Type Ia supernovae and the age of the component stars

[jnb]

As I understand it type Ia supernovae occur when a binary star system is such that matter from a red giant falls onto a white dwarf companion until its mass exceeds the Chandrasekhar limit and it erupts as a supernovae. So far so good and all quite simple.

If the components of the system are a white dwarf and a red giant then presumably the white dwarf is further along its evolution than the red giant. The white dwarf must be below 1.4 solar masses and if the red dwarf is not as evolved it must have a lower mass. The sun (1 solar mass obviously) will take about 10 billion years to reach a red giant phase. So surely any Type Ia supernovae should only occur in binary systems that are at least that age (possibly a little younger for stars between 1 and 1.4 solar masses).

So why can we see Type Ia supernovae early in the Universe?

 

[andrew s]

A white dwarf can form from a star with mass from about 0.5 to 10 solar masses ( see here https://en.wikipedia.org/wiki/White_dwarf#Formation)  so the companion only needs to be just a little in lower in mass to be going through its giant phase at the right time to accrete mass onto the white dwarf.

Another possible mechanism is that a less massive companion captures mass from the other star (which is on route to being the white dwarf) during it's giant phase so it's evolution is accelerated by the additional mass it captures.

Regards Andrew

[Knight of Clear Skies]

Also, when stars leave the main sequence they lose a considerable amount of mass in the red giant phase and when evolving into planetary nebulae. For example, our Sun is expected to to leave behind a 0.6 solar mass white dwarf. Larger stars lose an even larger fraction of their mass, so are much younger their mass suggests. Larger stars burn their way through their fuel much faster due to the higher pressure at their cores, an 8 solar mass star might only last 50 or 60 million years and leave behind a white dwarf close to the  Chandrasekhar limit, ripe for popping.