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HIGHLIGHTS: this week in A&A

Volume 507-2 (November IV 2009)


In section 6. Interstellar and circumstellar matter

“PROSAC: a submillimeter array survey of low-mass protostars. II.”, by J.K. Jorgensen et al., A&A 507, p. 861

A critical question facing theorists and observers studying the formation of low-mass stars is to understand at what point in the evolution of a protostar, disks become important as an intermediary in the accretion of matter onto the central object. It has seemed likely for some time that a small disk is present quite early on in protostellar evolution because disks are probably at the origin of the outflows observed in young "Class 0" objects that  still have much of their envelope to accrete. However, the SMA survey of 20 nearby protostars by Jorgensen et al. highlighted in this issue finds that disks are present and quite massive very early on in protostellar evolution and in fact do not change in mass between the "Class 0 " and "Class I" phases. Thus, disk evolution starts very early and does so in the presence of accretion from envelope to disk.  


In section 9. The Sun

“Magnetic flux emergence into the solar photosphere and chromosphere”, by A. Tortosa-Andreu and F. Moreno-Insertis, A&A 507, p. 949

In this manuscript, the emergence of magnetic flux through the solar surface into the photosphere and chromosphere is investigated by numerical 3D MHD experiments. Their results resemble recent observations quite closely and their analysis elucidates the physical processes leading to the heating of the quiet Sun chromosphere. 

In section 10. Planets and planetary systems

“Meteosat observation of the atmospheric entry of 2008 TC3 over Sudan and the associated dust cloud”, by J. Borovicka and Z. Charvat, A&A 507, p. 1015

On Oct. 6, 2008, R. Kowalski at the Catalina Sky Survey discovered 2008 TC3, an asteroid of several meters in diameter heading towards the Earth. The atmospheric entry occurred on Oct. 7, 2:46 UTC over Northern Sudan. The fireball was seen by numerous eyewitnesses from Egypt to Chad. In Dec. 2008, the first meteorites, named Almahata Sitta, were recovered in the Nubian desert. Borovicka and Charvat present an analysis of serendipitous observations of the fireball and the associated dust cloud by Meteosat weather satellites. The infrared spectra of the fresh dust clouds show a 10 micron SiO band and then absorption due to crystallite silicates at temperatures exceeding 1000K. Based on the heights of asteroid fragmentations, they infer that the bulk porosity of 2008 TC3 was about 50%, i.e. higher than the porosity of the recovered meteorites Almahata Sitta. This analysis is thus an important step in linking the actual meteoritic fall to the astronomical parent body.
In section 5. Galactic structure, stellar clusters, and populations

“The stellar content of the Hamburg/ESO survey. V. The metallicity distribution function of the Galactic halo”, by T. Schörck et al., A&A 507, p. 817

Understanding the properties and the formation epoch of the first stars in the Universe is one of the major challenges in present-day cosmology. We know from several lines of evidence that star formation activity must have begun within the first cosmic billion years. In addition, these stars must have a composition that is close to the primordial one produced by the Big Bang, that is, free  of heavy element. Because detecting these stars directly at high redshifts is currently unfeasible, the most promising alternative to studying them directly is to search for their relics in the halo of the Milky Way. These searches are currently witnessing a great boost, and this paper presents the best attempt ever to analyze a large sample of (1638) stars selected from the Hamburg/ESO dedicated survey. The authors obtain the metallicity distribution function for these stars and compare it with the predictions by the most sophisticated theoretical models.


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© Astronomy & Astrophysics 2009