HD 20781

HD 20781
HD 20781
ClassificazioneNana gialla
Classe spettraleG9.5V[1]
Distanza dal Sole116 anni luce[1]
CostellazioneFornace
Coordinate
(all'epoca J2000.0)
Ascensione retta03h 20m 02,943s
Declinazione-28° 47′ 01,80″
Parametri orbitali
Sistema planetario
Dati fisici
Raggio medio0,85[2] R
Massa
Periodo di rotazione46,8 giorni[3]
Temperatura
superficiale
  • 5256±29 K[3] (media)
Luminosità
0,49[3]
Indice di colore (B-V)0,82[3]
Metallicità78% rispetto al Sole[3]
Dati osservativi
Magnitudine app.8,44[1]
Magnitudine ass.5,70[3]
Parallasse28,27 mas
Moto proprioAR: 348,97 mas/anno
Dec: -67,69 mas/anno
Velocità radiale39,6 km/s
Nomenclature alternative
V376 Pegasi, BD +18°4917, SAO 107623, HIP 108859

HD 20781 è una stella di 8ª magnitudine distante circa 116 anni luce dalla Terra, situata nella costellazione della Fornace. Fa parte di un sistema binario largo assieme a HD 20782; entrambe le stelle, nane gialle di tipo solare, ospitano un sistema planetario. Ha una grande separazione angolare dalla compagna, di 252 secondi d'arco, che corrispondono a 9080 UA alla distanza in cui si trova il sistema[4].

Due pianeti extrasolari sono stati annunciati attorno a HD 20781 nel 2011; si tratta di giganti gassosi simili a Nettuno che orbitano piuttosto in prossimità della stella, tanto da essere definiti pianeti nettuniani caldi[5].

Nel 2019 sono stati scoperti altri due pianeti del tipo super Terra che orbitano più internamente rispetto ai primi due.[3]

Caratteristiche fisiche

HD 20781 è una nana giallo-arancione meno massiccia e luminosa della compagna. Di tipo spettrale G9,5V, ha una massa ed un raggio rispettivamente pari al 70% e all'84% di quelli della nostra stella. La sua temperatura superficiale è di poco più di 5200 K e la luminosità è la metà circa di quella del Sole. Rispetto alla compagna pare essere più povera di metalli, con un'abbondanza di elementi più pesanti dell'elio del 78% di quella del Sole[6].

Sistema planetario

I pianeti scoperti nel 2011 hanno masse rispettivamente 12 e 16 volte quella della Terra e ruotano attorno alla stella in un periodo di 29 e 85 giorni rispettivamente. L'eccentricità orbitale non è elevatissima come per il pianeta di HD 20782, anche se è comunque alta nel caso di HD 20781 c.

Meno massicci e probabilmente di tipo roccioso sono i due pianeti più interni la cui conferma della scoperta è giunta nel 2019: hanno periodi orbitali di 5,3 e 14 giorni circa e masse minime da 2 a 5 volte quella terrestre. Essendo stati scoperti con il metodo della velocità radiale e non essendo stati osservati transiti di tutti i pianeti non sono noti i raggi.[3]

Prospetto

Segue un prospetto con i principali dati del pianeta orbitante[1].

PianetaTipoMassaPeriodo orb.Sem. maggioreEccentricitàScoperta
dSuper Terra1,93 M5,31 giorni0,0529 UA0,102019
eSuper Terra5,33 M13,89 giorni0,1004 UA0,092019
bGigante gassoso10,61 M29,16 giorni0,1647 UA0,112011
cGigante gassoso14,03 M85,51 giorni0,3374 UA0,062011

Note

  1. ^ a b c d J. Schneider, HD 20781 e, su exoplanet.eu, Enciclopedia dei pianeti extrasolari. URL consultato il 21 agosto 2021.
  2. ^ Masana et al., Effective temperatures and radii of stars, su vizier.u-strasbg.fr, VizieR, 2006. URL consultato il 31 luglio 2013.
  3. ^ a b c d e f g h i S. Udry et al., The HARPS search for southern extra-solar planets (PDF), in Astronomy and Astrophysics, vol. 622, A37, febbraio 2019, pp. 29.
  4. ^ S. Desidera, M. Barbieri, Properties of planets in binary systems, in Astronomy & Astrophysics, vol. 462, 2007, pp. 345-353, DOI:10.1051/0004-6361:20066319. URL consultato il 31 luglio 2013.
  5. ^ M. Mayor, M. Marmier, C. Lovis, S. Udry, D. Ségransan, F. Pepe, W. Benz, J.-L. Bertaux, F. Bouchy, X. Dumusque, G. Lo Curto, C. Mordasini, D. Queloz, N. C. Santos, The HARPS search for southern extra-solar planets XXXIV. Occurrence, mass distribution and orbital properties of super-Earths and Neptune-mass planets (PDF), settembre 2011.arΧiv:1109.2497
  6. ^ M. Tsantaki, S. G. Sousa, V. Zh. Adibekyan, N. C. Santos, A. Mortier, G. Israelian, Deriving precise parameters for cool solar-type stars. Optimizing the iron line list, in Astronomy & Astrophysics, vol. 555, A150, luglio 2013.arΧiv:1304.6639

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