A zombie white dwarf is a supernova remnant

Artist's concept of a white dwarf stealing gas from a companion star (Image courtesy Russell Kightley)
Artist’s concept of a white dwarf stealing gas from a companion star (Image courtesy Russell Kightley)

An article published in the journal “Science” describes a research on the remnants of a strange supernova cataloged as LP 40-365. According to a team led by Stephane Vennes of the Astronomical Institute of Czech Academy of Sciences in Ondrejov, Czech Republic, that’s a type Iax supernova. It only partially destroyed a white dwarf leaving a part that moves at a very high speed and has an abnormal composition for that dead star type.

Supernovae that have a white dwarf at their origin are classified as type Ia but in 2013 a kind of extension of this type called Iax was proposed to identify some less energetic ones that don’t completely destroy the white dwarf. Over the course of these four years more than 50 objects have been identified that show the characteristics of remnants of type Iax supernovae and LP 40-365 seems one of them.

The mechanism that triggers it is one of the two possible for types Ia supernovae, with a white dwarf stealing gas from a companion star until it reaches the critical mass of about 1.4 solar masses called Chandrasekhar limit. At that point the supernova is triggered but sometimes it’s not so super and leaves a part of the white dwarf instead of completely destroying it, a kind of zombie. The other mechanism, caused by the merging of two white dwarfs, causes their complete destruction so at least for the moment it doesn’t seem possible for this mechanism to create a type Iax supernova.

In the case of the supernova that generated LP 40-365, what’s left has a mass that is about a seventh of the Sun’s in a volume that is just over eight times the Earth’s. To have a comparison, a typical white dwarf is Sirius B, which has a mass similar to the Sun’s compacted into a volume similar to the Earth’s. There are also other differences between LP 40-365 and a normal white dwarf, in fact Stephane Vennes’s team found a number abnormalities examining this strange object.

The white dwarf LP 40-365 has an atmosphere of mostly oxygen and neon with traces of elements such as aluminum and silicon. Elements such as hydrogen, helium and carbon weren’t detected and this suggests that the original outer layers were ejected together with them. Carbon may have been fused into heavier elements or could have sunk deep under the surface.

Another abnormality of the white dwarf LP 40-365 is its speed, higher than the Milky Way escape velocity. It can also be reached by an object that passes near the galactic core receiving gravitational thrust with a sort of slingshot effect but in this case it’s definitely more likely a consequence of the abnormal supernova. The researchers selected the white dwarf LP 40-365 exactly for its speed.

The research on type Iax supernovae is just beginning so the study of the white dwarf LP 40-365 is important. It confirms the existence of that kind of supernovae, also called subluminous, and offers more data such as those on the elements present in its atmosphere that are useful for the research follou-ups. The researchers hope they can also provide new information on type Ia supernovae.

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