How stellar death can lead to twin celestial jets

Astronomers know that while large stars can end their lives as violently cataclysmic supernovae, smaller stars end up as planetary nebulae — colourful, glowing clouds of dust and gas. In recent decades these nebulae, once thought to be mostly spherical, have been observed to often emit powerful, bipolar jets of gas and dust. But how do spherical stars evolve to produce highly aspherical planetary nebulae?In a theoretical paper published this week in the Monthly Notices of the Royal Astronomical Society, a University of Rochester professor and his undergraduate student conclude that only “strongly interacting” binary stars — or a star and a massive planet — can feasibly give rise to these powerful jets.When these smaller stars run out of hydrogen to burn they begin to expand and become Asymptotic Giant Branch (AGB) stars. This phase in a star’s life lasts at most 100,000 years. At some point some of these AGB stars, which represent the distended last spherical stage in the lives of low mass stars, become “pre-planetary” nebula, which are aspherical.”What happens to change these spherical AGB stars into non-spherical nebulae, with two jets shooting out in opposite directions?” asks Eric Blackman, professor of physics and astronomy at Rochester. “We have been trying to come up with a better understanding of what happens at this stage.”For the jets in the nebulae to form, the spherical AGB stars have to somehow become non-spherical and Blackman says that astronomers believe this occurs because AGB stars are not single stars but part of a binary system. The jets are thought to be produced by the ejection of material that is first pulled and acquired, or “accreted,” from one object to the other and swirled into a so-called accretion disk. There are, however, a range of different scenarios for the production of these accretion disks. All these scenarios involve two stars or a star and a massive planet, but it has been hard to rule any of them out until now because the “core” of the AGBs, where the disks form, are too small to be directly resolved by telescopes. Blackman and his student, Scott Lucchini, wanted to determine whether the binaries can be widely separated and weakly interacting, or whether they must be close and strongly interacting.By studying the jets from pre-planetary and planetary nebulae, Blackman and Lucchini were able to connect the energy and momentum involved in the accretion process with that in the jets; the process of accretion is what in effect provides the fuel for these jets. …

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Heavy metal in the early cosmos: Simulations shed light on formation and explosion of stars in earliest galaxies

Ab initio: “From the beginning.” It’s a term used in science to describe calculations that rely on established mathematical laws of nature, or “first principles,” without additional assumptions or special models. But when it comes to the phenomena that Milos Milosavljevic is interested in calculating, we’re talking really ab initio, as in from the beginning of time onward.Things were different in the early eons of the universe. The cosmos experienced rapid inflation; electrons and protons floated free from each other; the universe transitioned from complete darkness to light; and enormous stars formed and exploded to start a cascade of events leading to our present-day universe.Working with Chalence Safranek-Shrader and Volker Bromm at The University of Texas at Austin, Milosavljevic recently reported the results of several massive numerical simulations charting the forces of the universe in its first hundreds of millions of years using some of the world’s most powerful supercomputers, including the National Science Foundation-supported Stampede, Lonestar and Ranger (now retired) systems at the Texas Advanced Computing Center.The results, described in the Monthly Notices of the Royal Astronomical Society in January 2014, refine how the first galaxies formed, and in particular, how metals in the stellar nurseries influenced the characteristics of the stars in the first galaxies.”The universe formed at first with just hydrogen and helium,” Milosavljevic said. “But then the very first stars cooked metals and after those stars exploded, the metals were dispersed into ambient space.”Eventually the ejected metals fell back into the gravitational fields of the dark matter haloes, where they formed the second generation of stars. However, the first generation of metals ejected from supernovae did not mix in space uniformly.”It’s as if you have coffee and cream but you don’t stir it, and you don’t wait for a long enough time,” he explained. “You would drink some cream and coffee but not coffee with cream. There will be thin sheets of coffee and cream.”According to Milosavljevic, subtle effects like these governed the evolution of early galaxies. Some stars formed that were rich in metals, while others were metal-poor. Generally there was a spread in stellar chemical abundances because of the incomplete mixing.Another factor that influenced the evolution of galaxies was how the heavier elements emerged from the originating blast. Instead of the neat spherical blast wave that researchers presumed before, the ejection of metals from a supernova was most likely a messy process with blobs of shrapnel shooting in every direction.”Modeling these blobs properly is very important for understanding where metals ultimately go,” Milosavljevic said.Predicting future observationsIn astronomical terms, early in the universe translates to very far away. …

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Astronomer uncovers the hidden identity of an exoplanet

July 1, 2013 — Hovering about 70 light-years from Earth — that’s “next door” by astronomical standards — is a star astronomers call HD 97658, which is almost bright enough to see with the naked eye. But the real “star” is the planet HD 97658b, not much more than twice Earth’s diameter and a little less than eight times its mass. HD 97658b is a super-Earth, a class of planet for which there is no example in our home solar system.While the discovery of this particular exoplanet is not new, determining its true size and mass is, thanks to Diana Dragomir, a postdoctoral astronomer with UC Santa Barbara’s Las Cumbres Observatory Global Telescope (LCOGT). As part of her research, Dragomir looked for transits of this exoplanet with Canada’s Microvariability & Oscillations of Stars (MOST) space telescope. The telescope was launched in 2003 to a pole-over-pole orbit about 510 miles high. Dragomir analyzed the data using code written by LCOGT postdoctoral fellow Jason Eastman. The results were published online today in the Astrophysical Journal Letters.A super-Earth is an exoplanet with a mass and radius between those of Earth and Neptune. Don’t be fooled by the moniker though. Super-Earth refers to the planet’s mass and does not imply similar temperature, composition, or environment to Earth. The brightness of HD 97658 means astronomers can study this star and planet in ways not possible for most of the exoplanet systems that have been discovered around fainter stars.HD 97658b was discovered in 2011 by a team of astronomers using the Keck Observatory and a technique sometimes called Doppler wobble. …

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