* Astronomy

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RE: AB Aurigae

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Title: The circumstellar disc of AB Aurigae: evidence for envelope accretion at late stages of star formation?
Authors: Ya-Wen Tang, Stephane Guilloteau, Vincent Pietu, Anne Dutrey, Nagayoshi Ohashi, Paul T. P. Ho

The circumstellar disc of AB Aurigae has garnered great attentions due to the apparent existence of spirals at a relatively young stage and also the asymmetric disc traced in thermal dust emission. However, the physical conditions of the spirals are still not well understood. The origin of the asymmetric thermal emission is unclear.
We observe the disc at 230 GHz (1.3 mm) in both continuum and the spectral line ^12 CO 2-1 with IRAM 30 m, the PdBI and the SMA to sample all spatial scales from 0.37" to about 50". To combine the data obtained from these telescopes, several methods and calibration issues have been checked and discussed.
The 1.3 mm continuum (dust) emission is resolved into inner disc and outer ring. The emission from the dust ring is highly asymmetric in azimuth, with intensity variations by a factor 3. Molecular gas at high velocities traced by the CO line is detected aside the stellar location. The inclination angle of the disc is found to decrease toward the center. At larger scale, based on the intensity weighted dispersion and the integrated intensity map of ^12 CO 2-1, four spirals are identified, where two of them are also detected in the near-infrared. The total gas mass of the 4 spirals (M_spiral) is ~10^-6 solar masses, which is 3 orders of magnitude smaller than the mass of the gas ring. Surprisingly, the CO gas inside the spiral is apparently counter-rotating with respect to the CO disc and exhibits only small radial motion.
Although an hypothetic fly-by cannot be ruled out, the most likely explanation of the AB Aurigae system may be inhomogeneous accretion well above/or below the main disc plane from the remnant envelope, which can explain both the rotation and large scale motions detected with the 30-m image.

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AB Aurigae is a star in the Auriga constellation. It is better known for hosting a dust disk that may harbour a condensing planet or brown dwarf. The star could host a possible substellar companion in wide orbit.
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Absolute magnitude (MV): 7.18

Google earth file: AB Aurigae.kmz (1kb, kmz)

Position (J2000): R.A. 04 55 45.8445 | Dec. +30° 33' 04.292''

Blobrana · L

Title: Direct Imaging of Fine Structures in Giant Planet Forming Regions of the Protoplanetary Disk around AB Aurigae
Authors: J. Hashimoto, M. Tamura, T. Muto, T. Kudo, M. Fukagawa, T. Fukue, M. Goto, C. A. Grady, T. Henning, K. Hodapp, M. Honda, S. Inutsuka, E. Kokubo, G. Knapp, M. W. McElwain, M. Momose, N. Ohashi, Y. K. Okamoto, M. Takami, E. L. Turner, J. Wisniewski, M. Janson, L. Abe, W. Brandner, J. Carson, S. Egner, M. Feldt, T. Golota, O. Guyon, Y. Hayano, M. Hayashi, S. Hayashi, M. Ishii, R. Kandori, N. Kusakabe, T. Matsuo, S. Mayama, S. Miyama, J.-I. Morino, A. Moro-Martin, T. Nishimura, T.-S. Pyo, H. Suto, R. Suzuki, N. Takato, H. Terada, C. Thalmann, D. Tomono, M. Watanabe, T. Yamada, H. Takami, T. Usuda

We report high-resolution 1.6 \micron polarised intensity (PI) images of the circumstellar disk around the Herbig Ae star AB Aur at a radial distance of 22 AU (0."15) up to 554 AU (3."85), which have been obtained by the high-contrast instrument HiCIAO with the dual-beam polarimetry. We revealed complicated and asymmetrical structures in the inner part (\lesssim 140 AU) of the disk, while confirming the previously reported outer (r \gtrsim 200 AU) spiral structure. We have imaged a double ring structure at ~40 and ~100 AU and a ring-like gap between the two. We found a significant discrepancy of inclination angles between two rings, which may indicate that the disk of AB Aur is warped. Furthermore, we found seven dips (the typical size is ~45 AU or less) within two rings as well as three prominent PI peaks at ~40 AU. The observed structures, including a bumpy double ring, a ring-like gap, and a warped disk in the innermost regions, provide essential information for understanding the formation mechanism of recently detected wide-orbit (r >20 AU) planets.

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Title: The Solar-System-Scale Disk Around AB Aurigae
Authors: Ben R. Oppenheimer (1), Douglas Brenner (1), Sasha Hinkley (2), Neil Zimmerman (2), Anand Sivaramakrishnan (1), Remi Soummer (1), Jeffrey Kuhn (3), James R. Graham (4), Marshall Perrin (4), James P. Lloyd (5), Lewis C. Roberts, Jr. (6), David M. Harrington (3) ((1)AMNH, (2) Columbia, (3) IfA Hawaii, (4) UC Berkeley, (5) Cornell, (6) Boeing)

The young star AB Aurigae is surrounded by a complex combination of gas-rich and dust dominated structures. The inner disk which has not been studied previously at sufficient resolution and imaging dynamic range seems to contain very little gas inside a radius of least 130 astronomical units (AU) from the star. Using adaptive-optics coronagraphy and polarimetry we have imaged the dust in an annulus between 43 and 302 AU from the star, a region never seen before. An azimuthal gap in an annulus of dust at a radius of 102 AU, along with a clearing at closer radii inside this annulus, suggests the formation of at least one small body at an orbital distance of about 100 AU. This structure seems consistent with crude models of mean motion resonances, or accumulation of material at two of the Lagrange points relative to the putative object and the star. We also report a low significance detection of a point source in this outer annulus of dust. This source may be an overdensity in the disk due to dust accreting onto an unseen companion. An alternate interpretation suggests that the object's mass is between 5 and 37 times the mass of Jupiter. The results have implications for circumstellar disk dynamics and planet formation.

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Credit Lyot Project

The red spot at the centre of this image may be the location of an infalling dust cloud that surrounds an exoplanet.

Blobrana · L

No one has ever obtained an undisputed image of a planet around an ordinary star. One confirmed planet called 2M1207b has previously been photographed in orbit around a brown dwarf. A brown dwarf is more massive than a planet but too puny to shine the way ordinary stars do, by sustaining the nuclear fusion of hydrogen in their cores.
Although the new observations are not sensitive enough to see the suspected planet directly, they show a small bright patch that may be a cloud of gas and dust falling onto a developing planet.
The bright spot is next to a star called AB Aurigae, which is 2.4 times the mass of the Sun and about 3 million years old. That is about the age when planets are thought to form, so further observations could reveal unprecedented details about the early stages of the process.

Credit Lyot Project

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Blobrana · L

Title: THE SOLAR-SYSTEM-SCALE DISK AROUND AB AURIGAE
Authors: Ben R. Oppenheimer, Douglas Brenner, Sasha Hinkley, Neil Zimmerman, Anand Sivaramakrishnan, Remi Soummer, Jeffrey Kuhn, James R. Graham, Marshall Perrin, James P. Lloyd, Lewis C. Roberts, Jr., and David M. Harrington

The young star AB Aurigae is surrounded by a complex combination of gas-rich and dust dominated structures. The inner disk which has not been studied previously at sufficient resolution and imaging dynamic range seems to contain very little gas inside a radius of least 130 astronomical units (AU) from the star. Using adaptive-optics coronagraphy and polarimetry we have imaged the dust in an annulus between 43 and 302 AU from the star, a region never seen before. An azimuthal gap in an annulus of dust at a radius of 102 AU, along with a clearing at closer radii inside this annulus, suggests the formation of at least one small body at an orbital distance of about 100 AU. This structure seems consistent with crude models of mean motion resonances, or accumulation of material at two of the Lagrange points relative to the putative object and the star. We also report a low significance detection of a point source in this outer annulus of dust. This source may be an overdensity in the disk due to dust accreting onto an unseen companion. An alternate interpretation suggests that the object's mass is between 5 and 37 times the mass of Jupiter. The results have implications for circumstellar disk dynamics and planet formation.

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Title: TEXES Observations of Pure Rotational H2 Emission From AB Aurigae
Authors: M.A. Bitner, M.J. Richter, J.H. Lacy, T.K. Greathouse, D.T. Jaffe, G.A. Blake

We present observations of pure rotational molecular hydrogen emission from the Herbig Ae star, AB Aurigae. Our observations were made using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility and the Gemini North Observatory. We searched for H2 emission in the S(1), S(2), and S(4) lines at high spectral resolution and detected all three. By fitting a simple model for the emission in the three transitions, we derive T = 670 ±40 K and M = 0.52 ±0.15 earth masses for the emitting gas. Based on the 8.5 km/s FWHM of the S(2) line, assuming the emission comes from the circumstellar disk, and with an inclination estimate of the AB Aur system taken from the literature, we place the location for the emission near 18 AU. Comparison of our derived temperature to a disk structure model suggests that UV and X-ray heating are important in heating the disk atmosphere.

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ESA's X-ray observatory XMM-Newton has revealed evidence for a magnetic field in space where astronomers never expected to find one. The magnetic field surrounds a young star called AB Aurigae and provides a possible solution to a twenty-year-old puzzle.
At 2.7 times the mass of the Sun, AB Aurigae is one of the most massive stars in the Taurus-Auriga star-forming cloud. Although amongst nearly 400 smaller stars, its ultraviolet radiation plays a key role in shaping the cloud. Its massive status puts it in a class known as Herbig stars, named after their discoverer George Herbig.

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Credits: M. Guedel/ESA

As part of a large programme to survey Taurus-Auriga at X-ray wavelengths, XMM-Newton systematically targeted AB Aurigae and the other young stars in this region, using its European Photon Imaging Camera (EPIC). AB Aurigae stood out brightly in the image, indicating that it was releasing X-rays.
X-rays are expected to come from young stars with strong magnetic fields but computer calculations have repeatedly suggested that Herbig stars do not have the correct internal conditions to generate an appreciable magnetic field. Yet for twenty years, astronomers have been detecting X-ray emission from them.
Where could the X-rays be coming from? Some astronomers suggested that Herbig stars could have a smaller companion star in orbit around them and the X-rays are coming from the companion.
However, when an international team led by Manuel Güdel and his graduate student Alessandra Telleschi, of the Paul Scherrer Institut, Switzerland, analysed the AB Aurigae data, they found that the temperature of the gas producing the X-rays lay between one and five million degrees centigrade.

"That was suspiciously low," Güdel says. Young sun-like stars possess gaseous atmospheres that are heated to 10 million degrees and higher, by their magnetic field.
Güdel and his team found another clue that the X-rays must be coming from AB Aurigae itself: the X-rays varied in intensity every 42 hours. This is a magic number for the star because astronomers know that the optical and ultraviolet light from AB Aurigae also varies by this same amount.

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