ESA XMM-Newton telescope uncovers a celestial Rosetta stone

[bystander]

XMM-Newton telescope uncovers a celestial Rosetta stone
Astronomy.com - September 3, 2009

The mysterious object is a white dwarf, the dead heart of a star, shining X-rays into space.

The European Space Agency's (ESA) XMM-Newton orbiting X-ray telescope captured the first close-up of a white dwarf star, circling a companion star, that could explode into a particular kind of supernova in a few million years. These supernovae are used as beacons to measure cosmic distances and ultimately understand the expansion of our universe.

Astronomers have been on the trail of this mysterious object since 1997 when they discovered that something was giving off X-rays near the bright star HD 49798. Now, thanks to XMM-Newton's superior sensitivity, the mysterious object has been tracked along its orbit. The observation has shown it to be a white dwarf, the dead heart of a star, shining X-rays into space.

... this is no ordinary white dwarf. They measured its mass and found it to be more than twice what they were expecting. Most white dwarfs pack 0.6 solar masses into an object the size of Earth. This particular white dwarf contains at least double that mass but has a diameter just half that of Earth. It also rotates once every 13 seconds, the fastest of any known white dwarf.

The mass determination is reliable because the XMM-Newton tracking data allowed the astronomers to use the most robust method for 'weighing' a star, one that uses the gravitational physics devised by Isaac Newton in the 17th century. Most likely, the white dwarf has grown to its unusual mass by stealing gas from its companion star, a process known as accretion. At 1.3 solar masses, the white dwarf is now close to a dangerous limit.

When it grows larger than 1.4 solar masses, a white dwarf is thought either to explode or collapse to form an even more compact object called a neutron star. The explosion of a white dwarf is the leading explanation for type Ia supernovae, bright events that are used as standard beacons by astronomers to measure the expansion of the universe. Until now, astronomers have not been able to find an accreting white dwarf in a binary system where the mass could be determined so accurately.

"This is the Rosetta stone of white dwarfs in binary systems," said Mereghetti. "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."

XMM-Newton weighs up a rare white dwarf and finds it to be a heavyweight
ESA XMM-Newton Mission News - 04 Sep 2009

XMM-Newton uncovers a celestial Rosetta stone
Science Centric - 4 September 2009

[harry]

G'day Bystander

It is common for people to say that a white dwarf is a dead Star.

In acttual fact it's a phase in the regeneration of the Star.

What we see is actually the dynamo of the inner and outer core of the Star without the solar evelope that went nova.

In time it may merge or attract matter to it, the dipole jets will aid in creating turbulence in creating a new solar envelope.

The problem becomes complicated once Neutron matter dominates the inner core, even more so when it becomes a hybrid with a quark inner core.

[bystander]

A white dwarf is essentially dead. The material in the star no longer undergoes fusion reactions. Although very hot when formed, the white dwarf has no source of energy and will eventually radiate away all of its heat, unless it accretes material from a companion, in which case it may reach a more spectacular end.

[Chris Peterson]

It is common for people to say that a white dwarf is a dead Star.

Because that's what it is. No more fusion, no more dynamo, just a lump of non-reacting matter. It will continue to get cooler forever. Game over.

If it is part of a binary system, it may accrete matter and produce periodic bursts (although that isn't really the star itself doing anything). But that's a special case.

In acttual fact it's a phase in the regeneration of the Star.

Whatever was cast off during its formation may go on to be incorporated in new stars, but the material in the white dwarf is done participating in the recycling program. It isn't going to regenerate into anything.

What we see is actually the dynamo of the inner and outer core of the Star without the solar evelope that went nova.

Not really. First, there is no dynamo anymore. Second, the material is not in the same state it was in while the star was active. It has dropped into a much lower energy state.

In time it may merge or attract matter to it, the dipole jets will aid in creating turbulence in creating a new solar envelope.

There is no evidence at all that such a thing can happen, either theoretically or observationally.

[neufer]

A white dwarf is essentially dead. The material in the star no longer undergoes fusion reactions. Although very hot when formed, the white dwarf has no source of energy and will eventually radiate away all of its heat, unless it accretes material from a companion, in which case it may reach a more spectacular end.

<<A white dwarf will eventually cool and become a non-radiating black dwarf
in approximate thermal equilibrium with its surroundings and with the cosmic
background radiation. However, no black dwarfs are thought to exist yet.>>

<<The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported against gravitational collapse by the heat generated by fusion. It is supported only by electron degeneracy pressure, causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a nonrotating white dwarf, the Chandrasekhar limit—approximately 1.4 solar masses—beyond which it cannot be supported by degeneracy pressure. A carbon-oxygen white dwarf that approaches this mass limit, typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation. (SN 1006 is thought to be a famous example.)

• ------------------------
  d(Temperature)/dt = -C * (Temperature)^4
  (Temperature) = 1 / [ cube root(3Ct) ]
  ------------------------

Most observed white dwarfs have relatively high surface temperatures, between 8,000 K and 40,000 K. A white dwarf, though, spends more of its lifetime at cooler temperatures than at hotter temperatures, so we should expect that there are more cool white dwarfs than hot white dwarfs. Once we adjust for the selection effect that hotter, more luminous white dwarfs are easier to observe, we do find that decreasing the temperature range examined results in finding more white dwarfs. This trend stops when we reach extremely cool white dwarfs; few white dwarfs are observed with surface temperatures below 4,000 K, and one of the coolest so far observed, WD 0346+246, has a surface temperature of approximately 3,900 K. The reason for this is that, as the Universe's age is finite, there has not been time for white dwarfs to cool down below this temperature. The white dwarf luminosity function can therefore be used to find the time when stars started to form in a region; an estimate for the age of the Galactic disk found in this way is 8 billion years. >>