New Theories on Stellar Winds – Pulsating Magnetically Driven Radiative Energy

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A new study of the mechanism that drives stellar winds from the upper atmosphere of a star has shed new light. Astronomers think there are three possibilities: radiative, in which the pressure of the light pushes out the grains, magnetically driven, in which the stellar magnetic field plays a role in powering the flow, and pulsation driven, in which a periodic build-up of radiative energy in the stellar interior is suddenly released.

A new study of the mechanism that drives stellar winds from the upper atmosphere of a star has shed new light. Astronomers think there are three possibilities: radiative, in which the pressure of the light pushes out the grains, magnetically driven, in which the stellar magnetic field plays a role in powering the flow, and pulsation driven, in which a periodic build-up of radiative energy in the stellar interior is suddenly released.

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The winds of stars more evolved than the Sun (like the so-called giant stars that are cooler and larger in diameter than the Sun) often contain dust particles which enrich the interstellar medium with heavy elements. These winds also contain small grains on whose surfaces chemical reactions produce complex molecules. The dust also absorbs radiation and obscures visible light. Understanding the mechanism(s) that produce these winds in evolved stars is important both for modeling the wind and the character of the stellar environment, and for predicting the future evolution of the star.

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Nearly all stars have winds. The Sun’s wind, which originates from its hot outer layer (corona), contains charged particles emitted at a rate equivalent to about one-millionth of the moon’s mass each year. Some of these particles bombard the Earth, producing radio static, auroral glows, and (in extreme cases) disrupted global communications.

NASA'S Chandra Finds Fastest Wind From Stellar-Mass Black Hole
NASA’S Chandra Finds Fastest Wind From Stellar-Mass Black Hole

Over the years scientific opinion has varied among these alternatives, depending on each particular stellar example. Harvard–Smithsonian Center for Astrophysics Chris Johnson, and his colleagues explored the problem of wind-driving mechanism in giant stars by measuring the motion of the outflowing CO (carbon monoxide) gas around one the nearest and brightest giant stars, EU Del, which is only about 380 light-years away and shines with 1600 solar-luminosities.

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New Equation:
Increase Charged Particles and Decreased Magnetic Field → Increase Outer Core Convection → Increase of Mantle Plumes → Increase in Earthquake and Volcanoes → Cools Mantle and Outer Core → Return of Outer Core Convection (Mitch Battros – July 2012)

Its radius, if the star were placed at the position of the Sun, would extend past the orbit of Venus. EU Del is known to be a semi-regular variable star which pulses every sixty days or so (but with some secondary periods as well), and infrared observations suggest it has a circumstellar dust shell.

The astronomers used the submillimeter APEX (Atacama Pathfinder Experiment) telescope to look at warm CO gas in the wind, making EU Del one of the first stars of its class to be studied with this relatively new tool. The team reports finding the CO moving at about ten kilometers per second (twenty two thousand miles per hour) with a total mass-loss rate equal to about the mass of the Moon each year.

BREAKING NEWS: Supernova Showered Earth with Radioactive Debris

An international team of scientists has found evidence of a series of massive supernova explosions near our solar system, which showered the Earth with radioactive debris. The scientists found radioactive iron-60 in sediment and crust samples taken from the Pacific, Atlantic and Indian Oceans.

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Some theories suggest cosmic rays from the supernova could have increased cloud cover. The scientists believe the supernova in this case were less than 300 light years away; close enough to be visible during the day and comparable to the brightness of the Moon.

The supernova explosions create many heavy elements and radioactive isotopes which are strewn into the cosmic neighborhood. Although Earth would have been exposed to an increased cosmic ray bombardment, the radiation would have been too weak to cause direct biological damage or trigger mass extinctions.

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Any iron-60 dating from the Earth’s formation more than four billion years ago has long since disappeared. The iron-60 atoms reached Earth in minuscule quantities and so the team needed extremely sensitive techniques to identify the interstellar iron atoms.

The team from Australia, the University of Vienna in Austria, Hebrew University in Israel, Shimizu Corporation and University of Tokyo, Nihon University and University of Tsukuba in Japan, Senckenberg Collections of Natural History Dresden and Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany, also found evidence of iron-60 from an older supernova around eight million years ago, coinciding with global faunal changes in the late Miocene.

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The iron-60 was concentrated in a period between 3.2 and 1.7 million years ago, which is relatively recent in astronomical terms, said research leader Dr Anton Wallner from The Australian National University (ANU).

“We were very surprised that there was debris clearly spread across 1.5 million years,” said Dr Wallner, a nuclear physicist in the ANU Research School of Physics and Engineering. “It suggests there were a series of supernova, one after another. “It’s an interesting coincidence that they correspond with when the Earth cooled and moved from the Pliocene into the Pleistocene period.”

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The dating showed the fallout had only occurred in two time periods, 3.2 to 1.7 million years ago and eight million years ago. Current results from TU Munich are in line with these findings.

A possible source of the supernova is an ageing star cluster, which has since moved away from Earth, independent work led by TU Berlin has proposed in a parallel publication. The cluster has no large stars left, suggesting they have already exploded as supernova, throwing out waves of debris.

New Study Shows How Black Holes and Galaxies Formed

Until recently, many researchers thought supermassive black holes were seeded by the collapse of some of the first stars. But modeling work by several groups has suggested that this process would only lead to small black holes.

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Kentaro Nagamine at Osaka University’s Department of Earth and Space Science, Isaac Shlosman at the University of Kentucky and co-workers simulated a different situation, in which supermassive black holes are seeded by clouds of gas falling into potential wells created by dark matter – the invisible matter that astronomers believe makes up 85% of the mass of the Universe.

Simulating the dynamics of huge gas clouds is extremely complex, so the team had to use some numerical tricks called ‘sink particles’ to simplify the problem.

“Although we have access to extremely powerful supercomputers at Osaka University’s Cybermedia Center and the National Astronomical Observatory of Japan, we can’t simulate every single gas particle,” explains Nagamine. “Instead, we model small spatial scales using sink particles, which grow as the surrounding gas evolves. This allows us to simulate much longer timescales than was previously possible.”

The researchers found that most seed particles in their simulations did not grow very much, except for one central seed, which grew rapidly to more than 2 million Sun-masses in just 2 million years, representing a feasible path toward a supermassive black hole. Moreover, as the gas spun and collapsed around the central seed it formed two misaligned accretion discs, which have never been observed before.

In other recent work, Nagamine and co-workers described the growth of massive galaxies that formed around the same time as supermassive black holes. “We like to push the frontier of how far back in time we can see,” says Nagamine. The researchers hope their simulations will be validated by real data when NASA’s James Webb Space Telescope, due to be launched in 2018, observes distant sources where direct gas collapse is happening.