The chemical history of molecules in circumstellar disks

Schematic view of the history of H2O gas and ice throughout the disk. The main oxygen reservoir at t(acc) is indicated for each zone; the histories are described in the text. The percentages indicate the fraction of the disk mass contained in each zone. Note the disproportionality of the R and z axes. The colours have no specific meaning other than to distinguish the different zones

Context: The chemical composition of a molecular cloud changes dramatically as it collapses to form a low-mass protostar and circumstellar disk. Two-dimensional (2D) chemodynamical models are required to properly study this process.
Aims: The goal of this work is to follow, for the first time, the chemical evolution in two dimensions all the way from a pre-stellar core into a circumstellar disk. Of special interest is the question whether the chemical composition of the disk is a result of chemical processing during the collapse phase, or whether it is determined by in situ processing after the disk has formed.
Methods: Our model combines a semi-analytical method to get 2D axisymmetric density and velocity structures with detailed radiative transfer calculations to get temperature profiles and UV fluxes. Material is followed in from the core to the disk and a full gas-phase chemistry network — including freeze-out onto and evaporation from cold dust grains — is evolved along these trajectories. The abundances thus obtained are compared to the results from a static disk model and to observations of comets.
Results: The chemistry during the collapse phase is dominated by a few key processes, such as the evaporation of CO or the photodissociation of H2O. At the end of the collapse phase, the disk can be divided into zones with different chemical histories. The disk is not in chemical equilibrium at the end of the collapse, so care must be taken when choosing the initial abundances for stand-alone disk chemistry models. Our model results imply that comets must be formed from material with different chemical histories: some of it is strongly processed, some of it remains pristine. Variations between individual comets are possible if they formed at different positions or different times in the solar nebula.

http://arxiv.org/abs/1109.1741

The Star That Should Not Exist

A team of European astronomers has used ESO’s Very Large Telescope (VLT) to track down a star in the Milky Way that many thought was impossible. They discovered that this star is composed almost entirely of hydrogen and helium, with only remarkably small amounts of other chemical elements in it. This intriguing composition places it in the “forbidden zone” of a widely accepted theory of star formation, meaning that it should never have come into existence in the first place. The results will appear in the 1 September 2011 issue of the journal Nature.

A faint star in the constellation of Leo (The Lion), called SDSS J102915+172927 [1], has been found to have the lowest amount of elements heavier than helium (what astronomers call “metals”) of all stars yet studied. It has a mass smaller than that of the Sun and is probably more than 13 billion years old.

“A widely accepted theory predicts that stars like this, with low mass and extremely low quantities of metals, shouldn’t exist because the clouds of material from which they formed could never have condensed,” [2] said Elisabetta Caffau (Zentrum für Astronomie der Universität Heidelberg, Germany and Observatoire de Paris, France), lead author of the paper. “It was surprising to find, for the first time, a star in this ‘forbidden zone’, and it means we may have to revisit some of the star formation models.”

The team analysed the properties of the star using the X-shooter and UVES instruments on the VLT [3]. This allowed them to measure how abundant the various chemical elements were in the star. They found that the proportion of metals in SDSS J102915+172927 is more than 20 000 times smaller than that of the Sun [4][5].

“The star is faint, and so metal-poor that we could only detect the signature of one element heavier than helium — calcium — in our first observations,” said Piercarlo Bonifacio (Observatoire de Paris, France), who supervised the project. “We had to ask for additional telescope time from ESO’s Director General to study the star’s light in even more detail, and with a long exposure time, to try to find other metals.”

Cosmologists believe that the lightest chemical elements — hydrogen and helium — were created shortly after the Big Bang, together with some lithium [6], while almost all other elements were formed later in stars. Supernova explosions spread the stellar material into the interstellar medium, making it richer in metals. New stars form from this enriched medium so they have higher amounts of metals in their composition than the older stars. Therefore, the proportion of metals in a star tells us how old it is.

“The star we have studied is extremely metal-poor, meaning it is very primitive. It could be one of the oldest stars ever found,” adds Lorenzo Monaco (ESO, Chile), also involved in the study.

Also very surprising was the lack of lithium in SDSS J102915+172927. Such an old star should have a composition similar to that of the Universe shortly after the Big Bang, with a few more metals in it. But the team found that the proportion of lithium in the star was at least fifty times less than expected in the material produced by the Big Bang.

“It is a mystery how the lithium that formed just after the beginning of the Universe was destroyed in this star.” Bonifacio added.

The researchers also point out that this freakish star is probably not unique. “We have identified several more candidate stars that might have metal levels similar to, or even lower than, those in SDSS J102915+172927. We are now planning to observe them with the VLT to see if this is the case,” concludes Caffau.

Notes
[1] The star is catalogued in the Sloan Digital Sky Survey or SDSS. The numbers refer to the object’s position in the sky.

[2] Widely accepted star formation theories state that stars with a mass as low as SDSS J102915+172927 (about 0.8 solar masses or less) could only have formed after supernova explosions enriched the interstellar medium above a critical value. This is because the heavier elements act as “cooling agents”, helping to radiate away the heat of gas clouds in this medium, which can then collapse to form stars. Without these metals, the pressure due to heating would be too strong, and the gravity of the cloud would be too weak to overcome it and make the cloud collapse. One theory in particular identifies carbon and oxygen as the main cooling agents, and in SDSS J102915+172927 the amount of carbon is lower than the minimum deemed necessary for this cooling to be effective.

[3] X-shooter and UVES are VLT spectrographs — instruments used to separate the light from celestial objects into its component colours and allow detailed analysis of the chemical composition. X-shooter can capture a very wide range of wavelengths in the spectrum of an object in one shot (from the ultraviolet to the near-infrared). UVES is the Ultraviolet and Visual Echelle Spectrograph, a high-resolution optical instrument.

[4] The star HE 1327-2326, discovered in 2005, has the lowest known iron abundance, but it is rich in carbon. The star now analysed has the lowest proportion of metals when all chemical elements heavier than helium are considered.

[5] ESO telescopes have been deeply involved in many of the discoveries of the most metal-poor stars. Some of the earlier results were reported in eso0228 and eso0723 and the new discovery shows that observations with ESO telescopes have let astronomers make a further step closer to finding the first generation of stars.

[6] Primordial nucleosynthesis refers to the production of chemical elements with more than one proton a few moments after the Big Bang. This production happened in a very short time, allowing only hydrogen, helium and lithium to form, but no heavier elements. The Big Bang theory predicts, and observations confirm, that the primordial matter was composed of about 75% (by mass) of hydrogen, 25% of helium, and trace amounts of lithium.
http://www.eso.org/public/news/eso1132/

Life in the Universe by Stephen Hawking

In this talk, I would like to speculate a little, on the development of life in the universe, and in particular, the development of intelligent life. I shall take this to include the human race, even though much of its behaviour through out history, has been pretty stupid, and not calculated to aid the survival of the species. Two questions I shall discuss are, ‘What is the probability of life existing else where in the universe?’ and, ‘How may life develop in the future?’

It is a matter of common experience, that things get more disordered and chaotic with time. This observation can be elevated to the status of a law, the so-called Second Law of Thermodynamics. This says that the total amount of disorder, or entropy, in the universe, always increases with time. However, the Law refers only to the total amount of disorder. The order in one body can increase, provided that the amount of disorder in its surroundings increases by a greater amount. This is what happens in a living being. One can define Life to be an ordered system that can sustain itself against the tendency to disorder, and can reproduce itself. That is, it can make similar, but independent, ordered systems. To do these things, the system must convert energy in some ordered form, like food, sunlight, or electric power, into disordered energy, in the form of heat. A laptopIn this way, the system can satisfy the requirement that the total amount of disorder increases, while, at the same time, increasing the order in itself and its offspring. A living being usually has two elements: a set of instructions that tell the system how to sustain and reproduce itself, and a mechanism to carry out the instructions. In biology, these two parts are called genes and metabolism. But it is worth emphasising that there need be nothing biological about them. For example, a computer virus is a program that will make copies of itself in the memory of a computer, and will transfer itself to other computers. Thus it fits the definition of a living system, that I have given. Like a biological virus, it is a rather degenerate form, because it contains only instructions or genes, and doesn’t have any metabolism of its own. Instead, it reprograms the metabolism of the host computer, or cell. Some people have questioned whether viruses should count as life, because they are parasites, and can not exist independently of their hosts. But then most forms of life, ourselves included, are parasites, in that they feed off and depend for their survival on other forms of life. I think computer viruses should count as life. Maybe it says something about human nature, that the only form of life we have created so far is purely destructive. Talk about creating life in our own image. I shall return to electronic forms of life later on…… Continue reading Life in the Universe by Stephen Hawking

Extraterrestrial dust reveals asteroid’s past and future

The 500-metre-long Itokawa

Talk about seeing a world in a grain of sand. A sprinkling of asteroid dust that slipped into Japan’s Hayabusa probe when it touched down on the asteroid Itokawa six years ago has revealed surprising details about the space rock’s past and its likely future.

Hayabusa was meant to land on the 500-metre-wide asteroid in 2005 and fire projectiles into its surface, scooping up the resulting debris for later return to Earth. But the projectiles never fired, and team members had to wait five long years for the probe, which suffered numerous equipment failures, to limp back to EarthMovie Camera (see a picture of it after it touched down in Australia).

They hoped that some asteroid dust had managed to find its way into Hayabusa’s collection chamber when the probe touched Itokawa, but even when they saw dust there they were sceptical. “It was me who opened the sample catcher,” says Tomoki Nakamura of Tohoku University in Sendai. “I could not believe they were real Itokawa samples.”

Now, he and dozens of other researchers are reporting their analyses of the samples, which comprise more than 1500 rocky particles from Itokawa, all smaller than 0.2 millimetres across.

Larger parent

The studies suggest that Itokawa was once part of a much larger asteroid. The conclusion is based on the fact that the particles show a range of minerals that must have reached 800 °C to form (see picture). The decay of radioactive aluminium isotopes could have created that much heat if the parent body was at least 20 kilometres across – 40 times its current size, Nakamura and his collaborators say.

“The body needs to be big; otherwise, it would lose the temperature too quickly for these processes to occur,” says Trevor Ireland of the Australian National University in Canberra, who analysed some of the samples.

Indeed, data from Hayabusa itself had already determined that Itokawa has had its share of knocks in the past. The asteroid’s gravitational pull on the probe revealed that Itokawa has a very low density, suggesting it is a rocky rubble pile that probably coalesced after its parent body was smashed in an impact.

Dust that slipped into Japan's Hayabusa probe when it touched down on the asteroid Itokawa in 2005 pieces together its past and foretells its bleak future

Solar wind

The samples also hint at a bleak future for Itokawa. Keisuke Nagao of the University of Tokyo and colleagues studied noble gases, such as neon, in three grains. These gases can be implanted by charged particles streaming in from the solar wind and even from beyond the solar system.

The results suggest that the grains have been exposed to these charged particles for no more than 8 million years. This implies either that Itokawa coalesced 8 million years ago, or that it loses tens of centimetres of material from its surface every million years, exposing new layers of rubble in the process.

This could occur if dust grains lifted off the surface after micrometeoroid impacts simply float away from the lightweight asteroid. If Itokawa is losing its surface at that rate, it may completely disappear in less than a billion years.

Disappearing space rock

“It’s a bit sad to think it will eventually disappear,” says Scott Sandford of NASA’s Ames Research Center in California, who analysed some of the samples.

But he adds: “The ‘disappearance’ of Itokawa has its compensations. For one thing, I’d rather Itokawa be whittled away then to have it run into the Earth as a single object, which would cause some serious issues for the Earth if it happened. Also, one should remember that it is the process of whittling away at asteroids like Itokawa that produces the smaller meteoroids that ultimately land on the Earth as meteorites.”

This particular finding about its future may actually have been helped by Hayabusa’s failure to fire projectiles into the asteroid. “Solar wind penetrates only 100 nanometres or so into rock,” says Ireland. “The gun firing would have meant that the projectile penetrated into the asteroid, pushing out chips and fragments.” That would have made it “very hard to isolate surface pieces, which is where the solar wind resides”, he adds.

Link confirmed

The samples’ composition matches that of the most common type of meteorite found on Earth, called ordinary chondrites. Since Itokawa is classified as a stony S-type asteroid – the most common kind in the inner asteroid belt, the analyses confirm that ordinary chondrites come from S-type asteroids.

The studies also highlight the importance of returning samples from extraterrestrial bodies to Earth for study. “The analysis apparatus is too heavy to carry to an asteroid,” Nakamura told New Scientist.

Alexander Krot at the University of Hawaii at Manoa, who was not part of the team, says two other sample-return missions – Japan’s Hayabusa-2 and NASA’s OSIRIS-Rex – will blast off in 2014 and 2016 to collect samples from asteroids rich in minerals that formed in water.

These could reveal clues about “one of the most outstanding questions in planetary science – the origin of Earth’s water”, he says in a commentary in Science. Studies of hydrogen isotopes in the water-formed minerals in the target asteroids could help determine if Earth’s water was delivered by asteroids or comets or simply came from the dust from which the planet formed.

Journal reference: Science, DOI: 10.1126/science.1207758, 10.1126/science.1207776, 10.1126/science.1207865, 10.1126/science.1207794, 10.1126/science.1207807, 10.1126/science.1207785, 10.1126/science.1207758
http://www.newscientist.com/article/dn20834-extraterrestrial-dust-reveals-asteroids-past-and-future.html

Molecules in supernova ejecta

The fir rst molecules detected at infrared wavelengths in the ejecta of a Type II supernova, namely SN1987A, consisted of CO and SiO. Since then, con rmation of the formation of these two species in several other supernovae a few hundred days after explosion has been obtained. However, supernova environments appear to hamper the synthesis of large, complex species due to the lack of microscopically-mixed hydrogen deep in supernova cores. Because these environments also form carbon and silicate dust, it is of importance to understand the role played
by molecules in the depletion of elements and how chemical species get incorporated into dust grains. In the present paper, we review our current knowledge of the molecular component of
supernova ejecta, and present new trends and results on the synthesis of molecules in these harsh, explosive events….

Read more: http://arxiv.org/PS_cache/arxiv/pdf