Cosmology: The oldest cosmic light
Nature 518, 7538 (2015). doi:10.1038/518170a
Authors: David Spergel & Brian Keating
The cosmic microwave background is a faint glow of light left over from the Big Bang. It fills the entire sky and records the Universe's early history. Two independent experts outline what we know about this ancient light, both theoretically and observationally.
Star formation: Sibling rivalry begins at birth
Nature 518, 7538 (2015). doi:10.1038/518173a
Authors: Kaitlin M. Kratter
High-resolution astronomical observations of a nearby molecular gas cloud have revealed a quadruplet of stars in the act of formation. The system is arguably the youngest multiple star system detected so far. See Letter p.213
The formation of a quadruple star system with wide separation
Nature 518, 7538 (2015). doi:10.1038/nature14166
Authors: Jaime E. Pineda, Stella S. R. Offner, Richard J. Parker, Héctor G. Arce, Alyssa A. Goodman, Paola Caselli, Gary A. Fuller, Tyler L. Bourke & Stuartt A. Corder
The initial multiplicity of stellar systems is highly uncertain. A number of mechanisms have been proposed to explain the origin of binary and multiple star systems, including core fragmentation, disk fragmentation and stellar capture. Observations show that protostellar and pre-main-sequence multiplicity is higher than the multiplicity found in field stars, which suggests that dynamical interactions occur early, splitting up multiple systems and modifying the initial stellar separations. Without direct, high-resolution observations of forming systems, however, it is difficult to determine the true initial multiplicity and the dominant binary formation mechanism. Here we report observations of a wide-separation (greater than 1,000 astronomical units) quadruple system composed of a young protostar and three gravitationally bound dense gas condensations. These condensations are the result of fragmentation of dense gas filaments, and each condensation is expected to form a star on a timescale of 40,000 years. We determine that the closest pair will form a bound binary, while the quadruple stellar system itself is bound but unstable on timescales of 500,000 years (comparable to the lifetime of the embedded protostellar phase). These observations suggest that filament fragmentation on length scales of about 5,000 astronomical units offers a viable pathway to the formation of multiple systems.
Comet 67P/Churyumov-Gerasimenko sheds dust coat accumulated over the past four years
Nature 518, 7538 (2015). doi:10.1038/nature14159
Authors: Rita Schulz, Martin Hilchenbach, Yves Langevin, Jochen Kissel, Johan Silen, Christelle Briois, Cecile Engrand, Klaus Hornung, Donia Baklouti, Anaïs Bardyn, Hervé Cottin, Henning Fischer, Nicolas Fray, Marie Godard, Harry Lehto, Léna Le Roy, Sihane Merouane, François-Régis Orthous-Daunay, John Paquette, Jouni Rynö, Sandra Siljeström, Oliver Stenzel, Laurent Thirkell, Kurt Varmuza & Boris Zaprudin
Comets are composed of dust and frozen gases. The ices are mixed with the refractory material either as an icy conglomerate, or as an aggregate of pre-solar grains (grains that existed prior to the formation of the Solar System), mantled by an ice layer. The presence of water-ice grains in periodic comets is now well established. Modelling of infrared spectra obtained about ten kilometres from the nucleus of comet Hartley 2 suggests that larger dust particles are being physically decoupled from fine-grained water-ice particles that may be aggregates, which supports the icy-conglomerate model. It is known that comets build up crusts of dust that are subsequently shed as they approach perihelion. Micrometre-sized interplanetary dust particles collected in the Earth’s stratosphere and certain micrometeorites are assumed to be of cometary origin. Here we report that grains collected from the Jupiter-family comet 67P/Churyumov-Gerasimenko come from a dusty crust that quenches the material outflow activity at the comet surface. The larger grains (exceeding 50 micrometres across) are fluffy (with porosity over 50 per cent), and many shattered when collected on the target plate, suggesting that they are agglomerates of entities in the size range of interplanetary dust particles. Their surfaces are generally rich in sodium, which explains the high sodium abundance in cometary meteoroids. The particles collected to date therefore probably represent parent material of interplanetary dust particles. This argues against comet dust being composed of a silicate core mantled by organic refractory material and then by a mixture of water-dominated ices. At its previous recurrence (orbital period 6.5 years), the comet’s dust production doubled when it was between 2.7 and 2.5 astronomical units from the Sun, indicating that this was when the nucleus shed its mantle. Once the mantle is shed, unprocessed material starts to supply the developing coma, radically changing its dust component, which then also contains icy grains, as detected during encounters with other comets closer to the Sun.