Wednesday, July 30, 2014

Largest Mass of Water in the Known Universe

In July 2011, a team of astronomers reported on the discovery of the largest reservoir of water ever detected in the universe in a distant quasar identified as APM 08279+5255. A quasar is an extremely luminous object that is powered by a supermassive black hole accreting material at a stupendous rate. APM 08279+5255 sits in the core of a giant elliptical galaxy that is located at a distance of 12 billion light years, near the “edge” of the known universe. What this means is that light observed from APM 08279+5255 left it 12 billion years ago, at a time when the universe is still relatively young since the universe itself is only 13.8 billion years old.

Figure 1: Artist’s impression of a quasar, similar to APM 08279+5255. Image credit: NASA/ESA.

APM 08279+5255 is incredibly luminous. Its radiant power is estimated to be roughly equivalent to a thousand trillion Suns. At its heart is a supermassive black hole whose mass is estimated to be a whopping ~20 billion times the Sun’s mass. Observations of APM 08279+5255 were performed in the millimetre waveband using the Z-Spec instrument at the Caltech Submillimeter Observatory (CSO) and the Combined Array for Research Millimetre-Wave Astronomy (CARMA). The observations revealed an enormous mass of water vapour swirling around this cosmic monstrosity. The mass of water vapour is distributed around the supermassive black hole in a gaseous region spanning a few hundred light years across. Measurements show that the total mass of water vapour around the quasar is at least ~100,000 times the mass of the Sun. That is over ~100 trillion times the amount of water in Earth’s oceans.

In a typical galaxy like the Milky Way, most of the water is frozen as ice. In the case of APM 08279+5255, the immense amount of energy being put out by the quasar keeps the water in its gaseous phase. Water-ice sublimates into water vapour when it is heated above ~100 K. In fact, the water vapour in APM 08279+5255 is observed to have temperatures ranging from 100 to 650 K. The energised mass of water vapour is continuously cooling; generating a total observed cooling luminosity of at least ~6.5 billion times the Sun’s luminosity. This discovery shows that water is common throughout the universe and it can occur in ginormous masses even in the early universe.

Figure 2: Comparison of the water spectrum in Mrk 231 with that in APM 08279+5255, as measured with Z-Spec. Bradford et al. (2011).

- Bradford et al. (2011), “The Water Vapor Spectrum of APM 08279+5255: X-Ray Heating and Infrared Pumping over Hundreds of Parsecs”, arXiv:1106.4301 [astro-ph.CO]
- Riechers et al. (2008), “Imaging the Molecular Gas in a z=3.9 Quasar Host Galaxy at 0.3" Resolution: A Central, Sub-Kiloparsec Scale Star Formation Reservoir in APM 08279+5255”, arXiv:0809.0754 [astro-ph]

Tuesday, July 29, 2014

Observations of a Low-Gravity Brown Dwarf

Brown dwarfs occupy the mass range between the most massive planets and the least massive stars. They are not massive enough to sustain hydrogen fusion in their cores and so they cool gradually with time. As a brown dwarf cools, it contracts, evolving from a low to a high “surface” gravity. The word “surface” is shown in quotation because a brown dwarf does not have a solid surface. Instead, a brown dwarf’s “surface” simply refers to its observable photosphere. Brown dwarfs are gaseous throughout. They are primarily composed of hydrogen and helium, with trace amounts of heavier elements.

Figure 1: Artist impression of a brown dwarf that is still glowing red-hot from heat acquired during its formation.

For a young brown dwarf that has yet to cool and contract to its final radius, it can display low-gravity features that can be identified from observations in the near-infrared waveband. This is because the low-gravity atmosphere of a young brown dwarf drives the formation of thicker than normal clouds in the brown dwarf’s photosphere. The thicker clouds give rise to a redder near-infrared spectrum because shorter wavelength light (i.e. bluer light) is attenuated and scattered by clouds more than longer wavelength light (i.e. redder light). As a result, a brown dwarf with a redder near-infrared spectrum can signify its youthfulness.

Gagné et al. (2014) present the discovery of SIMP J2154-1055 - an L4 spectral type brown dwarf displaying signs of low-gravity in its near-infrared spectrum. SIMP J2154-1055 has a redder near-infrared spectrum compared with other known L4 brown dwarfs. This is consistent with the presence of thicker than normal clouds in a low-gravity photosphere. SIMP J2154-1055 has a good probability of being part of the Argus Association, a loose group of stars with similar ages. If it is indeed a member of the Argus Association, SIMP J2154-1055 would be roughly 30 to 50 million years old and its mass would be ~10 times the mass of Jupiter, indicating that SIMP J2154-1055 is a relatively young and low-mass brown dwarf.

Figure 2: Near-infrared spectrum of SIMP J2154-1055 compared with other known L4 brown dwarfs. SIMP J2154-1055 is the reddest L4 brown dwarf yet identified. All spectra are normalized to their median in the 1.27 to 1.33 μm range. Gagné et al. (2014).

Gagné et al. (2014), “SIMP J2154-1055: A New Low-Gravity L4β Brown Dwarf Candidate Member of the Argus Association”, arXiv:1407.5344 [astro-ph.SR]

Monday, July 28, 2014

Discovery of a Stellar Behemoth in Westerhout 49

Very massive stars with masses exceeding ~100 times the Sun’s mass are incredibly rare. Nevertheless, these stars have strong influence on their environment through their powerful winds and prodigious amounts of ionizing radiation. The existence of very massive stars with reported masses of up to ~300 times the Sun’s mass indicate that there may not be a ‘real’ upper mass limit for very massive stars. These very massive stars are found in massive star-forming clusters such as NGC 3603, the Arches cluster and R136 in the Large Magellanic Cloud.

Artist’s impression of the Sun in comparison to R136a1 - a very massive star with an estimated ~300 times the Sun’s mass at birth. Image credit: ESO/M. Kornmesser.

A study by Shiwei Wu et al. (2014) presents the spectroscopic identification of a very massive star in the heart of the star-forming region Westerhout 49, hereafter, referred to as W49. The central cluster of W49 consists of dozens of massive OB-type stars. Intervening interstellar dust greatly obscures the massive stars in W49. As a result, these stars can only be observed in the near-infrared waveband. Referred to as W49 nr1, this very massive star resides in the heart of W49. It was spectroscopically identified using instruments on three telescopes - the Very Large Telescope (VLT) at Paranal in Chile, the New Technology Telescope (NTT) at La Silla in Chile, and the Large Binocular Telescope (LBT) at Mount Graham in Arizona.

W49 nr1 is the brightest star in the central cluster of W49 and it is classified as an O2-3.5If* star. Its estimated effective temperature is between 40,000 and 50,000 K, and its estimated luminosity is between 1.7 million and 3.1 million times the Sun’s luminosity. W49 nr1is more luminous than tens of billions of the coolest red dwarf stars put together. Stellar evolutionary models of W49nr1 suggest an initial mass of 100 to 180 times the Sun’s mass. If more variations are included in the models, the initial mass range is 90 to 250 times the Sun’s mass. Very massive stars live fast and die young. W49 nr1 is believed to be no more than 3 million years old.

Shiwei Wu et al. (2014), “The Discovery of a Very Massive Star in W49”, arXiv:1407.4804 [astro-ph.SR]

Sunday, July 27, 2014

An LBV Masquerading as a Cool Hypergiant

The search for supernovae (plural for supernova) has led to the discovery of a population of “supernova impostors”. These outbursts appear like supernovae, but exhibit much lower luminosities and ejecta velocities. A study by Mauerhan et al. (2014) presents observations of a supernova impostor identified as SN Hunt 248. The outburst associated with SN Hunt 248 was observed in May to June 2014 and it occurred in two stages. Between May 21and June 3, the source brightened slowly. On June 4, it began to brighten rapidly, reaching a peak on June 16. The source then plateaued for ~10 days at peak brightness before fading away.

Analysis of the photometric and spectroscopic data indicates that SN Hunt 248 is consistent with an outburst from a massive star. Archival images from the Hubble Space Telescope between 1997 and 2005 reveal that the precursor star is a cool hypergiant with ~400,000 times the Sun’s luminosity and ~32 times the Sun’s mass. SN Hunt 248 is believed to be the first outburst observed from a cool hypergiant that is similar to the giant eruptions typical for luminous blue variable stars (LBVs).

Figure 1: Artist’s impression of a hypergiant.

Figure 2: Light curve of SN Hunt 248 compared with other supernova impostors, including SN 1997bs, SN 2006jc, SN 2002bu, SN 2009ip and SN 2008S. The vertical axis indicates the absolute magnitude and the horizontal axis indicates the number of days relative to peak brightness. Mauerhan et al. (2014).

The slow rise in brightness followed by an episode of rapid brightening indicates that something sudden took place. LBVs and cool hypergiants are known to have strong winds that launch stellar material off them, causing them to be surrounded by circumstellar material of their own. In the case for SN Hunt 248, the initial phase of slow brightening is from the initial outburst. Subsequently, ejecta from the initial outburst collide into the circumstellar material around the cool hypergiant, resulting in the sudden conversion of kinetic energy into radiation. This explains the rapid brightening following the initial phase of slow brightening. Rough estimates suggest an ejecta mass of ~1 times the Sun’s mass and an ejecta velocity of ~1000 km/s.

In fact, the precursor star of SN Hunt 248 is probably a LBV masquerading as a cool hypergiant. This is because there seems to be an absence of LBVs with effective temperatures in the range between 15,000 K to 21,000 K (Figure 3). It has been proposed that at these temperatures, the stellar winds emanating from LBVs become considerably denser. The density can be high enough to produce an opaque pseudo-photosphere around the LBV, allowing the LBV to masquerade as a cool hypergiant with an effective temperature of less than ~8,500 K. If such a connection between LBVs and cool hypergiants is true, then one might expect some cool hypergiants to also exhibit giant outbursts like LBVs do.

Figure 3: Hertzsprung-Russell (HR) diagram for LBVs and related stars, including SN Hunt 248 (purple square). The diagonal and vertical grey strips illustrate the regions of the S Doradus instability strip and the minimum temperature strip for classical LBVs near visual maximum. Mauerhan et al. (2014).

Mauerhan et al. (2014), “SN Hunt 248: a super-Eddington outburst from a massive cool hypergiant”, arXiv:1407.4681 [astro-ph.SR]

Saturday, July 26, 2014

Signature of a Giant Planet’s Rocky Core

Stars in binary systems generally have identical chemical compositions since they formed from the same natal cloud of material. Nevertheless, small differences in chemical composition can exist between a pair of stars in a binary system and one explanation is the process of planet formation. When planets form around a star, it can cause the star to be slightly depleted in heavy elements (i.e. elements heavier than hydrogen and helium) compared to its companion star.

Observations of 16 Cygni, a binary system comprised of two stars 16 Cygni A and 16 Cygni B (hereafter components A and B), reveal that component B has a giant planet with at least 1.5 times Jupiter’s mass. The giant planet, identified as 16 Cygni Bb, orbits its host star in a highly-eccentric 800-day orbit. At its minimum and maximum distances from its host star, the giant planet receives, respectively, 4.4 and 0.16 times the amount of insolation Earth gets from the Sun. Being a giant planet, 16 Cygni Bb is composed primarily of hydrogen and helium, much like Jupiter.

Figure 1: Artist’s impression of a giant planet with a system of moons around it.

Figure 2: Differences in heavy element abundance between components A and B versus condensation temperature. The dashed line is the average of the volatiles and the solid line is the average of the refractories. The dot dashed line is the mean trend for 11 Sun-like stars compared to the Sun. Maia et al. (2014).

A study by Maia et al. (2014) show a small difference in the abundance of heavy elements (i.e. metallicity) between components A and B of 16 Cygni. Component A has an overall metallicity (i.e. abundance of heavy elements) that is 0.047 ± 0.005 dex higher than the metallicity of component B. The abundance differences range from 0.03 dex for volatiles (i.e. elements such as carbon and oxygen), and up to 0.06 dex for refractories (i.e. elements such as iron, vanadium and magnesium). The lower abundance of heavy elements in component B is likely due to the formation of the giant planet that is presently in orbit around it, where the “missing” heavy elements were used to form the giant planet.

The higher deficiency in refractories compared to volatiles in component B means that the giant planet, 16 Cygni Bb, has a corresponding excess of refractories. This suggests that 16 Cygni Bb formed by the core accretion mechanism where an initial rocky core, comprised primarily of refractories, becomes massive enough to start accreting hydrogen, helium and other volatiles to form a giant planet. Estimates of the initial rocky core of 16 Cygni Bb places it at around 1.5 to 6 times the mass of Earth, consistent with Jupiter’s core mass. These findings validate the core accretion model for the formation of 16 Cygni Bb and offer yet another means to examine the relationship between a star and its planet.

Maia et al. (2014), “High precision abundances in the 16 Cyg binary system: a signature of the rocky core in the giant planet”, arXiv:1407.4132 [astro-ph.SR]

Friday, July 25, 2014

Globular Clusters and Dark Satellite Galaxies

Globular clusters are dense spherical collections of stars. Every large galaxy, such as the Milky Way, contains a system of globular clusters. Observations of globular clusters show that they do not contain gravitationally bound dark matter. Most of the matter in the universe is in the form of dark matter. Dark matter neither emits nor absorbs light, and its presence can only be inferred from its gravitational effects on normal matter and radiation. Nevertheless, the existence of dark matter is important because it provides the gravitational framework for normal matter to come together to form galaxies and clusters of galaxies. As a result, it remains a challenge to explain how normal matter could gravitate so tightly together to form globular clusters.

The globular cluster NGC 1806 located within the Large Magellanic Cloud as observed by the Hubble Space Telescope. Image credit: ESA/Hubble & NASA.

A study by Noaz & Narayan (2014) suggests that globular clusters can form naturally whenever there is some relative velocity between normal matter and dark matter. In this scenario, the formation of a globular cluster begins with a collapsing clump of normal matter in a dark matter halo which is itself also collapsing. The gravity that is driving the collapse comes mostly from dark matter. However, the collapsing clump of normal matter eventually finds itself outside the dark matter halo due to the relative velocity between the normal matter and dark matter components. If the relative velocity is small, then the clump of normal matter remains in the dark matter halo and forms a typical dwarf galaxy with somewhat comparable proportions of normal matter and dark matter, albeit more dark matter.

As a consequence of the relative velocity between the normal matter and dark matter components, the collapsing clump of normal matter becomes a long-lived dark matter-free gravitationally self-bound object (i.e. a globular cluster). Such a clump of normal matter can have a mass ranging from roughly a hundred thousand to a few million times the Sun’s mass, consistent with the masses of present-day globular clusters. On the contrary, the corresponding dark matter halo, depleted of normal matter, could become a dark satellite galaxy or an ultra-faint satellite galaxy. Such a galaxy would be comprised almost entirely of dark matter and would contain extremely few stars, possibly none at all, since stars are made of normal matter.

Noaz & Narayan (2014), “Globular Clusters and Dark Satellite Galaxies through the Stream Velocity”, arXiv:1407.3795 [astro-ph.GA]

Thursday, July 24, 2014

Ultra-Dense Ocean on a Neutron Star

A neutron star is an ultra-dense remnant core leftover from the violent demise of a massive star. It packs roughly as much mass as the Sun in an incredibly tiny volume measuring just several kilometres across. A spoonful of its material would contain a mass of roughly a billion tons. If the neutron star has a sufficiently close stellar companion, it can strip material from the companion in a process known as accretion. The accreted material can lead to the formation of an ocean on the neutron star. This ultra-dense and exotic ocean is comprised of elements with atomic number Z = 6 and larger. Most of these elements are formed from nuclear burning of the accreted hydrogen and helium from the companion star. Here, the ions behave like a liquid, hence the term “ocean”. Nonetheless, it is in no way like the oceans on Earth. The densities, pressures and temperatures are so extreme that they are only comprehensible numerically.

Figure 1: Artist’s impression of an accreting neutron star. Material stripped from the companion star forms an accretion disk around the neutron star. Image credit: NASA / Goddard Space Flight Centre / Dana Berry.

The ability to observe the sky in X-rays using space-based instruments has led to the discovery of superbursts. These energetic outbursts recur on timescales of years and are believed to be driven by the unstable ignition of a carbon-enriched layer on a neutron star. To ignite a superburst, a carbon-enriched layer needs to contain a carbon mass fraction of roughly 20 percent. However, such a carbon-enriched layer is difficult to produce in most theoretical models. Besides requiring enough carbon, models for superbursts also require large ocean temperatures of roughly 600 million K. Such high temperatures are difficult to attain from standing heating models of neutron stars.

A study by Medin & Cumming (2011) suggests that the preferential freezing of heavier elements at the base of the ocean on an accreting neutron star can substantially enrich the ocean with lighter elements such as oxygen and carbon. At the base of the ocean, the increasing pressure from the continuous accretion of material onto the neutron star forces the preferential freezing of heavier elements. The separation of lighter elements from heavier elements releases energy and provides an additional source of heating for the ocean. After the preferential freeze-out of heavier elements, the remaining fluid becomes lighter than the fluid immediately above it and acts as a source of buoyancy which drives convective mixing of the ocean. Convection distributes the heat throughout the ocean in the form of a convective flux. The extra heat input can raise the temperature of the ocean up to the required ignition temperature of around 500 to 600 million K to produce a superburst.

In the study, a 300 million K ocean consisting of a mixture of iron (Z = 26) and selenium (Z = 34), and a mixture of oxygen (Z = 8) and selenium (Z = 34) is examined. At the base of the ocean, the preferential freezing of heavier elements enhances the abundances of lighter elements in the ocean. For example, a mixture of oxygen and selenium with initial 2 percent oxygen by mass can be enriched to almost 40 percent oxygen by mass. Although oxygen was chosen as the light element in this study, models with carbon (Z = 6) were also investigated and shown to yield similar enrichment results. The carbon mass fraction can be brought up by enrichment to the required ~20 percent for superburst ignition.

Figure 2: Phase diagram for crystallization of an iron/selenium mixture (top panel) and an oxygen/selenium mixture (bottom panel) in a 300 million K ocean on a neutron star. The stable liquid region of each phase diagram is labelled as “L”, the stable solid region(s) are labelled as “S” or “S1” and “S2”, and the unstable region is filled with plus symbols. Additionally, in each panel the composition at the top of the ocean is marked by a vertical dashed line, the ocean-crust boundary is marked by a horizontal dotted line, the composition of the liquid at the base of the ocean is marked by a filled square, and the composition of the solid(s) in the outer crust are marked by filled circles. Medin & Cumming (2011).

Figure 3: Thermal profile of an ocean on an accreting neutron star. The ocean is composed of a mixture of oxygen and selenium. The solid line represents the thermal profile when the convective flux (i.e. energy released at the base of the ocean from the separation of lighter elements from heavier elements) is included in the total heat flux. The dashed line represents the thermal profile when the convective flux is ignored (i.e. the total heat flux is due only to the heat emanating from the neutron star’s interior). Medin & Cumming (2011).

Medin & Cumming, “Compositionally Driven Convection in the Oceans of Accreting Neutron Stars”, ApJ 730:97 (10pp), 2011 April 1.

Wednesday, July 23, 2014

Could it be a “Q-Star” instead of a Black Hole?

Compact objects fall under two categories - neutron stars or black holes. Neutron stars are the ultra-dense, compact remnant cores of massive stars. They are made almost entire of neutrons and have densities comparable to the density of an atomic nucleus. These neutrons are held together and kept from transmuting back into normal matter by the neutron star’s intense gravity which arises from its extraordinary compactness. A teaspoon of neutron star material would contain a mass of roughly a billion tons. The minimum and maximum mass possible for any neutron star is between ~0.1 and ~3 times the Sun’s mass. Below the minimum mass, the neutron star’s gravity is too weak to hold the star together and the star “decompresses” into normal matter. Above the maximum mass, the neutron star’s gravity becomes sufficiently strong to crush it into a black hole.

Figure 1: Artist’s impression of a neutron star whose intense gravity is lensing light from the background.

Nevertheless, the physics of matter at ultra-high densities remains poorly understood. Bahcall, Lynn & Selipsky (1990) propose that the same type of matter found in a neutron star could be stably confined by an alternative means other than gravity. Such a form of matter, though still considered ultra-dense, would have densities far below what is found in a neutron star. The outcome is that a compact object made of such a form of matter could exceed 3 times the Sun’s mass and would not collapse into a black hole under its own gravity since it is not as compact as a neutron star. These objects are termed “Q-stars”.

Theoretical models by Miller, Shahbaz & Nolan (1997) show Q-stars can be up to several times the Sun’s mass, far above the maximum mass for neutron stars. Furthermore, Q-stars that are several times the Sun’s mass can have radii less than 1.5 times the event horizon radius of a black hole of corresponding mass. Basically, a black hole’s event horizon is a non-physical boundary around a black hole, and within it, gravity is strong enough to keep even light from escaping. Since a black hole does not have a true surface, its event horizon could be regarded as its “surface”.

Figure 2: Radius of a Q-star plotted as a function of its mass. Miller, Shahbaz & Nolan (1997).

A non-rotating Q-star with 12 times the Sun’s mass can have a radius as small as ~52 km. In comparison, a black hole of the same mass would have an event horizon radius of 36 km. This difference is less than a factor of 1.5 and shows that a Q-star can be comparable in size to the event horizon of a black hole of corresponding mass. As a consequence, it may be difficult to observationally determine whether a high-mass compact object with several times the Sun’s mass is a black hole or a Q-star.

One possible method to distinguish a black hole from a Q-star would be to observe the accretion of material by the high-mass compact object. If the object were a Q-star, the accretion flow would eventually intersect the surface. If the accretion flow extends further inwards, closer than what would otherwise be the surface of the Q-star, it would be good evidence that the high-mass compact object is a black hole rather than a Q-star. An example of a known high-mass compact object that could turn out to be a Q-star is V404 Cygni - an object currently thought to be a black hole with ~12 times the Sun’s mass. Even so, one should be mindful that Q-stars are purely theoretical constructs and they may not exist at all.

- Bahcall, Lynn & Selipsky, “New Models for Neutron Stars”, ApJ (1990) 362, 251.
- Miller, Shahbaz & Nolan, “Are Q-stars a serious threat for stellar-mass black hole candidates”, MNRAS (1990) 294: L25-L29.

Tuesday, July 22, 2014

Formation of Binary Giant Planets

Giant planets seem to be ubiquitous around Sun-like stars. Our Solar System has two giant planets - Jupiter and Saturn. Both planets are primarily composed of hydrogen and helium. Jupiter and Saturn have 318 and 95 times the mass of Earth, respectively. Beyond Saturn, the planets Uranus and Neptune are generally classified as “ice giants” because they have much smaller masses and differ considerably in composition compared to Jupiter and Saturn. The orbits of Jupiter and Saturn form a 5:2 orbital resonance. For every five times Jupiter circles the Sun, Saturn would circle the Sun twice. On the whole, the orbits of Jupiter and Saturn are stable over the entire age of our Solar System.

In a planetary system with two giant planets, such as our Solar System, energy and angular momentum are conserved between the two giant planets, and the planetary system is stable. Instability only occurs if the orbits of the two giant planets bring them very close to one another. Exoplanet discoveries over the years have revealed a remarkable diversity of planetary systems. A number of studies have shown that planetary systems with three or more giant planets tend to be unstable. For such a planetary system, perturbations by the additional giant planet(s) tend to destabilise the system.

Figure 1: Artist’s impression of a pair of binary giant planets.

Figure 2: Artist’s impression of a giant planet.

When a planetary system consisting of three or more giant planets is destabilised, it can lead to a number of interesting outcomes. Ochiai et al. (2014) show that gravitationally bounded pairs of giant planets (i.e. binary giant planets) can form via planet-planet scattering during the destabilisation of a planetary system with three giant planets. In their study, N-body simulations of planetary systems with three Jupiter-mass giant planets were performed. The N-body simulations show that as much as ~10 percent of the planetary systems result in the formation of binary giant planets.

During the destabilization of a planetary system with three giant planets, the possible outcomes are - ejection of a planet, planet-planet collision, planet-star collision, formation of a hot-Jupiter and formation of a pair of binary giant planets. A hot-Jupiter forms when a giant planet is thrown inwards to its star whereby planet-star tidal interactions can circularise the orbit of the giant planet into a close-in orbit around the star, leading to the formation of a hot-Jupiter. As for binary giant planets, such a pair could form when two giant planets pass sufficiently close to one another that enough tidal dissipation occurs between them to form a gravitationally bound pair.

In their N-body simulations of planetary systems with three giant planets, Ochiai et al. (2014) used four sets of 100 simulation runs corresponding to the four different initial stellarcentric semimajor axes - 1, 3, 5 and 10 AU for the innermost giant planet. In the nomenclature, “stellarcentric semimajor axis” refers to the average distance of the giant planet from its host star and 1 AU is a unit of measurement equal to the average Earth-Sun separation distance. For the two outer giant plants, their semimajor axes are, respectively, factors of 1.45 and 1.9 times the semimajor axis of the innermost giant planet. The four sets of 100 runs follow the evolution of the planetary system over a period of 10 million years.

The results from the 400 simulation runs show that the formation rate of binary giant planets is ~10 percent and nearly independent of the stellarcentric semimajor axis. Binary giant planets generally form near their initial orbits because the period when they form is normally during the early stages of orbital instability. Regardless of the initial stellarcentric semimajor axes, the distribution of the semimajor axes of the binary giant planets (i.e. average distance between the two giant planets in the binary) show a peak at 2 to 4 times the combined planetary radii of the two giant planets in the binary. Also, the 400 simulation runs show that ejection rates increase and collision rates decrease as stellarcentric semimajor axis increases.

Figure 3: Distribution of the semimajor axes of the binary giant planets obtained from the 400 simulation runs. For each pair of binary giant planets, the semimajor axis is expressed as a ratio to the combined planetary radii of the two giant planets in the binary. Ochiai et al. (2014).

Figure 4: Results obtained from the 400 simulation runs for the four different initial stellarcentric semimajor axes - 1, 3, 5 and 10 AU. The colours represent binary giant planets (red), planet-planet or planet-star collisions (light green), hot-Jupiters (blue), ejections (magenta), and three giant planets still remaining after 10 million years (light blue). Ochiai et al. (2014).

Binary giant planets are expected to be stable over the long-term. If the stellarcentric semimajor axis of a pair of binary giant planets is larger than ~0.3 AU, the system is stable for ~10 billion years, which is similar in duration to the main-sequence lifespan of a Sun-like star. Interestingly, binary giant planets can have moons with wide orbits that circumscribe both planets. A loosely bound moon around one of the two giant planets has a roughly 20 percent chance of surviving the formation process leading to a pair of binary giant planets. Additionally, binary giant planets can also capture large moons into orbit around them, much like how Neptune captured its large moon Triton. Current planet detection methods might be able to detect binary giant planets.

Ochiai et al., “Extrasolar Binary Planets. I. Formation by Tidal Capture during Planet-Planet Scattering”, ApJ 790:92 (10pp), 2014 August 1

Monday, July 21, 2014

Kepler-421b: A Uranus-Sized Planet near the Snow-Line

“In future, children won’t perceive the stars as mere twinkling points of light: they’ll learn that each is a ‘Sun’, orbited by planets fully as interesting as those in our Solar System.”
- Martin Rees

A protoplanetary disk is a circumstellar disk of material around a young star in which the formation of planets occurs. The snow-line marks the distance from the central star where the protoplanetary disk becomes cool enough for volatiles such as water to condense into solid ice grains. By analysing publicly available data from NASA’s Kepler space telescope, Kipping et al. (2014) present the discovery of a cold transiting planet near the snow-line. This planet, identified as Kepler-421b, is the first of its kind to be discovered. It is similar in size to Uranus and it circles a star that is slightly cooler than the Sun in a nearly-circular orbit with an orbital period of 704.2 days. Kepler-421b is the longest period transiting planet discovered to date.

Figure 1: Artist’s impression of a Uranus-like planet with a large moon in orbit around it.

Figure 2: Transit light curve of Kepler-421b. Based on how much light it blocks when it passes in front its parent star, Kepler-421b is estimated to be ~4 times the Earth’s diameter, roughly the size of Uranus. Kipping et al. (2014).

“Finding Kepler-421b was a stroke of luck,” says lead author David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA). “The farther a planet is from its star, the less likely it is to transit the star from Earth’s point of view. It has to line up just right.” Kepler-421b is ~1.2 AU from its parent star. At that distance, the planet is closer to its parent star than Mars is from the Sun. Since its parents star is only ~40 percent as luminous as the Sun, Kepler-421b receives only ~64 percent of the insolation Mars gets from the Sun, or ~28 percent of the insolation Earth gets from the Sun. Kepler-421b receives the same amount of insolation as an object at ~2 AU from the Sun. If Kepler421b has a Uranus-like albedo, the planet’s effective temperature would be ~180 K. For comparison, Earth has a mean surface temperature of 288 K, or 15°C.

Assuming Kepler-421b has a Uranus-like composition (i.e. an ice giant), the planet probably formed at its current distance from its parent star (i.e. in situ formation). At that distance, it is cool enough for icy planetesimals to form in the protoplanetary disk, eventually leading to the creation of an ice giant. The large orbital period of 704.2 days means that transits of Kepler-421b are relatively infrequent. In fact, only two transits have been observed so far and those were sufficient to result in its initial detection. Unfortunately, the 3rd transit occurred in March 2014, after Kepler’s primary mission. Nevertheless, the 4th transit opportunity is in February 2016. Kepler-412b is the first known transiting Uranus-sized planet in a long-period orbit. Determining its mass and finding more of its kind would be the next logical steps. Finally, the large distance of Kepler421b from its parent star makes it an appealing target in the search for exomoons.

Kipping et al. (2014), “Discovery of a Transiting Planet Near the Snow-Line”, arXiv:1407.4807 [astro-ph.EP]