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Vega
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NASA, ESA Telescopes Find Evidence For Asteroid Belt Around Vega

Astronomers have discovered what appears to be a large asteroid belt around the star Vega, the second brightest star in northern night skies. The scientists used data from NASA's Spitzer Space Telescope and the European Space Agency's Herschel Space Observatory, in which NASA plays an important role.
The discovery of an asteroid belt-like band of debris around Vega makes the star similar to another observed star called Fomalhaut. The data are consistent with both stars having inner, warm belts and outer, cool belts separated by a gap. This architecture is similar to the asteroid and Kuiper belts in our own solar system.

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Vega Debris Disk
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Title: Confirming the Primarily Smooth Structure of the Vega Debris Disk at Millimetre Wavelengths
Authors: A. M. Hughes (UC Berkeley), D. J. Wilner (CfA), B. Mason (NRAO), J. M. Carpenter (Caltech), R. Plambeck (UC Berkeley), H.-F. Chiang (IfA, U Illinois), S. M. Andrews (CfA), J. P. Williams (IfA), A. Hales (NRAO), K. Su (U Arizona), E. Chiang (UC Berkeley), S. Dicker (U Penn), P. Korngut (U Penn), M. Devlin (U Penn)

Clumpy structure in the debris disk around Vega has been previously reported at millimetre wavelengths and attributed to concentrations of dust grains trapped in resonances with an unseen planet. However, recent imaging at similar wavelengths with higher sensitivity has disputed the observed structure. We present three new millimetre-wavelength observations that help to resolve the puzzling and contradictory observations. We have observed the Vega system with the Submillimeter Array (SMA) at a wavelength of 880 um and angular resolution of 5"; with the Combined Array for Research in Millimetre-wave Astronomy (CARMA) at a wavelength of 1.3 mm and angular resolution of 5"; and with the Green Bank Telescope (GBT) at a wavelength of 3.3 mm and angular resolution of 10". Despite high sensitivity and short baselines, we do not detect the Vega debris disk in either of the interferometric data sets (SMA and CARMA), which should be sensitive at high significance to clumpy structure based on previously reported observations. We obtain a marginal (3-sigma) detection of disk emission in the GBT data; the spatial distribution of the emission is not well constrained. We analyse the observations in the context of several different models, demonstrating that the observations are consistent with a smooth, broad, axisymmetric disk with inner radius 20-100 AU and width >50 AU. The interferometric data require that at least half of the 860 um emission detected by previous single-dish observations with the James Clerk Maxwell Telescope be distributed axisymmetrically, ruling out strong contributions from flux concentrations on spatial scales of <100 AU. These observations support recent results from the Plateau de Bure Interferometer indicating that previous detections of clumpy structure in the Vega debris disk were spurious.

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RE: Vega
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Title: First evidence of pulsations in Vega? Results of today's most extensive spectroscopic search
Authors: Torsten Böhm, François Lignières, Gregg Wade, Pacal Petit, Michel Aurière, Wolfgang Zima, Aurélie Fumel, Evelyne Alecian

The impact of rapid rotation on stellar evolution theory remains poorly understood as of today. Vega is a special object in this context as spectroscopic and interferometric studies have shown that it is a rapid rotator seen nearly pole one, a rare orientation particularly interesting for seismic studies. In this paper we present a first systematic search for pulsations in Vega. The goal of the present work is to detect for the first time pulsations in a rapidly rotating star seen nearly pole-on. Vega was monitored in quasi-continuous high-resolution echelle spectroscopy. A total of 4478 spectra were obtained within 3 individual runs in 2008, 2009 and 2010 at high resolution. This data set should represent the most extensive high S/N, high resolution quasi-continuous survey obtained on Vega as of today. Equivalent photospheric absorption profiles were calculated for the stellar spectrum, but also for the telluric lines acting as a radial velocity reference. Residual velocities were analysed and periodic low amplitude variations, potentially indicative of stellar pulsations, detected. All three data sets revealed the presence of residual periodic variations: 5.32 and 9.19 c/d, (A approx 6 m/s) in 2008, 12.71 and 13.25 c/d, (A approx 8 m/s) in 2009 and 5.42 and 10.82 c/d, (A approx 3-4 m/s) in 2010. A Lomb-Scargle periodogram of each velocity bin of the equivalent profile was performed for the 2010 run, not showing the presence of any higher order nrp mode. It is too early to conclude that the variations are due to stellar pulsations, and a confirmation of the detection with a highly stable spectrograph is a necessary next step. If pulsations are confirmed, their very small amplitudes show that the star would belong to a category of very "quiet" pulsators.

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Title: Long-term magnetic field stability of Vega
Authors: D. Alina, P. Petit, F. Lignières, G.A. Wade, R. Fares, M. Aurière, T. Böhm, H. Carfantan

We present new spectropolarimetric observations of the normal A-type star Vega, obtained during the summer of 2010 with NARVAL at Telescope Bernard Lyot (Pic du Midi Observatory). This new time-series is constituted of 615 spectra collected over 6 different nights. We use the Least-Square-Deconvolution technique to compute, from each spectrum, a mean line profile with a signal-to-noise ratio close to 20,000. After averaging all 615 polarized observations, we detect a circularly polarized Zeeman signature consistent in shape and amplitude with the signatures previously reported from our observations of 2008 and 2009. The surface magnetic geometry of the star, reconstructed using the technique of Zeeman-Doppler Imaging, agrees with the maps obtained in 2008 and 2009, showing that most recognizable features of the photospheric field of Vega are only weakly distorted by large-scale surface flows (differential rotation or meridional circulation).

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Gaze high in the eastern sky this week after dark and you'll find the bright summertime star Vega. Our solar system is speeding roughly in its direction at 12 miles per second. No need to worry about a collision, though; even at this remarkable speed, we'd need 5,300 human lifetimes to reach the star.
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Magnetic field on bright star Vega
Astronomy & Astrophysics publishes the first detection of a magnetic field on the bright star Vega. Using the NARVAL spectropolarimeter of the Bernard-Lyot telescope on top of the Pic du Midi (France), astronomers clearly observe the magnetically-induced effect in the spectrum of Vega, thereby showing that the star possesses a magnetic field, something unknown so far.

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Le champ magnétique de l'étoile Véga enfin détecté
La première détection d'un champ magnétique sur l'étoile Véga, vient d'être réalisée. Cette mesure a été obtenue par une équipe d'astronomes du Laboratoire d'Astrophysique de Toulouse-Tarbes (LATT : INSU-CNRS, Université Paul Sabatier, Observatoire Midi Pyrénées) qui ont utilisé le télescope de 2 m de diamètre Bernard Lyot (INSU-CNRS) du Pic du Midi équipé du spectropolarimètre NARVAL(1). Cette découverte, qui arrive après plusieurs tentatives infructueuses, devrait marquer un tournant dans notre compréhension du magnétisme stellaire et de son influence sur l'évolution des étoiles. Ce résultat est publié dans la revue Astronomy and Astrophysics et fait l'objet d'un communiqué de presse.

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Vega Dust Ring
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Title: Images of Vega Dust Ring at 350 and 450 microns: New Clues to the Trapping of Multiple-Sized Dust Particles in Planetary Resonances
Authors: K. A. Marsh, C. D. Dowell, T. Velusamy, K. Grogan, C. A. Beichman

Researchers have used the SHARC II camera at Caltech Submillimeter Observatory to make 350 and 450 micron images of the Vega dust disk at spatial resolutions (FWHM) of 9.7" and 11.1", respectively. The images show a ring-like morphology (radius \~ 100 AU) with inhomogeneous structure that is qualitatively different from that previously reported at 850 microns and longer wavelengths.
They attribute the 350/450 micron emission to a grain population whose characteristic size (~ 1 mm) is intermediate between that of the cm-sized grains responsible for emission longward of 850 microns and the much smaller grains (less than 18 microns) in the extensive halo, visible at 70 microns, discussed by Su et al. (2005).
They have combined their submillimeter images with Spitzer data at 70 microns to produce 2-d maps of line-of-sight optical depth (relative column density).
These "tau maps" suggest that the mm-sized grains are located preferentially in three symmetrically-located concentrations. If so, then this structure could be understood in terms of the Wyatt (2003) model in which planetesimals are trapped in the mean motion resonances of a Neptune-mass planet at 65 AU, provided allowance is made for the spatial distribution of dust grains to differ from that of the parent planetesimals. The peaks of the tau maps are, in fact, located near the expected positions corresponding to the 4:3 resonance. If this identification is confirmed by future observations, it would resolve an ambiguity with regard to the location of the planet.

Vega dust disk

Observed and photosphere-subtracted images of Vega at 350 µm and 450 µm.
The intensity scale (shown by the horizontal bar at the top of each image) is in units of mJy arcsec-2 and the orientation is such that north is up and east is to the left. The RMS measurement noise on the images is 0.032 and 0.026 mJy arcsec^-2 at 350 µm and 450 µm, respectively.



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RE: Vega
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Title: Thermal Infrared Constraint to a Planetary Companion of Vega with the MMT Adaptive Optics System
Authors: Philip M. Hinz, A. N. Heinze, Suresh Sivanandam, Douglas L. Miller, Matthew A. Kenworthy, Guido Brusa, Melanie Freed, J.R.P. Angel

Vega may have a massive companion in a wide orbit, as evidenced by structure in its cold dust debris. We have tested this hypothesis by direct imaging with adaptive optics in the M band. The observations were made with a newly commissioned thermal infrared camera, Clio, on the 6.5 MMT AO system with low-background deformable secondary. The observations constrain a planet to be less than 7 Jupiter masses at the approximate position angle expected from the dust structure and at a radius > 20AU (2.5 arcsec) . This result is more stringent than similar previous near-infrared observations of Vega, that achieve limits of 20 and 10 Jupiter masses at separations of 7 arcsec.
The higher sensitivity is due both to the more favourable contrast of gas giant planets at M band and to the higher Strehl and more stable point spread function at longer wavelengths. Future L' or M band observations could provide a powerful approach for wide separation planet detection, especially for cooler, and thus older or less massive planets. The natural best targets are nearby stars where planets in the range of 5-15 Jupiter masses and as old as several billion years are expected to be detectable with this technique.

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The observation of the immediate vicinity of a star other than the Sun has just been carried out for the first time. A debris disc made up of hot (1300 degrees) dust grains, residues of comet evaporation and collisions between asteroids, was indeed detected for the first time around Vega. This discovery is the work of an international team, which includes researchers of Paris Observatory (LESIA).

Vega is an important star in astronomy in more than one way: the fifth brightest star of the night sky, one of the three "beauties of summer" (with Deneb and Altair) which forms a large triangle at the zenith of our latitudes on festival evenings; it was a long time considered as a reference star, to which is compared the brightness of all the other stars. Located at 25 light-years, therefore relatively near the Sun, it is approximately three times larger and more massive, and 60 times more luminous than the Sun, and much younger (350 million years against 4.5 billion).

An international team detected in the vicinity of Vega a weak infra-red flux (78 times less important than that of the star) which would come from particles heated by the star to temperatures close to 1300 degrees C. It seems that these particles have a chemical composition different from those of the solar system, with a dominant carbonaceous material (like graphite), whereas our zodiacal cloud contains essentially silicates.
They would be also on average smaller (of a diameter lower than the micron, equivalent to particles constituting cigarette smoke). Such small grains should normally be driven out by the pressure created by the intense radiation of Vega. Their abundance thus proves that they are produced in permanence, probably in a phase of intense meteoritic and cometary bombardment like those experienced by the Earth at the origins of the solar system. The dust production rate would correspond in the daily passing of 13 large comets in the environment of Vega.


Credit: Olivier Absil, Université de Liège

The presence of cold dust around Vega (-190 C), located at a distance three times larger than the orbit of Pluto, was known for a long time.
This phenomenon is found also around a large number on stars similar to Vega. However, nothing was known on the internal part of these debris discs, where planets similar to the Earth are supposed to be formed. The observation of the internal part of the disc of Vega was made possible thanks to the array of CHARA (Centre for High Angular Resolution Astronomy) of the State University of Georgia, which allows, from six telescopes of 1 meter distributed on Mount Wilson in California, to simulate a giant telescope of almost 330 m, and thus to distinguish details of only 200 microseconds of arc, hardly larger than a soccer ball seen from the Moon! The light collected by CHARA is recombined by the instrument FLUOR (Fiber Linked Unit for Optical Recombination), developed by the LESIA (CNRS, Observatoire of Paris, Université of Paris VI and VII).

This device also allowed to observe the atmosphere of Vega, and to confirm some astonishing properties of the star: its high speed of rotation on itself (12.5 hours) indeed confers it a lenticular form, flattened at the poles, the latter being brighter and hotter by 2300 C than the equator.

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