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Stellar Activity in Exoplanet Hosts

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Stellar Activity in Exoplanet Hosts Stellar activity in exoplanet hosts Enrique Herrero Casas Aquesta tesi doctoral està subjecta a la llicència Reconeixement- NoComercial – CompartirIgual 3.0. Espanya de Creative Commons. Esta tesis doctoral está sujeta a la licencia Reconocimiento - NoComercial – CompartirIgual 3.0. España de Creative Commons. This doctoral thesis is licensed under the Creative Commons Attribution-NonCommercial- ShareAlike 3.0. Spain License. Stellar activity in exoplanet hosts Mem`oria presentada per Enrique Herrero Casas per optar al grau de Doctor per la Universitat de Barcelona Barcelona, juliol de 2014 2 . 3 Programa de doctorat en F´ısica Stellar activity in exoplanet hosts Mem`oria presentada per Enrique Herrero Casas per optar al grau de Doctor per la Universitat de Barcelona Directors: Dr. Ignasi Ribas Dra. Carme Jordi Tutora: Dra. Carme Jordi June 22, 2014 4 res Agra¨ıments Abans d’entrar en mat`eria, voldria expressar el meu m´es sincer agra¨ıment a totes les persones que han contribu¨ıt, de manera directa o indirecta, al treball que aqu´ıes presenta. Primer de tot vull destacar els meus directors de tesi, l’Ignasi i la Carme. He tingut la sort de poder treballar acompanyat per la seva motivaci´o i els seus coneixements i experi`encia en el camp dels exoplanetes i la f´ısica estel·lar. Sense tot aix`o aquest treball no hagu´es estat possible. Tamb´e vull agrair el bon ambient i hospitalitat que m’han acompanyat durant els anys de doctorat per part dels companys, tant durant les temporades al Departament d’Astronomia i Meteorologia (UB) com a l’Institut de Ci`encies de l’Espai (CSIC- IEEC). He gaudit sempre de bona companyia i de suport en els meus projectes. Guardo especial bon record de la meva estada a l’Osservatorio Astrofisico di Catania (INAF) pels consells i el bon fer de Nuccio Lanza, amb qui vaig poder aprofundir en les t`ecniques de modelitzaci´o d’estrelles i obtenir alguns dels resultats importants d’aquest treball. Un agra¨ıment molt especial ha de ser per a la meva fam´ılia, per al meu germ`a, per a l’Elena i per a tots els meus amics propers, ja que sense saber-ne gaire o gens de f´ısica estel·lar, han escrit bona part d’aquest treball quan han suportat els meus bons i mals moments, la meva pres`encia o abs`encia. Estic molt content pels resultats assolits i perqu`e tots ells n’hagin estat coautors. Perultim, ´ no em vull oblidar d’enviar un agra¨ıment tamb´e als meus enemics. N’hi ha pocs, potser es compten amb els dits d’una m`a, per`o sovint s’han esfor¸cat per posar-me els pals a la rodes que despr´es han estat el meu incentiu per fer les coses ben fetes. Aquest treball ha comptat amb el suport econ`omic d’una beca JAE Pre-Doc (CSIC). Tamb´e ha estat parcialment finan¸cat pels Plan Nacional de Astronom´ıa yAstrof´ısica (AYA2009-06934, AYA2012-39612-C03-01, AYA2012-39551-C02-01 i AYA2013-48318-C2-1-R) del Ministerio de Econom´ıa y Competitividad. 5 6 Resum Actualment, la major part dels esfor¸cos en la cerca i caracteritzaci´o d’exoplanetes de tipus terrestre es centren en aquells que orbiten estrelles de baixa massa. Aquestes constitueixen la gran majoria de la poblaci´o de la nostra gal`axia (aproximadament un 75% de les estrelles s´on de massa inferior a la solar) i a m´es el seu inter`esrauenel fet que el senyal fotom`etric i espectrosc`opic dels planetes de tipus terra que hi orbiten ´es major degut a la menor difer`encia amb la mida i la massa de l’estrella. D’altra banda, algunes caracter´ıstiques importants de l’estructura i processos d’aquest tipus d’estrelles s´on encara poc coneguts, i per tant ´es essencial una bona caracteritzaci´o pr`evia d’aquestes, en especial de les estrelles de tipus M, com un dels propers passos en el camp de l’exoplanetologia. En concret, els fen`omens d’activitat estel·lar, degut alapres`encia de taques i f`acules on el perfil espectral i el comportament convectiu ´es diferent al de la fotosfera, introdueixen variacions en les mesures fotom`etriques i espectrosc`opiques amb una periodicitat modulada per la rotaci´o de l’estrella. La seva caracteritzaci´o´es essencial per a l’estudi d’exoplanetes. L’abast dels grans aven¸cos en les t`ecniques observacionals est`a facilitant que l’astronomia amateur faci una important aportaci´oal’observaci´o fotom`etrica de tr`ansits d’exoplanetes. En aquest treball, la col·laboraci´o amb l’astr`onom amateur Ramon Naves (Observatori Montcabrer) ha conduit al descobriment i l’an`alisi de variabilitat fotom`etrica a l’estrella WASP-33 (Herrero et al. 2011), hostatgera de tipus A5 d’un planeta gegant amb tr`ansits amb un per´ıode d’1,21986 dies, esdevenint la primera estrella coneguda de tipus δ Scuti amb un planeta. L’an`alisi realitzada a partir de periodogrames Lomb-Scargle i diversos m`etodes de pesat de les dades va desvetllar la possible commensurabilitat de les oscil·lacions fotom`etriques amb el per´ıode orbital del planeta, suggerint algun tipus d’interacci´o d’aquest amb l’estrella hostatgera. Es tracta d’oscil·lacions d’una escala d’amplitud i periodicitat similars a aquelles relacionades amb l’activitat estel·lar, per`o vinculades a un fenomen de pulsacions, en aquest cas, en una estrella massiva. L’estudi dels fen`omens d’activitat a la fotosfera d’estrelles de baixa massa s’ha realitzat a trav´es de diverses t`ecniques que ens han perm`es modelitzar o b´esimular relacions entre les propietats estel·lars i les dades observables. L’activitat estel·lar alafotosferaest`a relacionada amb la intensitat del camp magn`etic, i aquest amb la velocitat de rotaci´o de l’estrella en termes de l’efecte dinamo. Diverses relacions emp´ıriques per a estrelles de tipus G, K i M, on existeix un embolcall convectiu, per- meten establir correlacions entre determinats indicadors d’activitat, com log(RHK), ielper´ıode de rotaci´o. Aquestes correlacions ens han perm`es generar mostres es- tad´ıstiques d’estrelles amb distribucions estoc`astiques pel que fa a les seves propietats i a la seva disposici´ogeom`etrica, poblant aix´ı el diagrama activitat - v sin i iperme- 7 8 tent realitzar estimacions de l’angle de l’eix de rotaci´o de l’estrella, i, quan existeixen mesures de log(RHK) i de la velocitat de rotaci´o projectada, v sin i. Assumint sempre una alineaci´o entre els eixos de l’`orbita i de la rotaci´o, les estrelles amb inclinacions properes a 90◦ representarien les millors candidates per a una cerca de planetes per tr`ansit. Aquest estudi estad´ıstic s’ha aplicat a una mostra real de 1200 estrelles de baixa massa provinents de diversos cat`alegsambmesuresdelog(RHK)iv sin i, obtenint-ne un subconjunt d’un 10% per al qual l’efici`encia d’una cerca de planetes per tr`ansits es podria veure incrementada entre un factor 2 i 3 respecte a una cerca sobre el conjunt total (Herrero et al. 2012). La fotometria de gran precisi´o de les recents missions espacials Kepler i CoRoT ha perm`es, a banda del descobriment d’alguns centenars de sistemes planetaris, l’estudi de la variabilitat estel·lar a gaireb´e totes les escales de temps on els fen`omens d’activitat hi tenen alguna influ`encia. Les t`ecniques de modelitzaci´o de taques, tant per m`etodes anal´ıtics com per m`etodes d’integraci´odesuperf´ıcie, permeten obtenir informaci´of´ısica de la superf´ıcie de l’estrella a partir de s`eries temporals fotom`etriques i espectrosc`opiques. Durant aquest treball hi ha hagut una estreta col·laboraci´o amb el Grup d’Estrelles Actives i Sistemes de l’OACT (INAF), en con- cret amb A. F. Lanza, per a l’an`alisi de fotometria d’estrelles actives de tipus M observades per Kepler a trav´es de t`ecniques d’integraci´odesuperf´ıcie i un model de m`axima entropia per al mapejat de la distribuci´o de regions actives. Ens hem centrat en l’objecte LHS 6343 A, un dels components d’una parellaoptica ` de dues estrelles M que mostra eclipsis produ¨ıts per una companya nana marr´o cada 12,718 dies. A m´es, tamb´e hi apareix una oscil·laci´o fotom`etrica d’aquest mateix periode desfassada ∼ 100◦ en relaci´o als eclipsis i persistent al llarg dels m´es de 500 dies d’observaci´o. La modelitzaci´o acurada de les dades ha perm`es explicar aquestes oscil·lacions amb la pres`encia de regions actives que apareixen preferentment a una longitud fixa en relaci´o a la posici´odelacompanyananamarr´o, assenyalant una possible con- nexi´o magn`etica entre els dos components (Herrero et al. 2013). Les regions actives mostrarien una deriva sistem`atica, indicant un periode de rotaci´odelaprim`aria de 13, 14 ± 0, 02 dies. D’altra banda, s’ha estudiat tamb´e una explicaci´oalternativa per les oscil·lacions fotom`etriques a LHS 6343 A en termes de l’efecte ”Doppler- beaming”. Hem utilitzat una aproximaci´oBayesianaatrav´es de m`etodes de Monte Carlo per tal d’ajustar l’efecte de modulaci´oel·lipsoidal i de ”Doppler-beaming”, mostrant que aquest darrer ´es el major causant de les oscil·lacions persistents obser- vades a la fotometria.
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