The Formax Galaxy

The center of the Fornax galaxy cluster is shown in this Chandra image. The galaxy NGC 1404 is shown below and to the left of the center. Hot gas is clearly visible in the center of the cluster and around NGC 1404. (Credit: NASA/CXC/Columbia U./C.Scharf et al.)


Using data from NASA's Chandra X-ray Observatory, scientists have reported the possible detection of a binary star system that was later destroyed in a supernova explosion. The new method they used provides great future promise for finding the detailed origin of these important cosmic events.

In an article on the February 14th issue of the journal Nature, Rasmus Voss of the Max Planck Institute for Extraterrestrial Physics in Germany and Gijs Nelemans of Radboud University in the Netherlands searched Chandra images for evidence of a much sought after, but as yet unobserved binary system - one that was about to go supernova. Near the position of a recently detected supernova, they discovered an object in Chandra images taken more than four years before the explosion.

The supernova, known as SN 2007on, was identified as a Type Ia supernova. Astronomers generally agree that Type Ia supernovas are produced by the explosion of a white dwarf star in a binary star system. However, the exact configuration and trigger for the explosion is unclear. Is the explosion caused by a collision between two white dwarfs, or because a white dwarf became unstable by pulling too much material off a companion star?
[+/-] Click here to expand

Answering such questions is a high priority because Type Ia supernovas are major sources of iron in the Universe. Also, because of their nearly uniform intrinsic brightness, Type Ia supernova are used as important tools by scientists to study the nature of dark energy and other cosmological issues.

"Right now these supernovas are used as black boxes to measure distances and derive the rate of expansion of the universe," said Nelemans. "What we're trying to do is look inside the box."

If the supernova explosion is caused by material being pulled off a companion star onto the white dwarf, fusion of this material on the surface of the star should heat the star and produce a strong source of X-radiation prior to the explosion. Once the supernova explosion occurs, the white dwarf is expected to be completely destroyed and then would be undetectable in X-rays. In the merger scenario, the intensity of X-ray emission prior to the explosion is expected to be much weaker.

Based on the detection of a fairly strong X-ray source at approximately the position of SN 2007on 4 years before the explosion, Voss and Nelemans conclude that the data support the scenario where matter is pulled off a companion star. The small number of X-ray sources in the field implies that there is only a small chance of an unrelated source being so close by coincidence. Also, the X-ray source has similar properties to those expected for fusion on a white dwarf, unlike most X-ray sources in the sky.

However, in follow-up studies, Voss, Nelemans and colleagues Gijs Roelofs (Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.) and Cees Bassa (McGill University, Canada) used higher-quality optical images to better determine the supernova's position. This work, which is not yet published, shows a small, but significant difference in the measured positions of the supernova and the X-ray source, suggesting the source may not be the progenitor.

Follow-up Chandra observations hint that the X-ray object has disappeared, but further observations are needed to finally decide whether the source was the progenitor or not.

The team is also applying this new method to other supernovas and has high hopes that they will eventually succeed in identifying the elusive cause of at least some of these explosions.

"We're very excited about opening up a new way of studying supernovas, even though we're not sure that we've seen this particular stellar bomb before it exploded," said Gijs Roelofs. "We're very confident that we'll learn a lot more about these important supernovas in the future."

Voss agrees that, even if the X-ray source is not found to be the progenitor of SN 2007on, the hunt is worth the effort.

"Finding the progenitor to one of these Type Ia supernovas is a great chase in astronomy right now," he said. "These supernovas are great tools for studying dark energy, but if we knew more about how they form they might become even better tools."

___________________________________________________________
___________________________________________________________

Colliding Supergiants

This false-color image from the Curtis Schmidt Telescope in Chile shows a large star-forming region in the Large Magellanic Cloud. The binary system LH54-425 is arrowed. It is located in the star cluster LH54.
Credit: Chris Smith and the University of Michigan Curtis Schmidt Telescope at CTIO.

Using NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite and ground-based telescopes, astronomers have determined, for the first time, the properties of a rare, extremely massive, and young binary star system.

The merger of two massive stars to make a single super star of over 80 suns could lead to an object like Eta Carinae, which might have looked like LH54-425 one million years ago.

Finding stars this massive so early in their life is very rare. These results expand our understanding of the nature of very massive binaries, which was not well understood. The system will eventually produce a very energetic supernova.

The system, known as LH54-425, is located in the Large Magellanic Cloud, a satellite galaxy of our Milky Way. The binary consists of two O-stars, the most massive and luminous types of stars in the Universe.
[+/-] Click here to expand

Spectra obtained by Georgia State University astronomer Stephen Williams at the 1.5-meter (4.9 foot) telescope at the Cerro Tololo Inter-American Observatory in Chile show that the two stars contain about 62 and 37 times the mass of our Sun. “The stars are so close to each other - about one-sixth the average Earth-Sun distance - that they orbit around a common center of mass every 2.25 days,” says Douglas Gies of Georgia State University, Atlanta. With a combined mass of about 100 suns, the system is one the most extreme binaries known. The stars are probably less than 3 million years old.

Each star blows off a powerful stellar wind, and FUSE’s observations have provided the first details of what happens when the two supersonic winds collide. The wind collision zone wraps around the smaller star and produces a curved surface of superheated gases that emit X-rays and far-ultraviolet radiation. FUSE is ideal for these measurements because the lines that best indicate the properties of stellar winds show up in the far ultraviolet part of the spectrum, where FUSE is most sensitive.


FUSE project scientist George Sonneborn of NASA Goddard Space Flight Center, presented these results today in a poster at the spring 2007 American Astronomical Society meeting in Honolulu, Hawaii.

The more massive star is shedding material at a rate of 500 trillion tons per second (about 400 times greater than the rate the sun loses mass through the solar wind), with a speed of 5.4 million miles per hour. The smaller star is ejecting mass at about one-tenth the rate of its sibling. The mass loss rate of both stars is consistent with other single stars having the same temperature and luminosity.

As the stars age and swell in size, they will begin to transfer substantial amounts of mass to each other. This process could begin in a million years. The stars are orbiting so close to each other that they are likely to merge as they evolve, producing a single extremely massive star like the more massive member of the Eta Carinae binary system. Eta Carinae is one of the most massive and luminous stars in the Milky Way Galaxy, with perhaps 100 solar masses.

NASA's FUSE Satellite Catches Collision of Titans
by Bob Naeye - Goddard Space Flight Center.
________________________________________________________
________________________________________________________

Cataclysmic Variable

_______________________________________________________
Artist's impression of SDSS 1035+0551. Credit: Stuart Littlefair/Science.

The hot white dwarf is the same size as the Earth, yet weighs the same as our Sun. The brown dwarf is about the same size as Jupiter, but much more massive. The gravity of the white dwarf pulls gas from the brown dwarf; this gas spirals down onto the surface of the white dwarf, like water down a plughole, forming an "accretion disc" of hot gas around the white dwarf.

Where the falling gas from the brown dwarf hits the accretion disc, it creates a hot spot. The position of this hot spot depends on the masses of the two stars. The authors precisely timed when the white dwarf and hot spot were eclipsed by the brown dwarf. This allowed them to measure the location of the hot spot, and infer the masses of the two stars.

A long-standing and unverified prediction of binary star evolution theory is the existence of a population of white dwarfs accreting from substellar donor stars. Such systems ought to be common, but the difficulty of finding them, combined with the challenge of detecting the donor against the light from accretion, means that no donor star to date has a measured mass below the hydrogen burning limit.

A binary star system in which a white dwarf accretes material from a companion is called a cataclysmic variable (CV). Every kilogram of material that falls onto the white dwarf gains the energy equivalent of a few kilotons of TNT. Much of this energy is released as ultraviolet or x-ray radiation. Many CVs have been identified from this highly variable, short-wavelength light produced by rapid mass transfer onto the white dwarf.

However, most CVs should have evolved through this violent phase to become a "dead CV" with a low-mass companion that can support only weak mass transfer. Extensive efforts to confirm this long-standing prediction have failed to identify any CVs that have clearly survived the rapid mass transfer phase of their evolution. Now, a team of astronomers report in Science (see reference below) the unambiguous detection of a dead CV from a direct mass measurement of the low-mass companion in the CV SDSS 103533.03+055158.4 (SDSS 1035 for short).

A typical CV is smaller than the Sun, so there is a good chance that the orientation of the binary is such that the companion eclipses the white dwarf once every orbit as seen from Earth. This will lead to an apparent dimming of the CV every orbit during the few minutes that the companion blocks the light from the white dwarf. SDSS 1035 is an eclipsing CV, so there is a wealth of information to be gleaned from the changes in brightness during the eclipse. These show, for example, that the mass transferred from the low-mass companion forms a disc around the white dwarf with a bright spot on its outer edge due to the inflowing material.

The orbital period of SDSS 1035 is only 82 min, so small features such as the white dwarf are eclipsed in less than a minute. To accurately measure these rapid changes in brightness in such a faint star, Littlefair et al. used the William Herschel Telescope and ULTRACAM, an instrument they designed that uses CCDs to measure the brightness of CVs and other rapidly varying stars.

The data quality is impressive and leads to a mass for the companion accurate to about 4%. This is good enough to show convincingly that they are observing a genuine dead CV because the companion is well below the limit of 0.072 solar masses which a star cannot sustain nuclear reactions in its core. Objects that are born with masses below this limit are known as brown dwarfs.

Source ING Isaac Newton Group of Telescopes
ULTRACAM Obtains the First Observational Evidence of a Dead Cataclysmic Variable
More information:
A Ghostly Star Revealed in Silhouette Science, 314, 1550. Pierre F. L. Maxted.
Cataclysmic Cannibals in the Sky Science Press Release, 7 December 2006
_______________________________________________________
Binary Stars & Terrestial Worlds by Centauri Dreams
Colliding White Dwarfs could create a supergiant by Universe Today
_______________________________________________________
_______________________________________________________