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RE: Fomalhaut
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Our own solar system has a similar ring called the Kuiper Belt, but it is estimated to have a thousand times less material than Fomalhaut's ring.
Dust rings surrounding stars of a certain age are thought to be made by comet and asteroid collisions.

"In some ways, the Fomalhaut system is a bit like our own. For example, the team have found that the dust is lying in a very flat ring, which is the way our comets lie in our own Kuiper Belt. But the whole thing we see around Fomalhaut is on a much grander scale. And what's so striking is that the inner edge of this dust ring is so sharp - that really is very strong evidence that the inner edge is just being trimmed off, like with scissors, by an orbiting planet." - Jane Greaves, School of Physics and Astronomy at the University of St Andrews in Scotland.

Our Kuiper Belt lies about 30 to 50 AU from the sun. The inner edge of the Kuiper Belt is sharp due to Neptune's orbit at about 30 AU.
Neptune knocks against the dust directly, pushing it back away from the sun.
But in the case of Fomalhaut, the planet acts as a brake rather than a barrier, slowing the rate of dust falling in towards the star.

"You have material flowing in towards the star, but it encounters a resonance with a planet that's much closer in. So the material gets delayed in its journey towards the star. And as it gets delayed, it's essentially just like a traffic jam on the highway, you see an over-density of dust" - Paul Kalas, University of California, Berkeley, lead author of the Nature paper.

While the dust ring lies about 133 to 158 AU from the star, they believe the planet is much closer in, orbiting somewhere between 50 to 70 AU.
This places the planet far outside the star's habitable zone - the region where water could remain as a liquid on a planet's surface. The calculated habitable zone of Fomalhaut is thought to be between 5 to 10 AU.

The habitable zone in our solar system is about .95 to 1.37 AU, or roughly between the orbits of Venus and Mars. Fomalhaut's habitable zone is farther out because it is an A-class star, much brighter and hotter than our sun, a G-class star.
They estimate the planet orbiting Fomalhaut could be 5 times the mass of Jupiter or smaller. Fomalhaut is about 200 million years old, and if the history of our solar system is typical, planets would still be in the process of forming around such a young star.

These young planets would be hot glowing embers, and detectable by infrared or by visible light shifted to the red portion of the spectrum. Hubble should have been able to see any planet greater than 5 Jupiter masses orbiting between 50 to 100 AU.
While they only detected 1.4 lunar masses worth of material in Fomalhaut's dust disk, the scientists estimate it could contain as much as 50 to 100 Earth masses. They base this estimate on the idea that such belts should contain larger objects than the sand-grain-sized dust they detected.
"The Kuiper Belt objects appear as little point sources in the sky moving in reference to the background stars. The dust is almost too faint to be seen in our Kuiper Belt. But it should be there, because these Kuiper Belt objects collide, and theorists produce simulations of our Kuiper Belt that show dust must be produced. Ironically, what we see around Fomalhaut is the dust and not the parent objects" - Paul Kalas.

Fomalhaut's disk reflects 10 percent of light that's incident on it, similar to the Kuiper Belt, and so presumably they have similar compositions.
These disks are common around A-class stars; more than half of them may have dusty rings. Vega, a hotter and more massive A-class star than Fomalhaut, has a dust disk, and the scientists say there is some evidence that Vega also may have a planet shepherding that dust around.

While there is plenty of raw material in Fomalhaut's ring to construct planets, we know very little about the planetary systems of A-class stars. There are no radial velocity planet searches for these types of stars because they have a featureless spectrum.
Dust disks are less common around G-class stars like our sun, only about 1 in 10 are thought to have them. Presumably, the older the star is, the less apt it is to harbour a dusty disk.

"When we look at very old stars, we tend to find dust less often. But there are examples like Tau Ceti, which is almost a twin to the sun apart from being twice as old, it still has 10 times more orbiting comets than our sun does. This has really overthrown our idea that dust goes away and then planets sit there happily and life evolves."- Jane Greaves

Life on Earth is believed to have begun more than 800 million years after the formation of the sun and the planets, prevented from flourishing earlier due to the period of heavy asteroid bombardment.
A-class stars like Fomalhaut only live for about 713 million years, so if a planet does orbit that star, life would have a relatively short time to develop before the star turns into a red giant.

"But we don't really know. Maybe life could take a different path in this different environment." - Jane Greaves.

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RE: TW Piscis Austrini
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The nearby orange-red (K4-5) dwarf star TW Piscis Austrini (HR 8721) located about 24.9 light-years (ly) away, has been determined to be a distant physical companion of Fomalhaut (HR 8728).
It too is about 100 300 million years old and shares the common proper motion.
As a flare star, TW Piscis Austrini is its variable star designation.


Position(2000): RA 22:56:24.1, Dec -31:33:56.0

This star may have around 81 percent of Sol's mass, 76 - 85 percent of its diameter, and 12 - 13 percent of its visual luminosity.

The Yale Bright Star Catalogue's entry notes for TW Piscis Austrini indicate that it is a BY Draconi-type variable that varies in apparent visual magnitude from 6.44 to 6.49 over 10 days.

Through proper motion studies, another K5 dwarf (LTT 8273) is believed to be an optical companion.
These two stars , including Fomalhaut, Vega, and Castor may be the remaining member of a low-density star cluster that has gradually dispersed over 100 to 300 million year period.


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This image taken by NASA's Hubble Space Telescope is the most detailed visible-light image ever taken of a narrow, dusty ring around the nearby star Fomalhaut (HD 216956).


Position(2000): R.A. 22h 57m 39s.0465 Dec. -29� 37' 20".050

The image offers the strongest evidence yet that an unruly and unseen planet may be gravitationally tugging on the ring.
Part of the ring (at left) is outside the telescope's view. Hubble unequivocally shows that the centre of the ring is a whopping 2.25 billion kilometres (15 astronomical units) away from the star.
This is a distance equal to nearly halfway across our solar system. The geometrically striking ring, tilted obliquely toward Earth, would not have such a great offset if it were simply being influenced by Fomalhaut's gravity alone.

The view at bottom points out important features in the image, such as the ring's inner and outer edges. Astronomers used the Advanced Camera for Surveys' (ACS) coronagraph aboard Hubble to block out the light from the bright star so they could see the faint ring.

The dot near the ring's centre marks the star's location. Despite the coronagraph, some light from the star is still visible in this image, as can be seen in the wagon wheel-like spokes that form an inner ring around Fomalhaut


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The Fomalhaut system is thought to resemble our own solar system when it was about 200 million years old. Astronomers have a theoretical model for how the planets formed, but only by looking at young solar systems can they confirm that the process played out as expected.
The ring around Fomalhaut resembles our solar system`s Kuiper Belt, a region of comet-like objects beyond Neptune. Astronomers think that as planets, asteroids and comets develop out of dust, a lot of material is kicked either inward to the star our outward into a leftover ring of debris.


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Importantly, the theory holds that the region near a newborn star, such as the location of Earth, is bereft of water in a solar system`s early years. Some of icy material that develops farther out is, however, expected to be booted inward after planet formation, bringing precious water ice to nascent planets and providing the ingredients for life.




NASA's Hubble Chases Unruly Planet

A detailed image from NASA's Hubble Space Telescope offers the strongest evidence yet that an unruly and unseen planet may be gravitationally tugging on a dusty ring around the nearby star Fomalhaut (HD 216956).

The most detailed visible light image ever taken of a narrow, dusty ring unequivocally shows the centre is a whopping 1.4 billion miles away from the star; a distance nearly halfway across our solar system. The most plausible explanation is an unseen planet, moving in an elliptical orbit, is reshaping the ring with its gravitational pull. The geometrically striking ring, tilted toward Earth, would not have such a great offset if it were only being influenced by Fomalhaut's gravity.

An offset of the ring centre from the star has been inferred from previous lower resolution submillimetre wavelength telescope observations; and by applying theoretical modelling and physical assumptions. Hubble`s sharp images directly reveal the ring`s offset from Fomalhaut. The observations provide strong evidence at least one unseen planetary mass object is orbiting the star. If the orbiting object were larger than a planet, such as a brown dwarf star, Hubble would have detected it.



"Our new images confirm those earlier hypotheses that proposed a planet was perturbing the ring" - astronomer Paul Kalas of the University of California at Berkeley. The ring is similar to our solar system's Kuiper Belt, a vast reservoir of icy material left over from the formation of our solar system planets.

The ring's inner edge is sharper than its outer edge, a telltale sign that an object is gravitationally sweeping out material like a plough clearing away snow. Another classic signature of a planet's influence is the ring's relatively narrow width, about 2.3 billion miles. Without an object to gravitationally keep the ring material intact, the particles would spread out much wider.

The suspected planet may be orbiting far away from Fomalhaut, inside the dust ring's inner edge, between 4.7 billion and 6.5 billion miles from the star. The ring is approximately 12 billion miles from Fomalhaut, much farther than our outermost planet Pluto is from the sun. These observations do not directly detect the planet, so astronomers cannot measure its mass. They will use computer simulations of the ring's dynamics to estimate its mass.

Fomalhaut, a 200-million-year-old star, is a mere infant compared to our own 4.5-billion-year-old sun. It is 25 light-years from the sun in the constellation Piscis Austrinus (the Southern Fish). The Fomalhaut ring is 10-times as old as debris disks previously seen around the stars AU Microscopii and Beta Pictoris, where planets may still be forming. If our solar system is any example, planets should have formed around Fomalhaut within tens of millions of years after the birth of the star.

"The size of Fomalhaut's dust ring suggests not all planetary systems form and evolve in the same way -- planetary architectures can be quite different from star to star. While Fomalhaut's ring is analogous to the Kuiper Belt, its diameter is four times greater." - Paul Kalas.

Kalas and his collaborators used Hubble over a five-month period in 2004 to map the ring's structure. They used the Advanced Camera for Surveys' (ACS) coronagraph to block out light from the bright star, so they could see details in the faint ring. One side of the faint ring has yet to be imaged, because it extended beyond the ACS field of view. Astronomers plan to map the entire ring later this summer.

source

-- Edited by Blobrana at 22:47, 2005-06-22

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Image of Fomalhaut Dust Ring at 350 Microns: Relative Column Density Map Shows Pericenter-Apocenter Asymmetry

Authors:
K. A. Marsh, T. Velusamy, C. D. Dowell, K. Grogan, C. A. Beichman


The researchers have imaged the circumstellar disk of Fomalhaut at 350 microns wavelength, using SHARC II at the Caltech Submillimeter Observatory.
The spatial resolution of the raw images (9 arcsec) has been enhanced by a factor of three using the HiRes deconvolution procedure.
They find that at this wavelength and signal to noise ratio (~ 12), the observed morphology is that of a simple inclined ring (i ~ 70 deg), with little or no other apparent structure; this is the first observation that shows clearly the ring morphology of the disk.
They combined their 350 micron data with Spitzer Space Telescope images at 24, 70, and 160 microns in order to estimate the 2-dimensional spatial variation of relative column density ("tau map") using their DISKFIT procedure.

The tau map is based on the following physical assumptions:
(1) the wavelength variation of opacity is the same throughout the disk,
(2) the radial variation of dust temperature is dictated by the energy balance of individual grains in the stellar radiation field, and
(3) the vertical scale height of the disk follows a power-law radial variation.



The results confirm the ring-like morphology, but also show that the geometric centre is displaced from the star by about 8 AU and that the ring has an apocentric enhancement of approximately 14% in integrated column density.

If we interpret the displacement in terms of elliptical orbital motion due to gravitational perturbation by an unseen planet, then the implied forced eccentricity is ~ 0.06; dynamical modelling then predicts an apocentric density enhancement consistent with that inferred from the tau map.

Read More (PDF)


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New observations from NASA's Hubble Space Telescope (HST) will be presented by astronomers during a media teleconference at 1 p.m. EDT, Wednesday, June 22.

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A picture taken by Hubble has provided the most detailed visible-light image ever of a narrow, dusty ring around the nearby star Fomalhaut. It offers the strongest evidence yet, that an unruly and unseen planet could be gravitationally tugging on the ring.
The findings will appear in the June 23 issue of Nature.



Fomalhaut (Alpha Piscis Austrinus) is the first magnitude star in the constellation Piscis Austrinus, the Southern Fish. (Arabic Fam al-Hut, "Mouth of the fish").



The star is 25.07 light-years away.
Fomalhaut is a white main sequence dwarf star of spectral and luminosity type A3 V. According to various estimates, the star has about 2.3 times Sol's mass, 1.85 times its diameter, and about 14 to 17.6 times its luminosity.
It is thought that Fomalhaut is between 100 - 300 million years old

In 1983 an orbiting satellite called IRAS discovered infrared radiation coming from a huge disk of matter about four times the size of our own planetary system. The disk, with inner and outer radii of 100 and 140 AU, respectively, is thought to be made of icy dust particles that have been warmed by the star.

This doughnut sliced in half "sub-millimetre" image shows emissions from millimetre sized dust particles surrounding Fomalhaut. Yellow to red areas of the image indicate the highest concentrations of cold dust, while blue to black areas suggest very little dust.
The short-sub millimetre wavelengths data also suggested the presence of a "warp" in the dust disk.


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The dust disk has a hole in the centre comparable to the size of Sol's planetary system. The inner hole is relatively empty, with dust density that is less than 10 percent of the dust density in the torus. An inner cavity about 100 AU may indicate the presence of a planet.



A Saturn-sized object with about 30 percent of Jupiter's mass may lie in the gap in the torus just inside the inner edge of the dust disk.

"We were amazed to find that the disk is actually bent about the star. This strongly suggests there is an orbiting giant planet shaping the dust we see" - Dr. Wayne Holland, UK Astronomy Technology Centre (UK ATC) in Edinburgh.



The dust is densest at a distance from Fomalhaut around the Kuiper Belt's location in the Solar System.
The distance from Fomalhaut where an Earth-type planet would be "comfortable" with liquid water is around 2.11 - 6.58 AU , centred between the Main Asteroid Belt and Jupiter in the Solar System. At that distance from the star, such a planet would have an orbital period of about 5.67 Earth years.

"We believe Fomalhaut looks quite similar to our own solar system when it was only 200 million years old. At that age, a planetary system would already have formed. We would have trouble seeing it with optical telescopes because of the shroud of dust." - Ben Zuckerman of the University of California at Los Angeles.


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Position (J2000): RA: 22h57m39.05s Dec: -29d37m20.05s
Magnitude: 1.16

All image credits caltec/nasa/ UK ATC

-- Edited by Blobrana at 04:43, 2005-06-18

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