Researchers at the Yunnan Observatory of the Chinese Academy of Sciences have come to a conclusion as to why some Type Ia supernovae explode earlier than expected. Observations have shown that about half of all Type Ia supernovae occur less than 100 million years after their galaxies' formations - much earlier than deemed possible by previous causal models. Dr. Bo Wang and his team believe these early supernovae are the result of a white dwarf star siphoning matter from a nearby helium star.
Images of Type Ia supernova 2005ke
exploding in optical (left), ultraviolet (center),
and x-ray (right) wavelengths
(Credit: NASA/Swift/S. Immler
)
One of the standard models describing the cause of Type Ia supernovae posits that they are the result of dense white dwarf stars – stars massing less than 1.4 solar masses that have ceased nuclear fusion and reached the end of their normal life span – pulling mass from nearby stars. When the resulting combined mass reaches 1.4 solar masses (also called the Chandrasekhar Limit), the white dwarf becomes too massive to remain stable. A fusion reaction is triggered and the star quickly explodes. However, the second star in the system has until now been typically modeled as either another white dwarf or as a main sequence star.
The team at Yunnan Observatory looked at 2600 white dwarfs and nearby companion star pairs using the Eggleston stellar evolution modeling system. They discovered that the white dwarfs reach the critical mass for fusion and the subsequent explosion within an earlier time frame when the companion star is a helium star.
Such early supernovae formation is important because it affects the amount of iron and other heavy metals found in the interstellar medium, subsequently changing the expected chemical balance. This in turn alters models of galactic evolution which are based on combination of the system's dynamics and chemistry. The abundance of different chemical elements is also used by cosmologists as a distance indicator, and therefore changing the chemical model may lead to significant changes in our understanding of large scale structures of the universe.
According to the scientists, the next step in their research is modeling the properties of companion helium stars, particularly after the supernovae explode. Wang’s team intends to work on mathematical models of the stars’ behavior, which hopefully could later be verified by the Large Sky Area Multi-Object Fiber Spectral Telescope (LAMOST).
Detailed information regarding the discovery can be obtained in the full text version of Dr. Wang's paper (PDF) at Cornell University Library's archive of scientific papers.
