Universe Today

UV image of supernova in spiral galaxy M100. Image credit: ESA/NASA/Immer et al. Click to enlarge
Scientists have found that a star that exploded in 1979 is as bright today in X-ray light as it was when it was discovered years ago, a surprise finding because such objects usually fade significantly after only a few months.

Using ESA?s XMM-Newton space observatory, a team of astronomers has discovered that this supernova, called SN 1979C, shows no sign of fading. The scientists can document a unique history of the star, both before and after the explosion, by studying rings of light left over from the blast, similar to counting rings in a tree trunk.
?This 25-year-old candle in the night has allowed us to study aspects of a star explosion never before seen in such detail,? said Dr Stefan Immler, leader of the team, from NASA?s Goddard Space Flight Center, USA. ?All the important information that usually fades away in a couple of months is still there.?

Among the many unique finds is the history of the star?s stellar wind dating back 16 000 years before the explosion. Such a history is not even known about our Sun. Also, the scientists could measure the density of the material around the star, another first. The lingering mystery, though, is how this star could fade away in visible light yet remain so radiant in X-rays.

Without fuel and thus energy to support their gravity, such stars first implode. The core reaches a critical density, and much of the collapsing matter gets bounced back out violently into space by powerful shockwaves.

Supernovae can outshine an entire galaxy and are often easily seen in neighbouring galaxies with simple amateur telescopes. Supernovae are typically half as bright after about ten days and fade steadily after that, regardless of the wavelength.

SN 1979C has in fact faded in optical light by a factor of 250 becoming barely visible with a good amateur telescope. In X-rays, however, this supernova is still the brightest object in its host galaxy, M100, in the constellation ?Coma Berenices?.

In identifying the history of the star that created SN 1979C, the team found that this star, about 18 times more massive than our Sun, produced fierce stellar winds. That material was flung into space for millions of years, creating concentric rings around the star.

The X-rays – produced after the explosion when the supernova shock caught up with the stellar wind and heated it to a temperature of several million degrees – illuminated 16 000 years? worth of stellar activity.

?We can use the X-ray light from SN 1979C as a ?time machine? to study the life of a dead star long before it exploded,? said Immler.

The detailed analysis was only possible because SN 1979C has not yet faded away. Scientists have 25 years? worth of data in a variety of wavelengths, from radio waves through to optical/ultraviolet and X-rays. They speculate that the abundance of stellar wind has provided ample material to keep SN 1979C glowing so brightly.

The team also captured a rare glimpse of the ultraviolet radiation from the supernova using XMM-Newton. The ultraviolet image independently confirms what the X-ray analysis found: that the circumstellar material ? covering a region 25 times larger than our Solar System – has a relatively high density of 10 000 atoms per cubic centimetre, or about 1000 times denser than the wind from our Sun. The ultraviolet image also shows galaxy M100 in detail never seen before.

?XMM-Newton is known among scientists as a superior X-ray observatory, but the study of SN 1979 demonstrates the importance of the satellite’s simultaneously observing ultraviolet and optical telescope,? said Dr Norbert Schartel, XMM-Newton Project Scientist at ESA’s European Space Astronomy Centre (ESAC) in Spain.

Original Source: ESA Portal

Artist’s conception of a possible collision around BD +20 307. Image credit: Gemini Observatory/Jon Lomberg. Click to enlarge
A relatively young star located about 300 light-years away is greatly improving our understanding of the formation of Earth-like planets.

The star, going by the unassuming name of BD +20 307, is shrouded by the dustiest environment ever seen so close to a Sun-like star well after its formation. The warm dust is believed to be from recent collisions of rocky bodies at distances from the star comparable to that of the Earth from the Sun. The results were based on observations done at the Gemini and W.M. Keck Observatories, and were published in the July 21 issue of the British science journal Nature.

This finding supports the idea that comparable collisions of rocky bodies occurred early in our solar system’s formation about 4.5 billion years ago. Additionally, this work could lead to more discoveries of this sort which would indicate that the rocky planets and moons of our inner solar system are not as rare as some astronomers suspect.

?We were lucky. This set of observations is like finding the proverbial needle in the haystack,? said Inseok Song, the Gemini Observatory astronomer who led the U.S.-based research team. ?The dust we detected is exactly what we would expect from collisions of rocky asteroids or even planet-sized objects, and to find this dust so close to a star like our Sun bumps the significance way up. However, I can’t help but think that astronomers will now find more average stars where collisions like these have occurred.”

For years, astronomers have patiently studied hundreds of thousands of stars in the hopes of finding one with an infrared dust signature (the characteristics of the starlight absorbed, heated up and reemitted by the dust) as strong as this one at Earth-to-Sun distances from the star. “The amount of warm dust near BD+20 307 is so unprecedented I wouldn’t be surprised if it was the result of a massive collision between planet-size objects, for example, a collision like the one which many scientists believe formed Earth’s moon,” said Benjamin Zuckerman, UCLA professor of physics and astronomy, member of NASA’s Astrobiology Institute, and a co-author on the paper. The research team also included Eric Becklin of UCLA and Alycia Weinberger formerly at UCLA and now at the Carnegie Institution.

BD +20 307 is slightly more massive than our Sun and lies in the constellation Aries. The large dust disk that surrounds the star has been known since astronomers detected an excess of infrared radiation with the Infrared Astronomical Satellite (IRAS) in 1983. The Gemini and Keck observations provide a strong correlation between the observed emissions and dust particles of the size and temperatures expected by the collision of two or more rocky bodies close to a star.

Because the star is estimated to be about 300 million years old, any large planets that might orbit BD +20 307 must have already formed. However, the dynamics of rocky remnants from the planetary formantion process might be dictated by the planets in the system, as Jupiter did in our early solar system. The collisions responsible for the observed dust must have been between bodies at least as large as the largest asteroids present today in our solar system (about 300 kilometers across). “Whatever massive collision ocurred, it managed to totally pulverize a lot of rock,” said team member Alycia Weinberger.

Given the properties of this dust, the team estimates that the collisions could not have occurred more than about 1,000 years ago. A longer history would give the fine dust (about the size of cigarette smoke particles) enough time to be dragged into the central star.

The dusty environment around BD +20 307 is thought to be quite similar, but much more tenuous than what remains from the formation of our solar system. “What is so amazing is that the amount of dust around this star is approximately one million time greater than the dust around the Sun,” said UCLA team member Eric Becklin. In our solar system the remaining dust scatters sunlight to create an extremely faint glow called the zodiacal light (see image above). It can be seen under ideal conditions with the naked eye for a few hours after evening or before morning twilight.

The team?s observations were obtained using Michelle, a mid-infrared spectrograph/imager built by the UK Astronomy Technology Centre, on the Frederick C. Gillette Gemini North Telescope, and the Long Wavelength Spectrograph (LWS) at the W.M. Keck Observatory on Keck I.

Original Source: Gemini Observatory News Release