Highlights - Volume 466-1 (April IV 2007)

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HIGHLIGHTS: this week in A&A

Volume 466-1 (April IV 2007)


Reconnection between emerging flux systems in the Sun

Emergence and interaction of twisted flux tubes in the Sun”, by V. Archontis, A.W Hood, and C. Brady A&A 466, p. 367

Magnetic field concentrations on the surface of the Sun such as sunspots are a manifestation of the dynamo process generating magnetic fields in the interior of our star. While it is generally agreed that these sunspots show the cross sections of large magnetic flux tubes sticking out of the surface layer, the detailed processes of the emergence of magnetic flux at the surface are still being discussed.
In this paper the authors present the first 2.5D simulations in which two flux tubes emerge from the surface layer. While previous studies used an imposed magnetic field in the atmosphere of the Sun, this new investigation studies two flux tubes: the first expands intothe atmosphere and establishes a more realistic environment for the second to expand into. Through this a new level of self-consistency in modeling the emergence of magnetic flux is reached. Even though the energeticsof the solar atmosphere cannot be treated properly yet, these simulations show that eruptive events in the corona are ultimately related to the emergence of magnetic flux from the interior.


Near-IR imaging of the cluster Westerlund 2

Near-infrared imaging of Galactic massive clusters: Westerlund 2", by J. Ascenso et al. A&A 466, p. 137

Infrared imaging is essential for working out the physics of star formation, as it allows us to see young stellar clusters throughtheir dust shrouds; thereby deriving the mass function (MF) of the newly-formed stars. Using deep near IR imaging of the Westerlund2 cluster, the authors find its distance and its age by comparison with theoretical isochrones. They then derive the MF and offer interesting evidence of radial stellar-mass segregation.


In section 5. Galactic structure, stellar clusters, and populations

“Whiting 1: the youngest globular cluster associated with the Sagittarius dwarf spheroidal galaxy”, by G. Carraro, R. Zinn, and C. Moni Bidin A&A 466, p. 181

Whiting 1 is a recently identified halo globular cluster, which is only 6.5 Gyr-old, hence much younger than the bulk of the galactic halo. The authors present radial velocities for 3 giant members and new deep photometry, and demonstrate that Whiting 1 is part of the trailing stream of theSagittarius dwarf spheroidalgalaxy. This reinforces the picture where (much of) the galactic halo develops through the accretion of small galaxies. 
In section 8. Stellar atmospheres

“Discovery of photospheric argon in very hot central stars of planetary nebulae and white dwarfs”, by K. Werner, T. Rauch, and J.W. Kruk A&A 466, p. 317

This paper reports the first discovery of photosphericargon in the atmospheres of hot central stars of planetary nebulae and hot white dwarfs, and presents an abundance analysisof argon in the atmospheres of 10 of these stars. The authors compare the observed Arabundance to the prediction of stellar nucleosynthesisand to the radiativeequilibrium theory. This contribution is significant for figuring out how elements heavier than hydrogen or helium are maintained in the atmospheres of these stars.  


© Astronomy & Astrophysics 2007