Thursday, March 28, 2013

Vega’s Asteroid Belt

Our solar system consists of a warm-and-cold-belt architecture. The warm belt corresponds to the asteroid belt and the cold belt corresponds to the Kuiper belt. Both belts are separated by a large gap that is populated by the giant planets - Jupiter, Saturn, Uranus and Neptune. The asteroid belt has a characteristic temperature of ~170 K and is located between the orbits of Mars and Jupiter. The more outlying Kuiper belt has a characteristic temperature of ~50 K and it extends beyond the orbit of Neptune. It is worth considering if such warm-and-cold-belt architectures also exist around other stars, if they might be common and if such belts may serve as clues to the existence of planets.

Spectroscopic studies were performed using the infrared spectrograph instrument on NASA’s Spitzer space telescope on Vega - a relatively nearby star about 25 light-years away. The observations revealed a mid-infrared excess which corresponds to material with a characteristic temperature of ~170 K. This indicates the presence of an asteroid belt at a distance of ~14 AU around Vega, analogous to the asteroid belt in our solar system. Since Vega is 37 times more luminous than our Sun, the asteroid belt around Vega is located much further out from the star than the asteroid belt in our solar system which is ~2.7 AU from our Sun.

In addition to an asteroid belt, Vega is also known to have a cold belt exterior to the asteroid belt. This cold belt is likely to be analogous to our solar system’s Kuiper belt and it reveals itself as an excess in the far-infrared, corresponding to material having a characteristic temperature of ~50 K. This suggests that Vega possesses a warm-and-cold-belt architecture like our Sun.

Observations have also revealed the presence of hot dust in the close vicinity of Vega. The dust shows up as an excess of short-infrared radiation corresponding to a characteristic temperature of ~1500 K. Based on this temperature, the dust is expected to exist within a distance of ~0.2 AU from Vega. The dust is proposed to be nano-size metal oxides coming from the sublimation of silicate-rich planetesimals. These nano-size metal oxides become charged either via the photoelectric effect or the stellar wind. Once charged, they become trapped in the star’s magnetic field at close proximity to the star.

Similar to the asteroid belt in our solar system, the asteroid belt around Vega is also located at the water-frost line. During the formation of planets in a protoplanetary disk, the water-frost line demarks a boundary around a star where temperatures become cool enough for grains of water ice to form. As a result, the density of solid particles in a protoplanetary disk increases abruptly by a factor of a few, beginning at the water-frost line and extending outwards. In addition to longer dynamical timescales, the increase in the amount of solid material favours the formation of giant planets exterior to the water-frost line. Our solar system is a good example where 4 giant planets exist beyond the asteroid belt. Besides our Sun, another example is a young star called HR 8799 which has a similar two-belt configuration with 4 giant planets between the inner and outer belts. These giant planets dominate the dynamics of the inner and outer belts, and help keep the large gap between the two belts relatively clear.

The large gap separating the warm and cold belts around Vega hints that Vega could be surrounded by multiple undiscovered planets. In fact, a number of searches have been conducted to look for planets around Vega. Although no planets have been discovered around Vega so far, these searches have placed strong limits on the maximum mass that planets around Vega could have. This limiting mass is no more than a few times the mass of Jupiter. As a result, the presence of one or two very massive planets with several times the mass of Jupiter is unlikely to be responsible for the large gap separating the inner and outer belts around Vega. Instead, the presence of multiple Jupiter-mass planets is sufficient to maintain the gap and keep within the planetary mass constrains. Better observations will be required to detect the presence of these planets.

Vega has an inner warm belt and an outer cold belt. The two belts are separated by a large gap that is likely to be populated by multiple planets. Besides our Sun, Vega and HR 8799, a number of other stars are also known to posses warm-and-cold-belt architectures. These stars include Fomalhaut and Epsilon Eridani.

Su et al. (2013), “Asteroid Belts in Debris Disk Twins: VEGA and FOMALHAUT”, arXiv:1301.1331 [astro-ph.EP]

Sunday, March 24, 2013

Very Low Energy Supernovae

Massive stars with more than ~8 times the Sun’s mass are known to end their lives in energetic core-collapse supernovae explosions. During the final evolutionary phases of a massive star, silicon in the star’s core undergoes fusion and produces an iron-rich core. When the iron-rich core grows above a certain mass, it can no longer support itself against the crushing force of gravity and undergoes catastrophic collapse. The collapsing core eventually comes to a halt, causing still infalling matter to rebound. This process launches a powerful outward propagating shock wave. By itself, the shock wave does not have sufficient energy to destroy the star in a supernova explosion. Nevertheless, as the core collapses to form a protoneutron star, copious amounts of neutrinos are produced. Neutrinos rarely interact with normal matter and most of the neutrinos that are produced simply escape out of the dying star at the speed of light. However, a small proportion of the neutrinos interacts with and reenergizes the shock wave, providing it with enough energy to cause the star to explode as a supernova.

For stars with more than ~20 times the Sun’s mass, a fraction of them do not explode as supernovae. Instead, the protoneutron star produced from core-collapse becomes massive enough to collapse further and form a black hole. This causes a portion of the neutrinos that would have been emitted to end up inside the black hole. As a result, there are insufficient neutrinos to produce a supernova. The rest of the star collapses directly into the black hole and simply “disappears” without a supernova. Even without a supernova, the star’s demise might still produce an observable signature. This is because the protoneutron star emits a considerable fraction of its mass-energy as neutrinos before eventually collapsing to form a black hole. As the neutrinos escape the dying star at the speed of light, the energy carried away by them is seen as an abrupt loss of gravitational mass which can amount to 0.2 - 0.5 times the Sun’s mass. In response to this, a shock is formed which propagates towards the outer layers of the dying star.

Although the shock is nowhere as energetic as the shock driving a core-collapse supernova, it is still expected to be energetic enough to eject the hydrogen envelop of the dying star to produce a transient event signalling the star’s demise. The hydrogen envelop is ejected with a low velocity of ~100 km/s and the ejecta temperature is likely to be very cool, on the order of ~3000 K. Most of the emitted energy is from the recombination of hydrogen where electrons and protons come together to form electrically neutral hydrogen atoms. This transient event is basically a massive hydrogen envelope being ejected at low energies. In a way, such a transient event is like a very weak supernova that is orders of magnitude less luminous and less energetic than a core-collapse supernova.

During their final stellar evolutionary phases, massive stars that are more than ~10 times the Sun’s mass will expand to become red supergiants. These stars are the largest known stars in the universe by volume. A survey that monitors red supergiants might catch such anomalous transient events. The transient event would appear as a sudden brightening of the red supergiant. The brightening will last for a year or so before dimming and eventually disappearing entirely. A transient event like this signals the birth of a black hole. If the shock is not sufficiently energetic to eject the hydrogen envelop, a transient event may still be produced. In this case, after the rest of the star has collapsed to form a black hole, the hydrogen envelop eventually falls back towards the black hole. This infalling matter forms an accretion disk around the black hole and powers a long-duration gamma-ray transient event.

Elizabeth Lovegrove and Stan Woosley (2013), “Very Low Energy Supernovae from Neutrino Mass Loss”, arXiv:1303.5055 [astro-ph.HE]

Thursday, March 21, 2013


Solar flares are the most energetic explosions in our Sun’s atmosphere. A typical solar flare releases anywhere between ~10 thousand up to ~100 million times the world’s annual energy consumption in a matter of hours. The most powerful solar flare in recorded history was the Carrington Event which took place in the year 1859. This solar flare created a large geomagnetic storm on Earth which damaged telegraph systems and produced dramatic aurorae all around the world, visible even as far as the tropical latitudes. Should such an event occur in the present day, it would be very damaging to our technological society. Even so, flares that are many times more energetic than the Carrington Event are known to occur on other Sun-like stars. These flares are known as superflares and they are typically ~10 to ~1000 times more energetic than the Carrington Event. Whether our present Sun is capable of launching a superflare is not just of astrophysical important, but also of sociological importance.

Kepler is a space telescope designed to detect Earth-like planets around other stars. It constantly monitors the brightness of over a hundred thousand stars to a high level of precision and look for periodic dips in brightness that could be indicative of a planet crossing in front of its parent star. Using data collected by Kepler from April 2009 to December 2009, a study by Maehara et al. (2012) discovered 365 superflares on 148 solar type stars. Among them, 14 superflares came from Sun-like stars with rotational periods longer than 10 days and surface temperatures of 5,600 K £ T £ 6,000 K. Note that our Sun has a rotational period of over 25 days and a surface temperature of 5778 K. Based on this, it is estimated for a Sun-like star that a superflare 100 times more energetic than the Carrington Event occur once ever 800 years and a superflare 1000 times more energetic than the Carrington Event occur once ever 5000 years. If this occurrence rate is applicable to our Sun, then sometime during the next few hundred or thousand years, a superflare could pose a serious threat to our technology dependant civilization.

The study by Maehara et al. (2012) shows that the occurrence rate of superflares on slowly rotating stars (rotational periods longer than 10 days) is much lower than on rapidly rotating stars as most of the 365 superflares observed by Kepler occurred on stars that rotated in less than 10 days. This is because the energy of stellar flares depends on stellar magnetic activity and more rapidly rotating stars have higher magnetic activity. However, the maximum energy of a superflare remains unchanged regardless of whether the star is slowly or rapidly rotating. This means that superflares from slowly rotating stars are just as powerful as those from rapidly rotating stars.

It has been proposed that the presence of a close-in hot-Jupiter around a star affects the star’s magnetic activity and superflares can only occur on stars with hot-Jupiters. This suggests that our Sun is unlikely to have a superflare since there is no hot-Jupiter orbiting our Sun. However, the study by Maehara et al. (2012) paints a different picture. Given that the probability for a transit of a hot-Jupiter in front of its parent star is about 10 percent and assuming that the superflares on all 148 stars were caused by hot-Jupiters, then Kepler should have already detected about 15 hot-Jupiters. Instead, not a single hot-Jupiter was found around the 148 solar type stars with superflares. Since Kepler has almost completed its survey of hot-Jupiters, the non-detection of hot-Jupiters shows that it is not necessary to have a hot-Jupiter for the production of superflares.

There is no record of any superflare from our Sun over the last few thousand years. The largest solar flare known to date is still the Carrington Event in the year 1859. Nevertheless, there are a number of curious events in history that might be attributed to superflares from our Sun even though there is a lack of evidence at present to form any meaningful conclusion. One such event was published in a study by Miyake et al. (2012) where they discovered a rapid increase in cosmic-rays from AD 774 to 775. This was inferred from a spike in concentration of carbon-14 found in the tree rings of Japanese cedar trees. Carbon-14 is a radioactive isotope of carbon produced when high energy radiation enters the Earth’s atmosphere. If a superflare from our Sun is responsible, the flare will need to be ~1000 times more energetic than the Carrington Event to produce the spike in carbon-14.

1. Maehara et al., “Superflares on solar-type stars”, Nature 485, 478-481 (24 May 2012)
2. Miyake et al., “A signature of cosmic-ray increase in AD 774-775 from tree rings in Japan”, Nature 486, 240-242 (14 June 2012)

Sunday, March 17, 2013

Storms on Jupiter

Jupiter is the largest of all the solar system’s planets, more than ten times bigger and three hundred times as massive as Earth. Jupiter is so immense it could swallow all the other planets easily. Its Great Red Spot, a storm that has raged for centuries, is itself wider than Earth. And the Spot is merely one feature visible among the innumerable vortexes and streams of Jupiter’s frenetically racing cloud tops. Yet Jupiter is composed mainly of the lightest elements, hydrogen and helium, more like a star than a planet. All that size and mass, yet Jupiter spins on its axis in less than ten hours, so fast that the planet is clearly not spherical: Its poles are noticeably flattened. Jupiter looks like a big, colourfully striped beach ball that’s squashed down as if some invisible child were sitting on it. Spinning that fast, Jupiter’s deep, deep atmosphere is swirled into bands and ribbons of multihued clouds: pale yellow, saffron orange, white, tawny yellow-brown, dark brown, bluish, pink and red. Titanic winds push the clouds across the face of Jupiter at hundreds of kilometres per hour.
- Ben Bova, Jupiter (2000)

Figure 1: A true colour mosaic of Jupiter constructed from images taken by the narrow angle camera onboard NASA’s Cassini spacecraft on December 29, 2000. Credit: (NASA/JPL/Space Science Institute)

On 4 May 1999, NASA’s Galileo spacecraft was in orbit around Jupiter and it took a series of images of a region within the giant planet’s South Equatorial Belt. A total of three sets of images were taken. The 1st and 2nd sets of images were taken when the region was in daylight while the 3rd set of images was taken when the region was experiencing night. In the images, two storm centres (latitude 14 S, longitude 268 W and latitude 15 S, longitude 263 W) can be clearly seen. The night time images do not show any cloud features, but they show bright spots due to lightning that are situated at where the two storm centres are.

Figure 2: On the left are images of the mapped region, showing the presence of two storm centres. Colour image of the mapped region (top left), night time image showing the lightning storms as black spots over the storm centres (middle left) and map of wind velocity vectors with the longest vector representing 70 m/s (bottom left). The expanded view of the storm (top right) is colour coded where white regions represent high level clouds and red regions represent deep clouds. A sketch of the storm’s cross-sectional profile (bottom right) shows the haze layers as dotted regions and optically thick clouds as solid outlines. The cloud base is likely to be deeper than indicated. Credit: Gierasch, P.J. et al. (2000)

Further analysis was performed for the more western storm (latitude 14 S, longitude 268 W). This active storm system has a length of 4000 kilometres. The bulk of the storm consists of an optically thick cloud which extends from below the 3 bar pressure level up to at least the 0.5 bar pressure level. As a result, the storm cloud has an estimated vertical extent of no less than 50 kilometres. At the cooler upper levels of the storm cloud, condensation is likely to consist of a combination of water, ammonia and ammonium hydrosulphide. Beneath the upper levels, the rest of the storm cloud is expected to consist of only water as a condensable species. This is because temperatures deeper in the atmosphere are too high for other volatile species to condense. Water is also expected to be the primary agent for cloud electrification. Apart from the primary storm cloud, images taken by the Galileo spacecraft also reveal the presence of background clouds in the vicinity. The components of these background clouds are consistant with a sheet of intermittent clouds (possibly ammonia ice) at the 0.9 bar level, a haze layer between the 0.9 bar and 0.2 bar levels and a more rarefied haze layer extending above the 0.2 bar level.

This storm is a form of moist convective updraft. On Jupiter, the temperature excess necessary to power an updraft up to such a vertical extent is estimated to be about 5 degrees Kelvin. This allows the total power output of the storm to be estimated at 5×1015 watts. In October and November of 1997 the Galileo spacecraft detected lightning from 26 storms on the night side of Jupiter. Based on this data, the estimated frequency of lightning storms occurring on Jupiter is 0.66×10-9 per km2. If all lightning storms on Jupiter come from such moist convective storms, then the heat flux curried by these storms when averaged over the entire surface of Jupiter turns out to be 3.3 W/m2.

For over four billion years, Jupiter has been slowly contracting under its own immense gravitational grip. As the stupendous bulk of the giant planet is squeezed, gravitational potential energy is converted into heat. This causes Jupiter to radiate more heat than is receives from the Sun. Jupiter’s internal heat flux averaged over its entire surface, is about 5.4 W/m2. The energy that drives these moist convective storms largely comes from the giant planet’s own internal heat flux. Comparing the internal heat flux with the heat flux carried by moist convective storms, it appears that these storms can effectively carry a large portion of the giant planet’s internal heat flux out into the upper atmosphere. This suggests that moist convection plays an important role in the dynamics of Jupiter’s atmosphere.

1. Gierasch, P.J. et al. 2000, “Observation of moist convection in Jupiter’s atmosphere”, Nature 403, 628-630
2. Little, B. et al. 1999, “Galileo images of lightning on Jupiter”, Icarus 142, 306-323

Friday, March 15, 2013

A World Like No Other

He could smell the earth and the trees around the shallow lake beneath the balcony. It was a cloudy night and very dark, just a hint of glow directly above, where the clouds were lit by the shining Plates of the Orbital's distant daylight side. Waves lapped in the darkness, loud slappings against the hulls of unseen boats. Lights twinkled round the edges of the lake, where low college buildings were set amongst the trees. The party was a presence at his back, something unseen, surging like the sound and smell of thunder from the faculty building; music and laughter and the scents of perfumes and food and exotic, unidentifiable fumes.
- Iain M. Banks, the Player of Games (1988)

An Orbital is a rotating megastructure consisting of an enormous band of material arranged into a ribbon-like ring measuring millions of kilometres in diameter. The entire megastructure is spun to create day-night cycles and produce artificial gravity on the structure’s inner surface. As the Orbital spins, centrifugal forces hold the lithosphere, hydrosphere and atmosphere against the structure’s inner surface to support the desired type of ‘planetary’ environment. In this article, I will describe an archetypical Orbital whose entire inner surface is made to duplicate roughly the same conditions as on the Earth’s surface.

To satisfy the first condition necessary to support an Earth-like environment, the Orbital has to orbit the Sun at around the same distance as Earth is from the Sun. This ensures the insolation received is just right. The next condition is to provide a 24 hour day-night cycle and an Earth-like gravity on the structure’s inner surface. This requires the Orbital to have a diameter of 3.71 million kilometres and the whole structure must be spinning at a rate of once every 24 hours. At that rate of spin, the structure’s rim is moving at 135 km/s. An Earth-like atmosphere is held against the structure’s inner surface by spin-induced centrifugal forces. Walls that are a few hundred kilometres high line the edges of the Orbital’s inner surface. These gigantic walls along the structure’s edges keep the atmosphere within the Orbital’s inner surface by preventing it from slipping off the edges into space.

Constructing the Orbital will be a challenge because there is still no known material strong enough to withstand the titanic stresses found within the structure of the Orbital. This yet-to-be-discovered material or “unobtanium” will be required to construct the Orbital’s stress-carrying structure. The lithosphere, hydrosphere and atmosphere will all be laid onto the inner surface of this unobtanium-based stress-carrying structure. Other structures such as the towering walls along the edges of the Orbital’s inner surface can be made using known materials with extremely low density and very high strength such as self-supporting diamondoid foam. In addition, diamondoid foam can also be used to sculpture the desired topography for the environment on the structure’s inner surface.

The entire circumference of the Orbital measures 11.66 million kilometres and it takes light almost 40 seconds to traverse that distance. If the Orbital has a width of 40000 kilometres, the total habitable area on the Orbital’s inner surface will be a mind-boggling 466.4 billion square kilometres. An area like this is equivalent to 912 times the total surface area of the Earth (including the oceans) or 47500 times the area of the United States of America. If each square meter of surface area requires 12800 metric tons of material, the entire Orbital will be about as massive as the Earth. This is equivalent to a 1.63 kilometre column of solid iron for each square meter of surface. Nevertheless, such a structure will have 912 times more habitable surface area per unit mass than a planet like Earth.

An observer standing on the Orbital’s inner surface will see a sky that is rather similar to one seen from Earth’s surface as the atmosphere overhead is entirely open to the vacuum of space. However, the observer will be constantly aware of a marvellous sight where the world at both spinward and anti-spinward horizons will appear to curve upwards and eventually join overhead at a great distance of 3.71 million kilometres away. As the structure rotates once every 24 hours, the observer will be able to see the approach of dawn and dusk along the great arc of the Orbital.

Nights on the inner surface of the Orbital will be spectacular as ‘ring shine’ from the illuminated portion of the Orbital will appear thousands of times brighter than the full moon on Earth. Due to the large amount of ‘ring shine’, astronomers on the structure’s inner surface will have a great difficulty trying to observe the nigh sky. Then again, the dark and pristine vacuum of space is never too far away as astronomical observations can be performed from the outer surface of the Orbital, which is just tens of kilometres ‘underground’ from the habitable inner surface.

The Orbital sweeps across the entire sky like a grand firmament. Observing it should be an interesting pastime for surface inhabitants. Looking overhead with unaided eyes at the great arc of the Orbital, the innumerable oceans and continents will appear as mere speckles. Even a full scale recreation of all of Earth’s continents will be barely noticeable on such an enormous scale. Given the sheer immensity of the Orbital, the effect of surface curvature is a lot less than on any planet-size globe. A surface inhabitant can see all the way to the base of distant mountains, unlike on a planet where the base of distant mountains tends to get hidden away by the planet’s curvature.

To create day-night cycles, the Orbital is tilted at an angle with respect to the Sun. If the Orbital has no tilt, it will eclipse the Sun all the time from the perspective of an observer on the structure’s inner surface. The tilt also creates two warm seasons and two cool seasons each year. The middle of each warm season is marked by a midsummer eclipse of the Sun and the middle of each cool season is marked by the Sun’s lowest position in the noon sky. Unlike on Earth, the length of daylight on the Orbital will not vary, resulting in no long summer days or long winter nights. If the orbit of the Orbital around the Sun has some eccentricity, it can cause one warm season to be warmer than the other and one cool season to be cooler than the other.

Areas adjacent to the towering walls along the edges of the structure’s inner surface will experience a more pronounced seasonal variation from the tilt of the Orbital. As the Orbital goes around the Sun, the area adjacent to one rim wall will be in the wall’s shadow for approximately half a year while the area adjacent to the other rim wall will receive extra light from sunlight reflected off the rim wall itself. This means that during first half of the year, the area adjacent to one rim wall will tend to be the coolest place on the Orbital and during the second half of the year; the same area will tend to be the warmest place on the Orbital. The opposite is true for the area adjacent to the other rim wall.

Overall, seasonal variations on the Orbital will not be as large as those that occur on the Earth. The Coriolis Effect will not be significant on the Orbital because the only form of Coriolis Effect is the rise and fall of air within the troposphere where most of the weather occurs. With a thickness of around 10 kilometres or so, the depth of the troposphere is insignificant compared to the 3.75 million kilometres diameter of the Orbital. Without major temperature gradients and without the Coriolis Effect, the weather on the habitable inner surface of the Orbital will be gentler and more localized than weather on Earth.

Without a moon, tidal effects on the Orbital will be weaker than those on Earth since the only form of tides on the Orbital will be those generated by the Sun. With no contrasting cold polar oceans and warm equatorial oceans like those found on Earth, natural circulation between surface waters and deep ocean waters cannot be established. Without such a circulation system, deep ocean waters will become anoxic. In order to maintain rich oxygen-bearing waters throughout the entire depth of the oceans, artificial heating can be applied to the deep ocean waters at specific locations on the ocean floor to keep the circulation running. Additionally, the underwater topology can be carefully sculptured to enable the mixing of surface water with deep ocean water from Sun-driven tidal effects alone.

Towering walls similar to those found rimming the structure’s edges or exceedingly high mountain ranges called ‘bulkhead ranges’ can be used to contain and isolated alien environments and ecosystems that are very different from the standard Earth-like environment on the Orbital. This is because the immense heights of these walls or ‘bulkhead ranges’ keep in the atmosphere that they surround. Entire pristine prehistoric worlds that are populated by once extinct creatures can also be enclosed within these colossal barriers. Panoramic views of the surroundings from these ‘bulkhead ranges’ will be especially breathtaking as these mountains tower hundreds of kilometres above the surroundings. In comparison, Earth’s Mount Everest is only 8848 meters in height. On the immense scale of the Orbital, mountains that far surpass the height of Earth’s Mount Everest or Mars’ Olympus Mons will appear as almost indistinguishable bumps on such an exceedingly vast landscape.

‘Bulkhead ranges’ and other mountains of comparable heights rise far above the atmosphere and their summits are exposed to the silent vacuum of space. These mountains are rather interesting as they rise through the full extent of the troposphere, stratosphere and mesosphere. In fact, these mountains are so high that they rise well above the ozone layer and even above the high-flying noctilucent clouds. The summit environments of these mountains are basically bare rock exposed to the vacuum of space. In order to reduce the mass of material required for such mountains, the interior bulk of these mountains can be made mostly hollow and be supported by ultra-strong diamondoid foam or other forms of exotic materials that have very low densities and very high strengths.

Journeying to a distant part of the Orbital will be a challenge due to the sheer size of the Orbital. Even for someone cruising at a speed of 10 km/s onboard a high speed vacuum tube maglev train, it will still take almost 2 weeks to circumnavigate the entire Orbital. As a result, rapid transit to far-off places on the Orbital will require technologies similar to those employed for large scale commercial interplanetary space travel. Furthermore, interplanetary space voyages disembarking from the Orbital’s rim will be much simpler since a spacecraft released from the outer surface of the Orbital will already be travelling at a speed of 135 km/s.

An advanced technological civilization is likely to view the construction of an Orbital as a very attractive mega-engineering project. Apart from the stunning vistas, an Orbital provides hundreds of times more habitable area than an Earth-size planet for an Earth-mass worth of building material. This makes an Orbital a lot more ‘efficient’ than a planet since it offers many times more habitable area per unit mass of material.

Although building such a megastructure is far beyond current technology, the search for Orbital-like megastructures around other stars can be a form of non-conventional search for extraterrestrial intelligence. NASA’s Kepler space telescope is a planet-hunting instrument which monitors the brightness of over a hundred thousand stars simultaneously and looks for tiny dips in a star’s brightness that may be indicative of a transiting planet. An Orbital-like structure transiting a star should produce a transit lightcurve that is different from one produced by a transiting planet.

Thursday, March 14, 2013

Dual-Shock Quark Nova

A super-luminous supernova (SLSN) is a class of supernova whose peak luminosity is several times larger than a typical supernova. Mechanisms that can give rise to a SLSN include the explosion of a very massive star in a pair-instability supernova, the interaction of supernova ejecta with circumstellar matter or a dual-shock quark nova (dsQN) event. SN 2006oz is currently the only SLSN known to exhibit a double-humped lightcurve that is consistant with a dsQN model. The lightcurve of SN 2006oz can be explained by a quark nova occurring 6.5 days after a core-collapse supernova explosion. A dsQN event like SN 2006oz is very rare since it is estimated to occur at a rate of 1 in every 10,000 core-collapse supernovae.

In a dsQN model, a massive star explodes in normal core-collapse supernova and leaves behind a rapidly-spinning, high-mass neutron star. As the neutron star spins down, its central density gradually increases. This eventually leads to a detonative phase transition known as a quark nova, where the neutron star violently converts into a quark star. During this process, the neutron star’s outer layer is ejected at ultra-relativistic velocities. An enormous amount of kinetic energy is carried away since the quark nova ejecta consists of ~100 Earth masses of material travelling close to the speed of light.

Although the quark nova occurs several days after the core-collapse supernova, ejecta from the quark nova travel many times faster than the supernova ejecta. The quark nova ejecta rapidly catch up and collide with ejecta from the preceding supernova. This re-shocks the supernova ejecta and leads to a rise in luminosity over an extended period of time. As a result, a SLSN consisting of a normal core-collapse supernova followed by a quark nova is characterised by a double-humped lightcurve. The fainter first hump corresponds to the core-collapse supernova while the brighter second hump corresponds to the re-shocked supernova ejecta.

A double-humped lightcurve indicative of a dsQN is only produced when the quark nova happens ~10 days after the core-collapse supernova. If the time interval between the supernova and quark nova is too long, the supernova ejecta would have dissipated so much that the quark nova basically occurs in isolation. In contrast, if the time interval is too short, the two lightcurves would overlap and prevent a distinct double-hump.

1. D. Leahy and R. Ouyed (2013), “Double-humped Super-luminous Supernovae”, arXiv:1303.2047 [astro-ph.HE]
2. Giorgos Leloudas et al. (2012), “SN 2006oz: rise of a super-luminous supernova observed by the SDSS-II SN Survey”, arXiv:1201.5393 [astro-ph.SR]

Tuesday, March 12, 2013

A Nearby Pair of Brown Dwarfs

NASA’s Wide-field Infrared Survey Explorer (WISE) is a space telescope which performed an all-sky astronomical survey in the infrared-wavelength. One of its mission objectives was to detect a class of cool and dim objects known as brown dwarfs. In fact, data from WISE has allowed the discovery of a bonanza of such objects. A brown dwarf is more massive than a planet but is short of being a “true” star since it is not massive enough to fuse hydrogen in its core to produce energy. Brown dwarfs do not produce much visible light. Instead, they are warm and glow in the infrared. Because brown dwarfs are extremely faint, some of them may lie as close as the nearest stars and still remain undiscovered.

A paper published in the Astrophysical Journal Letters announced the discovery of a pair of brown dwarfs located at a mere 6.5 light years from the Sun. This binary brown dwarf system is known as WISE 1049-5319 and the discovery was made by Kevin Luhman, an associate professor of astronomy and astrophysics at Pennsylvania State University. WISE 1049-5319 is the third closest star system to us, after the Alpha Centauri triple-star system at about 4.3 light years away and Barnard’s Star at about 6 light years away.

 This diagram illustrates the locations of the star systems that are closest to the Sun. Credit: Janella Williams, Penn State University.

The closeness of WISE 1049-5319 was revealed by its large proper motion. Since proper motion is the observed change in position of an object over time, nearby stars have large proper motions while stars further away have smaller proper motions. This is like looking out from a car window and seeing nearby trees fly by while distant mountains appear almost motionless. The large proper motion of WISE 1049-5319 was revealed in older images taken between the years 1978 to 1999 from the Digitized Sky Survey (DSS), the Two Micron All-Sky Survey (2MASS) and the Deep Near-Infrared Survey of the Southern Sky (DENIS).

WISE 1049-5319 shows up as a single object in the WISE data and in older images. Its binary nature was only revealed from follow-up observations using the large Gemini telescope on the night of 23 February 2013. The sharper image from Gemini shows WISE 1049-5319 as two brown dwarfs separated from each other by approximately 3 times the Earth-Sun distance. It is estimated that the two brown dwarfs circle each other every 25 years or so.

Image of WISE 1049-5319 from the WISE satellite and the Gemini imagery (inset) that revealed it to be a binary system. Credit: NASA/JPL/Gemini Observatory/AURA/NSF

The close proximity of WISE 1049-5319 makes it a unique target for the detection of planets around it. Furthermore, the signature of a planet orbiting a brown dwarf is more easily detectable than for a planet orbiting a star because a brown dwarf is much fainter and less massive than a star. In the distant future, WISE 1049-5319 may turn out to be one of the first few destinations for interstellar exploratory missions beyond our solar system. The presence of planets around WISE 1049-5319 will make it an even more interesting target. Recall that in late 2012, an Earth-sized planet was discovered in a close-in orbit around one star in the Alpha Centauri triple-star system.

K. L. Luhman (2013), “Discovery of a Binary Brown Dwarf at 2 Parsecs from the Sun”, arXiv:1303.2401 [astro-ph.GA]

Sunday, March 10, 2013

Detecting the Constructs of Extraterrestrial Civilizations

A planet that happens to pass in front of its parent star along an observer’s line-of-sight will block a small fraction of the star’s light and cause a dip in the observed brightness of the star. As a result, planets can be detected around a star by precisely monitoring the star’s brightness to search for periodic dips in the star’s brightness that may signal the presence of a planet in orbit around the star. This method of planet detection is known as the transit method and it is used by space telescopes such as Kepler and Corot to search for planets around other stars. It becomes reasonable to consider that the transit method can also be employed to search for artificial planetary-sized objects in orbit around other stars.

An artificial planetary-sized object can take on any shape and should produce a transit light curve with a profile that is different from one produced by a spherical object such as a planet. The detection of an artificial transit can serve as definite proof for the existence of an advanced technological civilization. Such a large object can be constructed by an advanced civilization to communicate its existence or for an entirely different purpose (e.g. a large collecting surface to harness energy from its parent star). Artificial transits are an ideal method through which an advanced civilization is likely to communicate its existence because such an object is potentially able to last a lot longer than the lifetime of the civilization itself.

Multiple artificial planetary-sized objects in transit around a single star may turn out to be much more attention-grabbing than the transit of a single object. To create multiple transits, the objects involved do not need to have non-spherical shapes to communicate their artificial nature. This is because the transit of each object can be orchestrated in a remarkably regular fashion (e.g. grouped in prime numbers) to create a series of transits whose artificial nature is obvious. As a result, a technological civilization that wants to communicate its presence may find it more appropriate to construct a series of smaller spherical objects than a single large non-spherical object.

In October 2012, astronomer Geoff Marcy was awarded a grant to search through data collected from the Kepler space telescope with the aim of finding possible signatures of megastructures created by advanced technological civilizations. This fits well with what astronomer Jill Tarter mentioned in a paper published in 2001: “An advanced technology trying to attract the attention of an emerging technology, such as we are, might do so by producing signals that will be detected within the course of normal astronomical explorations of the cosmos. Sooner or later the emerging technology will build the proper instruments to observe their surroundings and capture the signal.”

1. Arnold L. (2005), “Transit Light-Curve Signatures of Artificial Objects”, Astrophysical Journal, 627:534-539
2. Jill Tarter, “The Search for Extraterrestrial Intelligence (SETI)”, Annual Review of Astronomy and Astrophysics, Vol. 39: 511-548 (September 2001)

Saturday, March 9, 2013

The Universe’s Brightest Explosions

Supernovae (plural of supernova) are the most luminous stellar explosions in the universe. Over a period of several days to several months, a typical supernova can produce as much energy as the Sun emits during its entire multi-billion year life span. A supernova can be created either by runaway nuclear fusion in a white dwarf star or by the sudden gravitational collapse of the core of a massive star. There is a rare type of supernova known as a pair-instability supernova and such a supernova can blaze up to around 100 times more luminous than a typical supernova. This makes pair-instability supernovae the brightest stellar explosions known in the universe.

Pair-instability supernovae only happen for very massive stars (140 to 260 solar mass) that have a very low abundance of elements heavier than hydrogen and helium. The low abundance of heavier elements reduces mass loss and keeps these stars sufficiently massive to eventually explode as pair-instability supernovae. Pair instability occurs when the thermal energy in the core of a massive star becomes large enough to produce electron-positron pairs. This process reduces radiation pressure that supports the star and causes the star to contract. The situation rapidly runs out of control as the contraction triggers an explosive thermonuclear burning of oxygen and silicon in the star’s core which occurs over a span of just a few seconds. More thermal energy is released than the star’s own gravitational binding energy and this eventually destroys the star, leaving no black hole or any other remnant object behind.

During the explosive thermonuclear burning event, a significant fraction of the star’s core is transformed into radioactive nickel-56. For the most massive stars, up to 40 solar mass of nickel-56 can be synthesized. Nickel-56 is a radioactive isotope which decays with a half-life of 6.1 days into cobalt-56 which then further decays with a half-life of 77.2 days into stable iron-56. The exceptionally large amount of nickel-56 being produced in a pair-instability supernova means that the radioactive decay of nickel-56 into iron-56 powers a very luminous light curve which lasts for a few hundred days. In comparison, the luminosity of a typical supernova only lasts for up to 100 days or so.

For a star between 100 to 140 solar mass, a true pair-instability supernova does not occur. Instead, the star undergoes a “pulsational pair-instability supernova”. In this case, the explosive thermonuclear burning is insufficient to completely unbind the star is it produces less energy than the star’s gravitational binding energy. Nevertheless, many solar masses of material are still ejected from the star. The core of the star then contracts into a stable burning state before the next explosion occurs. A “pulsational pair-instability supernova” can be extremely luminous as the highly energetic ejecta from the second supernova plows into the ejecta from the first supernova. Finally, a star with over 260 solar mass does not explode as a pair-instability supernova since the temperature in the core of such a star becomes high enough for alpha particles to photo-disintegrate into free nucleons. This process consumes as much energy as that produced by all the preceding thermonuclear burning. As a result, gravity prevails and the star collapses into a black hole.

Population III stars are the first stars to form in the universe. These stars consist only of hydrogen and helium since heavier elements have yet to be synthesized by nuclear fusion in stars. The absence of heavier elements makes it easier to form stars of higher masses than those existing in the current universe. Population III stars between 140 to 260 solar mass serve as suitable progenitors of pair-instability supernovae. Since Population III stars are expected to exist only during the beginning of the universe, they can only be found at the edge of the observable universe. As a result, observations of pair-instability supernovae from Population III stars will be affected by a large amount of cosmological time dilation. Together with the intrinsically slow light curve evolution of pair-instability supernovae, observations of these enormous stellar explosions will require multi-year baselines.

Friday, March 1, 2013

Measuring a Black Hole’s Spin

NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency’s XMM-Newton are two X-ray space telescopes that have teamed up to measure the spin of a 2 million solar mass supermassive black hole at the centre of the galaxy NGC1365. The observations were conducted simultaneously on July 2012 and provided the first ever definitive measurement of a supermassive black hole’s spin. In the region near the supermassive black hole in NGC1365, there is a pair of bipolar jets consisting of energetic particles that travel at very close to the speed of light. At the base of each jet, X-rays is produced which reflects off the surrounding accretion disk and makes the accretion disk observable in X-rays. By measuring how fast matter is swirling around the black hole in the inner region of the accretion disk, the spin rate of the black hole was determined.

Figure 1: This artist’s concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Supermassive black holes are enormously dense objects buried at the hearts of galaxies. Image credit: NASA/JPL-Caltech

The gravitational radius or the event horizon of a black hole is a region of space around a black hole where the gravity is sufficiently strong to prevent even light from escaping. This region can be seen as a “point of no return” for any object that enters. The innermost stable circular orbit around a black hole defines the inner edge of the black hole’s accretion disk. How close the inner edge of an accretion disk can get to a black hole’s gravitational radius depends on the black hole’s spin rate. For a black hole with a faster spin rate, the inner edge of its accretion disk can exist closer to its gravitational radius.

The incredible spin of the supermassive black hole in NGC1365 allows the innermost edge of the accretion disk to exist within just 2.5 gravitational radii. Being so close to the black hole’s gravitational radius, the intense gravity warps the fabric of space-time and distorts the X-ray emission from the accretion disk. This distortion is observable and is what allows the spin rate of the supermassive black hole to be measured. From the observed distortion in X-ray emission, the spin of the supermassive black hole in NGC1365 is estimated to be 84 percent as fast as Einstein’s theory of general relativity would allow. Although the spin of a black hole does not translate effectively into units of speed like kilometres per hour, it is safe to say that this supermassive black hole is spinning incredibly rapidly and twisting the fabric of space-time around it.

Figure 2: An illustration showing how the spin and intense gravity of a black hole warps the fabric of space-time and distorts the X-ray emission from the accretion disk. Image credit: NASA/JPL-Caltech

Supermassive black holes are known to acquire most of their rotation as they grow and studying how they spin allows their growth and evolution to be better constrained. The ultra-fast spin of the supermassive black hole in NGC1365 shows that it did not grow from the capture of smaller black holes in randomly-oriented orbits as this is very unlikely to have spun up the black hole in the same direction. Instead, the supermassive black hole is likely to have acquired its ultra-fast spin from a merger event with a comparable mass black hole or the accretion of material from a disk around it. Both scenarios would tend to spin up the black hole in the same direction.

G. Risaliti et al., “A rapidly spinning supermassive black hole at the centre of NGC 1365”, Nature 28 February 2013; doi:10.1038/nature11938
Volonteri, M., Madau, P., Quataert, E. & Rees, M. J., “The distribution and cosmic evolution of massive black hole spins”, Astrophys. J. 620, 69–77 (2005)