Vampire star as a ticking stellar time bomb

Astronomers discover candidates for type Ia supernova

The expanding matter shell around V445 Puppis © PA Woudt / ESO
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With the aid of the Very Large Telescope of the European Southern Observatory (ESO), astronomers have for the first time succeeded in creating a time-lapse film of matter ejected from a "vampire star". The star had been sucking in matter from a close companion for a long time, before it came to an outbreak in November 2000, the course of which has followed astronomers for years.

It is a promising candidate for a class of objects that astronomers have been looking for for some time now: the precursors of so-called Ia supernova explosions. Such explosions play a key role in the exploration of the dark energy that fills our cosmos, according to researchers in the journal Astrophysical Journal.

"Unpleasant knowledge gap"

"What types of star system explode as type Ia supernovae? This is one of the important open questions in modern astrophysics, "says Patrick Woudt of the University of Cape Town, first author of the article. "On the other hand, as these supernovae play a key role in the studies demonstrating that the expansion of our universe is accelerating, this is an awkward knowledge gap."

Target of the new detailed study was the object named V445 Puppis in the constellation "Aft Deck of the Ship" in the southern sky. The object was already in November 2000 as so-called Nova conspicuous. Novae are events in which a star temporarily increases in brightness, in the case of V445 by a remarkable factor of 250. They occur in star systems in which a white dwarf and a companion star orbit.

The White Dwarf removes matter from his companion, who gathers around the Dwarf star, becoming unstable over time and exploding. Large quantities of matter are thrown into space. display

Surrounding the Nova V445 Puppis © ESO / Digitized Sky Survey 2

First nova without hydrogen

Usually, the collected matter is primarily hydrogen. V445 Puppis is the first nova in which no hydrogen has been detected. This circumstance refers to a class of even more powerful stellar explosions: supernovae of the type Ia.

"That's a key finding, because we know that hydrogen is also missing in type Ia supernovae, " says co-author Danny Steegs of the University of Warwick in the UK. The companion star of V445 Puppis also fits into the scheme, as it also lacks hydrogen. Instead, especially helium flows from this companion to the White Dwarf star

Two-part matter floor

Woudt's research team used the NACO instrument, a device on ESO's Very Large Telescope (VLT), which uses the adaptive optics method. This can be extremely sharp images of astronomical objects win. With NACO, the researchers observed V445 Puppis over a two-year period. The pictures show a two-part material case, with an initially very pronounced constriction in the middle. At the outer ends of the hull, there are also two compactions that appear to be moving outward at a speed of around 30 million kilometers per hour.

The matter itself - also observed for the first time in a nova - moves at a speed of around 24 million kilometers per hour, according to the astronomers. The two stars themselves are covered by a thick dust disk, which should also have been created in the course of the last eruption.

Adaptive optics gives good results

That we can see in these pictures incredibly fine details - their apparent size is about a hundred millibose seconds, equivalent to one from a distance of 40 kilometers considered one euro coin - is Thanks to adaptive optics, such as those used on large ground-based telescopes such as the ESO VLT, says Steeghs.

White dwarf sucks matter from red giant star ESO

Supernovae as "standard candles"

Supernovae are huge explosions at the end of the life of certain stars. Type Ia supernovae are of particular interest because they can be used as "standard candles" and allow large distances to be determined in the cosmos. Such measurements are based on investigations to accelerate the cosmic expansion, which in the current cosmological models is based on the presence of so-called dark energy.

Similar to novae, supernovae of type Ia are also believed to occur in binary systems where one of the stars is a white dwarf. If such stars attract too much additional matter, they will become unstable and explode. Especially in the case of type Ia supernovae, we are dealing with a rather complicated process. The matter of the companion star initially forms an additional layer on the surface of the White Dwarf. If this layer becomes too dense, a nova explosion occurs - a comparatively weak eruption in which some of the accumulated matter is thrown into space.

The key question is: what has to happen so that a white dwarf can accumulate enough more matter in spite of this protective mechanism, so that in the end it explodes completely in an even more massive type Ia supernova explosion? According to astronomers, there are some indications that the nature of the accumulated matter plays an important role: A distinguishing feature of type Ia supernovae is the absence of hydrogen, the most abundant chemical element in the Universe, as determined by spectrum studies.

White dwarf star with very high mass

With the help of NACO images and additional data from other telescopes, astronomers were able to determine the system's distance from Earth - around 25, 000 light-years - and its intrinsic brightness - 10, 000 times brighter than the Sun. According to the researchers, this brightness suggests that the white dwarf star in this system already has a very high mass, just below the critical limit at which it explodes, but on the other hand, considerable additional amounts of material of the companion on the dwarf star stream.

"Whether V445 Puppis will eventually explode as a supernova, or whether the system has thrown out enough matter in the nova just watched to avoid this fate, is uncertain, " says Woudt: "But no doubt we have one promising candidate for a future one Do type Ia supernova. "

(idw - Max Planck Institute for Astronomy, 18.11.2009 - DLO)