentrations of toroidal field, buoyant loops can emerge " Nelson!et!al.!(2014)! ! They do not rise to the surface to create well defined spots yet, this has to be modeled independently "

Solar>like!stars:!buoyant!loops!rise! ! Toroidal!flux!tube!introduced!at!the!base!of!the!convec1on!zone! ! Influence!of!the!Coriolis!force!and!! convec1on!introduce!asymetries!and!! modula1on!in!longitude! ! ! Jouve!et!al.!2013! ! ! ! ! Bipolar!magne1c!regions!emerge,!with!proper1es!close!to!the!observed!ones! 8!

Solar>like!stars:!magne1c!cycles,!2D!models! • !Mean>field!induc1on!equa1on!only! • !Babcock>Leighton!dynamo!model! • !2!coupled!PDEs! Standard!model:!! single>celled!! meridional!! circula1on! Is!this!solar!model! applicable!for! rapidly>rota1ng! Cyclic!field! solar>like!stars?! Bumerfly!diagram!close!to!observa1ons!

Solar>like!stars:!prescrip1ons!from!3D!models! Dikpa1!et!al.!2001!assumed!Vp!~ Ω" Charbonneau!&!Saar!2001! assumed!Vp!α! Ω !or!log( Ω )! Scaling!of!MC!deduced!from! Brown!et!al.!2008:!Vp!α! Ω >0.9! ΔΩ " increases!with! Ω !!

Solar>like!stars:!applying!solar!models!to!other!stars! 0.5! Ω sol! !!!!Stronger!Btor! compared!to!Bpol! 1me! 5! Ω sol! Pcyc!=!20!yr! Slower!cycle!when!! !!!!! Ω !increased! 1me! Jouve,!Brown,!Brun!2010!

Solar>like!stars:!applying!solar!models!to!other!stars! Can!we!reconcile!this!model!with!stellar!! !!!!!!data!using!a!more!complex!MC?! !Mul1cell!! meridional! !!!!flow! Jouve,!Brown,!Brun!2010! 5! Ω sol,!Pcyc!=!5.2!yr,!bemer!agreement! 1me!

Models!of!fully!convec1ve!stars! ! Weakly!stra1fied!(Dobler!2005)! ! More!strongly!stra1fied!(Browning!2008)! Mul1polar! Dipolar! ! Systema1c!study!(Gas1ne!et!al.!2012)! ! Most!recent!(Yadav!et!al.!2015)! Aspect!ra1o,! density!contrast! and!influence!of! Small!and!large!! rota1on!(Rossby! !!scales!coexist! number)!varied! !

Fully!convec1ve!stars:!Rossby!and!bistability! ! Change!in!Rossby! !!!!!(also!seen!in!planetary!dynamos)! ! Christensen!&!Aubert!(2006)! Ordering!role!of!Coriolis=dipolar! ! Iner1a!becomes!dominant=mul1polar! ! ! ! ! Two!regimes!for!low!Rossby!numbers! !!!!!(also!seen!in!planetary!dynamos)! Schrinner!et!al.!(2012)! ! Strong!ini1al!field=no!shear! ! (no!role!of!shear!in!dynamo)! Weak!ini1al!field=shear! ! (shear!plays!a!role:!Parker!waves?)! ! ! ! Strong!stra1fica1on!leads!to!! !!!!!mul1polar!fields!

Fully!convec1ve!stars:!comparison!to! observa1ons! Yadav!et!al.!(2015)! Gas1ne!et!al.!(2013)! ! Dipolarity!lost!for!Ro!>!0.1! ! Bistability!for!low!Ro! ! Variability!for!very!low!Ro:!! compa1ble!with!Parker!waves?! ! ZDI!reconstruc1on:!! > Field!geometry!recovered!(large>scale!only)! > Field!strength!underes1mated!

Conclusions! ! Dynamo!models!of!solar>like!stars:! ! !>!Role!of!the!tachocline:!building!organised!field! !>!Is!a!tachocline!necessary!for!buoyant!loops!genera1on?! !>!What!is!missing!in!3D!models!to!actually!produce!spots?! !>!Models!commonly!applied!to!the!Sun!are!challenged!by!other!stars! ! ! ! ! Dynamo!models!of!fully!convec1ve!stars:! ! !>!Change!of!geometry!with!Rossby!number!(or!with!internal!structure?)! !>!Bistable!regime!for!late!M! !>!Temporal!variability!for!mul1polar!fields?! !>!Can!dipoles!(and!thus!bistability)!resist!strong!stra1fica1ons?! !