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Topic Name: Researchers Find the Black Hole's Gravitational Pull on the White Dwarf would Cause Tidal Forces Sufficient to a Supernova Explosion

Category: STAR (Space, Telecommunications & Radioscience)

Research persons: Enrico Ramirez-Ruiz

Location: University of California, Santa Cruz, United States

Details

A strange and violent fate awaits a white dwarf star that wanders too close to a moderately massive black hole. According to a new study, the black hole's gravitational pull on the white dwarf would cause tidal forces sufficient to disrupt the stellar remnant and reignite nuclear burning in it, giving rise to a supernova explosion with an unusual appearance. Observations of such supernovae could confirm the existence of intermediate-mass black holes, currently the subject of much debate among astronomers.

"Our supercomputer simulations show a peculiar supernova that would be a unique signature of an intermediate-mass black hole," said Enrico Ramirez-Ruiz, assistant professor of astronomy and astrophysics at the University of California, Santa Cruz.

Ramirez-Ruiz and his collaborators--Stephan Rosswog of Jacobs University in Bremen, Germany, and William Hix of Oak Ridge National Laboratory--used detailed computer simulations to follow the entire process of tidal disruption of a white dwarf by a black hole. Their simulations included gas dynamics, gravity, and nuclear physics, requiring weeks of computer time to simulate events that would take place in a fraction of a second. A paper describing their results has been accepted for publication in Astrophysical Journal Letters, and a preprint is currently available online.

"Every star that is not too massive ends up as a white dwarf, so they are very common. We were interested in whether tidal disruption can bring this stellar corpse to life again," said Rosswog, the first author of the paper.

A white dwarf can explode as a "type Ia" supernova if it accumulates enough mass by siphoning matter away from a companion star. When it reaches a critical mass (about 1.4 times the mass of the Sun), the white dwarf collapses and explodes. Astronomers use these type Ia supernovae as "standard candles" for cosmic distance measurements because their brightness evolves over time in a predictable manner.

The new paper describes a distinctly different mechanism for igniting a white dwarf, in which tidal disruption by a black hole causes drastic compression of the stellar material. The white dwarf is flattened into a pancake shape aligned in the plane of its orbit around the black hole. As each section of the star is squeezed through a point of maximum compression, the extreme pressure causes a sharp increase in temperatures, which triggers explosive burning.

The explosion ejects more than half of the debris from the disrupted star, while the rest of the stellar material falls into the black hole. The infalling material forms a luminous accretion disk that emits x-rays and should be detectable by the Chandra X-ray Observatory, the researchers said.

"This is a new mechanism for ignition of a white dwarf that results in a very different type of supernova than the standard type Ia, and it is followed by an x-ray source," Ramirez-Ruiz said.

He estimated that this type of event would occur about 100 times less frequently than the standard type Ia supernovae, but should be detectable by future surveys designed to observe large numbers of supernovae. The Large Synoptic Survey Telescope (LSST), planned for completion in 2013, is expected to discover hundreds of thousands of type Ia supernovae per year.

"These exotic creatures will start showing up in the data from the LSST," Ramirez-Ruiz said. "We want to predict the light curves so we can look for them in the survey data."

The mechanism described in the paper requires a black hole that is neither too small nor too big. Such intermediate-mass black holes (500 to 1,000 times the mass of the Sun) may reside in some globular star clusters, but there is much less evidence for their existence than there is for the relatively small stellar black holes (tens of times the mass of the Sun) or for supermassive black holes (a few million times the mass of the Sun), found at the centers of galaxies.

The new paper describes in detail the disruption of a white dwarf with two-tenths the mass of the Sun by a black hole 1,000 times the mass of the Sun. The researchers also found that they can vary the mass of the white dwarf and still get the same outcome--tidal disruption and ignition of the white dwarf.

"We can ignite the whole mass range of white dwarfs if they get close enough to the black hole," Rosswog said.

Note for White Dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. As white dwarfs have mass comparable to the Sun's and their volume is comparable to the Earth's, they are very dense. Their faint luminosity comes from the emission of stored heat. They comprise roughly 6% of all known stars in the solar neighborhood. The unusual faintness of white dwarfs was first recognized in 1910 by Henry Norris Russell, Edward Charles Pickering and Williamina Fleming;, p. 1 the name white dwarf was coined by Willem Luyten in 1922.
White dwarfs are thought to be the final evolutionary state of all stars whose mass is not too high—over 97% of the stars in our Galaxy., §1. After the hydrogen-fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core by the triple-alpha process. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf. Usually, therefore, white dwarfs are composed of carbon and oxygen. It is also possible that core temperatures suffice to fuse carbon but not neon, in which case an oxygen-neon-magnesium white dwarf may be formed. Also, some helium white dwarfs appear to have been formed by mass loss in binary systems.

Note for Type Ia Supernova
A Type Ia supernova is a sub-category of cataclysmic variable stars that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion. However, white dwarfs of the common carbon-oxygen variety are capable of further fusion reactions that release a great deal of energy if their temperatures rise high enough.
Physically, white dwarfs with a low rate of rotation are limited to masses that are below the Chandrasekhar limit of about 1.38 solar masses. This is the maximum mass that can be supported by electron degeneracy pressure. Beyond this limit the white dwarf would begin to collapse. If a white dwarf gradually accretes mass from a binary companion, its core is believed to reach the ignition temperature for carbon fusion as it approaches the limit. If the white dwarf merges with another star (a very rare event), it will momentarily exceed the limit and begin to collapse, again raising its temperature past the nuclear fusion ignition point. Within a few seconds of initiation of nuclear fusion, a substantial fraction of the matter in the white dwarf undergoes a runaway reaction, releasing enough energy (1-2 × 1044 joules) to unbind the star in a supernova explosion.
This category of supernovae produces consistent peak luminosity because of the uniform mass of white dwarfs that explode via the accretion mechanism. The stability of this value allows these explosions to be used to measure the distance to their host galaxies because the visual magnitude of the supernovae depends primarily on the distance.

Note for Tidal
Tides are the rising and falling of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans. Tidal phenomena can occur in any object that is subjected to a gravitational field that varies in time and space, such as the Earth's land masses.
Tides noticeably affect the depth of marine and estuarine water bodies and produce oscillating currents known as tidal streams, making prediction of tides very important for coastal navigation. The strip of seashore that is submerged at high tide and exposed at low tide, the intertidal zone, is an important ecological product of ocean tides.
The changing tide produced at a given location is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the effects of Earth rotation and the local shape of the sea floor. Sea level measured by coastal tide gauges may also be strongly affected by wind. A tide is a repeated cycle of sea level changes in the following stages:
Over several hours the water rises or advances up a beach in the flood 
The water reaches its highest level and stops at high tide. Because tidal currents cease this is also called slack water or slack tide. The tide reverses direction and is said to be turning. 
The sea level recedes or falls over several hours during the ebb tide. 
The level stops falling at low tide. This point is also described as slack or turning.

Note for Stellar Black Hole
A stellar black hole is a black hole formed by the gravitational collapse of a massive star (20 or more solar masses, though the exact amount of mass needed has not been determined and may depend on many parameters) at the end of its lifetime. The process is observed as a supernova explosion or as a gamma ray burst. The largest known stellar black hole (as of 2007) is 15.65±1.45 solar masses. Additionally, there is evidence that the IC 10 X-1 X-ray source is a stellar black hole with a probable mass of 24-33 solar masses.
A black hole could exist of any mass in theory (general relativity). The lower the mass, the higher the density of matter has to be in order to form a black hole (see e.g. the discussion in Schwarzschild radius, the radius of a black hole). There are no known processes that can produce black holes with mass less than a few times the mass of the Sun. If they exist, they are most likely primordial black holes.
The collapse of a star is a natural process to produce a black hole. It is inevitable at the end of the life of a star, when all stellar energy sources are exhausted. If the mass of the collapsing part of the star is below a certain critical value, the end product is a compact star, either a white dwarf or a neutron star. Both these stars have a maximum mass. So if the collapsing star has a mass exceeding this limit, the collapse will continue forever (catastrophic gravitational collapse) and form a black hole. The maximum mass of a neutron star is not well known, but is believed to be about 3 solar masses.
There is observational evidence for two other types of black holes, which are much more massive than stellar black holes. They are intermediate-mass black holes (in the centre of globular clusters) and supermassive black holes in the centre of the Milky Way and active galaxies.

Note for Supermassive Black Hole
A supermassive black hole is a black hole with a mass of an order of magnitude between 105 and 1010 (hundreds of thousands and tens of billions) of solar masses. It is currently thought that most, if not all galaxies, including the Milky Way, contain supermassive black holes at their galactic centers. There is also evidence that two supermassive black holes can co-exist in the same galaxy for a certain amount of time.
Supermassive black holes have properties which distinguish them from their relatively low-mass cousins:
The average density of a supermassive black hole (measured as the mass of the black hole divided by its Schwarzschild volume) can be very low, and may actually be lower than the density of air. This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, and mass merely increases linearly, the volume increases at a greater rate than mass. Thus, density decreases for increasingly larger radii of black holes.
The tidal forces in the vicinity of the event horizon are significantly weaker. Since the central singularity is so far away from the horizon, a hypothetical astronaut travelling towards the black hole center would not experience significant tidal force until very deep into the black hole.

About Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the mass limit for white dwarf stars to become neutron stars. "Chandra" also means "moon" or "luminous" in Sanskrit.
Chandra Observatory is the third of NASA's four Great Observatories. The first was Hubble Space Telescope; second the Compton Gamma Ray Observatory, launched in 1991; and last is the Spitzer Space Telescope. Prior to successful launch, the Chandra Observatory was known as AXAF, the Advanced X-ray Astrophysics Facility. AXAF was assembled and tested by TRW (now Northrop Grumman Space Technology) in Redondo Beach, California.
Since the Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes, requiring a space-based telescope to make these observations.

About Large Synoptic Survey Telescope
The Large Synoptic Survey Telescope (LSST) is a planned wide-field "survey" reflecting telescope that will photograph the available sky every three nights. Construction should start in 2010 with first light in 2015.
The telescope will be located on the El Peñón peak of Cerro Pachón, a 2682 metre high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.
Particular scientific goals of the LSST include:
Measuring weak gravitational lensing in the deep sky to detect dark energy and dark matter. 
Mapping small objects in the solar system, particularly Near-Earth asteroids and Kuiper belt objects. 
Detecting transient optical events such as Novae and Supernovae. 
Mapping the Milky way.

This research was supported by the Department of Energy's Program for Scientific Discovery through Advanced Computing.

In figure 1, A schematic of the Earth-Moon system (not to scale), showing the entire Earth following the motion of its center of gravity.

In figure 2, Image of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint dot to the lower left of the much brighter Sirius A.

In figure 3, Chandra X-ray Observatory

In figure 4, Large Synoptic Survey Telescope

In figure 5, An artist's conception of a supermassive black hole accreting from a disk

In figure 6, Multiwavelength X-ray image of SN 1572 or Tycho's Nova, the remnant of a Type Ia supernova