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Een nieuw type supernova ontdekt

Geplaatst op 26-12-2009 om 18u00 door Bert Carrein - 2051 keer gelezen
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Science meldt een nieuw soort supernova-explosie. Radboud-astronoom Gijs Nelemans is niet verrast. Hij voorspelde — samen met een collega - het type explosie twee jaar geleden al. ‘De .la supernova’s zijn niet langer hypothetische fenomenen’.

‘Twee jaar geleden berekenden we dat er een nieuw soort supernova- explosie plaats kon vinden, minder helder en korter zichtbaar dan de al bekende supernova's,’ zegt Gijs Nelemans van de Radboud Universiteit in Nijmegen. Hij werkt al jaren samen met Lars Bildsten van het Kavli Institute for Theoretical Physics (KITP) in Santa Barbara. Het onderzoek, in samenwerking met Ken Shen en Nevin Weinberg, destijds promovendus en post-doc in Santa Barbara, leidde twee jaar geleden tot de spectaculaire voorspelling.

‘In de afgelopen jaren hebben we ons nieuwe idee op veel plekken gepresenteerd, maar steeds werden we erop gewezen dat dergelijke explosies nog nooit waren gezien’ aldus Lars Bildsten, eerste auteur van het artikel in 2007. ‘Zo gek was dat niet overigens’ licht Gijs Nelemans toe ‘want we hadden berekend dat ze minder vaak voorkomen dan andere supernova's en ook nog eens minder helder zijn en dus moeilijker waar te nemen. Maar het was een kwestie van tijd voordat de eerste gevonden zou worden’. Met het verschijnen van het artikel van de hand van Dovi Poznanski en collega's uit Berkeley in Science deze week lijkt dat nu het geval.

astronomie sterrenkunde ruimtevaart nieuwsNet als de zon eindigen de meeste sterren hun leven als een witte dwerg, de uitgebrande kern van een ster, waarin de massa vergelijkbaar met die van de zon in een volume zo klein als de aarde is geperst. Aangezien veel sterren in het heelal voorkomen als dubbelster, zijn ook veel witte dwergen onderdeel van een paar. In bijzondere gevallen is ook de begeleider een witte dwerg en draaien de sterren zo dicht om elkaar heen dat de buitenste lagen van de ene witte dwerg op de andere witte dwerg vallen.

Deze zogenaamde AM Canum Venaticorum dubbelsterren worden in Nijmegen onderzocht in de onderzoeksgroep van Paul Groot en Gijs Nelemans van het Institute for Mathematics, Astrophysics and Particle Physics. Bildsten en Nelemans ontdekten dat uiteindelijk een dusdanig dikke laag helium van de begeleider op de witte dwerg terecht kan komen dat kernreacties tot een explosie kunnen leiden, waarin de hele laag helium de ruimte ingeslingerd wordt.

Explosies van witte dwergen door kernreacties zijn al veel langer bekend, en staan bekend als type la supernova's. In die gevallen gaat het echter om de explosie van de hele witte dwerg. De nieuwe soort explosie is ongeveer tien keer zo zwak en verdwijnt ook weer tien keer zo snel als een normale type la supernova, wat Chris Stubbs (Harvard) ertoe bracht ze .la (punt één a, naar het Engelse decimale teken) te noemen.

Interessant genoeg dateert de deze week in Science gerappor-teerde explosie al uit 2002. Dovi Poznanski ontdekte dat de super-nova destijds verkeerd geclassifi-ceerd was en dat het om een vrij zwakke, snel verdwijnende super-nova ging. Ook de andere kenmerken van de explosie wijzen in de richting van een .la supernova. Met deze ontdekking lijkt het erop dat de .Ia supernova's niet langer hypothetische fenome-nen zijn. Maar er zijn ook nog open vragen, met name over de precieze omstandigheden waarin de explosies voorkomen en over het lot van de witte dwerg.

Bron: The Guidestar: http://www.astro-event-group.be

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