本帖最后由 细胞海洋 于 2012-9-29 10:57 编辑 4 o+ y- K I0 A# V; B, n! U ; s: k# M! {! q |6 G; r: R: s* F1 ?' m5 L0 l" y
Type Ia supernovae are thought to occur in binaries containing a white dwarf and a companion that is a red giant, subgiant, main-sequence star or another white dwarf. In the last of these options, the ‘double-degenerate‘ case, the two white dwarfs would merge before the explosion and no companion would be left. In a single-degenerate system the former companion star would survive. Previous searches for remnant companions have revealed one contested case for the Tycho Brahe supernova SN 1572. More recently, observations have restricted surviving companions to small, main-sequence stars, ruling out giant companions. Jonay González Hernández et al. report the result of a search for surviving companions to the progenitor of SN 1006 and find no sign of a former companion. Together with previous results, this suggests that fewer than 20% of type Ia supernovae occur through the single-degenerate channel, the slow accumulation of mass from a large star. The more common trigger, it seems, is the rapid break-up of a smaller orbiting white dwarf. The cover shows a composite image of the SN 1006 supernova remnant. A twisting ribbon of light seen by the Hubble Space Telescope reveals where the expanding blast wave is sweeping into the tenuous surrounding gas. (Cover: NASA/CXC/Rutgers/G. Cassam-Chenaï, J. Hughes et al.; Radio: NRAO/AUI/NSF/GBT/VLA/Dyer, Maddalena & Cornwell; Optical: Middlebury College/F. Winkler, NOAO/AURA/NSF/CTIO Schmidt & DSS.): a& ]9 L) q6 k( w& \2 m. h# F& Z. v
. @; j+ N, B2 q, {3 |$ N
6 G; r: R: s* F1 ?' m5 L0 l" y