Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae1,2. Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The ‘old’ environment near the supernova location, and the very low derived ejected mass (∼0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous3,4 or a regular1 type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive 44Ti.
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We thank P. Podsiadlowski, E. Nakar and D. Maoz for comments. We acknowledge observations with the Liverpool Telescope, and various telescopes at the Lick, Palomar and Keck Observatories. We are grateful to the staffs of these observatories, as well as to the institutions, agencies and companies funding these facilities. This research also made use of the NASA/IPAC Extragalactic Database (NED). H.B.P. acknowledges the ISF/FIRST and Ilan Ramon-Fulbright Fellowships, and is a Harvard-Smithsonian Center for Astrophysics Fellow. The collaborative work of A.G.-Y. and P.A.M. is supported by a Weizmann-Minerva grant. A.G.-Y. acknowledges further support by the Israeli Science Foundation, an EU Seventh Framework Programme Marie Curie IRG Fellowship, the Benoziyo Center for Astrophysics, and the Peter and Patricia Gruber Awards. A.V.F. is grateful for the support of the US National Science Foundation, the US Department of Energy, Gary and Cynthia Bengier, the Richard and Rhoda Goldman Fund, the Sylvia & Jim Katzman Foundation, and the TABASGO Foundation. R.J.F. is a Clay Fellow.
The authors declare no competing financial interests.
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