* Astronomy

Title: The sudden appearance of dust in the early Universe
Author: Lars Mattsson

Observations suggest that high-redshift galaxies are either very dusty or essentially dust free. The evolution from one regime to the other must also be very fast, since evolved and dusty galaxies show up at redshifts corresponding to a Universe which is only about 500 Myr old. In the present paper models which predicts the existence of an apparent dichotomy between dusty and dust-free galaxies at high redshift are considered. Galaxies become dusty as soon as they reach an evolved state and the transition is very rapid. A special case suggests that while stellar dust production is overall relatively insignificant -- contrary to what has been argued recently -- it can at the same time be consistent with efficient dust production in supernovae in the local Universe. Special attention will be given to the recent discovery of a dusty normal galaxy (A1689-zD1) at a very high redshift z = 7.5 ± 0.2.

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Pitch Black: Cosmic Clumps Cast the Darkest Shadows

Astronomers have found cosmic clumps so dark, dense and dusty that they throw the deepest shadows ever recorded. Infrared observations from NASA's Spitzer Space Telescope of these blackest-of-black regions paradoxically light the way to understanding how the brightest stars form.
The clumps represent the darkest portions of a huge, cosmic cloud of gas and dust located about 16,000 light-years away. A new study takes advantage of the shadows cast by these clumps to measure the cloud's structure and mass.
The dusty cloud, results suggest, will likely evolve into one of the most massive young clusters of stars in our galaxy. The densest clumps will blossom into the cluster's biggest, most powerful stars, called O-type stars, the formation of which has long puzzled scientists. These hulking stars have major impacts on their local stellar environments while also helping to create the heavy elements needed for life.
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Title: A close halo of large transparent grains around extreme red giant stars
Authors: Barnaby R. M. Norris, Peter G. Tuthill, Michael J. Ireland, Sylvestre Lacour, Albert A. Zijlstra, Foteini Lykou, Thomas M. Evans, Paul Stewart, Timothy R. Bedding

Intermediate-mass stars end their lives by ejecting the bulk of their envelope via a slow dense wind back into the interstellar medium, to form the next generation of stars and planets. Stellar pulsations are thought to elevate gas to an altitude cool enough for the condensation of dust, which is then accelerated by radiation pressure from starlight, entraining the gas and driving the wind. However accounting for the mass loss has been a problem due to the difficulty in observing tenuous gas and dust tens of milliarcseconds from the star, and there is accordingly no consensus on the way sufficient momentum is transferred from the starlight to the outflow. Here, we present spatially-resolved, multi-wavelength observations of circumstellar dust shells of three stars on the asymptotic giant branch of the HR diagram. When imaged in scattered light, dust shells were found at remarkably small radii (<~ 2 stellar radii) and with unexpectedly large grains (~300 nm radius). This proximity to the photosphere argues for dust species that are transparent to starlight and therefore resistant to sublimation by the intense radiation field. While transparency usually implies insufficient radiative pressure to drive a wind, the radiation field can accelerate these large grains via photon scattering rather than absorption - a plausible mass-loss mechanism for lower-amplitude pulsating stars.

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Ferocious superwind will seal sun's doom

Superwinds were already known to exist, but it was not clear how they could be powered. One idea was that iron-tainted dust might absorb starlight to be pushed outwards. The problem with that model is that the dust would heat up and vaporise before it could really get moving and drive the wind.
Zijlstra's team used the Very Large Telescope in Chile to look at three red giant stars that are in this late stage of life. By using polarisers and masking off parts of the telescope, they could pick out the polarised light coming from shells of dust around each star.
The dust was so close to these stars that it would certainly be vaporised if it were absorbing light strongly. So it must be made of transparent crystals. The team suspect it is made of clear silicate minerals, similar to ordinary terrestrial sand grains.
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