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TOPIC: tau Bootis A


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Title: A magnetic cycle of tau Bootis? The coronal and chromospheric view
Authors: K. Poppenhaeger, H.M. Guenther, J.H.M.M. Schmitt

Tau Bootis is a late F-type main sequence star orbited by a Hot Jupiter. During the last years spectropolarimetric observations led to the hypothesis that this star may host a global magnetic field that switches its polarity once per year, indicating a very short activity cycle of only one year duration. In our ongoing observational campaign, we have collected several X-ray observations with XMM-Newton and optical spectra with TRES/FLWO in Arizona to characterize tau Boo's corona and chromosphere over the course of the supposed one-year cycle. Contrary to the spectropolarimetric reconstructions, our observations do not show indications for a short activity cycle.

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Tau Bootis
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Title: Photospheric and coronal abundances in solar-type stars: the peculiar case of Tau Bootis
Authors: A. Maggio, J. Sanz-Forcada, L. Scelsi
(Version v2)

Chemical abundances in solar-type stars are a much debated topic. Planet-hosting stars are known to be metal-rich, but whether or not this peculiarity applies also to the chemical composition of the outer stellar atmospheres is still to be clarified. More in general, coronal and photospheric abundances in late-type stars appear to be different in many cases, but understanding how chemical stratification effects work in stellar atmospheres requires an observational base larger than currently available. We obtained XMM-Newton high-resolution X-ray spectra of Tau Bootis, a well known nearby star with a Jovian-mass close-in planet. We analysed these data with the aim to perform a detailed line-based emission measure analysis and derive the abundances of individual elements in the corona with two different methods applied independently. We compared the coronal abundances of Tau Bootis with published photospheric abundances based on high-resolution optical spectra and with those of other late-type stars with different magnetic activity levels, including the Sun. We find that the two methods provide consistent results within the statistical uncertainties for both the emission measure distribution of the hot plasma and for the coronal abundances, with discrepancies at the 2-sigma level limited to the amount of plasma at temperatures of 3-4 MK and to the O and Ni abundances. In both cases, the elements for which both coronal and photospheric measurements are available (C, N, O, Si, Fe, and Ni) result systematically less abundant in the corona by a factor 3 or more, with the exception of the coronal Ni abundance, which is similar to the photospheric value. Comparison with other late-type stars of similar activity level shows that these coronal/photospheric abundance ratios are peculiar to Tau Bootis and possibly related to the characteristic over-metallicity of this planet-hosting star.

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Title: Magnetic cycles of the planet-hosting star Tau Bootis: II. a second magnetic polarity reversal
Authors: R. Fares, J.-F. Donati, C. Moutou, D. Bohlender, C. Catala, M. Deleuil, E. Shkolnik, A.C. Cameron, M.M. Jardine, G.A.H. Walker

In this paper, we present new spectropolarimetric observations of the planet-hosting star Tau Bootis, using ESPaDOnS and Narval spectropolarimeters at Canada-France-Hawaii Telescope (CFHT) and Telescope Bernard Lyot (TBL), respectively. We detected the magnetic field of the star at three epochs in 2008. It is a weak magnetic field of only a few Gauss, oscillating between a predominant toroidal component in January and a dominant poloidal component in June and July. A magnetic polarity reversal was observed relative to the magnetic topology in June 2007. This is the second such reversal observed in two years on this star, suggesting that Tau Boo has a magnetic cycle of about 2 years. This is the first detection of a magnetic cycle for a star other than the Sun. The role of the close-in massive planet in the short activity cycle of the star is questioned.
Tau Boo has strong differential rotation, a common trend for stars with shallow convective envelope. At latitude 40 deg., the surface layer of the star rotates in 3.31 d, equal to the orbital period. Synchronization suggests that the tidal effects induced by the planet may be strong enough to force at least the thin convective envelope into corotation. Tau Boo shows variability in the Ca H & K and Halpha throughout the night and on a night to night time scale. We do not detect enhancement in the activity of the star that may be related to the conjunction of the planet. Further data is needed to conclude about the activity enhancement due to the planet.

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An international group of astronomers that includes the University of Hawaii's Evgenya Shkolnik reported today that they have discovered that the sun-like star tau Bootis flipped its magnetic field from north to south sometime during the last year.
It has been known for many years that the Sun's magnetic field changes its direction every 11 years, but this is the first time that such a change has been observed in another star. The team of astronomers, who made use of Canada-France-Hawaii Telescope atop Mauna Kea, are now closely monitoring tau Bootis to see how long it will be before the magnetic field reverses again.

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Title: Magnetic cycles of the planet-hosting star tauBootis
Authors: J. F. Donati, C. Moutou, R. Fares, D. Bohlender, C. Catala, M. Deleuil, E. Shkolnik, A. C. Cameron, M. M. Jardine, G. A. H. Walker

We have obtained new spectropolarimetric observations of the planet-hosting star tauBootis, using the ESPaDOnS and NARVAL spectropolarimeters at the Canada-France-Hawaii Telescope and Telescope Bernard-Lyot. With this data set, we are able to confirm the presence of a magnetic field at the surface of tauBoo and map its large-scale structure over the whole star. The overall polarity of the magnetic field has reversed with respect to our previous observation (obtained a year before), strongly suggesting that tauBoo is undergoing magnetic cycles similar to those of the Sun. This is the first time that a global magnetic polarity switch is observed in a star other than the Sun; we speculate that the magnetic cycle period of tauBoo is much shorter than that of the Sun.
Our new data also allow us to confirm the presence of differential rotation from the latitudinal shearing that the magnetic structure is undergoing. The differential rotation surface shear that tauBoo experiences is found to be 6 to 10 times larger than that of the Sun. We propose that the short magnetic cycle period is due to the strong level of differential rotation. With a rotation period of 3.0 and 3.9 d at the equator and pole respectively, tauBoo appears as the first planet-hosting star whose rotation (at intermediate latitudes) is synchronised with the orbital motion of its giant planet (period 3.3 d). Assuming that this synchronisation is not coincidental, it suggests that the tidal effects induced by the giant planet can be strong enough to force the thin convective envelope (though not the whole star) into co-rotation and thus to play a role in the activity cycle of tauBoo.

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Les étoiles comme le Soleil cachent sous leur surface un impressionnant mécanisme : la génération de leur champ magnétique. Ces champs magnétiques ressemblent à ceux de simples aimants, si ce n'est qu'ils se retournent régulièrement en échangeant leurs pôles Nord et Sud, tous les 11 ans environ dans le cas du Soleil. Pour la première fois, une équipe internationale d'astrophysiciens, conduite par des chercheurs du CNRS, vient de surprendre une autre étoile - tau Bootis A - en train de faire sa galipette magnétique. Pour tau Bootis A, cette bascule magnétique semble être plus fréquente que pour le Soleil. La planète géante en orbite rasante qu'elle héberge, est-elle à l'origine de cet emballement ? Cette découverte, publiée dans les Monthly Notices of the Royal Astronomical Society, devrait nous aider à mieux comprendre le mécanisme des cycles magnétiques dans les étoiles comme le Soleil.

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Magnetic field detected on star
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Astronomers using the Canada-France-Hawaii Telescope on Mauna Kea for the first time have directly measured magnetism on a star.
The magnetic field on Tau Bootis, 50 light-years away, is 100 times weaker than a typical refrigerator magnet, said Evgenya Shkolnik, astronomer with the NASA Astrobiology Institute at the Institute for Astronomy.

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Using the ESPaDOnS spectropolarimeter installed on the Canada-France-Hawaii telescope (Mauna Kea, Hawaii), an international team of researchers, led by two french astronomers (LESIA, Observatoire de Paris and LATT, Observatoire Midi-Pyrénées), has just discovered a magnetic field on tau Bootis, a star orbited by a giant planet on a close-in orbit: the first ever detection of this kind! Up to now, only indirect clues pointed to the presence of magnetic fields on stars hosting giant extra-solar planets. This result opens major prospects, in particular the study of the interaction between the planet and the magnetosphere of its star. This discovery is published in a Letter to the Journal MNRAS (Monthly Notices of Royal Astronomical Society).
This first measurement of a magnetic field in a planet-hosting star has been obtained by an international team of astronomers with the ESPaDOnS spectropolarimeter located on the Canada-France-Hawaii telescope. They detected the magnetic field of tau Bootis, a one billion year old star, having a mass of one and a half solar masses and located at nearly 50 light years from the Earth. This cool and weakly active star, orbited by a giant planet with 4.4 Jupiter masses on a very close-in orbit at 0.049 AU (i.e. 5% of the Sun-Earth distance), possesses a magnetic field of a few gauss, just a little more than the Sun's, but showing a more complex structure.

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An extrasolar planet has been caught dragging the outer layers of its host star around with it as it orbits. The newly discovered “tail wagging the dog” phenomenon may be common among massive planets in close orbit around their stars.
Many celestial bodies begin their lives spinning quickly but decelerate over time because of the gravitational tugs of nearby objects. The Moon, for example, has slowed to the point that it rotates on its axis in the same amount of time it takes to orbit Earth.
But smaller bodies rarely dictate their hosts' rotation, unless the two objects are closely spaced and are relatively similar in mass - such as Pluto and its moon, Charon. Now, a team of astronomers has observed this "lock-step" phenomenon in a planet-star pair for the first time.
The team used Canada's MOST - Microvariability and Oscillations of Stars - satellite to search for reflected light from a planet first detected by the wobble it induces in its host star, tau Bootis A. The planet appears to be at least 3.87 times the mass of Jupiter and orbits eight times closer to its star than Mercury does from the Sun. It has one of the shortest known periods, only 3.31 days, and orbits at a close distance of 0.046 astronomical units from the star (6.9 million kilometres) in a nearly circular orbit, making it one of the several "hot Jupiter's known.



The team noticed the star's own brightness changes every 3.3 days - the same orbital period as the planet.
"The star is keeping pace with the planet." - Jaymie Matthews, MOST's leader at the University of British Columbia in Vancouver, Canada. He attributes the effect to the mass and proximity of the planet, which allow it to gravitationally yank on the star's outer atmosphere.
Tau Bootis, at magnitude 4.50, has been classed both as an F6 sub giant and an F7 dwarf. At a distance of 51 light years, it shines with 3.1 times the solar luminosity. Warmer than our Sun, the surface temperature fall around 6340 Kelvin. The star's higher luminosity and temperature show it to have a radius 1.46 times solar and a mass of 1.3 times that of the Sun. The star is most likely a core-hydrogen-fusing dwarf, but one with some age to it. Tau Bootis is a slight variable with a magnitude range of under 10 percent, which would make it unusual among stars with planets. More unusual, Tau Boo has a distant companion, a red class M dwarf of 0.4 solar masses that orbits far outside the planetary system at an average distance of 100 or 240 astronomical units with a revolutionary period 750 or 2600 years. Like most stars that have planets, this one is also metal rich, the iron abundance (relative to hydrogen) 1.7 times solar.

The discovery bolsters another recent finding of a star that heats up in synch with a closely orbiting planet - an effect that may be caused by the entanglement of the two objects' magnetic fields.
"We now seem to be finding evidence that planets interact with their stars to a degree we didn't think possible." - Guillermo Torres, astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US.
Indeed, over about a month of observations, the MOST team saw brightness changes on the order of a few tenths of a percent suggesting the star is magnetically active, with constantly varying "starspots" similar to those on the Sun. That activity, says Matthews, along with the rapid rotation of its outer layers, suggests the star is young - perhaps a few hundred million years old - and "rambunctious".
But Matthews says more study is needed to verify its age. "What we really don't know is if tau Bootis A is a young, fast-spinning star and this has simply fine-tuned it, or whether it's a much older star and this process has rejuvenated it and spun up its outer layers."
The answer to this question could shed light on how long it takes for gas giant planets - which probably form at greater distances from such stars - to arrive at their tight orbits.

Matthews says MOST is the only telescope in existence that could have made this discovery. The suitcase-sized telescope orbits above Earth's distorting atmosphere, which gives it about 100 times the precision of ground-based telescopes. And its orbit means it can observe a single object for about two months without interruption.
"We like to think of it as putting stars under a stakeout. You need this continuous coverage to get the complete story."
He presented the results on Monday at a meeting of the Canadian Astronomical Society in Montreal.

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-- Edited by Blobrana at 21:08, 2005-05-18

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