Posted on by Jon Voisey

Trojans May Yet Rain Down

It would be an interesting survey to catalog the initial reactions readers have to “Trojans”. Do you think first of wooden horses, or do asteroids spring to mind? Given the context of this website, I’d hope it’s the latter. If so, you’re thinking along the right lines. But how much do you really know about astronomical Trojans?

While most frequently used to discuss the set of objects in Jupiter’s orbital path that lie 60º ahead and behind the planet, orbiting the L4 and L5 Lagrange points, the term can be expanded to include any family of objects orbiting these points of relative stability around any other object. While Jupiter’s Trojan family is known to include over 3,000 objects, other solar system objects have been discovered with families of their own. Even one of Saturn’s moons, Tethys, has objects in its Lagrange points (although in this case, the objects are full moons in their own right: Calypso and Telesto).

In the past decade Neptunian Trojans have been discovered. By the end of this summer, six have been confirmed. Yet despite this small sample, these objects have some unexpected properties and may outnumber the number of asteroids in the main belt by an order of magnitude. However, they aren’t permanent and a paper published in the July issue of the International Journal of Astrobiology suggests that these reservoirs may produce many of the short period comets we see and “contribute a significant fraction of the impact hazard to the Earth.”

The origin of short period comets is an unusual one. While the sources of near Earth asteroids and long period comets have been well established, short period comets parent locations have been harder to pin down. Many have orbits with aphelions in the outer solar system, well past Neptune. This led to the independent prediction of a source of bodies in the far reaches by Edgeworth (1943) and Kuiper (1951). Yet others have aphelions well within the solar system. While some of this could be attributed to loss of energy from close passes to planets, it did not sufficiently account for the full number and astronomers began searching for other sources.

In 2006, J. Horner and N. Evans demonstrated the potential for objects from the outer solar system to be captured by the Jovian planets. In that paper, Horner and Evans considered the longevity of the stability of such captures for Jupiter Trojans. The two found that these objects were stable for billions of years but could eventually leak out. This would provide a storing of potential comets to help account for some of the oddities.

However, the Jupiter population is dynamically “cold” and does not contain a large distribution of velocities that would lead to more rapid shedding. Similarly, Saturn’s Trojan family was not found to be excited and was estimated to have a half life of ~2.5 billion years. One of the oddities of the Neptunian Trojans is that those few discovered thus far have tended to have high inclinations. This indicates that this family may be more dynamically excited, or “hotter” than that of other families, leading to a faster rate of shedding. Even with this realization, the full picture may not yet be clear given that searches for Trojans concentrate on the ecliptic and would likely miss additional members at higher inclinations, thus biasing surveys towards lower inclinations.

To assess the dangers of this excited population, Horner teamed with Patryk Lykawka to simulate the Neptunian Trojan system. From it, they estimated the family had a half life of ~550 million years. Objects leaving this population would then undergo several possible fates. In many cases, they resembled the Centaur class of objects with low eccentricities and with perihelion near Jupiter and aphelion near Neptune. Others picked up energy from other gas giants and were ejected from the solar system, and yet others became short period comets with aphelions near Jupiter.

Given the ability for this the Neptunian Trojans to eject members frequently, the two examined how many of the of short period comets we see may be from these reservoirs. Given the unknown nature of how large these stores are, the authors estimated that they could contribute as little as 3%. But if the populations are as large as some estimates have indicated, they would be sufficient to supply the entire collection of short period comets. Undoubtedly, the truth lies somewhere in between, but should it lie towards the upper end, the Neptunian Trojans could supply us with a new comet every 100 years on average.

Posted on by Jon Voisey

Does a “Rock Comet” Generate the Geminids?

Meteor
Geminid meteor shower

[/caption]

Many annual meteor showers have parent bodies identified. For example, the Perseids are ejecta from the comet, Swift-Tuttle and the Leonids from Tempel-Tuttle. Most known parent bodies are active comets, but one exception is the Geminid meteor shower that peaks in mid December. The parent for this shower is 3200 Phaethon. Observations of this object have shown it to be largely inactive pegging it as either a dead comet or an asteroid. But on June 20, 2009, shortly after perihelion, 3200 Phaethon brightened by over two magnitudes indicating this object may not be as dead as previously considered. A new paper considers the causes of the brightening and concludes that it could be a new mechanism leading to what the authors deem a “rock comet”.

David Jewett and Jing Li of UCLA, the authors of this new paper, consider several potential causes. Due to the size of 3200 Phaethon, they suggest that a collision is unlikely. One clue to the reason for the sudden change in brightness was a close link of a half of a day to a brightening in the solar corona. Given a typical solar wind speed and the distance of 3200 Phaethon at the time, this would put the Geminid parent just at the right range to be feeling the effects of the increase. However, the authors conclude that this cannot be directly responsible by imparting sufficient energy on the surface of the object to cause it to fluoresce due to an insufficient solar wind flux at that distance.

Instead, Jewett and Li consider more indirect explanations. Due to the temperature at 3200 Phaethon’s perihelion (0.14 AU) the presence of ices and other volatile gasses frozen solid and then blasting away as often happens in comets was ruled out as they would have been depleted on earlier orbits. However, the blow from the increased solar wind may have been sufficient to blow off loosely bound dust particles. While this is plausible, the authors note that the amount of mass lost if this were the case would be a paltry 2.5 x 108 kg. While it’s possible that this may have been the cause of this single brightening, this amount of mass loss to the overall stream of particles responsible for the Geminid shower would be insufficient to sustain the stream and similar losses would have to occur ~10 times per orbit of the body. Since this has not been observed, it is unlikely that this event was tied to the production of the meteors. Additionally, it is somewhat unlikely that it could even be the event for this sole case since repeated perihelions would slowly deplete the reservoir of available dust until the body was left with only a bare surface. Unlike active comets which continually free dust to be ejected through sublimation of ice, 3200 Phaethon has no such process. Or does it?

The novel proposition is that this object may have an unusual mechanism by which to continually generate and liberate dust particles of the size of the Geminids. The authors propose that the heating at perihelion causes portions of the rock to decompose. This process is greatly enhanced if the rock has water molecules bonded to it and lab experiments have shown that this can lead to violent fracturing. Such processes, if present, could easily lead to the production of new dust particles that would be liberated during close approach to the sun. This would make this object a “rock comet” in which the properties of a comet’s dust ejection via gasses would be carried out by rocks.

To confirm this hypothesis, future observations would be needed to search for subsequent brightening at perihelion. Similarly, it should be expected that such a process may make a faint cometary tail with only a dust component that may be visible as well, although the lack of any such detection so far, despite studies looking for cometary tails, casts some doubt on this process.

Posted on by Jon Voisey

Possibility for White Dwarf Pulsars?

AE Aquarii - A possible White Dwarf Pulsar
The white dwarf in the AE Aquarii system is the first star of its type known to give off pulsar-like pulsations that are powered by its rotation and particle acceleration.

[/caption]

Some satellites get all the glory. While Hubble, Chandra, and Spitzer frequently make headlines with their stunning images, many other space based observatories silently toil away. One of them, known as the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) has been in orbit since 2006, but rarely receives media attention although a stunning discovery has led to the publication of over 300 papers within a single year. A new paper in that onslaught has proposed an interesting new object: pulsars powered by white dwarfs.
PAMELA isn’t a satellite in its own right. It piggybacks on another satellite. Its mission is to observe high energy cosmic rays. Cosmic rays are particles, whether they be protons, electrons, nuclei of entire atoms, or other pieces, that are accelerated to high velocities, often from exotic sources and cosmological distances.

Among the types of particles PAMELA detects is the elusive positron. This anti-particle of the electron is quite rare due to the scarcity of anti-matter in general in our universe. However, much to the surprise of astronomers, in the range of 10 – 100 GeV, PAMELA has reported an abundance of positrons. In even higher ranges (100 GeV – 1 TeV) astronomers have found that there is a rise in both electrons and positrons. The conclusion from this is that something is able to actually create these particles in these energy ranges.

A flurry of papers went to publication to explain this unexpected finding. Explanations ranged from showers of particles created by even higher energy cosmic rays striking the interstellar medium, to the decay of dark matter, to neutron stars, pulsars, supernovae, and gamma ray bursts. Indeed, many events that produce high energies are sufficient to spontaneously produce matter from energy through the process of pair production. However, the range of these ejected particles would be limited. Effects, such as synchrotron and inverse Compton emission would drain their energy over large distances and as such, by the time they reached PAMELA’s detectors would be too low energy to account for the excesses in the observed energy ranges. From this, astronomers are presuming the culprits are in the local universe.

Joining the long list of candidates, a new paper has proposed a mundane object could be responsible for the high energy necessary to create these energetic particles, albeit with an unusual twist. Neutron stars, one of the potential objects formed in a supernova, are known to release large amounts of energies when spinning quickly while creating a strong magnetic field in the form of pulsars, but the authors propose that white dwarfs, the products of the slow death from stars not massive enough to result in a supernova, may be able to do the same thing. The difficulty in creating such a white dwarf pulsar is that, since white dwarfs don’t collapse to such a small size, they don’t “spin up” as much as they conserve angular momentum and shouldn’t have the sufficient angular velocity necessary.

The authors, led by Kazumi Kashiyama at Kyoto University propose that a white dwarf may reach the necessary rotational speed if they undergo a merger or accrete a sufficient amount of mass. This idea is not unheard of since white dwarf mergers and accretion are already implicated in Type Ia Supernovae. The combination of this with the expectation that around 10% of white dwarfs are expected to have magnetic fields of 106 Gauss, the steps necessary to produce a pulsar from a white dwarf seem to be in place. They note that since white dwarfs tend to have weaker magnetic fields, they shed their angular momentum more slowly and would last longer. Although this duration is still far longer than humans can possibly watch, this may indicate that many of the pulsars observed in our own galaxy are white dwarfs.

Next, the authors hope to conclusively identify such a star. The creation of each of these types of pulsars may provide a clue: Since neutron stars form from supernovae, they are surrounded by a shell of gas that contains a shock front from the supernova itself, which is more dense than the interstellar medium in general. As particles pass through this shock front, some of them would be lost. The same would not be said for white dwarfs which formed from a more gentle release and aren’t impeded by the relatively high density area. This shift in energy distributions may be one distinguishing characteristic.

Some stars have even been tentatively proposed as candidates for white dwarf pulsars. AE Aquarii was seen to give off some pulsar-like signals. EUVE J0317-855 is another white dwarf that appears to meet the qualifications, although no signals have been detected from this star. This new class of stars would be able to explain the excess signal in the higher energy range detected by PAMELA and will likely be the target of further observational searches in the future.

Posted on by Jon Voisey

Electric Resistance May Make Hot Jupiters Puffy

The Sun’s magnetic field

One of the surprises coming from the discoveries of the class of exoplanets known as “Hot Jupiters” is that they are puffed up beyond what would be expected from their temperature alone. The interpretation of these inflated radii is that extra energy must be being deposited in the regions of the atmosphere with large amounts of circulation. This extra energy would be deposited as heat, causing the atmosphere to expand. But from where was this extra energy coming? New research is suggesting that ionized winds passing through magnetic fields may create this process. Continue reading “Electric Resistance May Make Hot Jupiters Puffy”

Posted on by Jon Voisey

The Case of the Missing Bulges

The Hubble sequence is astronomer’s main tool for classifying galaxies. On one side, you have elliptical galaxies with defined structure. As you progress, the galaxies become more stretched out, but still lack definition until suddenly, there’s a bulge in the center and spiral arms! Oh yeah, and then there’s the cousins that no one really likes to hang out with, the “irregular” galaxies, hanging out in the corner.

But there’s another class of galaxies that seems to have fallen off the Hubble wagon. Some spiral galaxies seem to lack defined bulges. These oddities pose a challenge to our understanding of galactic formation.

The current understanding of galactic formation is one of hierarchical merging. Small dwarf galaxies form first, and then form bigger galaxies which merge and continue to eat more dwarf galaxies until a fully fledged galaxy is formed. However, the collisional nature of this formation tends to scatter stars, favoring random orbits towards the center of flattened galaxies, which should create a classical bulge. Galaxies that do not have a bulge, or have a “pseudobulge” (small bulges created by gravitational sorting of stars within an already formed galaxy) don’t seem to fit this picture.

A recent review suggests that galaxies without true bulges are in fact common and include many well-known galaxies such as M101 (the Pinwheel Galaxy) and M33. The team, led by John Kormendy of the University of Texas, Austin, conducted a survey of spiral galaxies in the Local Group to determine just how common they were. To determine the status of the bulge, the team analyzed the physical size of the bulge, its luminosity as a fraction of the overall light output, and the color/age of the stars therein. Bulges that were small, indistinct, and contained stars similar to the color/age of the stars found in the disk were considered examples of the psuedobulges. Ones with significant, bright, and distinctly redder/older bulges were indicative of what would be expected in the classical merger bulge.

The team determined that as much as 58-74% of their sample did not contain a classical bulge. Furthermore, they state, “Almost all of the classical bulges that we do identify – some with substantial uncertainty – are smaller than those normally made in simulations of galaxy formation.” Indeed, included among these galaxies is our own Milky Way which has a very odd, box shaped bulge. The team notes that the velocity distribution of the apparent bulge merges seamlessly into the disk portion of the galaxy as opposed to a discontinuous fit in classical bulges.

Kormendy’s team finds that one way to form such “pure-disk” galaxies is to allow for the possibility of early star formation. According to the paper, this would “give the halo time to grow without forming a classical bulge.”

These findings stand in strong contrast with a study published by the same group in 2009, analyzing the Virgo cluster of galaxies. In that study they found that classical bulge galaxies (including in this study, elliptical galaxies) seemed to dominate. As such, they suggest that the formation of bulges is somehow related to the local environment. Although the question cannot yet be answered, it begs the question for future study: What about our environment is so special that we can form galaxies in a non-merger process? The answer to this question will require further study.

Posted on by Jon Voisey

Disturbance in the Force – A Spatially Varying Fine Structure Constant

Illustration of the dipolar variation in the fine-structure constant, alpha, across the sky, as seen by the two telescopes used in the work: the Keck telescope in Hawaii and the ESO Very Large Telescope in Chile. IMAGE CREDIT: Copyright Dr. Julian Berengut, UNSW, 2010.

[/caption]

In order for astronomers to explore the outer reaches of our universe, they rely upon the assumption that the physical constants we observe in the lab on Earth are physically constant everywhere in the universe. This assumption seems to hold up extremely well. If the universe’s constants were grossly different, stars would fail to shine and galaxies would fail to coalesce. Yet as far we we look in our universe, the effects which rely on these physical constants being constant, still seem to happen. But new research has revealed that one of these constants, known as the fine structure constant, may vary ever so slightly in different portions of the universe.

Of all physical constants, the fine structure constant seems like an odd one to be probing with astronomy. It appears in many equations involving some of the smallest scales in the universe. In particular, it is used frequently in quantum physics and is part of the quantum derivation of the structure of the hydrogen atom. This quantum model determines the allowed energy levels of electrons in the atoms. Change this constant and the orbitals shift as well.

Since the allowed energy levels determine what wavelengths of light such an atom can emit, a careful analysis of the positioning of these spectral lines in distant galaxies would reveal variations in the constant that helped control them. Using the Very Large Telescope (VLT) and the Keck Observatory, a team from the University of New South Whales has analyzed the spectra of 300 galaxies and found the subtle changes that should exist if this constant was less than constant.

Since the two sets of telescopes used point in different directions (Keck in the Northern hemisphere and the VLT in the Southern), the researchers noticed that the variation seemed to have a preferred direction. As Julian King, one of the paper’s authors, explained, “Looking to the north with Keck we see, on average, a smaller alpha in distant galaxies, but when looking south with the VLT we see a larger alpha.”

However, “it varies by only a tiny amount — about one part in 100,000 — over most of the observable universe”. As such, although the result is very intriguing, it does not demolish our understanding of the universe or make hypotheses like that of a greatly variable speed of light plausible (an argument frequently tossed around by Creationists). But, “If our results are correct, clearly we shall need new physical theories to satisfactorily describe them.”

While this finding doesn’t challenge our knowledge of the observable universe, it may have implications for regions outside of the portion of the universe we can observe. Since our viewing distance is ultimately limited by how far we can look back, and that time is limited by when the universe became transparent, we cannot observe what the universe would be like beyond that visible horizon. The team speculates that beyond it, there may be even larger changes in this constant which would have large effects on physics in such portions. They conclude the results may, “suggest a violation of the Einstein Equivalence Principle, and could infer a very large or in finite universe, within which our `local’ Hubble volume represents a tiny fraction, with correspondingly small variations in the physical constants.”

This would mean that, outside of our portion of the universe, the physical laws may not be suitable for life making our little corner of the universe a sort of oasis. This could help solve the supposed “fine-tuning” problem without relying on explanations such as multiple universes.

Want some other articles on this subject? Here’s an article about there might be 10 dimensions.

Posted on by Jon Voisey

The Hercules Satellite – A Galactic Transitional Fossil

Smaller satellite galaxies caught by a spiral galaxy are distorted into elongated structures consisting of stars, which are known as tidal streams, as shown in this artist's impression. Credit: Jon Lomberg

[/caption]

On Friday, I wrote about the population of the thick disk and how surveys are revealing that this portion of our galaxy is largely made of stars stolen from cannibalized dwarf galaxies. This fits in well with many other pieces of evidence to build up the general picture of galactic formation that suggests galaxies form through the combination of many small additions as opposed to a single, gigantic collapse. While many streams of what is, presumably, tidally shredded galaxies span the outskirts of the Milky Way, and other objects exist that are still fully formed galaxies, few objects have yet been identified as a satellite that is undergoing the process of tidal disruption.

A new study, to be published in the October issue of the Astrophysical Journal suggests that the Hercules satellite galaxy may be one of the first of this intermediary forms discovered.

In the past decade, numerous minor stellar systems have been discovered in the halo of our Milky Way galaxy. The properties of these systems have suggested to astronomers that they are faint galaxies in their own right. Although many have elongated and elliptical shapes (averaging an ellipticity of 0.47; 0.15 higher than that of brighter dwarf galaxies that orbit further out), simulations have suggested that even these stretched dwarfs are still able to remain largely cohesive. In general, the galaxy will remain intact until it is stretched to an ellipticity of 0.7.  At this point, a minor galaxy will lose ~90% of its member stars and dissolve into a stellar stream.

In 2008, Munoz et al. reported the first Milky Way satellite that was clearly over this limit. The Ursa Major I satellite was shown to have an ellipticity of 0.8. Munoz suggested that this, as well as the Hercules and Ursa Major II dwarfs were undergoing tidal break up.

The new paper, by Nicolas Martin and Shoko Jin, further analyzes this proposition for the Hercules satellite by going further and examining the orbital characteristics to ensure that their passage would continue to distort the galaxy sufficiently. The system already contains an ellipticity of 0.68, which puts it just under the theoretical limit.

The team looked to see just how closely the satellite would pass to our own galactic center. The closer it passed, the more disruption it would feel. By projecting the orbit, they estimated the galaxy would come within ~6 kiloparsecs of the galactic center which is about 40% of the radius of the galaxy overall. While this may not seem especially close Martin and Jin report that they cannot conclude that it will be insufficient. They state that disruption would be dependent on “the properties of the stellar system at that time of its journey in the Milky Way potential and, as such, out of reach to the current observer.”

However, there were some telling signs that the dwarf may already be shedding stars. Along the major axis of the galaxy, deep imaging has revealed a smaller number of stars that does not appear to be bound to the galaxy itself. Photometry of these stars has shown that their distribution on a color-magnitude diagram is strikingly similar to that of the Hercules galaxy itself.

At this point, we cannot fully determine if the Hercules galaxy is doomed to become another stellar stream around the Milky Way, but if it is not truly in the process of breaking up, it seems to be on the very edge.

Posted on by Jon Voisey

Type II-P Supernovae as a New Standard Candle

Artist illustration of a supernova. Image credit: ESO

[/caption]

Much of astronomical knowledge is built on the cosmic distance ladder. This ladder is built to determine distances to objects in our sky. Low lying rungs for nearby objects are used to calibrate the methodology for more distant objects which are, in turn, used to calibrate for more distant objects and so on. One of the reason so many runs need to be added is that techniques often become difficult to impossible to used past a certain distance. Cepheid Variables are a fantastic object to allow us to measure distances, but their luminosity is only sufficient to allow us to detect them to a few tens of millions of parsecs. As such, new techniques, based on brighter objects must be developed.

The most famous of these is the use of Type Ia Supernovae (ones that collapse just pass the Chandrasekhar limit) as “standard candles”. This class of objects has a well defined standard luminosity and by comparing its apparent brightness to the actual brightness, astronomers can determine distance via the distance modulus. But this relies on the fortuitous circumstance of having such an event occur when you want to know the distance! Obviously, astronomers need some other tricks up their sleeve for cosmological distances, and a new study discusses the possibility of using another type of supernova (SN II-P) as another form of standard candles.

Type II-P supernovae are classical, core-collapse supernovae that occur when the core of a star has passed the critical limit and can no longer support the mass of the star. But unlike other supernovae, the II-P decays more slowly, leveling off for some time creating a “plateau” in the light curve (which is where the “P” comes from). Although their plateaus are not all at the same brightness, making them initially useless as a standard candle, studies over the past decade have shown that observing other properties may allow astronomers to determine what the brightness of the plateau actually is and making these supernovae “standardizable”.

In particular, discussion has been centering recently around possible connections between the velocity of ejecta and the brightness of the plateau. A study published by D’Andrea et al. earlier this year attempted to link the absolute brightness to the velocities of the Fe II line at 5169 Angstroms. However, this method left large experimental uncertainties which translated to an error of up to 15% of the distance.

A new paper, to be published in October’s issue of the Astrophysical Journal, a new team, led by Dovi Poznanski of the Lawrence Berkley National Laboratory attempts to reduce these errors by utilizing the hydrogen beta line. One of the primary advantages to this is that hydrogen is much more plentiful allowing the hydrogen beta line to stand out whereas the Fe II lines tend to be weak. This improves the signal to noise (S/N) ratio and improves overall data.

Using data from the Sloan Digital Sky Survey (SDSS), the team was able to decrease the error in distance determination to 11%. Although this was an improvement over the D’Andrea et al. study, it is still significantly higher than many other methods for distance determination at similar distances. Poznanski suggests that this data is likely skewed due to a natural bias towards brighter supernovae. This systematic error stems from the fact that the SDSS data is supplemented up with follow-up data which the team employed, but the follow-ups are only conducted if the supernova meets certain brightness criteria. As such, their method is not fully representative of all supernovae of this type.

To improve their calibration and hopefully improve the method, the team plans to continue their study with expanded data from other studies that would be free of such biases. In particular the team intends to use the Palomar Transient Factory to supplement their results.

As the statistics improve, astronomers will gain another rung on the cosmological distance ladder, but only if they’re lucky enough to find one of this type of supernova.

Posted on by Jon Voisey

The Thick Disk: Galactic Construction Project or Galactic Rejects?

Our Milky Way Gets a Makeover
Our Milky Way Gets a Makeover

[/caption]

The disk of spiral galaxies is comprised of two main components: The thin disk holds the majority of stars and gas and is the majority of what we see and picture when we think of spiral galaxies. However, hovering around that, is a thicker disk of stars that is much less populated. This thick disk is distinct from the thin disk in several regards: The stars there tend to be older, metal deficient, and orbit the center of the galaxy more slowly.

But where this population of the stars came from has been a long standing mystery since its identification in the mid 1970’s. One hypothesis is that it is the remainder of cannibalized dwarf galaxies that have never settled into a more standard orbit. Others suggest that these stars have been flung from the thin disk through gravitational slingshots or supernovae. A recent paper puts these hypothesis to the observational test.

At a first glance, both propositions seem to have a firm observational footing. The Milky Way galaxy is known to be in the process of merging with several smaller galaxies. As our galaxy pulls them in, the tidal effects shred these minor galaxies, scattering the stars. Numerous tidal streams of this sort have been discovered already. The ejection from the thin disk gains support from the many known “runaway” and “hypervelocity” stars which have sufficient velocity to escape the thin disk, and in some cases, the galaxy itself.

The new study, led by Marion Dierickx of Harvard, follows up on a 2009 study by Sales et al., which used simulations to examine the features stars would take in the thick disk should they be created via these methods. Through these simulations, Sales showed that the distribution of eccentricities of the orbits should be different and allow a method by which to discriminate between formation scenarios.

By using data from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), Dierickx’s team compared the distribution of the stars in our own galaxy to the predictions made by the various models. Ultimately, their survey included some 34,000 stars. By comparing the histogram of eccentricities to that of Sales’ predictions, the team hoped to find a suitable match that would reveal the primary mode of creation.

The comparison revealed that, should ejection from the thin disk be the norm there were too many stars in nearly circular orbits as well as highly eccentric ones. In general, the distribution was too wide. However, the match for the scenario of mergers fit well lending strong credence to this hypothesis.

While the ejection hypothesis or others can’t be ruled out completely, it suggests that, at least in our own galaxy, they play a rather minor role. In the future, additional tests will likely be employed, analyzing other aspects of this population.

Posted on by Jon Voisey

Follow-up Studies on June 3rd Jupiter Impact

Color image of impact on Jupiter on June 3, 2010. Credit: Anthony Wesley

[/caption]

Poor Jupiter just can’t seem to catch a break. Ever since 1994, when our largest planet was hit by Comet Shoemaker-Levy, detections of impacts on Jupiter have occurred with increasing regularity. Most recently, an impact was witnessed on August 20. On June 3rd of 2010, (coincidentally the same day pictures from Hubble were released from a 2009 impact) Jupiter was hit yet again. Shortly after the June 3rd impact, several other telescopes joined the observing.

A paper to appear in the October issue of The Astrophysical Journal Letters discusses the science that has been gained from these observations.

The June 3rd impact was novel in several respects. It was the first unexpected impact that was reported from two independent locations simultaneously. Both discoverers were observing Jupiter with aims of engaging in a bit of astrophotography. Their cameras were both set to take a series of quick images, each lasting a fifth to a tenth of a second. This short time duration is the first time astronomers have had the ability to recreate the light curve for the meteor. Additionally, both observers were using different filters (one red and one blue) allowing for exploration of the color distribution.

Analysis of the light curve revealed that the flash lasted nearly two seconds and was not symmetric; The decay in brightness occurred faster than the increase at onset. Additionally, the curve showed several distinct “bumps” which indicated a flickering that is commonly seen on meteors on Earth.

The light released in the burning up of the object was used to estimate the total energy-released and in turn the mass of the object.  The total energy released was estimated to be between roughly (1.0–4.0) × 1015 Joules (or 250–1000 kilotons).

Follow-up observations from Hubble three days later revealed no scars from the impact. In the July 2009 impact, a hole punched in the clouds remained for several days. This indicated the object in the June 3 impact was considerably smaller and burned up before it was able to reach the visible cloud decks.

Observations intended to find debris came up empty. Infrared observations showed that no thermal signature was left even as little as 18 hours following the discovery.

Assuming that the object was an asteroid with a relative speed of ~60 km/sec and a density of ~2 g/cm3, the team estimated the size of the object to be between 8 and 13 meters, similar to the size of the two asteroids that recently passed Earth. This represents the smallest meteor yet observed on Jupiter. An object of similar size was estimated to be responsible for the impact on Earth in 1994 near the Marshall Islands. Estimates “predict objects of this size to collide with our planet every 6–15 years” with significantly higher rates on Jupiter ranging from one to one hundred such events annually.

Clearly, amateur observations led to some fantastic science. Modest telescopes, “in the range 15–20 cm in diameter equipped with webcams and video recorders” can easily allow for excellent coverage of Jupiter and continued observation could help in determining the impact rate and lead to a better understanding of the population of such small bodies in the outer solar system.