Image of the Day: White Dwarf Ripped Apart by Black Hole

Image of the Day: White Dwarf Ripped Apart by Black Hole
Image of the Day: White Dwarf Ripped Apart by Black Hole The Daily Galaxy --Great Discoveries Channel: Sci, Space, TechFollow the Daily Galaxy  « Image of the Day: A Spiral Galaxy in Eridanus --'Supernova Mine Field' |Main| Search for Extraterrestrial Life Becomes More Complex --"Strategies Require Rethinking" »

August 02, 2012 Image of the Day: White Dwarf Ripped Apart by Black Hole

 

"We think these unusual signatures can be explained by a white dwarf that strayed too close to a black hole and was torn apart by the extreme tidal forces."

Joel Bregman of the University of Michigan.

Results from NASA's Chandra X-ray Observatory and the Magellan telescopes suggest that a dense stellar remnant has been ripped apart by a black hole a thousand times as massive as the Sun in NGC 1399, an elliptical galaxy about 65 million light years from Earth.In the image above, X-rays from Chandra Space Observatory are shown in blue and are overlaid on an optical image from the Hubble Space Telescope. The Chandra observations show that this object is a so-called ultraluminous X-ray source (ULX). ULXs emit more X-rays than stars, but less than quasars. Their exact nature has remained a mystery, but one suggestion is that some ULXs are black holes with masses between about a hundred and a thousands times that of the Sun.

If confirmed, this discovery would be a cosmic double play: it would be strong evidence for an intermediate mass black hole, which has been a hotly debated topic, and would mark the first time such a black hole has been caught tearing a star apart.

The intensity of the X-ray emission places the source in the "ultraluminous X-ray source" or ULX category, meaning that it is more luminous than any known stellar X-ray source, but less luminous than the bright X-ray sources (active galactic nuclei) associated with supermassive black holes in the nuclei of galaxies. The nature of ULXs is a mystery, but one suggestion is that some ULXs are black holes with masses between about a hundred and several thousand times that of the Sun, a range intermediate between stellar-mass black holes and supermassive black holes located in the nuclei of galaxies.

This ULX is in a globular cluster, a very old and crowded conglomeration of stars. Astronomers have suspected that globular clusters could contain intermediate-mass black holes, but conclusive evidence for this has been elusive.

"Astronomers have made cases for stars being torn apart by supermassive black holes in the centers of galaxies before, but this is the first good evidence for such an event in a globular cluster," said Jimmy Irwin of the University of Alabama who led the study.

Irwin and his colleagues obtained optical spectra of the object using the Magellan I and II telescopes in Las Campanas, Chile. These data reveal emission from gas rich in oxygen and nitrogen but no hydrogen, a rare set of signals from globular clusters. The physical conditions deduced from the spectra suggest that the gas is orbiting a black hole of at least 1,000 solar masses. The abundant amount of oxygen and absence of hydrogen indicate that the destroyed star was a white dwarf, the end phase of a solar-type star that has burned its hydrogen leaving a high concentration of oxygen. The nitrogen seen in the optical spectrum remains an enigma.

Theoretical work suggests that the tidal disruption-induced X-ray emission could stay bright for more than a century, but it should fade with time. So far, the team has observed there has been a 35 percent in X-ray emission from 2000 to 2009.

The Daily Galaxy via JPL/NASA

 

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The 'Vulpecula White Dwarf' --A Hot Zone for Habitable Planets?

The 'Vulpecula White Dwarf' --A Hot Zone for Habitable Planets?
The 'Vulpecula White Dwarf' --A Hot Zone for Habitable Planets?The Daily Galaxy --Great Discoveries Channel: Sci, Space, TechFollow the Daily Galaxy  « The Most Massive Black Hole in the Universe --6.6 Billion Solar Masses |Main| Image of the Day: A Spiral Galaxy in Eridanus --'Supernova Mine Field' »

August 01, 2012 The 'Vulpecula White Dwarf' --A Hot Zone for Habitable Planets?

 

       

Looking like an image out of a video game, M 27, or NGC 6853 is a planetary nebula in the constellation Vulpecula, at a distance of about 1,360 light years. Its central star, a white dwarf, is larger than any other known white dwarf in the universe. Also known by the absurd name, the Dumbbell, it contains knots marked by a central region of dark and bright cusped knots and their associated dark tails. The knots vary in appearance from symmetric objects with tails to rather irregular tail-less objects. Similarly to the Helix Nebula and the Eskimo Nebula, the heads of the knots have bright cusps which are local.

It could just be that the most productive to look for planets that can support life is around dim, dying stars called white dwarfs. Eric Agol, a University of Washington  professor of astronomy, has suggested that potentially habitable planets orbiting white dwarfs could be much easier to find than other exoplanets located so far.
White dwarfs, though born hot, typically have about 60 percent of the mass of the sun, but by volume they are only about the size of Earth. They eventually become cooler than the sun and emit just a fraction of its energy, so the habitable zones for their planets are significantly closer than Earth is to the sun.

"If a planet is close enough to the star, it could have a stable temperature long enough to have liquid water at the surface –- if it has water at all –- and that's a big factor for habitability," Agol said.

The nearest white dwarf to Earth is Sirius B at a distance of about 8.5 light years (a light year is about 6 trillion miles). It is believed to once have been five times more massive than the sun, but now it has about the same mass as the sun packed into the same volume as Earth.

Agol is proposing a survey of the 20,000 white dwarfs closest to Earth. Using a 1-meter ground telescope, he said, one star could be surveyed in 32 hours of observation. If there is no telltale dimming of light from the star in that time, it means no planet orbiting closely enough to be habitable is passing in front of the star so that it is easily observable from Earth. Ideally, the work could be carried out by a network of telescopes that would make successive observations of a white dwarf as it progresses through the sky.
"This could take a huge amount of time, even with such a network," he said.

The same work could be accomplished by larger specialty telescopes, such as the Large Synoptic Survey Telescope that is planned for operations later this decade in Chile, of which the UW is a founding partner. If it turns out that the number of white dwarfs with potential Earthlike planets is very small –- say one in 1,000 –- that telescope still would be able to track them down efficiently.

Finding an Earthlike planet around a white dwarf could provide a meaningful place to look for life, Agol said. But it also would be a potential lifeboat for humanity if Earth, for some reason, becomes uninhabitable.

"Those are the reasons I find this project interesting," he said. "And there's also the question of, 'Just how special is Earth?'"

The Daily Galaxy via astrobio.net and ESO.org

Image Credit: http://www.sc.eso.org/~tszeifer/

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