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Efter 100 år kender vi nu banen for Proxima Centauri
Proxima Centauri kredser faktisk om dobbeltstjerneparret Alpha Centauri, ser det ud til
22. december 2016
Interessen for vores nabostjernesystem Alpha Centauri har været særligt høj siden det blev offentliggjort, at der er en planet, kaldet Proxima b, med en masse nogenlunde som Jordens, som kredser om den tredje stjerne i dette flerstjernesystem. — Proxima Centauri er den stjerne, som befinder sig nærmest Solen. Det større dobbeltstjernepar, Alpha Centauri A and B, ser ud til at have en egenbevægelse på himlen, som meget ligner den, som den mindre og svagere Proxima har, men indtil nu har det ikke været muligt at vise, at de tre stjerner faktisk er bundet til hinanden af tyngdekræfterne imellem dem, så de danner et triple system.
Nu har de tre astronomer Pierre Kervella, Frédéric Thévenin og Christophe Lovis, fastslået, at de tre stjerner faktisk er bundet til hinanden. Det har i alle de hundrede år, der er gået siden Proxima Centauri blev opdaget, været meget svært at måle stjernens radialhastighed, fordi den er så svagtlysende. Radialhastigheden er den hastighed, hvormed stjernen bevæger sig imod os eller væk fra os på Jorden. ESOs planetjægerinstrument HARPS har nu givet os en meget præcis måling af Proximas radialhastighed, og det er blevet koordineret med andre korrektioner for en række mindre effekter[1].
Resultatet er, at astronomerne nu kan udlede meget enslydende værdier for radialhastighederne af Alpha Centauriparret og Proxima Centauri. Det underbygger ideen om, at de er bundet til hinanden. Med de nye målinger er der foretaget nye beregninger af banerne for de tre stjerner, og de viser, at de relative hastigheder imellem Proxima og Alpha-parret ligger markant under den tærskel, som bestemmer, om de tre stjerner ville være bundet til hinanden af tyngdekræfterne.
Det nye resultat betyder meget for vores forståelse af Alpha Centauri-systemet og planetdannelsen her. Alt tyder på, at de tre stjerner har samme alder (omkring 6 milliarder år), og det giver os så igen en god vurdering af alderen for planeten Proxima b.
Astronomerne overvejer, om planeten kan være dannet i en bane omkring Proxima Centauri, som var noget større end den nuværende. Derefter er den blevet sendt ind i sin nuværende meget nære bane som resultat af en tæt passage af Proxima Centauristjernen forbi søskendestjernerne i Alpha Centauri-parret. Et alternativ er, at planeten måske er dannet i kredsløb omkring parret, og så senere er indfanget af Proxima Centauri. Hvis en af disse ideer er korrekt, er der mulighed for, at planeten oprindeligt havde en iskappe, og at isen nu er smeltet, så der er flydende vand på dens overflade.
Note
[1] Stjernernes hastigheder er målt ved at observere udvalgte dele af deres lys; det, som kaldes spektrallinier. Der er nogle fysiske effekter, som kan påvirke disse målinger, ud over stjernernes bevægelse. Hvis stjernen har en ustabil overflade, kan det forårsage en konvektiv blåforskydning af spektrallinierne, og stjernens tyngdekraft kan bevirke a gravitationel rødforskydning.
Mere Information
Disse nye forskningsresultater offentliggøres i en artikel i tidsskriftet Astronomy and Astrophysics.
Forskerholdet består af P. Kervella, CNRS UMI 3386, University of Chile og LESIA, Paris Observatory; F. Thévenin, Côte d'Azur Observatory, Frankrig; og Christophe Lovis, Observatoire astronomique de l’Université de Genève, Schweiz.
Kontakter
Ole J. Knudsen
ESON-Danmark, Stellar Astrophysics Centre, Aarhus Universitet
telefon 8715 5597, mobil 4059 4520, e-mail eson-denmark@eso.org
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