How Do Black Holes Get Super Massive?

A binary black hole pair with an accretion disk inclined 45 degrees. Source: Nixon et al.

Since their discovery, supermassive black holes – the giants lurking in the center of every galaxy – have been mysterious in origin. Astronomers remain baffled as to how these supermassive black holes became so massive.

New research explains how a supermassive black hole might begin as a normal black hole, tens to hundreds of solar masses, and slowly accrete more matter, becoming more massive over time. The trick is in looking at a binary black hole system.  When two galaxies collide the two supermassive black holes sink to the center of the merged galaxy and form a binary pair.  The accretion disk surrounding the two black holes becomes misaligned with respect to the orbit of the binary pair. It tears and falls onto the black hole pair, allowing it to become more massive.

In a merging galaxy the gas flows are turbulent and chaotic. Because of this “any gas feeding the supermassive black hole binary is likely to have angular momentum that is uncorrelated with the binary orbit,” Dr. Chris Nixon, lead author on the paper, told Universe Today. “This makes any disc form at a random angle to the binary orbit.

Nixon et al. examined the evolution of a misaligned disk around a binary black hole system using computer simulations. For simplicity they analyzed a circular binary system of equal mass, acting under the effects of Newtonian gravity. The only variable in their models was the inclination of the disk, which they varied from 0 degrees (perfectly aligned) to 120 degrees.

After running multiple calculations, the results show that all misaligned disks tear. Watch tearing in action below:

In most cases this leads to direct accretion onto the binary.

“The gravitational torques from the binary are capable of overpowering the internal communication in the gas disc (by pressure and viscosity),” explains Nixon. “This allows gas rings to be torn off, which can then be accreted much faster.”

Such tearing can produce accretion rates that are 10,000 times faster than if the exact same disk were aligned.

In all cases the gas will dynamically interact with the binary.  If it is not accreted directly onto the black hole, it will be kicked out to large radii.  This will cause observable signatures in the form of shocks or star formation.  Future observing campaigns will look for these signatures.

In the meantime, Nixon et al. plan to continue their simulations by studying the effects of different mass ratios and eccentricities.  By slowly making their models more complicated, the team will be able to better mimic reality.

Quick interjection: I love the simplicity of this analysis. These results provide an understandable mechanism as to how some supermassive black holes may have formed.

While these results are interesting alone – based on that sheer curiosity that drives the discipline of astronomy forward – they may also play a more prominent role in our local universe.

Before we know it (please read with a hint of sarcasm as this event will happen in 4 billion years) we will collide with the Andromeda galaxy. This rather boring event will lead to zero stellar collisions and a single black hole collision – as the two supermassive black holes will form a binary pair and then eventually merge.

Without waiting for this spectacular event to occur, we can estimate and model the black hole collision.  In 4 billion years the video above may be a pretty good representation of our collision with the Andromeda galaxy.

The results have been published in the Astrophysical Journal Letters (preprint available here). (Link was corrected to correct paper on 8/15/2013).

The Latest from Mars: Dried up Riverbed May Have Flowed into an Ancient Ocean

An artist's rendition of what water on Mars may have looked like. Source Credit: NASA

When it comes to Mars, the hot topic of study is water – a prerequisite for life.

While liquid water is currently not stable on the surface of Mars, there is extensive evidence that it may have been in the past. Astronomers have discovered dried up riverbeds, lake deltas, and evidence of widespread glaciers – to name but a few examples.

However, evidence for a massive standing body of water, such as an ocean, is hard to come by. Early climate models struggle to create circumstances under which liquid water would be stable at all. Nonetheless, an ocean spanning the northern lowlands (approximately one third of the planet) has been long hypothesized.

Scientists at Caltech may have just now confirmed this long-held hope in finding recent evidence for a vast Martian ocean.

The region under investigation is known as Aeolis Dorsa – a plain located at the border between the northern lowlands and the southern highlands. This plain contains many ridges, which are interpreted as ancient river channels.

“These ‘inverted’ channels are now elevated because the coarse sand and gravel carried by the channels is more resistant to erosion than the surrounding mud and silt making up the floodplain material,” Dr. Roman DiBiase, lead author on the study, told Universe Today.

Satellite images of Aeolis Dorsa were collected using the HiRISE camera aboard the Mars Reconnaissance Orbiter.  The resolution was so precise scientists could distinguish features as small as 25 centimeters – an impressive feat even when compared to images of the Earth.

For certain locations “repeat pictures taken with a slight offset enable the creation of stereo-images from which we can determine the relative elevations of features on the planet’s surface,” explains DiBiase. This impressive technique led to high-resolution topographic models, allowing the team to analyze the geometry and patterns of these inverted channels in unprecedented detail.

Not only do the channels spread out toward the end, they also slope steeply downward, forming a delta – a sedimentary deposit that forms where rivers flow into lakes or oceans.

While deltas have been identified on Mars before, all lie within distinct topographic boundaries, such as an impact crater. This is the most compelling evidence for a delta leading into an unconfined region – an ocean.

Final proof of a Martian ocean will advance our knowledge of the intricate interplay between water, climate, and life. “The history of water on Mars has implications not only for the evolution of Martian climate, but also for learning about the early evolution of Earth and Earth’s climate,” explains DiBiase.

As always, further research is needed. Perhaps in the nearby future the Mars Reconnaissance Orbiter and Curiosity will compliment each other quite well – the orbiter taking images from above while Curiosity plays in the dirt, gathering samples in the riverbed.

The study was published in the Journal of Geophysical Research and may be found here.

Jets Boost — Not Hinder — Star Formation in Early Galaxies, New Study Suggests

An artist's conception of jets protruding from a quasar. Credit: ESO/M. Kornmesser

Understanding the formation of stars and galaxies early in the Universe’s history continues to be somewhat of an enigma, and a new study may have turned our current understanding on its head. A recent survey used archival data from four different telescopes to analyze hundreds of galaxies. The results provided overwhelming evidence that radio jets protruding from a galactic center enhance star formation – a result that directly contradicts current models, where star formation is hindered or even stopped.

All early galaxies consist of intensely luminous cores powered by huge black holes.  These so-called active galactic nuclei, or AGN for short, are still the topic of intense study. One specific mechanism astronomers are studying is known as AGN feedback.

“Feedback is the astronomer’s slang term for the way in which an AGN – with its large amount of energy release – influences its host galaxy,” Dr. Zinn, lead researcher on this study, recently told Universe Today. He explained there is both positive feedback, in which the AGN will foster the main activity of the galaxy: star formation, and negative feedback, in which the AGN will hinder or even stop star formation.

Current simulations of galaxy growth invoke strong negative feedback.

“In most cosmological simulations, AGN feedback is used to truncate star formation in the host galaxy,” said Zinn. “This is necessary to prevent the simulated galaxies from becoming too bright/massive.”

Zinn et al. found strong evidence that this is not the case for a large number of early galaxies, claiming that the presence of an AGN actually enhances star formation. In such cases the total star formation rate of a galaxy may be boosted by a factor of 2 – 5.

Furthermore the team showed that positive feedback occurs in radio-luminous AGN. There is strong correlation between the far infrared (indicative of star formation) and the radio.

Now, a correlation between the radio and the far infrared is no stranger to galactic astronomy. Stars form in extremely dusty regions. This dust absorbs the starlight and re-emits it in the far infrared. The stars then die in huge supernova explosions, causing powerful shock-fronts, which accelerate electrons and lead to the emission of strong synchrotron radiation in the radio.

This correlation however is a stranger to AGN studies. The key lies in the radio jets, which penetrate far into the host galaxy itself.  A “jet which is launched from the AGN hits the interstellar gas of the host galaxy and thereby induces supersonic shocks and turbulence,” explains Zinn. “This shortens the clumping time of gas so that it can condense into stars much more quick and efficiently.”

This new finding conveys that the exact mechanisms in which AGN interact with their host galaxies is much more complicated than previously thought. Future observations will likely shed a new understanding of the evolution of galaxies.

The team used data primarily from the Chandra Deep Field South image
but also data from Hubble, Herschel and Spitzer.

The results will be published in the Astrophysical Journal (preprint available here).

60 Billion Habitable Planets in the Milky Way Alone? Astronomers say Yes!

An artist's conception of how common exoplanets are throughout the Milky Way Galaxy. Image Credit: Wikipedia

A new study suggests that the number of habitable exoplanets within the Milky Way alone may reach 60 billion.

Previous research performed by a team at Harvard University suggested that there is one Earth-sized planet in the habitable zone of each red dwarf star. But researchers at the University of Chicago and Northwestern University have now extended the habitable zone and doubled this estimate.

The research team, lead by Dr. Jun Yang considered one more variable in their calculations: cloud cover. Most exoplanets are tidally locked to their host stars – one hemisphere continually faces the star, while one continuously faces away. These tidally locked planets have a permanent dayside and a permanent nightside.

One would expect the temperature gradient between the two to be very high, as the dayside is continuously receiving stellar flux, while the nightside is always in darkness. Computer simulations that take into account cloud cover show that this is not the case.

The dayside is covered by clouds, which lead to a “stabilizing cloud feedback” on climate.  It has a higher cloud albedo (more light is reflected off the clouds) and a lower greenhouse effect. The presence of clouds actually causes the dayside to be much cooler than expected.

“Tidally locked planets have low enough surface temperatures to be habitable,” explains Jang in his recently published paper. Cloud cover is so effective it even extends the habitable zone to twice the stellar flux. Planets twice as close to their host star are still cool enough to be habitable.

But these new statistics do not apply to just a few stars. Red dwarfs “represent about ¾ of the stars in the galaxy, so it applies to a huge number of planets,” Dr. Abbot, co-author on the paper, told Universe Today. It doubles the number of planets previously thought habitable throughout the entire galaxy.

Not only is the habitable zone around red dwarfs much larger, red dwarfs also live for much longer periods of time. In fact, the Universe is not old enough for any of these long-living stars to have died yet. This gives life the amount of time necessary to form. After all, it took human beings 4.5 billions years to appear on Earth.

Another study we reported on earlier also revised and extrapolated the habitable zone around red dwarf stars.

Future observations will verify this model by measuring the cloud temperatures. On the dayside, we will only be able to see the high cool clouds. A planet resembling this model will therefore look very cold on the dayside. In fact, “a planet that does show the cloud feedback will look hotter on the nightside than the dayside,” explains Abbot.

This effect will be testable with the James Webb Space Telescope.  All in all, the Milky Way is likely to be teeming with life.

The results will be published in Astrophysical Journal Letters (preprint available here).

The Hunt for Exomoons Begins!

An artist's conception of a potentially-habitable exomoon. Credit: NASA

The latest exciting undertaking in exoplanet research is the search for exomoons. A team led by Dr. David Kipping at the Harvard-Smithsonian Center for Astrophysics has jumped at this challenge. After having theoretically proven that detecting an Earth-sized exomoon is possible, the team carried out the first detailed search for an exomoon.

Are you leaning forward on the edge of your seat awaiting the results? Well here you go: the data show no evidence of a moon. That’s simply the luck of the draw. We didn’t discover an exoplanet on our first try either. I believe that this non-detection shows that we’re on the verge of our next greatest discovery.

The reasons for searching for exomoons are abundant. “Exomoons may be frequent, habitable abodes for life and so far we know next to nothing about the underlying frequency of such objects in the cosmos,” Dr. Kipping told Universe Today. “They also play an important role in the habitability of those planets which they orbit, for example the Moon is thought to stabilize the axial tilt of the Earth and so too the climate.”

The project titled “The Hunt of Exomoons with Kepler,” more commonly known as HEK, was formed with these reasons in mind. As such, the HEK project will search for exomoons that are likely to be habitable.

The first target is Kepler-22b – the first transiting exoplanet to have been detected in the habitable zone of its host star. At 2.4 Earth radii, it is too large to be considered an Earth-analog, but it could easily have an Earth-sized moon

There are currently two methods in which we may detect exomoons.

1.) Dynamic effects – the exomoon tugs the planet, which causes deviations in the times and durations of the host planet’s transits. This is similar to the radial velocity technique for detecting exoplanets.

2.) Transit effects – the exomoon may transit the star immediately before or just after the planet does. This will cause an added dip in the observed light. See this video for a great demonstration. This is similar to the light curve technique for detecting exoplanets.

The team modeled the initial transit light curves of Kepler-22b. They then injected an Earth-sized moon into the system in order to analyze the effects. While this caused clear variations in the light curve, such variations had to be above the level of noise.

As such, they also injected noise in the light curves, which mirrors that of the Kepler data. In the end, the variations in a star’s light curve due to the presence of an exomoon are much higher than the noise. The team is able to recover the correct answer with extremely high confidence.

Here Kipping et al. presents injected moon fits.  As an example, the upper left-hand figure shows an exoplanet transit, with a moon transiting as well. Here the moon transits first, causing the light to be blocked, then the planet follows, causing more of the light to be blocked.
Here Kipping et al. presents injected moon fits. As an example, the upper left-hand figure shows an exoplanet transit, with a moon transiting as well. Here the moon transits first, causing the light to be blocked, then the planet follows, causing more of the light to be blocked. Source: Kipping et al. 2013

The real data does not show deviations like the previous figure does. This non-detection implies that there is no moon with a mass greater than 0.54 times the mass of the Earth. While there is no Earth-analog in this system, there may be a smaller undetectable moon.

I asked Dr. Kipping about our chances of success in other systems. His answer: “That depends upon nature herself!” We have no idea how regularly nature produces moons in other solar systems. “There is nothing more exciting than working on a project where the answer is wholly unknown.”

But remember: two decades ago we were unsure if nature regularly produced planets. We have since observed them in abundance. I have to believe that with 168 moons in our solar system alone, we’re likely to find them in other systems.  We’re on the verge of the next greatest discovery. So stay tuned because I promise I’ll be writing about it when it happens.

Source: Kipping et al. 2013

Behind the Scenes at Kitt Peak Observatory: What is an Observing Run Really Like?

The view of Kitt Peak National Observatory, as seen from 1 mile below the summit.

Greetings, from the Kitt Peak National Observatory, in Arizona!  I’m here on a weeklong observing run, which is arguably the coolest and hardest part of the job.

Kitt Peak rests on the Quinlan Mountains, 6,880 feet above sea level and 55 miles southwest of Tucson. When you begin your drive up the mountain, you first see a beautiful panorama of glittering white domes. There are 26 telescopes on the Mountain.

The Mayall 4-meter telescope quickly catches your eye – the colossal giant that towers over the rest.  As you continue your drive, a radio telescope can be seen on the left, followed by various signs stating that cell phone use is strictly prohibited. Observing runs here require radio silence, and a great chance to escape.

At the top of the mountain, two telescopes stand apart from the rest – the McMath-Pierce Solar Telescope and the WIYN observatory.  The solar telescope reflects sunlight through a tunnel that leads underground.  The WIYN observatory has an octagonal shape for a dome.

This is my third trip to Kitt Peak, but my first chance to observe on the Mayall 4-meter telescope. The first thing to know about the 4-meter is that it is a colossal maze. Literally.  There are 16 stories of rooms, now obsolete and out of date, before reaching the base of the telescope itself.

The dome of the 4-meter Mayall telescope (left) as well as the telescope itself (right)
The dome of the 4-meter Mayall telescope (left) as well as the telescope itself (right).

These rooms include old darkrooms, instrument rooms, machine rooms, classrooms, dormitories, game rooms, and other mysteries.  We’ve been joking most of this week that Hollywood should rent out the 4-meter for a fantastic horror film. Just think: The Big Bang Theory meets Psycho.

On our first day here, my colleague and I managed to get pretty lost. To reach the telescope you have to take two different gated and locked elevators.  But when we finally made it to the control room, we realized that this room alone is much more of maze than the building.

The control room consists of 4 computers, 16 monitors, 3 personal laptops, 4 tv screens, and an array of controls that operate the telescope. Eventually we became very comfortable floating from monitor to monitor.

The control room for the 4-m Mayall telescope. Dr. Mike DiPompeo is taking images.
The control room for the 4-m Mayall telescope. Dr. Mike DiPompeo is taking images throughout the night.

Here is what a typical day on an observing run looks like.

We typically wake up a little after noon and grumpily head to the dining hall for coffee.  Breakfast (or lunch) runs until about 1 pm.

In the late afternoon, we take a few flat field calibrations – images of a white screen, which is uniformly lit up. Any variations in the final image are due to variations in the detector or distortions in the optical path. At the end of the day, you can divide your science images by your flat field images, in order to achieve much cleaner images.

Shortly thereafter, the dewar is filled with liquid nitrogen.  This keeps the instrument cool (approximately -100 degrees Fahrenheit), as any thermal current can cause added noise.

After a quick dinner we return to the telescope.  At this point sunset is approximately 2 hours away, but it’s already time to open the dome. When you’re standing next to the telescope, an opening dome sounds like a freight train screeching to a stop.  It’s slightly terrifying, but it is by far one of my favorite sounds. It signifies that for the rest of the night you’re in control of this phenomenal instrument, which has the power to discover the secrets of the Universe.

After two hours of various preparations – making sure the telescope is pointing correctly, guiding correctly, etc. – we “get on sky.”  Throughout the night the telescope operator controls the telescope, moving it to the fields we would like to observe, while we are in charge of taking the images by verifying the exposure time, filters to use, etc.

If everything goes smoothly the night is pretty easy.  The telescope operator moves from target to target while we continuously take images. This means that we end up sitting in front of a computer screen, pressing enter every 300 seconds in order to start a new exposure. That’s really all it takes! Of course you should keep checking on your images in order to verify that they look good.

Around midnight it’s time for night lunch, a packed lunch that the dining hall provides.  A little extra protein helps make the long nights more bearable.  And then you push through, making coffee if necessary. The challenging part is staying awake throughout the night. It’s amazing how hard simple calculations can be when dawn is approaching.

At the end of the night you step outside and save for the flickering glint of Tucson’s city lights, the only noticeable light is found by looking up into the night sky.  The stars here are brilliant, and the Milky Way is astonishing. After spending an entire evening stuck in a black box, it’s a wonderful reminder of what it’s all about: the night sky.

I observed with Dr. Mike DiPompeo, who concurred on what I noticed about the observing experience.

“When you first get into astronomy you’re in awe of the beauty of the night sky, compelled and driven by it,” DiPompeo told me. “But it can be easy to forget in the day to day business of being an astronomer – sitting at a computer, writing code, going through the data, reading papers – that your job is to understand that beauty. Observing reconnects you with the night sky.”

My favorite part of an observing run occurs in the morning – on the walk from the telescope to the dorm, when a yellow arch of light first appears above the horizon. Kitt Peak provides fantastic sunrises. And you really have to soak in every last ray of sun, before you crawl into bed in a very dark room.

One of many beautiful sunrises.
One of many beautiful sunrises.

Observing runs lie at the root of pure research.  You spend the long nights collecting data, then the months or years analyzing the data, and finally hope that a cool result comes from all the hard work.

A Rare Opportunity to Watch a Blue Straggler Forming

The globular cluster NGC 6388. Blue stragglers may clearly be seen around the edges. More are hidden within the central core. Credit: ESO

A unique and enigmatic variety of stars known as blue stragglers appear to defy the normal stellar aging process. Discovered in globular clusters, they appear much younger than the rest of the stellar population. Since their discovery in 1953, astronomers have been asking the question: how do these stars regain their youth?

For years, two theories have persisted. The first theory suggests that two stars collide, forming a single more massive star. The second theory proposes that blue stragglers emerge from binary pairs. As the more massive star evolves and expands, it blows material onto the smaller star. In both theories, the star grows steadily more massive and bluer – it regains its youth.

But now, a surprising finding may lend credence to the second theory. Astronomers at the Nicolaus Copernicus Astronomical Center in Poland recently observed a blue straggler caught in the midst of forming!

The binary system that was studied, known as M55-V60, is located within the globular cluster M55. Dr. Michal Rozyczka, one of the research scientists on the project, told Universe Today, “The system is a showcase example of a blue straggler formed via the theoretically predicted peaceful mass exchange between its components.”

The team used both photometric (the overall light from the system) and spectroscopic (the light spread out into a range of wavelengths) observations. The photometric data revealed the light curve – the change in brightness due to one star passing in front of the other – of the system. This provided evidence that the astronomers were looking at a binary system.

From the spectroscopic data, shifts in wavelength reveal the velocity (along the line of sight) of a source. The research team noted that the system’s center of mass was moving with respect to the binary system. This will occur in a semi-detached binary system, where mass transfers from one star to the other. As it does this, the center of mass will follow the mass-transfer.

From both photometric and spectroscopic observations (which covered more than 10 years!) the team was able to verify that this object is not only a binary, but a semi-detached binary, residing at the edge of M55.

An artist's conception of how a blue straggler may form from a binary system. Credit:NASA/ESA
An artist’s conception of how a blue straggler may form from a binary system. Credit: NASA/ESA

“The system is semi-detached with the less massive (secondary) component filling its Roche lobe,” explained Dr. Rozyczka. “The secondary has a tearlike shape, with the tip of the tear directed toward the more massive primary. A stream of gas flows out of the tip along a curved path and hits the primary.”

How do we know that it is in fact a blue straggler? The simple answer is that the secondary star, with is gaining mass, appears bluer than normal. This blue straggler is clearly in the process of forming. It is the second observation of such a formation, with the first being V228 in the globular cluster: 47 Tuc.

This research verifies that semi-detached binaries are a viable formation mechanism for blue stragglers. The binary was discovered by happenstance, in a project aimed at determining accurate ages and distances of nearby clusters. It’s certainly a surprising result from the survey.

The results will be published in Acta Astronomica, a peer-reviewed scientific journal located in Poland (preprint available here).


How do Hypervelocity Stars End up Breaking The Speed Limit?

An artist's conception of a hypervelocity star that has escaped the Milky Way. Credit: NASA

The Sun is racing through the Galaxy at a speed that is 30 times greater than a space shuttle in orbit (clocking in at 220 km/s with respect to the galactic center). Most stars within the Milky Way travel at a relatively similar speed. But certain stars are definitely breaking the stellar speed limit. About one in a billion stars travel at a speed roughly 3 times greater than our Sun – so fast that they can easily escape the galaxy entirely!

We have discovered dozens of these so-called hypervelocity stars. But how exactly do these stars reach such high speeds? Astronomers from the University of Leicester may have found the answer.

The first clue comes in observing hypervelocity stars, where we can note their speed and direction. From these two measurements, we can trace these stars backward in order to find their origin. Results show that most hypervelocity stars begin moving quickly in the Galactic Center.

We now have a rough idea of where these stars gain their speed, but not how they reach such high velocities. Astronomers think two processes are likely to kick stars to such great speeds. The first process involves an interaction with the supermassive black hole (Sgr A*) at the center of our Galaxy. When a binary star system wanders too close to Sgr A*, one star is likely to be captured, while the other star is likely to be flung away from the black hole at an alarming rate.

The second process involves a supernova explosion in a binary system. Dr. Kastytis Zubovas, lead author on the paper summarized here, told Universe Today, “Supernova explosions in binary systems disrupt those systems and allow the remaining star to fly away, sometimes with enough velocity to escape the Galaxy.”

There is, however, one caveat. Binary stars in the center of our Galaxy will both be orbiting each other and orbiting Sgr A*. They will have two velocities associated with them. “If the velocity of the star around the binary’s center of mass happens to line up closely with the velocity of the center of mass around the supermassive black hole, the combined velocity may be large enough to escape the Galaxy altogether,” explained Zubovas.

In this case, we can’t sit around and wait to observe a supernova explosion breaking up a binary system. We would have to be very lucky to catch that! Instead, astronomers rely on computer modeling to recreate the physics of such an event. They set up multiple calculations in order to determine the statistical probability that the event will occur, and check if the results match observations.

Astronomers from the University of Leicester did just this. Their model includes multiple input parameters, such as the number of binaries, their initial locations, and their orbital parameters. It then calculates when a star might undergo a supernova explosion, and depending on the position of the two stars at that time, the final velocity of the remaining star.

The probability that a supernova disrupts a binary system is greater than 93%. But does the secondary star then escape from the galactic center? Yes, 4 – 25% of the time. Zubovas described, “Even though this is a very rare occurrence, we may expect several tens of such stars to be created over 100 million years.” The final results suggest that this model ejects stars with rates high enough to match the observed number of hypervelocity stars.

Not only do the number of hypervelocity stars match observations but also their distribution throughout space. “Hypervelocity stars produced by our supernova disruption method are not evenly distributed on the sky,” said Dr. Graham Wynn, a co-author on the paper. “They follow a pattern which retains an imprint of the stellar disk they formed in. Observed hypervelocity stars are seen to follow a pattern much like this.”

In the end, the model was very successful at describing the observed properties of hypervelocity stars. Future research will include a more detailed model that will allow astronomers to understand the ultimate fate of hypervelocity stars, the effect that supernova explosions have on their surroundings, and the galactic center itself.

It’s likely that both scenarios – binary systems interacting with the supermassive black hole and one undergoing a supernova explosion – form hypervelocity stars.  Studying both will continue to answer questions about how these speedy stars form.

The results will be published in the Astrophysical Journal (preprint available here)


Mysterious and Well-Preserved Oort Cloud Object Heading Into Our Solar System

An artist's conception of two tidally locked objects orbiting the Sun from afar. The system: 2010 WG9 may likely look like this. Credit: zmescience

What if we could journey to the outer edge of the Solar System – beyond the familiar rocky planets and the gas giants, past the orbits of asteroids and comets – one thousand times further still – to the spherical shell of icy particles that enshrouds the Solar System. This shell, more commonly known as the Oort cloud, is believed to be a remnant of the early Solar System.

Imagine what astronomers could learn about the early Solar System by sending a probe to the Oort cloud! Unfortunately 1-2 light years is more than a little beyond our reach. But we’re not entirely out of luck. 2010 WG9 – a trans-Neptunian object — is actually an Oort Cloud object in disguise. It has been kicked out of its orbit, and is heading closer towards us so we can get an unprecedented look.

But it gets even better! 2010 WG9 won’t get close to the Sun, meaning that its icy surface will remain well-preserved. Dr. David Rabinowitz, lead author of a paper about the ongoing observations of this object told Universe Today, “This is one of the Holy Grails of Planetary Science – to observe an unaltered planetesimal left over from the time of Solar System formation.”

Now you might be thinking: wait, don’t comets come from the Oort Cloud? It’s true; most comets were pulled out of the Oort cloud by a gravitational disturbance. But observing comets is extremely difficult, as they are surrounded by bright clouds of dust and gas. They also come much closer to the Sun, meaning that their ices evaporate and their original surface is not preserved.

So while there is a surprisingly high number of Oort cloud objects hanging out within the inner solar system, we needed to find one that is easy to observe and whose surface is well preserved. 2010 WG9 is just the object for the job! It is not covered by dust or gas, and is believed to have spent most of its lifetime at distances greater than 1000 AU. In fact, it will never approach closer than Uranus.

Astronomers at Yale University have observed 2010 WG9 for over two years, taking images in different filters. Just as coffee filters allow ground coffee to pass through but will block larger coffee beans, astronomical filters allow certain wavelengths of light to pass through, while blocking all others.

Recall that the wavelength of visible light relates to color. The color red, for example, has a wavelength of approximately 650 nm. An object that is very red will therefore be brighter in a filter of this wavelength, as opposed to a filter of, say, 475 nm, or blue. The use of filters allow astronomers to study specific colors of light.

Astronomers observed 2010 WG9 with four filters: B, V, R, and I, also known as blue, visible, red, and infrared wavelengths. What did they see? Variation – a change in color over the course of just days.

The likely source is a patchy surface. Imagine looking at the Earth (pretend there’s no atmosphere) with a blue filter. It would brighten when an ocean came into view, and dim when that ocean left the field of view. There would be a variation in color, dependent on the different elements located on the surface of the planet.

The dwarf planet Pluto has patches of methane ice, which also show up as color variations on its surface. Unlike Pluto, 2010 WG9 is relatively small (100 km in diameter) and cannot hold on to its methane ice. It’s possible that part of the surface is newly exposed after an impact. According to Rabinowitz, astronomers are still unsure what the color variations mean.

Rabinowitz was very keen to explain that 2010 WG9 has an unusually slow rotation. Most trans-Neptunian objects rotate every few hours. 2010 WG9 rotates on the order of 11 days! The best reason for this discrepancy is that it exists in a binary system. If 2010 WG9 is tidally locked to another body — meaning that the spin of each body is locked to the rate of rotation — then 2010 WG9 will be slowed down in its rotation.

According to Rabinowitz, the next step will be to observe 2010 WG9 with larger telescopes — perhaps the Hubble Space Telescope — in order to better measure the color variation. We may even be able to determine if this object is in a binary system after all, and observe the secondary object as well.

Any future observations will help us further understand the Oort cloud. “Very little is known about the Oort cloud – how many objects are in it, what are its dimensions, and how it formed,” Rabinowitz explained.  “By studying the detailed properties of a newly arrived member of the Oort cloud, we may learn about its constituents.”

2010 WG9 will likely hint at the origin of the Solar System in helping us further understand its own origin: the mysterious Oort cloud.

Source: Rabinowitz, et al. AJ, 2013

Blocking Light Sheds New Light on Exoplanet Atmospheres

Kepler-16b is but one example of an uncanny world. It orbits two suns. Credit: Discovery

Exoplanets are uncanny. Some seem to have walked directly out of the best science-fiction movies. For example, we’ve discovered a planet consisting purely of water (GJ 1214b) and one with two suns (Kepler 16b). Some planets nearly scrape their host stars once every orbit, while others exist in darkness without a host star at all. The field of exoplanet research is moving beyond detecting exoplanets to characterizing them – understanding which molecules are present and if they might possibly harbor life.

A key research element in characterizing these alien worlds is observing their atmospheres. But how exactly do astronomers do this? We can’t simply tug the planet toward us to get a closer look.  It’s also incredibly difficult to directly image their atmospheres from afar.  Why? Stars are incredibly bright in comparison to their puny, barely reflective, and nearby exoplanets. So a direct image of an exoplanet’s atmosphere seemed out of the question – until recently.

It may be tricky to directly image an exoplanet’s atmosphere, but astronomers always have quite a few tricks up their sleeves. The first one is in mounting an instrument called a coronagraph on your telescope.  This instrument blocks out the star’s light, leaving an image of the exoplanet alone.  Another trick, known as adaptive optics, is to send a laser beam through the atmosphere.  The changes in the laser allow us to monitor changes in the atmosphere, providing corrections to clean and smooth the image.

HR 8799, a large star orbited by four known giant planets, is relatively nearby (remember that ‘nearby’ is an astronomers way of saying that it is still pretty far, or in this case 130 light years away). In 2008, three of the planets were directly imaged using the Gemini and Keck telescopes on Mauna Kea, Hawaii.  In 2010, the fourth planet, which was closest to the star and therefore the most difficult to see was directly imaged by the Keck telescope.

Direct image of the HR 8799 system.  The star has been blocked and all four planets can clearby be seen. Credit: Oppenheimer et al. 2013
Direct image of the HR 8799 system. The star has been blocked and all four planets can clearby be seen. Credit: Oppenheimer et al. 2013

A direct image of an exoplanet’s atmosphere may tell us what color the atmosphere appears to be, and how thick the atmosphere is, but it gives us little more information.  We need to know the atmospheric composition – the specific molecules and their abundances that are present within the atmosphere itself.  If we’re looking at the question of habitability we need to know if there is water in the atmosphere or maybe carbon dioxide.

The key is in mounting a spectrograph on the telescope.  Instead of collecting the overall light from the planet, that light is broken up into a spectrum of wavelengths.  Imagine seeing a rainbow after a thunderstorm.  That rainbow is simply the light from the sun broken up across all visible wavelengths due to ice crystals in our atmosphere.  Molecules emit light at specific wavelengths, leaving well-known fingerprints that may be identified in a lab on Earth, in a rainbow in the sky, or in the spectrum of an exoplanet located 130 light years away.

When astronomers mounted their instrumentation (i.e. a coronagraph, an adaptive optics system, and a spectrograph) known as Project 1640 onboard the Palomar 5m Hale Telescope, they were able to shed new light on the HR 7899 system.  Only last month one of its exoplanets revealed a mixture of water vapor and carbon monoxide in its atmosphere, but the story has changed. See a previous article in Universe Today.

Project 1640 observed not one – but four atmospheres at once.  Gautam Vasisht of JPL explains, “in just one hour, we were able to get precise composition information about four planets around one overwhelmingly bright star.”  These four exoplanets are believed to be coeval, in that they formed from a protoplanetary disk at roughly the same time.  They also have the same luminosity and temperature, leading to the assumption that they are roughly similar to each other.  But results show that they all have radically different spectra, and therefore different chemical compositions!

More specifically, HR 8799 b and d contain carbon dioxide, b and c contain ammonia, d and e contain methane, and b, d, and e contain acetylene.  Noticing a few trends? There really aren’t any! Not only are these planets different from each other, they are also different from any other known objects. Acetylene, for example, has never been convincingly identified in a sub-stellar object outside the solar system.  While the varying spectra pose many questions, one thing is clear: the diversity of planets must be greater than previously thought!

This is only the first exoplanet system for which we’ve obtained direct spectra of all exoplanet atmospheres. Project 1640 will conduct a 3-year survey of 200 nearby stars. The hope is to find hot Jupiters located far from their host star.  While this is what the current technique allows astronomers to detect, it will also teach astronomers how Earth-like planets form.

“The outer giant planets dictate the fate of rocky ones like Earth. Giant planets can migrate in toward a star, and in the process, tug the smaller, rocky planets around or even kick them out of the system. We’re looking at hot Jupiters before they migrate in, and hope to understand more about how and when they might influence the destiny of the rocky, inner planets,” explained Vasisht.

In an attempt to understand our own blue marble, astronomers point their telescopes at uncanny worlds light years away. Project 1640 will block the light of distant stars in order to shed light on distant worlds as well as our own.

Sources: Jet Propulsion Laboratory, and B. R. Oppenheimer et al. 2013 ApJ 768 24