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Sirius B
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In 1844 the German astronomer Friedrich Bessel deduced from changes in the proper motion of Sirius that it had an unseen companion. Nearly two decades later, on January 31, 1862, American telescope-maker and astronomer Alvan Graham Clark first observed the faint companion, which is now called Sirius B, or affectionately "the Pup". This happened during testing of an 18.5-inch (470 mm) aperture great refractor telescope for Dearborn Observatory, which was the largest refracting telescope lens in existence at the time, and the largest telescope in America.
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Title: Piercing the Glare: Direct Imaging Search for Planets in the Sirius System
Authors: Christian Thalmann (1,2), Tomonori Usuda (3), Matthew Kenworthy (4), Markus Janson (5), Eric E. Mamajek (6), Wolfgang Brandner (2), Carsten Dominik (1,7), Miwa Goto (2), Yutaka Hayano (3), Thomas Henning (2), Phil M. Hinz (8), Yosuke Minowa (3), Motohide Tamura (9) ((1) Anton Pannekoek Institute, Amsterdam, (2) MPIA Heidelberg, (3) Subaru Telescope, (4) Leiden Observatory, (5) University of Toronto, (6) University of Rochester, (7) Radboud University, (8) University of Arizona, (9) NAOJ, Tokyo)

Astrometric monitoring of the Sirius binary system over the past century has yielded several predictions for an unseen third system component, the most recent one suggesting a \leq50 MJup object in a ~6.3-year orbit around Sirius A. Here we present two epochs of high-contrast imaging observations performed with Subaru IRCS and AO188 in the 4.05 \mum narrow-band Br alpha filter. These data surpass previous observations by an order of magnitude in detectable companion mass, allowing us to probe the relevant separation range down to the planetary mass regime (6-12 M_Jup at 1", 2-4 M_Jup at 2", and 1.6 M_Jup beyond 4"). We complement these data with one epoch of M-band observations from MMT/AO Clio, which reach comparable performance. No dataset reveals any companion candidates above the 5-sigma level, allowing us to refute the existence of Sirius C as suggested by the previous astrometric analysis. Furthermore, our Br alpha photometry of Sirius B confirms the lack of an infrared excess beyond the white dwarf's blackbody spectrum.

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Title: Sirius B Imaged in the Mid-Infrared: No Evidence for a Remnant Planetary System
Authors: Andrew J. Skemer, Laird M. Close

Evidence is building that remnants of solar systems might orbit a large percentage of white dwarfs, as the polluted atmospheres of DAZ and DBZ white dwarfs indicate the very recent accretion of metal-rich material. (Zuckerman et al. 2010). Some of these polluted white dwarfs are found to have large mid-infrared excesses from close-in debris disks that are thought to be reservoirs for the metal accretion. These systems are coined DAZd white dwarfs (von Hippel et al. 2007). Here we investigate the claims of Bonnet-Bidaud & Pantin (2008) that Sirius B, the nearest white dwarf to the Sun, might have an infrared excess from a dusty debris disk. Sirius B's companion, Sirius A is commonly observed as a mid-infrared photometric standard in the Southern hemisphere. We combine several years of Gemini/T-ReCS photometric standard observations to produce deep mid-infrared imaging in five ~10 micron filters (broad N + 4 narrowband), which reveal the presence of Sirius B. Our photometry is consistent with the expected photospheric emission such that we constrain any mid-infrared excess to <10% of the photosphere. Thus we conclude that Sirius B does not have a large dusty disk, as seen in DAZd white dwarfs.

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RE: Sirius
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Sirius is white because it's white-hot, much hotter than our sun, and that's the way it has been throughout human history. Like the sun, Sirius is a settled, stable, hydrogen-fusing star in the midst of its long prime of life. A million years is nothing to a star like this.
But for a while, historians of ancient Greece and Rome thought they had discovered otherwise.
Around 135 AD, the astronomer Claudius Ptolemy in Alexandria listed Sirius as one of six bright stars that have a "sub-red" or fiery character. The other five really do look fire-coloured, and this led to centuries of speculation that Sirius has changed colour within historical times. Several other ancient writers called it reddish, too, though others described it as white or blue-white, the way it looks now. Could the star have been a red giant just a couple of thousand years ago?

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The deepest infrared image around the brightest star.
 Sirius, the brightest star of the sky, is reported to have shown a change of colour, a possibility that has led some scientists to suspect a possible but yet undetected small companion.
Now using a specific mask and the modern technique of adaptive optics which allows to suppress most of the blurring of the atmosphere, Jean-Marc Bonnet-Bidaud and Eric Pantin of the C.E.A. Astrophysical Department have obtained the first and most sensitive image of the Sirius field, in the infrared domain.

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Canis Major
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The Canis Major constellation
Exposure 10 second taken at 1:48 UT on Wednesday 24 January 2007.

Canis Major

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This picture is an artist's impression showing how the binary star system of Sirius A and its diminutive blue companion, Sirius B, might appear.


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Sirius A and Sirius B.
The projected separation of Sirius A and Sirius B in this image is 6".10, which at the distance of Sirius is 16.1 Astronomical Units (AU). The semimajor axis of the relative orbit is 7".48, or 19.7 AU.
Credit: NASA, ESA and G. Bacon (STScI)


The large, bluish-white star Sirius A dominates the scene, while Sirius B is the small but very hot and blue white-dwarf star on the right. The two stars revolve around each other every 50 years. White dwarfs are the leftover remnants of stars similar to our Sun. The Sirius system, only 8.6 light-years from Earth, is the fifth closest stellar system known. Sirius B is faint because of its tiny size. Its diameter is only 7,500 miles, slightly smaller than the size of our Earth. The Sirius system is so close to Earth that most of the familiar constellations would have nearly the same appearance as in our own sky. In this rendition, we see in the background the three bright stars that make up the Summer Triangle: Altair, Deneb, and Vega. Altair is the white dot above Sirius A; Deneb is the dot to the upper right; and Vega lies below Sirius B. But there is one unfamiliar addition to the constellations: our own Sun is the second-magnitude star, shown as a small dot just below and to the right of Sirius A.

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A new study reveals that the brightest star in our sky, the brilliant blue-white Sirius, has a companion that’s smaller than Earth but about 98 percent as massive as the Sun, .

Astronomers already knew that Sirius had a stellar companion. But they didn’t know the object’s mass. The new measurements were done by an international team of astronomers using the Hubble Space Telescope.
Sirius, Alpha Canis Majoris, is one of the closest known stars at 8.6 light-years away. It is twice as massive as the Sun and has a surface temperature of 9,880 degrees Kelvin. It is white class A0mV - A1mV early dwarf Star and is "metal rich," its iron content triple that of the Sun, most likely from some sort of elemental diffusion.
The companion, called Sirius B, is known to be a white dwarf, and much hotter. It is the nearest white dwarf to the Sun. It was discovered in 1862 but observation is difficult because of the glare of the primary star. Sirius B is about 10,000 times fainter than Sirius.

The two orbit each other with a 50.1 year period at an average distance of 19.8 Astronomical Units, about Uranus's distance from the Sun, a large orbital eccentricity carrying them from 31.5 AU apart to 8.1 AU and back again. The two were closest in 1994 and will be again in 2044, while they will be farthest apart in 2019. From the orbit, we find that Sirius A and B have respective masses of 2.1 and 1.0 times that of the Sun (the radius of Sirius A coming in at 1.7 solar, in agreement with the measured angular diameter).

"Studying Sirius B has challenged astronomers for more than 140 years" - Martin Barstow of the University of Leicester, U.K.


Position(2000): RA = 06h45m08.9s Dec = -16°42'58"
This Hubble Space Telescope image shows Sirius A, the brightest star in our nighttime sky, along with its faint, tiny stellar companion, Sirius B. Astronomers overexposed the image of Sirius A [at centre] so that the dim Sirius B (tiny dot at lower left) could be seen.
Credit: NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester


The white dwarf’s mass was calculated by studying how its intense gravitational field alters the wavelengths of light emitted by the main star. The results are published in the Monthly Notices of the Royal Astronomical Society.
White dwarfs are involved in explosions called Type Ia supernovas, which are used to measure cosmological distances and the universe’s rate of expansion.

"Accurately determining the masses of white dwarfs is fundamentally important to understanding stellar evolution. Our Sun will eventually become a white dwarf. White dwarfs are also the source of Type Ia supernova explosions that are used to measure cosmological distances and the expansion rate of the universe. Measurements based on Type Ia supernovae are fundamental to understanding 'dark energy,' a dominant repulsive force stretching the universe apart. Also, the method used to determine the white dwarf's mass relies on one of the key predictions of Einstein's theory of General Relativity; that light loses energy when it attempts to escape the gravity of a compact star" - Martin Barstow


Sirius, commonly referred to as the "Dog Star", is currently found in the constellation Canis Major.

Sirius B, is just 12,000 kilometres in diameter, has a gravitational field that is 350,000 times greater than Earth's. A person weighing 68 kilograms on Earth would weigh 25 million kilograms standing on Sirius B.
Based on Einstein’s General Relativity theory of 1916, light from the surface of the hot white dwarf has to climb out of this gravitational field and is stretched to longer, redder wavelengths of light in the process. It is gravitationally redshifted
The new observations refined the measurement of Sirius B’s surface temperature to be 24,800 ±100 degrees Kelvin.

White dwarfs are the leftover remnants of stars similar to our Sun. They have exhausted their nuclear fuel sources and have collapsed down to a very small size.

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The white dwarf orbiting Sirius started as a blue star with five times the Sun's mass. If this blue star still shone today, Sirius would be so bright it would cast shadows on Earth.
Just 8.6 light-years away, Sirius is already the brightest nighttime star. It is double: a bright A-type main-sequence star and a faint white dwarf - the closest white dwarf to Earth.
Now James Liebert, David Arnett, Jay Holberg, and Kurtis Williams of the University of Arizona and Patrick Young of Los Alamos National Laboratory have studied Sirius's evolution.
They first had to know the stars' masses, because the more massive a star, the faster it evolves. The two stars orbit each other every fifty years. This orbital motion reveals that Sirius A, the brighter star, has 2.02 ± 0.03 solar masses and Sirius B, the white dwarf, has 1.00 ± 0.02 solar masses.

Liebert and his colleagues then determined the age of Sirius A. As a main-sequence star ages, its luminosity and diameter change. Recent interferometric work found Sirius A's diameter to be 71 percent greater than the Sun's.
By modelling the evolution of a star with Sirius A's mass, the astronomers find the star achieves its current luminosity and diameter 225 to 250 million years after birth. This age means Sirius has completed just one orbit around the Galaxy. In contrast, the Sun is 4.6 billion years old--twenty times older.

Next Liebert's team examined Sirius B. It was once a main-sequence star, like Sirius A. Then it swelled into a red giant and shrank into a white dwarf. White dwarfs cool and fade as they age. From Sirius B's mass and temperature (25,000 Kelvin), the astronomers estimate the star became a white dwarf around 124 million years ago. Thus, the star shone as a main-sequence star and then a red giant for 101 to 126 million years - the expected lifetime of a star born with about 5 solar masses.

A main-sequence star with this mass would have emitted a few times more light than Regulus in Leo, which a 2005 study found has 3.4 solar masses.
If Sirius B shone so brightly today, it would look brighter than Venus - bright enough to cast shadows. When Sirius B became a red giant, it must have lost 80 percent of its mass, because today it has the same mass as the Sun.

The new work casts further doubt on a long-held belief that Sirius moves through space with the five central stars of the Big Dipper. In 1909, Danish astronomer Ejnar Hertzsprung said Sirius probably belonged to the so-called Ursa Major moving group, whose core is 80 light-years away.
In 2003, however, Jeremy King of Clemson University and his colleagues questioned whether Sirius was truly a member of the group. They also put the group's age at 500 million years. This is twice the age Liebert and his colleagues find for Sirius, suggesting the two have nothing to do with each other.
The astronomers will publish their work in a future issue of Astrophysical Journal Letters.

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