Posted on by Jon Voisey

Do Stars Really Form in Clusters?

The long standing view on the formation of stars is that they form in clusters. This theory is supported by understanding of the formation process that requires large clouds of gas and dust to be able to condense. Small clouds with enough mass to only form one star just can’t meet the required conditions to condense. In a large cloud, where conditions are sufficient, once one star begins, the feedback effects from this star will trigger other star formation. Thus, if you get one, you’ll likely get lots.

But a new paper takes a critical look at whether or not all stars really form in clusters.

The main difficulty in answering this question boils down to a simple question: What does it mean to be “in” a cluster. Generally, members of a cluster are stars that are gravitationally bound. But as time passes, most clusters shed members as gravitational interactions, both internal and external, remove outer members. This blurs the boundary between being bound and unbound.

Similarly, some objects that can initially look very similar to clusters can actually be groups known as an association. As the name suggests, while these stars are in close proximity, they are not truly bond together. Instead, their relative velocities will cause the the group to disperse without the need for other effects.

As a result, astronomers have considered other requirements to truly be a member of a cluster. In particular for forming stars, there is an expectation that cluster stars should be able to interact with one-another during the formation process.

Its these considerations that this new team uses as a basis, led by Eli Bressert from the University of Exeter. Using observations from Spitzer, the team analyzed 12 nearby star forming regions. By conducting the survey with Spitzer, an infrared telescope, the team was able to pierce the dusty veil that typically hides such young stars.

By looking at the density of the young stellar objects (YSOs) in the plane of the sky, the team attempted to determine just what portion of stars could be considered true cluster members under various definitions. As might be expected, the answer was highly dependent on the definition used. If a loose and inclusive definition was taken, they determined that 90% of YSOs would be considered as part of the forming cluster. However, if the definition was drawn at the narrow end, the percentage dropped as low as 40%. Furthermore, if the additional criterion of needing to be in such proximity that their “formation/evolution (along with their circumstellar disks and/or planets) may be affected by the close proximity of their low-mass neighbours”, the percentage dropped to a scant 26%.

As with other definition boundaries, the quibbling may seem little more than a distraction. However, with such largely varying numbers attached to them, these triflings carry great significance since inconsistent definitions can greatly distort the understanding. This study highlights the need for clarity in definitions for which astronomers constantly struggle in a muddled universe full overlapping populations and shades of gray.

Posted on by Jon Voisey

Does Tidal Evolution Cause Stars to Eat Planets?

Artists impression of the 'hot Jupiter' HD209458b, which has incredible storms. Credit: ESO.

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With the success of the Kepler mission, the viability of looking for planets via transits has reached maturity. However, Kepler is not the first intensive study. Previously, other observatories have employed transit searches. To increase the chances of discovery, studies often concentrated on large clusters in which thousands of stars could be observed simultaneously. Based on the percentage of stars with super Jovian planets in the Sun’s vicinity, a Hubble observation run on the globular cluster 47 Tuc expected to find roughly 17 “hot Jupiters”. Yet not a single one was found. Follow-up studies on other regions of 47 Tuc, published in 2005, also reported a similar lack of signals.

Could the subtle effect of tidal forces have caused the planets to be consumed by their parent stars?

Within our solar system, the effects of tidal influences are more subtle than planetary destruction. But on stars with massive planets in tight orbits, the effects can be very different. As a planet would orbit its parent star, its gravitational pull would pull the star’s photosphere towards it. In a frictionless environment, the raised bulge would remain directly under the planet. Since the real world has real friction, the bulge will be displaced.

If the star rotates slower than the planet orbits (a likely scenario for close in planets since stars slow themselves via magnetic breaking during formation), the bulge will trail behind the planet since the pull has to compete against the photospheric material through which its pulling. This is the same effect that happens between the Earth-Moon system and is why we don’t have tides whenever the moon is overhead, but rather, the tides occur some time later. This lagging bulge creates a component of the gravitational force opposed to the direction of motion of the planet, slowing it down. As time goes on, the planet gets dragged closer to the star by this torque which increases the gravitational force and accelerating the process until the planet eventually enters the star’s photosphere.

Since transit discoveries rely on the planets orbital plane being exactly in line with its parent star and our planet, this favors planets in a very tight orbit since planets further out are more likely to pass above or below their parent star when viewed from Earth. The result of this is that planets that could potentially be discovered by this method are especially prone to this tidal slowing and destruction. This effect with the combination of the old age of 47 Tuc, may explain the dearth of discoveries.

Using a Monte-Carlo simulation, a recent paper explores this possibility and finds that, with the tidal effects, the non-detection in 47 Tuc is completely accounted for without the need to include additional reasons (such as metal deficiency in the cluster). However, to go beyond simply explaining a null result, the team made several predictions that would serve to confirm the destruction of such planets. If a planet were wholly consumed, the heavier elements should be present in the atmospheres of their parent star and thus be detectable via their spectra in contrast with the overall chemical composition of the cluster. Planets that were tidally stripped of atmospheres by filling their Roche Lobes could still be detected as an excess of rocky, super Earths.

Another test could inolve comparison between several of the open clusters visible in the Kepler study. Should astronomers find a decrease in the probability of finding hot Jupiters corresponding with a decrease with cluster age, this would also confirm the hypothesis. Since several such clusters exist within the area planned for the Kepler survey, this option is the most readily accessible. Ultimately, this result make sit clear that, should astronomers rely on methods that are best suited for short period planets, they may need to expand their observation window sufficiently since planets with a sufficiently short period may be prone to being consumed.

Posted on by Jon Voisey

The Other End of the Planetary Scale

A comparison of the size of Jupiter, a brown dwarf, a small star and the Sun (Gemini Observatory/Artwork by Jon Lomberg)

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The definition of a “planet” is one that has seen a great deal of contention. The ad-hoc redefinition has caused much grief for lovers of the demoted Pluto. Yet little attention is paid to the other end of the planetary scale, namely, where the cutoff between a star and a planet lies. The general consensus is that an object capable of supporting deuterium (a form of hydrogen that has a neutron in the nucleus and can undergo fusion at lower temperatures) fusion, is a brown dwarf while, anything below that is a planet. This limit has been estimated to be around 13 Jupiter masses, but while this line in the sand may seem clear initially, a new paper explores the difficulty in pinning down this discriminating factor. For many years, brown dwarfs were mythical creatures. Their low temperatures, even while undergoing deuterium fusion, made them difficult to detect. While many candidates were proposed as brown dwarfs, all failed the discriminating test of having lithium present in their spectrum (which is destroyed by the temperatures of traditional hydrogen fusion). This changed in 1995 when the first object of suitable mass was discovered when the 670.8 nm lithium line was discovered in a star of suitable mass.

Since then, the number of identified brown dwarfs has increased significantly and astronomers have discovered that the lower mass range of purported brown dwarfs seems to overlap with that of massive planets. This includes objects such as CoRoT-3b, a brown dwarf with approximately 22 Jovian masses, which exists in the terminological limbo.

The paper, led by David Speigel of Princeton, investigated a wide range of initial conditions for objects near the deuterium burning limit. Among the variables included, the team considered the initial fraction of helium, deuterium, and “metals” (everything higher than helium on the periodic table). Their simulations revealed that just how much of the deuterium burned, and how fast, was highly dependent on the starting conditions. Objects starting with higher helium concentration required less mass to burn a given amount of deuterium. Similarly, the higher the initial deuterium fraction, the more readily it fused. The differences in required mass were not subtle either. They varied by as much as two Jovian masses, extending as low as a mere 11 times the mass of Jupiter, well below the generally accepted limit.

The authors suggest that because of the inherent confusion in the mass limits, that such a definition may not be the “most useful delineation between planets and brown dwarfs.” As such, they recommend astronomers take extra care in their classifications and realize that a new definition may be necessary. One possible definition could involve considerations of the formation history of objects in the questionable mass range; Objects that formed in disks, around other stars would be considered planets, where objects that formed from gravitational collapse independently of the object they orbit, would be considered  brown dwarfs. In the mean time, objects such as CoRoT-3b, will continue to have their taxonomic categorization debated.

Posted on by Jon Voisey

Aesthetics of Astronomy

This Hubble image reveals the gigantic Pinwheel Galaxy (M101), one of the best known examples of "grand design spirals," and its supergiant star-forming regions in unprecedented detail. Astronomers have searched galaxies like this in a hunt for the progenitors of Type Ia supernovae, but their search has turned up mostly empty-handed. Credit: NASA/ESA
This Hubble image reveals the gigantic Pinwheel Galaxy (M101), one of the best known examples of "grand design spirals". Credit: NASA/ESA

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When I tell people I majored in astronomy, the general reaction is one of shock and awe. Although people don’t realize just how much physics it is (which scares them even more when they found out), they’re still impressed that anyone would choose to major in a physical science. Quite often, I’m asked the question, “Why did you choose that major?”

Only somewhat jokingly, I reply, “Because it’s pretty.” For what reasons would we explore something if we did not find some sort of beauty in it? This answer also tends to steer potential follow up questions to topics of images they’ve seen and away from topics from half-heard stories about black holes from sci-fi movies.

The topic of aesthetics in astronomy is one I’ve used here for my own devices, but a new study explores how we view astronomical images and what sorts of information people, both expert and amateur, take from them.

The study was conducted by a group formed in 2008 known as The Aesthetics and Astronomy Group. It is comprised of astrophysicists, astronomy image development professionals, educators, and specialists in the aesthetic and cognitive perception of images. The group asked to questions to guide their study:

  1. How much do variations in presentation of color, explanatory text, and illustrative scales affect comprehension of, aesthetic attractiveness, and time spent looking at deep space imagery?
  2. How do novices differ from experts in terms of how they look at astronomical images?

Data to answer this question was taken from two groups; The first was an online survey taken by volunteers from solicitations on various astronomy websites and included 8866 respondents. The second group was comprised of four focus groups held at the Harvard-Smithsonian Center for Astrophysics.

To analyze how viewers viewed color, the web study contained two pictures of the elliptical galaxy NGC 4696. The images were identical except for the colors chosen to represent different temperatures. In one image, red was chosen to represent hot regions and blue for cold regions. In the other version, the color scheme was reversed. A slight majority (53.3% to 46.7%) responded saying they preferred the version in which blue was assigned to be the hotter color. When asked which image they thought was the “hotter” image, 71.5% responded that the red image was hotter. Since astronomical images are often assigned with blue as the hotter color (since hotter objects emit shorter wavelength light which is towards the blue end of the visible spectrum), this suggests that the public’s perception of such images is likely reversed.

A second image for the web group divided the participants into 4 groups in which an image of a supernova remnant was shown with or without foreground stars and with or without a descriptive caption. When asked to rate the attractiveness, participants rated the one with text slightly higher (7.96 to 7.60 on a 10 point scale). Not surprisingly, those that viewed the versions of the image with captions were more likely to be able to correctly identify the object in the image. Additionally, the version of the image with stars was also more often identified correctly, even without captions, suggesting that the appearance of stars provides important context. Another question for this image also asked the size in comparison to the Earth, Solar System, and Galaxy. Although the caption gave the scale of the SNR in lightyears, the portion that viewed the caption did not fare better when asked to identify the size revealing such information is beyond the limit of usefulness.

The next portion showed an image of the Whirlpool galaxy, M51 and contained either, no text, a standard blurb, a narrative blurb, or a sectionized caption with questions as headers. Taking into consideration the time spent reading the captions, the team found that those with text spent more time viewing the image suggesting that accompanying text encourages viewers to take a second look at the image itself. The version with a narrative caption prompted the most extra time.

Another set of images explored the use of scales by superimposing circles representing the Earth, a circle of 300 miles, both, or neither onto an image of spicules on the Sun’s surface, with or without text. Predictably, those with scales and text were viewed longer and the image with both scales was viewed the longest and had the best responses on a true/false quiz over the information given by the image.

When comparing self-identified experts to novices, the study found that both viewed uncaptioned images for similar lengths of time, but for images with text, novices spent an additional 15 seconds reviewing the image when compared to experts. Differences between styles of presenting text (short blurb, narrative, or question headed), novices preferred the ones in which topics were introduced with questions, whereas experts rated all similarly which suggested they don’t care how the information is given, so long as it’s present.

The focus groups were given similar images, but were prompted for free responses in discussions.

<

p style=”padding-left: 30px;”>[T]he non-professionals wanted to know what the colors represented, how the images were made, whether the images were composites from different satellites, and what various areas of the images were. They wanted to know if M101 could be seen with a home telescope, binoculars, or the naked eye.

Additionally, they were also interested in historical context and insights from what professional astronomers found interesting about the images.

Professionals, on the other hand, responded with a general pattern of “I want to know who made this image and what it was that they were trying to convey. I want to judge whether this image is doing a good job of telling me what it is they

wanted me to get out of this.” Eventually, they discussed the aesthetic nature of the images which reveals that “novices … work from aesthetics to science, and for astrophysicists … work from science to aesthetics.”

Overall, the study found an eager public audience that was eager to learn to view the images as not just pretty pictures, but scientific data. It suggested that a conversational tone that worked up to technical language worked best. These findings can be used to improve communication of scientific objectives in museums, astrophotography sections of observatories, and even in presentation of astronomical images and personal conversation.

Posted on by Jon Voisey

Two New Asteroids to Pass Earth This Week

Orbits of 2010 RF12 and 2010 RX30

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Two newly discovered asteroids will pass the Earth this week. The asteroids were discovered on September 5th of this year by Andrea Boattini using the 1.5 metre reflector at Mount Lemmon in Arizona as part of the Mount Lemmon Survey.

These two new asteroids have been given the designations of 2010 RF12 and 2010 RX30. Both are small bodies, which is why they were not discovered until mere days before they would pass the Earth. Estimates put the size of RF12 at 5 – 15 meters with a best estimate being around 8 meters (26 ft). The larger, RX30 is estimated to be 12 meters (39 ft), but the range of estimates go from 7 – 25.

Due to the large range of estimates on sizes, as well as poorly constrained relative velocities and an unknown composition, it would be difficult to predict the damage an impact from these bodies could cause. The majority of the mass for such small objects would burn up in the atmosphere with only small fragments surviving to the ground. For comparison, the estimated size of the object that caused the Tunguska event was estimated to be at least a few tens of meters in diameter at the point it exploded in the atmosphere some few miles up. Since the diameter helps to determine the volume, and thus the mass and kinetic energy, this factor increases the potential damage rapidly. However, although the bodies were just discovered this week, their orbits have already been well established for the near future and neither will collide with Earth. Both are rated at a 0 on the Torino scale (data from NASA’s NEO Program for RF12 and RX30 can be seen here and here respectively).

Although both objects will pass closer to the Earth than the moon, due to their small size, neither will be visible to the naked eye. 2010 RF12 is expected to pass the Earth at 21% of the Earth-moon distance and at maximum brightness, reach only 14th magnitude, which is just over 600 times too faint to see with the unaided eye. RX30 will approach at 66% of the Earth-moon distance and is expected to reach a similar peak magnitude. For those interested in tracking or photographing these objects, the Fawkes Telescope Project has created a page dedicated to these two objects, including best exposure times and filters for cameras that can be found here. Ephemeris for RF12 and RX30 can be found here and here respectively.

Although both of these asteroids were discovered on the same day and will be approaching near the same time, their orbits do not appear to be related. RF12’s orbit extends from 0.82 to 1.17 AU and it orbits the Sun once every year. Predictions have shown it only passes near the Earth once every one hundred years. Initially, RX30 was thought to be rotate extremely fast, but revised observations have shown that it takes at least 6 hours to rotate about its axis.

Posted on by Jon Voisey

The Origin of Exoplanets

Artist's impression of the planet OGLE-TR-L9b. Credit: ESO/H. Zodet

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We truly live in an amazing time for exoplanet research. It was only 18 years ago the first planet outside our solar system was discovered. Fifteen since the first confirmation of one around a main sequence star. Even more recently, direct images have begun to sprout up, as well as the first spectra of the atmospheres of such planets. So much data is becoming available, astronomers have even begun to be able to make inferences as to how these extra solar planets could have formed.

In general, there are two methods by which planets can form. The first is via coaccretion in which the star and the planet would form from gravitational collapse independently of one another, but in close enough proximity that their mutual gravity binds them together in orbit. The second, the method through which our solar system formed, is the disk method. In this, material from a thin disk around a proto-star collapses to form a planet. Each of these processes has a different set of parameters that may leave traces which could allow astronomers to uncover which method is dominant. A new paper from Helmut Abt of Kitt Peak National Observatory, looks at these characteristics and determines that, from our current sampling of exoplanets, our solar system may be an oddity.

The first parameter that distinguishes the two formation methods is that of eccentricity. To establish a baseline for comparison, Abt first plotted the distribution of eccentricities for 188 main-sequence binary stars and compared that to the same type of plot for the only known system to have formed via the disk method (our Solar System). This revealed that, while the majority of stars have orbits with low eccentricity, this percentage falls off slowly as the eccentricity increases. In our solar system, in which only one planet (Mercury) has an eccentricity greater than 0.2, the distribution falls off much more steeply. When Abt constructed the distribution for the 379 planets with known eccentricity, it was nearly identical to that for binary stars.

A similar plot was created for the semi major axis of binary stars and our solar system. Again, when this was plotted for the known extra solar planets the distribution was similar to that of binary star systems.

Abt also inspected the configuration of the systems. Star systems containing three stars generally contained a pair of stars in a tight binary orbit with a third in a much larger orbit. By comparing the ratios of such orbits, Abt quantified the orbital spacing. However, instead of simply comparing to the solar system, he considered the analogous situation of formation of stars around the central mass of the galaxy and built a similar distribution in this manner. In this case, the results were ambiguous; Both modes of formation produced similar results.

Lastly, Abt considered the amount of heavy elements in the more massive body. It is widely known that most extra-solar planets are found around metal-rich stars. While there’s no reason planets forming in a disk couldn’t be formed around high mass stars, having a metal-rich cloud from which to form stars and planets is a requirement for the coaccretion model because it tends to accelerate the collapse process, allowing giant planets to fully form before the cloud was dissipated as the star became active. Thus, the fact that the vast majority of extra-solar planets exist around metal-rich stars favors the coaccretion hypothesis.

Taken together, this provides four tests for formation models. In every case, current observations suggest that the majority of planets discovered thus far formed from coaccretion and not in a disc. However, Abt notes that this is most likely due to statistical biases imposed by the sensitivity limits of current instruments. As he notes, astronomers “do not yet have the radial velocity sensitivity to detect disk systems like the solar system, except for single large planets, like Jupiter at 5 AU.” As such, this view will likely change as new generations of instruments become available. Indeed, as instruments improve to the point that three dimensional mapping becomes available, and orbital inclinations can be directly observed, astronomers will be able to add another test to determine the modes of formation.

EDIT: Following some confusion and discussion in the comments, I wanted to add one further note. Keep in mind this is only the average of all systems currently known that looks like coaccreted systems. While there are undoubtedly some in there that did form from disks, their rarity in the current data makes them not stand out. Certainly, we know of at least one system that fits a strong test for the disk method. This recent discovery by Kepler, in which three planets have been observed transiting their host star demonstrates that all of these planets must lie in a disk which does not conform to expectations of independent condensation. As more systems like this are discovered, we expect that the distributions of the tests described above will become bimodal, having components that match each formation hypothesis.

Posted on by Jon Voisey

The Black Hole/Globular Cluster Correlation

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Often in astronomy, one observable property traces another property which may be more difficult to observe directly; X-ray activity on stars can be used to trace turbulent heating of the photosphere. CO is used to trace cold H2. Sometimes these correlations make sense. Activities in stars produce the X-ray emissions. Other times, the tracer seems distantly related at best.

This is the case of a newly discovered correlation between the mass of the central black hole of galaxies and the number of globular clusters they contain. What can this relationship teach astronomers? Why does it hold for some types of galaxies better than others? And where does it come from in the first place.

The mass of a galaxy’s super massive black hole (SMBH) is known to have a strong relationship between many features of their host galaxies. It has identified to follow the range of velocities of stars in the galaxy, the mass and luminosity of the bulge of spiral galaxies, and the total amount of dark matter in galaxies. Because dark matter in the halo of galaxies and the luminosity have also been known to correspond to the number of globular clusters, Andreas Burkert of the Max-Planck-Institute for Extraterrestrial Physics in Germany, and Scott Tremaine at Princeton wondered if they could cut out the middlemen of dark matter and luminosity and still maintain a strong correlation between the central SMBH and the number of globular clusters.

Their initial investigation involved only 13 galaxies, but a follow-up study by Gretchen and William Harris and submitted to the Monthly Notices of the Royal Astronomical Society, increased the number of galaxies included in the survey to 33. The results of these studies indicated that for elliptical galaxies, the SMBH-GC relationship is evident. However, for lenticular galaxies there was no clear correlation. While there appeared to be a trend for classical spirals, the small number of data points (4) would not provide a strong statistical case independently, but did appear to follow the trend established by the elliptical galaxies.

Although the correlation appeared strong in most cases, about 10% of the galaxies included in the larger surveys were clear outliers. This included the Milky Way which has a SMBH mass that falls significantly short of the expectation from cluster number. One source of error the authors of the original study suspect is that it is possible that, in some cases, objects identified as globular clusters may have been misidentified and in actuality, be the cores of tidally stripped dwarf galaxies. Regardless, the relationship as it stands presently, seems to be quite strong and is even more tightly defined than that of the correlation between that of the SMBH mass and velocity dispersion that implied the potential relationship in the first place. The reason for the discordance in lenticular galaxies has not yet been explained and no reasons have yet been postulated.

But what of the cause of this unusual relation? Both sets of authors suggest the connection lies in the formation of the objects. While distinct in most respects, both are fed by major merger events; Black holes gain mass by accreting gas and globular clusters are often formed from the resulting shocks and interactions. Additionally, the majority of both types of objects formed at high redshifts.

Sources:

A correlation between central supermassive black holes and the globular cluster systems of early-type galaxies

The Globular Cluster/Central Black Hole Connection in Galaxies

Posted on by Jon Voisey

How to Crash Stars Together

Globular Cluster
A Hubble Space Telescope image of the typical globular cluster Messier 80, an object made up of hundreds of thousands of stars and located in the direction of the constellation of Scorpius. The Milky Way galaxy has an estimated 160 globular clusters of which one quarter are thought to be ‘alien’. Image: NASA / The Hubble Heritage Team / STScI / AURA. Click for hi-resolution version.

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The math is simple: Star + Other star = Bigger star.

While conceptually this works well, it fails to take into account the extremely vast distances between stars. Even in clusters, where the density of stars is significantly higher than in the main disk, the number of stars per unit volume is so low that collisions are scarcely considered by astronomers. Of course, at some point the stellar density must reach a point at which the chance for a collision does become statistically significant. Where is that tipping point and are there any locations that might actually make the cut?

Early in the development of stellar formation models, the necessity of stellar collisions to produce massive stars was not well constrained. Early models of formation via accretion hinted that accretion may be insufficient, but as models became more complex and moved into three dimensional simulations, it became apparent that collisions simply weren’t needed to populate the upper mass regime. The notion fell out of favor.

However, there have been two recent papers that have explored the possibility that, while still certainly rare, there may be some environments in which collisions are likely to occur. The primary mechanism that assists in this is the notion that, as clusters sweep through the interstellar medium, they will inevitably pick up gas and dust, slowly increase in mass. This increase mass will cause the cluster to shrink, increasing the stellar density. The studies suggest that in order for the probability of collision to be statistically significant, a cluster would be required to reach a density of roughly 100 million stars per cubic parsec. (Keep in mind, a parsec is 3.26 light years and is roughly the distance between the sun, and our nearest neighboring star.)

Presently, such a high concentration has never been observed. While some of this is certainly due to the rarity of such densities, observational constraints likely play a crucial role in making such systems difficult to detect. If such high densities were to be achieved, it would require extraordinarily high spatial resolution to distinguish such systems. As such, numerical simulations of extremely dense systems will have to replace direct observations.

While the density necessary is straightforward, the more difficult topic is what sorts of clusters might be capable of meeting such criteria. To investigate this, the teams writing the recent papers conducted Monte Carlo simulations in which they could vary the numbers of stars. This type of simulation is essentially a model of a system that is allowed to play forward repeatedly with slightly different starting configurations (such as the initial positions of the stars) and by averaging the results of numerous simulations, an approximate understanding of the behavior of the system is reached. An initial investigation suggested that such densities could be reached in clusters with as little as a few thousand stars provided gas accumulation were sufficiently rapid (clusters tend to disperse slowly under tidal stripping which can counteract this effect on longer timescales). However, the model they used contained numerous simplifications since the investigation into the feasibility of such interactions was merely preliminary.

The more recent study, uploaded to arXiv yesterday, includes more realistic parameters and finds that the overall number of stars in the clusters would need to be closer to 30,000 before collisions became likely. This team also suggested that there were more conditions that would need to be satisfied including rates of gas expulsion (since not all gas would remain in the cluster as the first team had assumed for simplicity) and the degree of mass segregation (heavier stars sink to the center and lighter ones float to the outside and since heavier ones are larger, this actually decreases the number density while increasing the mass density).  While many globular clusters can easily meet the requirement of number of stars, these other conditions would likely not be met. Furthermore, globular clusters spend little time in regions of the galaxy in which they would be likely to encounter sufficiently high densities of gas to allow for accumulation of sufficient mass on the necessary timescales.

But are there any clusters which might achieve sufficient density? The most dense galactic cluster known is the Arches cluster. Sadly, this cluster only reaches a modest ~535 stars per cubic parsec, still far too low to make a large number of collisions likely. However, one run of the simulation code with conditions similar to those in the Arches cluster did predict one collision in ~2 million years.

Overall, these studies seem to confirm that the role of collisions in forming massive stars is small. As pointed out previously, accretion methods seem to account for the broad range of stellar masses. Yet in many young clusters, still forming stars, rarely do astronomers find stars much in excess of ~50 solar masses. The second study this year suggests that this observation may yet leave room for collisions to play some unexpected role.

(NOTE: While it may be suggested that collisions could also be considered to take place as the orbit of binary stars decays due to tidal interactions, such processes are generally referred to as “mergers”. The term “collision” as used in the source materials and this article is used to denote the merging of two stars that are not gravitationally bound.)

Sources:

Stellar collisions in accreting protoclusters: a Monte Carlo dynamical study

Collisional formation of very massive stars in dense clusters

Posted on by Jon Voisey

Ultraluminous Gamma Ray Burst 080607 – A “Monster in the Dark”

Shedding Light on Dark Gamma Ray Bursts
Shedding Light on Dark Gamma Ray Bursts

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Gamma Ray Bursts (GRBs) are among the most energetic phenomena astronomers regularly observe. These events are triggered by massive explosions and a large amount of the energy if focused into narrow beams that sweep across the universe. These beams are so tightly concentrated that they can be seen across the visible universe and allow astronomers to probe the universe’s history. If such an event happened in our galaxy and we stood in the path of the beam, the effects would be pronounced and may lead to large extinctions. Yet one of the most energetic GRBs on record (GRB 080607) was shrouded in cloud of gas and dust dimming the blast by a factor of 20 – 200, depending on the wavelength.  Despite this strong veil, the GRB was still bright enough to be detected by small optical telescopes for over an hour. So what can this hidden monster tell astronomers about ancient galaxies and GRBs in general?

GRB 080607 was discovered on June 6, 2008 by the Swift satellite. Since GRBs are short lived events, searches for them are automated and upon detection, the Swift satellite immediately oriented itself towards the source. Other GRB hunting satellites quickly joined in and ground based observatories, including ROTSE-III and Keck made observations as well. This large collection of instruments allowed astronomers, led by D. A. Perley of UC Berkley, to develop a strong understanding of not just the GRB, but also the obscuring gas. Given that the host galaxy lies at a distance of over 12 billion light years, this has provided a unique probe into the nature of the environment of such distant galaxies.

One of the most surprising features was unusually strong absorption near 2175 °A. Although such absorption has been noticed in other galaxies, it has been rare in galaxies at such large cosmological distances. In the local universe, this feature seems to be most common in dynamically stable galaxies but tends to be “absent in more disturbed locations such as the SMC, nearby starburst galaxies” as well as some regions of the Milky Way which more turbulence is present. The team uses this feature to imply that the host galaxy was stable as well. Although this feature is familiar in nearby galaxies, observing it in this case makes it the furthest known example of this phenomenon. The precise cause of this feature is not yet known, although other studies have indicated “polycyclic aromatic hydrocarbons and graphite” are possible suspects.

Earlier studies of this event have shown other novel spectral features. A paper by Sheffer et al. notes that the spectrum also revealed molecular hydrogen. Again, such a feature is common in the local universe and many other galaxies, but never before has such an observation been made linked to a galaxy in which a GRB has occurred. Molecular hydrogen (as well as other molecular compounds) become disassociated at high temperatures like the ones in galaxies containing large amounts of star formation that would produce regions with large stars capable of triggering GRBs. With observations of one molecule in hand, this lead Sheffer’s team to suspect that there might be large amounts of other molecules, such as carbon monoxide (CO). This too was detected making yet another first for the odd environment of a GRB host.

This unusual environment may help to explain a class of GRBs known as “subluminous optical bursts” or “dark bursts” in which the optical component of the burst (especially the afterglow) is less bright than would be predicted by comparison to more traditional GRBs.

Sources:

Monster in the Dark: The Ultra Luminous GRB 080706 and its Dusty Environment

The Discovery of Vibrationally-Excited H2 In the Molecular Cloud Near GRB 080706

disassociated
Posted on by Jon Voisey

The Race to Stellar Formation

The Cosmic Web - NGC 2070 by Joseph Brimacombe

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Racing is rarely the term that comes to mind when one considers astronomy. However, many events are a race to reach stability before a system flies apart or implodes. The formation of stars from gigantic interstellar clouds is just such a race in which stars struggle to form before the cloud is dispersed. Although a rough estimation of the requirements for collapse are discussed in introductory astrophysics classes (See: Jeans Mass Criterion) this formulation leaves out several elements that come into play in the real universe. Unfortunately for astronomers, these effects can be subtle but significant but untangling them is the subject of a recent paper uploaded to the arXiv preprint server.

The Jeans Mass Criterion only takes into consideration a gas cloud in isolation. Whether or not it will collapse will depend on whether or not the density is sufficiently high. But as we know, stars don’t form in isolation; They form in stellar nurseries which form hundreds to thousands of stars. These forming stars contract under self gravity, and in doing so, heat up. This increases the local pressure and slows contraction as well as giving off additional radiation that also affects the cloud at large. Similarly, solar winds (particles streaming from the surface of formed stars) and supernovae can also disrupt further formation. These feedback mechanisms are the target of a new study by a group of astronomers led by Laura Lopez from the University of California Santa Cruz.

To investigate how each feedback mechanism operated, the group selected the Tarantula Nebula (aka, 30 Doradus or NGC 2070), one of the largest star forming regions easily accessible to astronomers since it resides in the Large Magellanic Cloud. This region was selected due to its large angular size which allowed the team to have good spatial resolutions (down to scales smaller than a parsec) as well as being well above the plane of our own galaxy to minimize interference from gas sources in our own galaxy.

To conduct their study, Lopez’s team broke 30 Dor into 441 individual regions to assess how each feedback mechanism worked in different portions of the nebula. Each “box” consisted of a column slicing through the nebula that was a mere 8 parsecs to a side to ensure sufficient quality of the data across the entire spectrum since observations were used from radio telescopes to X-ray and used data from Spitzer and Hubble.

Perhaps unsurprisingly, the team found that different feedback mechanisms played varying roles in different places. Close the the central star cluster (<50 parsecs), radiation pressure dominated the effects on the gas. Further out, pressure from the gas itself played the stronger role. Another potential feedback mechanism was that of “hot” gas being excited by X-ray emission. What the team uncovered is that, although there is a significant amount of this material, the nebula’s density is insufficient to entrap it and allow it to have a large effect on the overall pressure. Rather, they described this portion as “leaking out of the pores”.

This research is some of the first to observationally explore, on a large scale, many of the mechanisms that have been proposed by theorists in the past. Although such research may seem inconsequential, these feedback mechanisms will have large effects on the distribution of stellar masses (known as the Initial Mass Function). This distribution determines which the relative amounts of massive stars which help to create heavy elements and drive the chemical evolution of galaxies as a whole.