Saturday, May 23, 2015

Concept for a Human Mission to Callisto in the 2040s

In 2003, NASA did a conceptual study on a Human Outer Planet Exploration (HOPE) mission to Jupiter’s moon Callisto in the 2040s time frame. Jupiter is orbited by the four large Galilean satellites - Io, Europa, Ganymede and Callisto. Callisto is the third largest moon in the Solar System and the second largest in the Jovian system. With a diameter of 4,821 km, Callisto is nearly as large as the planet Mercury. The surface gravity on Callisto is 1/8th of Earth’s. Callisto is also thought to harbour an ocean of liquid water deep beneath its icy surface.

Figure 1: Artist’s impression of a base on the surface of Callisto. Image credit: William Black.

Figure 2: Artist’s impression of a base on the surface of Callisto. Image credit: William Black.

The HOPE mission to Callisto consists of a crew of six astronauts. It employs a split-mission approach involving the use of three separate vehicles - cargo, tanker and crew. Nuclear electric propulsion (NEP) is the method of propulsion for the mission. Each vehicle has an NEP system propelled by magnetoplasmadynamic (MPD) thrusters using hydrogen as propellant. Before departing to Callisto, the cargo, tanker and crew vehicles are 242, 244, and 262 tons, respectively, for a total mission mass of 748 tons. The autonomous cargo and tanker vehicles will depart first to deploy orbital and surface assets at Callisto before the crew vehicle departs for Callisto.

On each vehicle, the NEP system consists of a gas-cooled nuclear fission reactor fuelled with enriched uranium-235. A gaseous mixture consisting of helium and xenon is used as the coolant. The gas extracts thermal energy from the nuclear reactor and then expands through a turbine-alternator-compressor unit to generate electricity. Waste heat from the nuclear reactor is radiated into space using radiator panels on each vehicle. Each of the nuclear reactors on the cargo, tanker and crew vehicles generate ~6.4, ~6.4 and ~8.2 megawatts (MW) of electricity, respectively. Part of the electricity that is generated goes on to power the MPD thrusters on each vehicle.

The crew vehicle, also known as the Piloted Callisto Transfer Vehicle (PCTV) is designed to transport the crew of six for the round trip to Callisto. During the transit between Earth and Callisto, the crew lives inside an inflatable TransHab module. The TransHab module is position at one end of a dumb-bell shaped structure attached to the forward end of the PCTV. A number of small liquid hydrogen propellant tanks surround the TransHab module to reduce crew exposure to radiation in deep space. Four large liquid hydrogen propellant tanks are placed on the other end of the dumb-bell shaped structure to act as a counterbalance to the TransHab module.

Figure 3: Artist’s impression of the Piloted Callisto Transfer Vehicle (PCTV).

The entire dum-bell shaped structure rotates at ~4 revolutions per minute (rpm) to generate an artificial gravity environment with ~1/8th Earth’s surface gravity for the crew residing in the TransHab module. When the PCTV arrives at Callisto, it will remain in orbit around Callisto. Although the PCTV is designed for a round trip to Callisto, the amount of propellant it carries is sufficient only for its ~2.1 year outbound trip to Callisto. During its 120 day stay in orbit around Callisto, it will be re-supplied with propellant from the tanker vehicle for its ~2.1 year inbound trip back to Earth. The total mission duration for the PCTV with crew onboard is ~4.5 years.

The cargo vehicle delivers three different assets onto the surface of Callisto - a reusable crew descent/ascent module, a surface habitat and a propellant production plant. The reusable crew descent/ascent module allows the astronauts to travel between the orbiting PCTV and the surface habitat. Down on the surface of Callisto, the propellant production plant breaks down water-ice to produce liquid oxygen and liquid hydrogen for use as propellant for the reusable crew descent/ascent module. This will allow crew rotation and/or re-supply sortie missions between the orbiting PCTV and the surface habitat every ~30 days. The surface habitat can support three astronauts on the surface of Callisto for a minimum of 30 days.

When the tanker vehicle arrives, it stays in orbit around Callisto and subsequently re-supplies the PCTV with propellant when it gets there. Basically, the tanker vehicle delivers to Callisto the propellant required for the PCTV to return to Earth. Its payload consists of four 19 m long by 7.6 m diameter refrigerated tanks, each containing ~50 tons of liquid hydrogen propellant.

Figure 4: Schematic of the tanker vehicle. L. McGuire et al. (2003).

Figure 5: Cargo vehicle trajectory and mission mass details. L. McGuire et al. (2003).

Figure 6: Tanker vehicle trajectory and mission mass details. L. McGuire et al. (2003).

Figure 7: Crew vehicle (PCTV) trajectory and mission mass details. L. McGuire et al. (2003).

Callisto appears to be a suitable destination for exploration beyond Mars. Being the outermost of the four Galilean satellites, Callisto is situated beyond Jupiter’s main radiation belts. This makes its environment more conducive for human exploration. Settlements on the surface of Callisto will be easier and safer to construct since they require less radiation protection. Furthermore, Callisto is an icy, rocky world with abundant water-ice on its surface that can be exploited to produce rocket propellant. Callisto could serve as a base for exploring the other moons of Jupiter and even Jupiter itself. Moreover, Callisto can serve as a refuel and re-supply stop for spacecraft bound for further destinations such as Saturn or Neptune. 

References:
- A. Troutman et al., “Revolutionary Concepts for Human Outer Planet Exploration (HOPE)”, AIP Conf. Proc. 654, 821 (2003)
- L. McGuire et al., “High Power MPD Nuclear Electric Propulsion (NEP) for Artificial Gravity HOPE Missions to Callisto”, Space Technology and Applications International Forum (STAIF-2003)

Friday, May 22, 2015

Detection of an Oxygen Atmosphere around Callisto

Of all the moons in the Solar System, Saturn’s moon Titan has by far the densest atmosphere. The atmospheric pressure on the surface of Titan is 1.4 times greater than at sea-level on Earth. Titan’s atmosphere is so thick that it obscures its entire surface. From space, Titan appears as a fuzzy orange orb with no visible indication of any surface features. After Titan, the moon with the next thickest atmosphere is Neptune’s moon Triton. Triton’s atmosphere is so rarefied that it is only ~1/20,000th the density of Earth’s atmosphere. Nevertheless, its atmosphere is still thick enough to have winds, clouds and weather.

Like Earth, the atmospheres of Titan and Triton are thick enough that their gas molecules collide with one another before travelling any appreciable distance. Such atmospheres are known as collisional atmospheres. In contrast, some other moons in the Solar System, including Earth’s Moon, have extremely tenuous atmospheres known as exospheres. The gas particles in an exosphere are spaced so far apart that they rarely collide with each other. An exosphere is basically a non-collisional atmosphere.

Besides Titan and Triton, Jupiter’s moon Io is the third moon in the Solar System with a collisional atmosphere. The atmospheric pressure on Io is roughly a billion times less than at sea-level on Earth. Most of Io’s atmosphere is comprised of sulphur dioxide. Some of the sulphur dioxide comes directly from its constantly erupting volcanoes and the rest comes from sublimating sulphur dioxide frost on its dayside.

Figure 1: Artist’s impression of Jupiter’s moon Callisto.

A recent paper by Cunningham et al. (2015) reports the detection of an oxygen-dominated atmosphere around Jupiter’s moon Callisto. The detection was made using the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) and the observations were directed towards Callisto’s leading/Jupiter-facing hemisphere. Like the three other large Galilean moons, Callisto is also tidally-locked to Jupiter. This means the same hemisphere of Callisto is always oriented towards Jupiter. Furthermore, Callisto’s orbital motion around Jupiter means it has a leading hemisphere (forward-facing hemisphere) and a trailing hemisphere (aft-facing hemisphere).

Jupiter has four large Galilean satellites - Io, Europe, Ganymede and Callisto. Of the four Galilean satellites, Callisto is the outermost. Measurements by the COS instrument indicate that Callisto’s atmosphere at its leading/Jupiter-facing hemisphere has a column density of ~4×10^15 oxygen molecules per cm². This is dense enough for Callisto’s atmosphere to be collisional, making Callisto the fourth moon known in the Solar System that has a collisional atmosphere.

While the measurements are of Callisto’s leading/Jupiter-facing hemisphere, the column density over its trailing hemisphere may be ~10 times denser. In fact, Callisto has the second densest oxygen-rich collisional atmosphere in the Solar System, the densest one being Earth’s atmosphere. Since it is collisional, the atmosphere of Callisto should be able to support winds and other weather phenomena.

The oxygen molecules in Callisto’s atmosphere are produced when water molecules on Callisto’s icy surface dissociate into hydrogen and oxygen. The light hydrogen atoms escape into space while the heavy oxygen atoms remain behind, resulting in Callisto’s oxygen-dominated atmosphere. Europa and Ganymede also have oxygen-dominated atmosphere derived from the same processes as on Callisto. However, their atmospheres are several times more tenenuous Callisto’s. As a result, the atmospheres of Europa and Ganymede are non-collisional; hence they are exospheres, with no winds and no weather.

Figure 2: The four moons in the Solar System that are known to have collisional atmospheres. In order of decreasing atmospheric column density, they are: Saturn’s Titan, Neptune’s Triton, and Jupiter’s Io and Callisto. Image Credit: NASA / JPL / SSI / Ted Stryk / Jason Perry / Emily Lakdawalla.

Reference:
Cunningham et al. (2015), “Detection of Callisto’s oxygen atmosphere with the Hubble Space Telescope”, Icarus 254, 178-189

Thursday, May 21, 2015

Subsurface Ocean of Liquid Water on Callisto

Figure 1: Callisto (bottom left), Jupiter (top right) and Europa (below and left of Jupiter’s Great Red Spot) as viewed in 2000 by NASA’s Cassini spacecraft on its way to Saturn. Credit: NASA/JPL/University of Arizona.

Callisto was discovered by Galileo Galilei, an Italian astronomer, in January 1610 along with three other large moons around Jupiter - Io, Europa and Ganymede. These four large moons are known as the Galilean satellites of Jupiter and Callisto is the outermost member. With a diameter of 4,821 km, Callisto is the third largest moon in the Solar System and is nearly as large as the planet Mercury. Callisto orbits far enough from Jupiter that it does not participate in the orbital resonance that the three inner Galilean satellites are in. As a result, Callisto has never experienced any appreciable tidal heating.

With a mean density of 1.83 g/cm³, Callisto’s bulk composition is roughly half rocky material and half water-ice. Unlike the three other Galilean satellites, Callisto never got warm enough to fully differentiate into a rocky core and an icy mantle. Instead, Callisto is only partially differentiated, with the proportion of rocky material increasing with depth. Nevertheless, interior models of Callisto based on measurements of its density and moment of inertia do not rule out the existence of a tiny rocky core in the center. However, the size of such a rocky core cannot exceed a diameter of about 1,200 km.

Figure 2: A model of the internal structure of Callisto with an internal ocean. The icy crust is 135 to 150 km thick; the thickness of the water layer is ~120 to 180 km, and the total thickness of the water-ice shell is ~270 to 315 km. The solid curve and the dashed one are for densities of the rock-iron component equal to 3.62 and 3.15 g/cm³, respectively. Kuskov & Kronrod (2005).

When NASA’s Galileo spacecraft was in orbit around Jupiter, it detected the presence of an induced magnetic field around Callisto. This is evidence for the existence of a conducting layer just beneath the surface of Callisto. Such a conducting layer may be best explained by the presence of a salty subsurface ocean of liquid water. However, theoretical models have difficulties showing how such an ocean could have remained liquid till now. This is because Callisto does not experience any significant tidal heating. Furthermore, solid-state convection in the ice layer overlying the subsurface ocean would drive convective heat loss, leading to complete freezing of the ocean in ~100 million years.

One way such an ocean can be kept from freezing completely is the presence of substances that lower the melting temperature of water-ice. For example, the presence of ammonia can lower the melting temperature of water-ice to ~176 K and the presence of chloride salts or sulphuric acid can lower the melting temperature to ~210 K. A study by Javier Ruiz (2001) suggests that even without the presence of substances to depress the melting temperature of water-ice, the subsurface ocean on Callisto can be kept from freezing completely if the overlying ice layer is more rigid than commonly assumed, thereby greatly reducing the rate of heat loss through convection.

A rigid, non-convecting ice shell overlying an internal ocean of liquid water appears to be consistent with the ancient surface geology of Callisto. The surface of Callisto is one of the most heavily cratered in the Solar System and shows a lack of endogenic geological processes. A stable and non-convecting ice shell can explain Callisto’s geologically inactive surface. Callisto’s salty subsurface ocean of liquid water lies beneath an icy shell that is estimated to be between 100 to 200 kilometres in thickness. If icy shells are more rigid and more difficult to undergo convection than commonly assumed, then the existence of subsurface oceans of liquid water on the numerous icy worlds in the Solar System could be more common than thought.

References:
- Kuskov & Kronrod, “Models of the Internal Structure of Callisto”, Solar System Research, Vol. 39, No. 4, 2005, pp. 283-301
- K. Khurana et al., “Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto”, Nature 395, 777-780 (22 October 1998)
- Javier Ruiz, “The stability against freezing of an internal liquid-water ocean in Callisto”, Nature 412, 409-411 (26 July 2001)

Wednesday, May 20, 2015

Large Water-Rich Moons around Super-Jovian Planets

Large moons similar in size to the planet Mars (0.53 Earth radii) or Jupiter’s moon Ganymede (0.41 Earth radii) should be detectable in the available dataset of photometric measurements by NASA’s Kepler space telescope. Super-Jovian planets (i.e. planets more massive than Jupiter) appear to be abundant at ~1 AU around Sun-like stars (i.e. 1 AU is the distance of Earth from the Sun). At these distances, temperatures are just right for large moons around these planets to potentially support life. As a result, it is worth considering whether super-Jovian planets at ~1 AU around Sun-like stars can host large moons that are habitable.

Figure 1: Artist’s impression of a gas giant planet like Jupiter with a system of moons orbiting it.

A study by Heller & Pudritz (2015) predicts that water-rich Mars-mass moons can form around super-Jovian planets that lie beyond ~5 AU around Sun-like stars. Jupiter-mass planets that form closer than ~4.5 AU to Sun-like stars are unlikely to form water-rich moons because their accretion disks are less massive and depleted of water-ice. However, a super-Jovian planet with 12 times the mass of Jupiter can potentially form water-rich moons as close as ~3 AU to a Sun-like star as its accretion disk is significantly more massive.

A super-Jovian planet with 10 times the mass of Jupiter forming at 5.2 AU around a Sun-like star can form a moon system with a total mass of roughly 10 times the mass of Ganymede or 2 times the mass of Mars. Simulations indicate that this mass is distributed over 3 to 6 moons in ~90 percent of the cases. If the same planet formed at 9.6 AU, the distance of Saturn from the Sun, the total mass of its moon system can be twice as large.

Figure 2: Artist’s impression of a gas giant planet seen partly through the atmosphere of a large moon in orbit around it.

Figure 3: Artist’s impression of a large, potentially habitable moon in orbit around a gas giant planet.

Due to various interactions, planets like Jupiter tend to migrate away from where they initially form. It is more plausible that observed population of super-Jovian planets at ~1 AU around Sun-like stars formed further out and migrated inwards to their current positions. If these super-Jovian planets formed beyond about 3 to 4.5 AU, they should host water-rich Mars-sized moons. At ~1 AU from a Sun-like star, such a moon can be a potentially habitable ocean world. The abundance of super-Jovian planets at ~1 AU around Sun-like stars could mean that Mars-mass ocean worlds are a common type of habitable environment in the universe.

A super-Jovian planet hosting two or more habitable moons can have interesting astrobiological implications because life on these moons can share a common evolutionary tree of life. This is because a system of moons around a super-Jovian planet is much more compact than a system of planets around a star. As a consequence, it is much easier for the process of panspermia to transfer biological material from one habitable moon to another.

In the less plausible scenario that super-Jovian planets at ~1 AU around Sun-like stars formed in-situ, then they are less likely to host water-rich moons. The absence or presence of water-rich Mars-sized moons orbiting super-Jovian planets at ~1 AU around Sun-like stars can indicate whether these planets tend to form in-situ or tend to migrated to their current positions from further out.

Reference:
Heller & Pudritz (2015), “Conditions for water ice lines and Mars-mass exomoons around accreting super-Jovian planets at 1 - 20 AU from Sun-like stars”, arXiv:1504.01668 [astro-ph.EP]

Tuesday, May 19, 2015

Cloudy Mornings & Searing Afternoons on Hot-Jupiters

Hot-Jupiters are a class of planets that orbit very close to their host stars. As a result, these planets receive extremely intense irradiation and are expected to be tidally-locked, with one side experiencing permanent day and one side experiencing permanent night. The strong temperature contrast between the permanent day and night sides on a hot-Jupiter drives a powerful west-to-east circulation that goes around the planet.

If the orbit of the hot-Jupiter is observed edge-on, it will appear to pass behind its host star on every orbit. Such an event is known as an eclipse. The planet’s evening-side is more visible pre-eclipse (i.e. during the first half of the orbit), while the planet’s morning-side is more visible post-eclipse (i.e. during the second half of the orbit). As the planet circles around its host star, some fraction of its illuminated hemisphere will be visible depending on the position of the planet in its orbit. This generates a light curve where the combined brightness of the star and planet varies in a small and periodic fashion every orbit. The variation is small because the star is orders of magnitude brighter than the planet.

Figure 1: Artist’s impression of a hot-Jupiter that is glowing red hot.

Using data from NASA’s Kepler space telescope, a study by Esteves et al. (2015) uncovered weather cycles on 6 hot-Jupiters. The brightest spot on each planet appears offset from the substellar point. Basically, the substellar point on a hot-Jupiter is the spot on the planet where the planet’s host star is directly overhead and it is where the stellar irradiation is most intense. The 2 hotter planets (Kepler-76b and HAT-P-7b) appear brightest pre-eclipse (i.e. the brightest spot on the planet is east of the substellar point, towards the evening-side); while the 4 cooler planets (Kepler-7b, Kepler-8b, Kepler-12b and Kepler-41b) appear brightest post-eclipse (i.e. the brightest spot on the planet is west of the substellar point, towards the morning-side).

On these 6 hot-Jupiters, winds blow eastward from the substellar point towards the evening-side, around the night side and back towards the morning-side, before returning to the substellar point. The 4 cooler planets (Kepler-7b, Kepler-8b, Kepler-12b and Kepler-41b) all have temperatures under ~2,300 K. The best explanation why they appear brightest on the morning-side (i.e. post-eclipse) is because cool temperatures on the night side allow clouds to form by condensation and the west-to-east atmospheric circulation brings these reflective clouds to the morning-side, causing the morning-side to be the brightest region on the planet. Subsequently, these clouds continue on towards the substellar point where they dissipate due to the increased irradiation.

In contrasts, the 2 hotter planets (Kepler-76b and HAT-P-7b) have temperatures exceeding ~2,700 K and both planets appear brightest on the evening-side (i.e. pre-eclipse). The best explanation is that the brightness is dominated by thermal emission from a hot-spot that has been shifted east from the substellar point, towards the evening-side. Furthermore, the high temperatures make it difficult for clouds to form, resulting in little or no clouds on the morning-side that can reflect a sufficient amount of incoming stellar radiation to contribute significantly to the planet’s brightness.

For the 4 cooler planets, the brightest spot is shifted west (i.e. towards the morning-side) from the substellar point by over ~25° since the clouds are probably thickest and most reflectively near the day-night terminator on the morning-side. For the 2 hotter planets, the brightest spot is shifted by a much smaller amount of less than ~8°, this time the shift is eastwards (i.e. towards the evening-side). The brightest spot is closer to the substellar point for the 2 hotter planets due to the rapid re-emission of absorbed stellar energy after leaving the substellar point. This study supports the correlation between the position of a planet’s brightest spot and the planet’s temperature. The brightness of a hotter planet is likely to be dominated by thermal emission from a hot-spot that has been shifted east of the substellar point (i.e. evening-side), while the brightness of a cooler planet is likely to be dominated by reflected light from clouds west of the substellar point (i.e. morning-side).

Figure 2: Light curve of Kepler-8b, a hot-Jupiter with 1.42 times the radius and 0.59 times the mass of Jupiter, in a 3.52-day orbit around its host star. The left and right panels contain, respectively, the transit light curve (i.e. drop in the star’s brightness when the planet passes in front of its host star) and phase light curve (i.e. combined brightness of the star and planet as the planet orbits the star). On the right panel, the dip in the middle denotes the eclipse, whereby the planet passes behind its host star and is obscured. Esteves et al. (2015).

Figure 3: Light curve of Kepler-12b, a hot-Jupiter with 1.75 times the radius and 0.43 times the mass of Jupiter, in a 4.44-day orbit around its host star. Esteves et al. (2015).

Figure 4: Light curve of Kepler-41b, a hot-Jupiter with 1.04 times the radius and 0.56 times the mass of Jupiter, in a 1.86-day orbit around its host star. Esteves et al. (2015).

Figure 5: Light curve of Kepler-8b, a hot-Jupiter with 1.36 times the radius and 2.01 times the mass of Jupiter, in a 1.54-day orbit around its host star. Esteves et al. (2015).

Reference:
Esteves et al. (2015), “Changing Phases of Alien Worlds: Probing Atmospheres of Kepler Planets with High-Precision Photometry”, arXiv:1407.2245 [astro-ph.EP]

Monday, May 18, 2015

Tightly-Packed Multi-Planet Systems

Over the years, observations by NASA’s Kepler space telescope have shown that tightly-packed multi-planet systems are common around Sun-like stars. Each of these multi-planet systems contains 4 to 6 planets packed within a region of space much smaller than Mercury’s orbit around the Sun. In these tightly-packed multi-planet systems, the spacings between adjacent planets appear to cluster at ~12 mutual Hill radii. The Hill radius of a planet is the region of space around a planet where the planet’s gravity dominates. For a satellite to orbit the planet, it has to lie within the planet’s Hill radius. In these tightly-packed multi-planet systems, the planets tend to have almost circular orbits that lie on the same plane.

Figure 1: Artist’s impression of a multi-planet system seen from one of the planets.

Observations by Kepler have also turned up planetary systems that are sparsely populated, in particular, single or double planetary systems. These planets tend to have orbits that are more inclined and more eccentric. One hypothesis suggests that sparsely populated planetary systems could be the remnants of tightly-packed multi-planet systems that have undergone dynamic instability. Interactions during the period of dynamic instability can leave the remaining planets with more inclined and more eccentric orbits.

Simulations show that for a multi-planet system to remain stable for as long as a billion years, the required minimum spacing between adjacent planets is ~10 mutual Hill radii if all planets have orbits that are circular and lie on the same plane. However, if the orbits of the planets are slightly eccentric, then the required minimum spacing between adjacent planets is ~12 mutual Hill radii. The similarity between what is observed and what is theoretically predicted for tightly-packed multi-planet systems suggests that planetary systems tend to form with much tighter spacings between adjacent planets. Tightly-packed multi-planet systems with 5 or more planets are probably planetary systems that have remained stable since formation, while single or double planetary systems are the remnants of more tightly-packed multi-planet systems.

Figure 2: The observed K-distribution (i.e. distribution of the spacings between adjacent planets in units of mutual Hill radii) of multi-planet systems shown as solid histograms for the 4 planet and 5 planet + 6 planet systems. Pu & Wu (2015).

Reference:
Pu & Wu (2015), “Spacing of Kepler Planets: Sculpting by Dynamical Instability”, arXiv:1502.05449 [astro-ph.EP]

Sunday, May 17, 2015

A Way to Produce Neutron Star-Black Hole Binaries

Massive stars in binary systems can potentially evolve to form exotic compact binary objects such as neutron star-black hole (NS-BH) binaries or neutron star-neutron star (NS-NS) binaries. Consider a binary system with two massive stars closely orbiting one another. The more massive star evolves quicker and eventually runs out of nuclear fuel to support itself. Its core collapses to form a neutron star (NS) via a supernova explosion. Meanwhile, the less massive star continues to evolve and swells in size as it builds up a carbon-oxygen (CO) core in its center. The star’s outer layers eventually engulf the nearby NS and cause the NS to spiral inwards, towards the CO core.



As the NS spirals in, it transfers angular momentum to the star’s outer layers. This causes the star’s other layers, comprised mostly of hydrogen and helium, to be expelled into space, leaving behind a massive CO core. What was once a binary system consisting of two massive stars is now a tightly-bound binary system consisting of a massive CO core with ~2 times the Sun’s mass and a NS with more than ~2 times the Sun’s mass. Subsequently, the massive CO core also collapses in a supernova explosion, leaving behind a new neutron star (νNS) with ~1.5 times the Sun’s mass. The supernova explosion ejects ~0.5 Sun’s mass worth of material that was once part of the massive CO core.

The nearby NS grows in mass by accreting matter from the supernova ejecta. Soon, the NS becomes too massive and collapses gravitationally to form a black hole (BH) with at least ~2.5 times the Sun’s mass. An energetic gamma ray burst (GRB) is produced during the collapse. The tightly-bound νNS- BH binary system continues to evolve into an ever tighter configuration via the emission of gravitational waves. Such a νNS- BH binary system may be detectable from its gravitational wave signal. Within a span of 10,000 years or less, the νNS-BH binary eventually merge, driving an ultra-short duration GRB. Two supernovae (i.e. plural of supernova) and two GRBs later, a binary system of two massive stars is now a single black hole drifting through space.

Reference:
C. L. Fryer, F. G. Oliveira, J. A. Rueda, R. Ruffini (2015), “On the Neutron Star-Black Hole Binaries Produced by Binary-driven Hypernovae”, arXiv:1505.02809 [astro-ph.HE]

Saturday, May 16, 2015

Discovery of a Low-Mass Double-Lined Eclipsing Binary

Figure 1: Artist’s impression of a binary star system.

G. Zhou et al. (2015) present the discovery and characterisation of a binary system consisting of two low-mass red dwarf stars that periodically eclipse one another. The binary system is identified as HATS551-027. The more massive star is HATS551-027A and the less massive star is HATS551-027B, hereafter referred to as components A and B. HATS551-027 is also a double-lined binary system and this means that the spectral lines of both stars are visible. Double-lined eclipsing binary systems comprising of low-mass red dwarf stars serve as good natural laboratories for the precise measurements of fundamental stellar parameters (i.e. mass, radius, etc) of low-mass stars. These measurements can then be used to validate and improve models of low-mass stars.

The two components of HATS551-027 circle around one another ever ~4.1 days. Component A is estimated to have 0.24 times the mass and 0.26 times the radius of the Sun while component B is estimated to have 0.18 times the mass and 0.22 times the radius of the Sun. Compared to theoretical models, the radius of component A is consistent while the radius of component B is inflated by 9 percent at 2σ significance. Both components of HATS551-027 have masses low enough to support fully convective interiors. The estimated effective temperatures for components A and B are 3,190 ± 100 K and 2,990 ± 110 K, respectively. Both stars appear slightly cooler than predicted by theoretical models. HATS551-027 is the third well characterised double-lined eclipsing binary system with both components supporting fully convective interiors. The other two are CM Draconis and KOI-126.

Figure 2: Light curves of HATS551-027. The left panel corresponds to the primary eclipse (i.e. component B passing in front of component A) and the right panel corresponds to the secondary eclipse (i.e. component A passing in front of component B). G. Zhou et al. (2015).

Figure 3: Radial velocity curves of HATS551-027A (red) and B (black). As both components circle around one another, one component will appear to recede while the other component will appear to approach, and vice versa. The receding component will have its spectral lines red-shifted (i.e. positive radial velocity) while the approaching component will have its spectral lines blue-shifted (i.e. negative radial velocity). G. Zhou et al. (2015).

Figure 4: The measured masses, radii and temperatures of the well characterised double-line eclipsing binaries compared with theoretical models. G. Zhou et al. (2015).

Reference:
G. Zhou et al. (2015), “A 0.24+0.18 Msun double-lined eclipsing binary from the HATSouth survey”, arXiv:1505.02860 [astro-ph.SR]

Friday, May 15, 2015

Warm Neptunes with Helium Atmospheres

Warm Neptune-sized and sub-Neptune-sized exoplanets are a class of planets that orbit closer to their host stars than Mercury’s orbit around the Sun. Although these planets started out with hydrogen-helium atmospheres, the strong irradiation they receive due to their close proximity to their host stars can drive away the hydrogen and lead to the formation of helium-dominated atmospheres. If the planet started out with a hydrogen-helium atmosphere that is ~0.1 percent of the planet’s mass, the effect of intense stellar irradiation can drive off the hydrogen in the atmosphere in ~10 billion years. In a helium-dominated atmosphere, the scarcity of hydrogen causes the main molecular carrier of carbon to be carbon monoxide (CO) rather than methane (CH4).

GJ 436 b is a Neptune-sized planet with 23.2 times the mass and 4.22 times the diameter of Earth. It is in a close-in orbit around a red dwarf star and it may have a helium-dominated atmosphere. Observations of GJ 436 b indicate that the planet’s atmosphere is rich in carbon monoxide (CO) but depleted in methane (CH4). This is strange because if GJ 436 b has a hydrogen-helium atmosphere similar to Neptune’s, most of its carbon should be in the form of methane (CH4) and not carbon monoxide (CO). Nevertheless, a helium-dominated atmosphere can explain the observed atmospheric composition of GJ 436 b.


A helium-dominated atmosphere exhibits certain characteristics that distinguish it from a hydrogen-helium atmosphere. Helium has a much lower heat capacity compared to hydrogen because monatomic helium has three degrees of freedom while a hydrogen molecule has six degrees of freedom. As a result, the temperature gradient (i.e. the increase in temperature with depth) of a helium-dominated atmosphere is much larger compared to other planetary atmospheres because the temperature gradient of a planetary atmosphere is inversely proportional to its specific heat capacity.

Since warm Neptunes and sub-Neptunes orbit close to their parent stars, these planets are likely tidally-locked with the same hemisphere always facing the star, leading to permanent day and night sides. The spot on the dayside that receives the most intense irradiation is known as the substellar point. Here, the star is always directly overhead. The intense irradiation creates a hot spot around the substellar point. However, the presence of winds in the atmosphere tends to shift the hot spot away from the substellar point. For a warm Neptune or sub-Neptune with a hydrogen-helium atmosphere, the hot spot can shift far from the substellar point due to the larger heat capacity of hydrogen. In contrast, for a helium-dominated atmosphere, the low heat capacity of helium means that the hot spot is likely to remain at the substellar point because it cannot shift far enough without cooling significantly.

Reference:
Hu et al. (2015), “Helium Atmospheres on Warm Neptune- and Sub-Neptune-Sized Exoplanets and Applications to GJ 436 b”, arXiv:1505.02221 [astro-ph.EP]

Thursday, May 14, 2015

Two Temperate Super-Earths in a Five Planet System


Kepler-296 is a binary star system consisting of two red dwarf stars - Kepler-296A and Kepler-296B. NASA’s Kepler space telescope previously detected the presence of five transiting planets associated with the Kepler-296 system. However, it is unclear whether the five planets orbit Kepler-296A or Kepler-296B since both stars are quite close to one another. Kepler-296A (primary star) is more massive and more luminous than Kepler-296B (secondary star), and it is responsible for ~3/4 of the system’s total luminosity.

Using statistical and analytical analysis, Barclay et al. (2015) show the five planets most likely orbit Kepler-296A, the primary star. The five planets around Kepler-296A are termed “super-Earths” because they are somewhat larger in size than Earth. Moving out from Kepler-296A, the five planets are identified as Kepler-296Ac, b, d, e and f.

- Planet c: orbital period of 5.8 days, 2.00 times Earth’s diameter and 14.8 times Earth’s insolation.
- Planet b: orbital period of 10.9 days, 1.61 times Earth’s diameter and 6.5 times Earth’s insolation.
- Planet d: orbital period of 19.9 days, 2.09 times Earth’s diameter and 2.90 times Earth’s insolation.
- Planet e: orbital period of 34.1 days, 1.53 times Earth’s diameter and 1.41 times Earth’s insolation.
- Planet f: orbital period of 63.3 days, 1.80 times Earth’s diameter and 0.62 times Earth’s insolation.


One reason why the five planets most likely orbit only one of the two stars in the binary system (Kepler-296A in this case) is that the ratios of the orbital period between successive pairs of planets are all ~1.8 (1.88, 1.83, 1.71 and 1.86). This means that the planets underwent convergent migration in the past, and migrated into the current configuration of nearly equal orbital period ratios. If one or more planets orbit the other star Kepler-296B, such a chain of nearly equal orbital period ratios would be very unlikely.

Of the five planets, Kepler-296Ae and Kepler-296Af, respectively, receive 1.41 and 0.62 times the amount of insolation Earth gets from the Sun. This amount of insolation places both planets within or at least close to the habitable zone around Kepler-296A. Based on the definition by Kopparapu et al. (2013), Kepler-296f, the outermost of the five planets, is probably a better candidate for a habitable planet since it falls in the “conservative” habitable zone, while Kepler-296e falls into the “optimistic” habitable zone.

References:
- Barclay et al. (2015), “The Five Planets in the Kepler-296 Binary System All Orbit the Primary: A Statistical and Analytical Analysis”, arXiv:1505.01845 [astro-ph.EP]
- Kopparapu et al. (2013), “Habitable Zones Around Main-Sequence Stars: New Estimates”, arXiv:1301.6674 [astro-ph.EP]