Mystery of exploding stars yields to astrophysicists

A longstanding mystery about the tiny stars that let loose powerful explosions known as Type Ia supernovae might finally be solved.

A visualization of a computer simulation of a Type Ia supernova. Ejected material (brown) 
from an exploding white dwarf star crashes into its companion star (blue), 
yielding an ultraviolet pulse of light streaming from the c
ompanion star's location [Credit: Daniel Kasen]

For decades, astronomers have debated whether one white dwarf star, or two, is necessary for firing up this particular kind of supernova. The answer is not merely academic. Understanding the nitty-gritty physics and diversity of Type Ia supernovae will help illuminate our study of the evolution of galaxies and the strange cosmic force known as dark energy.

"It's about understanding one of the ultimate mysteries about stars," Laura Chomiuk, an Assistant Professor in Michigan State University's Department of Physics and Astronomy, told The Kavli Foundation in a roundtable discussion. "Something in the universe makes one percent of white dwarfs commit suicide and explode as supernovae. We are only now starting to have an idea of what causes this."

Chomiuk was one of four astrophysicist who came together to discuss the importance of two studies, published earlier this year in the journal Nature, that offer some of the strongest evidence to date that either one or two white dwarfs can trigger Type Ia supernovae.

"Previously, we all thought there was just going to be one scenario for Type Ia supernovae," said Andrew Howell, a Staff Scientist at Las Cumbres Observatory Global Telescope Network (LCOGT), an adjunct faculty member in the Physics Department at the University of California, Santa Barbara (UCSB), and an author of several recent papers. "But now we're starting to see that there are many different ways to make a Type Ia supernova."

The Nature studies relied on a new theory proposed by physicist Daniel Kasen. He predicted that the material hurled out from an exploding white dwarf "corpse" star should create an observable ultraviolet flash when it slams into a nearby, normal companion star. That companion star is a necessary ingredient in the single white dwarf scenario, as it provides the matter that triggers the white dwarf's thermonuclear explosion. An alternative scenario calls for two dwarfs to merge and blow up.

Observational astronomers, such as Howell, have recently seen Type 1a supernovae with and without the telltale ultraviolet flash predicted by Kasen.

"People have discussed over the years whether companion stars could survive the blast from a nearby supernova explosion. But nobody had asked, 'What would it look like to actually see that happen?'" said Kasen, an Associate Professor of Astronomy and Physics at the University of California, Berkeley, a faculty scientist at the Lawrence Berkeley National Laboratory, and a coauthor of one of the Nature papers. "So I did a back-of-the-envelope calculation to estimate how hot that collision would be, how bright it might be, and so on. Surprisingly, the collision turned out to be bright enough to be seen on Earth. That gave the observers an idea of what to look for."

These discoveries have come about in large part thanks to new telescope survey projects, using a suite of instruments available in space and on the ground, and advances in computer modeling that strengthen theoretical models.

"It's not just new capabilities but all these new capabilities working together that are allowing us to make such progress," said Lars Bildsten, the Director of the Kavli Institute for Theoretical Physics (KITP) and a Professor in the Physics Department at the University of California, Santa Barbara.

A more thorough knowledge of Type Ia supernovae will inform many areas of astrophysics and cosmology. For instance, the energy unleashed by supernovae can modify galaxies by dispersing the matter required by new stars to form.

Type Ia supernovae also explode with an intrinsic brightness, which scientists have long relied on to calculate cosmic distances. Such distance measurements are integral for tracking the acceleration of the expansion of the universe due to dark energy.

Amazingly, Type Ia supernova can outshine entire galaxies of billions of stars, making them easy to spot across cosmic history. Getting the mechanics right of these explosions will further explain how the deaths of certain stars make for such impressive stellar pyrotechnics.

Further information: http://www.kavlifoundation.org/science-spotlights/captured-last-tiny-stars-spark-fierce-supernovae-explosions

Source: The Kavli Foundation [August 19, 2015]

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“Planetary pebbles” were the building blocks for the largest planets

Researchers at Southwest Research Institute (SwRI) and Queen's University in Canada have unraveled the mystery of how Jupiter and Saturn likely formed. This discovery, which changes our view of how all planets might have formed, will be published in the Aug. 20 issue of Nature.

This artist’s concept of a young star system shows gas giants forming first, while the
 gas nebula is present. Southwest Research Institute scientists used computer simulations
 to nail down how Jupiter and Saturn evolved in our own solar system. These new
 calculations show that the cores of gas giants likely formed by gradually accumulating
 a population of planetary pebbles – icy objects about a foot in diameter 
[Credit: NASA/JPL-Caltech]

Ironically, the largest planets in the solar system likely formed first. Jupiter and Saturn, which are mostly hydrogen and helium, presumably accumulated their gasses before the solar nebula dispersed. Observations of young star systems show that the gas disks that form planets usually have lifetimes of only 1 to 10 million years, which means the gas giant planets in our solar system probably formed within this time frame. In contrast, the Earth probably took at least 30 million years to form, and may have taken as long as 100 million years. So how could Jupiter and Saturn have formed so quickly?

The most widely accepted theory for gas giant formation is the so-called core accretion model. In this model, a planet-sized core of ice and rock forms first. Then, an inflow of interstellar gas and dust attaches itself to the growing planet. However, this model has an Achilles heel; specifically, the very first step in the process. To accumulate a massive atmosphere requires a solid core roughly 10 times the mass of Earth. Yet these large objects, which are akin to Uranus and Neptune, had to have formed in only a few million years.

In the standard model of planet formation, rocky cores grow as similarly sized objects accumulate and assimilate through a process called accretion. Rocks incorporate other rocks, creating mountains; then mountains merge with other mountains, leading to city-sized objects, and so on. However, this model is unable to produce planetary cores large enough, in a short enough period of time, to explain Saturn and Jupiter.

"The timescale problem has been sticking in our throats for some time," said Dr. Hal Levison, an Institute scientist in the SwRI Planetary Science Directorate and lead author of the paper.

"It wasn't clear how objects like Jupiter and Saturn could exist at all," continued Levison. New calculations by the team show that the cores of Jupiter and Saturn could form well within the 10-million-year time frame if they grew by gradually accumulating a population of planetary pebbles -- icy objects about a foot in diameter. Recent research has shown that gas can play a vital role in increasing the efficiency of accretion. So pebbles entering orbit can spiral onto the protoplanet and assimilate, assisted by a gaseous headwind.

In their article, Levison, Kretke, and Duncan show that pebble accretion can produce the observed structure of the solar system as long as the pebbles formed slowly enough that the growing planets have time to gravitationally interact with one another.

"If the pebbles form too quickly, pebble accretion would lead to the formation of hundreds of icy Earths," said Kretke. "The growing cores need some time to fling their competitors away from the pebbles, effectively starving them. This is why only a couple of gas giants formed."

"As far as I know, this is the first model to reproduce the structure of the outer solar system, with two gas giants, two ice giants (Uranus and Neptune), and a pristine Kuiper belt," says Levison.

"After many years of performing computer simulations of the standard model without success, it is a relief to find a new model that is so successful," adds Duncan.

Levison is the principal investigator of the research, funded through a National Science Foundation Astronomy and Astrophysics Research Grant.

Source: Southwest Research Institute [August 19, 2015]

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Solar System formation don't mean a thing without that spin

New work from Carnegie's Alan Boss and Sandra Keiser provides surprising new details about the trigger that may have started the earliest phases of planet formation in our solar system. It is published by The Astrophysical Journal. For decades, it's been hypothesized that our Solar System's genesis was initiated by a shock wave from a supernova.

These images show the central plane of a rotating disk orbiting a newly formed 
protostar (dark dot) formed in a three-dimensional model of the shock-triggered 
collapse of a molecular cloud of gas and dust. Density is shown on the left, 
while the x velocity plot on the right shows how the shock (outer edge) has
 injected fingers with motions that are responsible for producing the spin
 of the disk around the central protostar [Credit: Alan Boss]

According to this theory, the wall of pressure formed by a shock wave from the exploding star smacked into a cloud of dust and gas, causing it to collapse and contract into the core of a new proto-star--our Sun. This young Sun was surrounded by a rotating disk of dust and gas that eventually aggregated to form the planets of our Solar System.

Boss and Keiser have explored this theory of cloud collapse--as opposed to a previous theory of shock wave-caused cloud shredding--using advanced 2-D and 3-D modeling for several years and have published a series of papers supporting it.

A crucial aspect of this line of research is the distribution of certain products of the explosion, called short-lived radioisotopes. Isotopes are versions of elements that contain the same number of protons, but different number of neutrons. Specific isotopes were formed during the supernova's explosion and distributed throughout the region that would eventually become our Solar System, before the isotopes had a chance to undergo radioactive decay. Their daughter products can be found today in samples of primitive meteorites.

Previous modeling work from Boss and Keiser demonstrated that a wave of pressure expanding out from a supernova's explosion could have struck a gas cloud, and formed little finger-like indentations in the cloud's surface, which injected the short-lived radioisotopes into the collapsing ball of gas and dust that would eventually become our Sun and its planets.

The new modeling work from Boss and Keiser shows that this shock injection could have been responsible not only for the distribution of the isotopes now found in primitive meteorites, but also for the spin of our Solar System. The angular momentum of the shock fingers appears to be what allowed the disk of gas and dust to form around the Sun, instead of collapsing into it.

"This was a complete surprise to me," Boss said, "the very fact that a rotating disk formed around our proto-Sun may have been a result of the spin induced by this shock front. Without spin, the cloud disappears into the proto-Sun. With spin, a disk suitable for planet formation is created."

Source: Carnegie Institution [August 18, 2015]

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