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New Search for Centauri Planets Begins

To the two ongoing hunts for planets around the Alpha Centauri stars we can now add a third. John Hearnshaw (University of Canterbury, Christchurch) reports in a recent post on Cosmic Diary that the university's Mt. John Observatory has begun a program to search for Earth-mass planets around Centauri A and B. Although the observatory is heavily invested in microlensing technologies (working with the Microlensing Observations in Astrophysics collaboration), the new efforts will put radial velocity methods to work using the Hercules spectrograph.
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The race to find Earth-like planets in their habitable zones, and the prospects for alpha Centauri
No-one should have any doubt about what is one of the most exciting and challenging projects in modern astronomy of the early 21st century. It is the race to find Earth-mass planets lying in their habitable zones. It is easy to understand why this topic has captured the imagination of astronomers, the media and the general public. First, because there exists a widely held belief amongst astronomers that nothing in the universe is unique. Wherever we look, the same types of objects are generally found in many places, although some objects are certainly rare.

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Using the light from the triple-star system Alpha Centauri, astronomers have figured out the chemical composition of any rocky planets that may orbit the stars.
Alpha Centauri is a group of three stars that includes our nearest neighbour, the faint Proxima Centauri, a mere 4.2 light-years (40 trillion km) away, as well as the bright, close binary of Alpha Centauri A and B.
Astronomer Charles Lineweaver from the Australian National University in Canberra presented his study of the Alpha Centauri triple-star system last week at the 9th Australian Space Sciences Conference at the University of Sydney.

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The nearest Earth out there in space? It might be right next door, galactically speaking.
Two teams of astronomers, one from the United States and one from Europe, are in a race to find a planet orbiting our near neighbours Alpha Centauri A and Alpha Centauri B, twin stars that appear from Earth as a single point of light.

"I'm betting that there are planets like Earth or Mars or Venus around either or both of those stars, and the only question is whether we'll be able to detect them" - Debra Fischer, an astronomer at San Francisco State University.

Backed with U.S. government funding, she is using a telescope in Chile to assemble 100,000 observations of the Centauri system.

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Title: The Alpha Centauri Binary System: Atmospheric Parameters and Element Abundances
Authors: G. F. Porto de Mello, W. Lyra, G. R. Keller
(Version v2)

The alpha Centauri binary system, owing to its duplicity, proximity and brightness, and its components' likeness to the Sun, is a fundamental calibrating object for the theory of stellar structure and evolution and the determination of stellar atmospheric parameters. This role, however, is hindered by a considerable disagreement in the published analyses of its atmospheric parameters and abundances. We report a new spectroscopic analysis of both components of the alpha Centauri binary system and compare published analyses of the system. The analysis is differential with respect to the Sun, based on high-quality spectra, and employed spectroscopic and photometric methods to obtain as many independent Teff determinations as possible. The atmospheric parameters are also checked for consistency against the results of the dynamical analysis and the positions of the components in a theoretical HR diagram. We discuss possible origins of discrepancies, concluding that the presence of NLTE effects is a probable candidate, but we note that there is as yet no consensus on the existence and cause of an offset between the spectroscopic and photometric Teff scales of cool dwarfs. The spectroscopic surface gravities also agree with those derived from directly measured masses and radii. The abundance pattern can be deemed normal in the context of recent data on metal-rich stars. The position of alpha Cen A in an up-to-date theoretical evolutionary diagrams yields a good match of the evolutionary mass and age with those from the dynamical solution and seismology.

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Title: Modelling the excitation of acoustic modes in Alpha Cen A
Authors: R. Samadi, K. Belkacem, M.J. Goupil, M.-A. Dupret, F. Kupka

We infer from different seismic observations the energy supplied per unit of time by turbulent convection to the acoustic modes of Alpha Cen A (HD 128620), a star which is similar but not identical to the Sun. The inferred rates of energy supplied to the modes (i.e. mode excitation rates) are found to be significantly larger than in the Sun. They are compared with those computed with an excitation model that includes two sources of driving, the Reynolds stress contribution and the advection of entropy fluctuations. The model also uses a closure model, the Closure Model with Plumes (CMP hereafter), that takes the asymmetry between the up- and down-flows (i.e. the granules and plumes, respectively) into account. Different prescriptions for the eddy-time correlation function are also confronted to observational data. Calculations based on a Gaussian eddy-time correlation underestimate excitation rates compared with the values derived from observations for Alpha Cen A. On the other hand, calculations based on a Lorentzian eddy-time correlation lie within the observational error bars. This confirms results obtained in the solar case. With respect to the helioseismic data, those obtained for Alpha Cen A constitute an additional support for our model of excitation. We show that mode masses must be computed taking turbulent pressure into account. Finally, we emphasize the need for more accurate seismic measurements in order to discriminate, in the case of Alpha Cen A, between the CMP closure model and the quasi-Normal Approximation as well as to confirm or not the need to include the excitation by the entropy fluctuations.

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Title: The Alpha Centauri Binary System: Atmospheric Parameters and Element Abundances
Authors: G. F. Porto de Mello, W. Lyra, G. R. Keller

The Alpha Centauri binary system, owing to its binarity, proximity and brightness, is a fundamental calibrating object for the theory of stellar structure and evolution. This role, however, is hindered by a considerable disagreement in the published analyses of its atmospheric parameters and abundances. We report a detailed spectroscopic analysis of both components of the Alpha Centauri binary system, differentially with respect to the Sun, based on high quality spectra (R = 35 000, S/N > 1000). The atmospheric parameters of the system are found to be Teff = 5820 K, [Fe/H] = +0.24, log g = 4.34 and xi = 1.46 km/s, for Alpha Cen A, and Teff = 5240 K, [Fe/H] = +0.25, log g = 4.44 and xi = 1.28 km/s for Alpha Cen B. The parameters were derived from the simultaneous excitation & ionisation equilibria of the equivalent widths of Fe I and Fe II lines, by fitting theoretical profiles to the Halpha line and from photometric calibrations, good agreement being reached between the criteria for both stars. We derived the abundances of Na, Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Y and Ba, concluding that the abundance pattern of the system is solar but for significant Na, Mn and Ni excesses, and a deficit of Ba. An analysis of the position of the two stars in up-to-date theoretical evolutionary diagrams yields masses and ages in good agreement with the dynamical and seismological data. Its abundance pattern can be deemed normal in the context of recent data of metal-rich stars.

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Another Earth may be orbiting the star next door, and we could detect its presence within a few years, a new study argues. A telescope trained permanently on Alpha Centauri should be able to pick up the slight stellar wobbles induced by a small, rocky, Earth-like planet.
Previous computer simulations suggested terrestrial planets probably formed around one or both stars. That is borne out by the work of Javiera Guedes at the University of California, Santa Cruz (UCSC), US, and colleagues, who have gone a step further and worked out how to detect such planets.

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Proxima Centauri
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Title: A possible activity cycle in Proxima Centauri
Authors: Carolina Cincunegui, Rodrigo F. Díaz, Pablo J. D. Mauas

Several late-type stars present activity cycles resembling the Solar one. This fact has been observed mostly in stars ranging from F to K, i.e., in stars with a radiative core and an outer convective layer. This work aims at studying whether an activity cycle can be detected in the dM5.5e star Proxima Centauri, which is supposed to be completely convective. We present periodical medium-resolution echelle observations covering the complete visual range, which were taken at the CASLEO Argentinean Observatory. These observations are distributed over 7 years. We discarded the spectra that present flare activity, and analyse the remaining activity levels using four different statistical techniques to look for a period of activity. We find strong evidence of a cyclic activity, with a period of around 442 days. We also estimate that the Ca II S index varies around 130% due to activity variations outside of flares.

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Title: Deep imaging survey of the environment of Alpha Centauri - II. CCD imaging with the NTT-SUSI2 camera
Authors: Pierre Kervella (LESIA), Frédéric Thévenin (OCA)

Context: The nearby pair of solar-type stars Alpha Centauri is a favourable target for an imaging search for extrasolar planets. Indications exist that the gravitational mass of Alpha Cen B could be higher than its modelled mass, the difference being consistent with a substellar companion of a few tens of Jupiter masses. However, Alpha Centauri usually appears in star catalogues surrounded by a large void area, due to the strong diffused light.
Aims: We searched for faint comoving companions to Alpha Cen located at angular distances of the order of a few tens of arcseconds, up to 2-3 arcmin. As a secondary objective, we built a catalogue of the detected background sources.
Methods: In order to complement our adaptive optics search at small angular distances (Paper I), we used atmosphere limited CCD imaging from the NTT-SUSI2 instrument in the Bessel V, R, I, and Z bands.
Results: We present the results of our search in the form of a catalogue of the detected objects inside a 5.5 arcmin box around this star. A total of 4313 sources down to mV~24 and mI~22 were detected from this wide-field survey. We extracted the infrared photometry of part of the detected sources from archive images of the 2MASS survey (JHK bands). We investigate briefly the nature of the detected sources, many of them presenting extremely red colour indices (V-K > 14).
Conclusions: We did not detect any companion to Alpha Centauri between 100 and 300 AU, down to a maximum mass of ~15 times Jupiter. We also mostly exclude the presence of a companion more massive than 30 MJup between 50 and 100 AU.

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