I try to run interesting astronomical art wherever I can find it, but the image that accompanies this ESA news release on the discovery of an interesting white dwarf just doesn’t cut it. So use your imagination as I describe the results of a study using data from ESA’s XMM-Newton X-ray telescope, which have given us something we’ve long lacked — highly accurate mass information for an accreting white dwarf in a binary system, one that is growing close to the point of becoming a supernova.
Something in the vicinity of HD 49798 has been known to be giving off X-rays since 1997, but it has taken XMM-Newton to nail the culprit. The white dwarf near the larger star is twice as massive as expected, cramming about 1.3 solar masses into an object with a diameter about half that of our planet. Rotating every thirteen seconds, this object boasts the fastest white dwarf rotation known.
Why the larger mass? We’re looking at a white dwarf that is pulling gaseous material out of its companion star. The process points to an interesting future: When a white dwarf in this situation reaches 1.4 solar masses, it will doubtless explode, becoming what is known as a type Ia supernova. Such events are the ‘standard candles’ that astronomers use to study the expansion of the universe, and here we see a type Ia supernova in the process of developing.
Sandro Mereghetti (INAF–IASF Milan) describes the find in terms of a useful historical analogy:
“This is the Rosetta stone of white dwarfs in binary systems. Our precise determination of the masses of the two stars is crucial. We can now study it further and try to reconstruct its past, so that we can calculate its future.”
Not that we need wait up for this one. A supernova near HD 49798 shouldn’t occur for a few million years, and even when it does, it poses no danger to Earth. That will be an interesting event, visible in broad daylight with the naked eye, but for today’s astronomical purposes, this white dwarf tells us much about the supernovae so critical in our understanding of a universe whose expansion seems to be accelerating.
The paper is Mereghetti et al., “An ultra massive fast-spinning white dwarf in a peculiar binary system,” Science Vol. 325, No. 5945 (4 September 2009), pp. 1222-1223 (abstract).