Sunday, July 5, 2015

Giant Planet around Aldebaran

Aldebaran is an orange giant star located approximately 65 light years away in the constellation Taurus. It is the brightest star in the constellation Taurus and is also one of the brightest stars in the night sky. Aldebaran is currently in an advance stage of stellar evolution and has expanded to 44.2 times the Sun’s diameter. The surface temperature of Aldebaran is about 4,000 K and the star shines with ~500 times the Sun’s luminosity.

Figure 1: Artist’s impression of a giant planet.

Using new radial velocity measurements combined with data from observations stretching back 30 years, Hatzes et al. (2015) present the detection of a giant planet around Aldebaran. The amplitude of the radial velocity curve of Aldebaran indicates how much back and forth motion the star is experiencing due to the gravitational tugging from a companion planet. This enables the mass of the planetary companion around Aldebaran to be determined. Assuming Aldebaran has 1.13 ± 0.11 times the Sun’s mass, its planetary companion is estimated to have at least 6.47 ± 0.53 times the mass of Jupiter. With such a mass, the planetary companion is a giant planet. It orbits Aldebaran at a distance of 1.46 ± 0.27 AU, and has an orbital period of 628.96 ± 0.9 days.

Aldebaran is a giant star and long-lived features on the star’s surface may create radial velocity signatures that can mimic the presence of a planet. These surface features can last for up to several years. As a result, three decades of radial velocity data is needed to show that the period of 628.96 ± 0.9 days in the radial velocity curve is consistent enough to be attributed to a giant planet. This newly discovered giant planet around Aldebaran resembles the giant planets found around other giant stars. These planets tend to have between 3 to 14 times the mass of Jupiter and have orbital radii of roughly 2 AU.

Figure 2: Radial velocity measurements for Aldebaran from 7 data sets spanning three decades. Hatzes et al. (2015).

Figure 3: Radial velocity measurements for Aldebaran from 7 data sets phased to the orbital period of the giant planet. Hatzes et al. (2015).

Reference:
Hatzes et al. (2015), “Long-lived, long-period radial velocity variations in Aldebaran: A planetary companion and stellar activity”, arXiv:1505.03454 [astro-ph.SR]

Saturday, July 4, 2015

A Low-Density Transiting Super-Neptune


Bayliss et al. (2015) present the discovery of a low-density transiting super-Neptune identified as HATS-8b. This planet is in a close-in 3.58-day orbit around a Sun-like star. Photometric and spectroscopic observations indicate that HATS-8b has 0.873 times the radius and 0.138 times the mass of Jupiter, resulting in a remarkably low bulk density of only 0.259 g/cm³. HATS-8b is termed a super-Neptune as its mass is roughly halfway between that of Neptune and Saturn. Due to its large radius and low density, the gravity on HATS-8b is less than half the surface gravity on Earth and its atmospheric scale height (i.e. increase in altitude over which the atmospheric pressure decreases by a factor of 2.718) is expected to be almost 1,000 km.

HATS-8b is similar in mass as Kepler-101b, which is slightly more massive at 0.16 times the mass of Jupiter. However, the size of Kepler-101b is only 0.515 times the radius of Jupiter, giving it a much higher bulk density of 1.45 g/cm³, a factor of 5.6 times the bulk density of HATS-8b. The large range of densities amongst super-Neptunes suggests a huge compositional diversity and very different formation scenarios. HATS-8b is close enough to its host star that its dayside is heated to roughly 1,300 K. The orbit of HATS-8b around its host star is probably the limit for super-Neptunes. If it were any closer to its host star, evaporation would have reduced it to a super-Earth class planet.

Light curve showing the transit of HATS-8b in front of its host star. The transit depth indicates that HATS-8b has 0.873 times the radius of Jupiter. Bayliss et al. (2015).

Radial velocity curve showing the perturbation HATS-8b exerts on its host star. The amplitude of the radial velocity curve indicates that HATS-8b has 0.138 times the mass of Jupiter. Bayliss et al. (2015).

Reference:
Bayliss et al. (2015), “HATS-8b: A Low-Density Transiting Super-Neptune”, arXiv:1506.01334 [astro-ph.EP]

Friday, July 3, 2015

Case for a Low Mass Black Hole

Neutron stars and black holes are two types of compact stellar remnants. The most massive known neutron star is about 2 times the Sun’s mass, while the least massive known black hole is about 4 times the Sun’s mass. There appears to be an absence of compact stellar remnants between 2 to 4 times the Sun’s mass. Neutron stars are unlikely to populate this mass gap since a neutron star more than twice the Sun’s mass is expected to gravitationally collapse into a black hole. However, black holes with 2 to 4 times the Sun’s mass are possible.


Low mass X-ray binaries (LMXB) are a class of binary systems that are comprised of compact stellar remnants, either a neutron star or a black hole, that accretes material from a low mass companion star. Observations of a particular LMXB known as V1408 Aquilae show that the compact stellar remnant in this binary system is quite certainly a low mass black hole. Material stripped from the companion star forms an accretion disk around the black hole. Irradiation from the black hole’s accretion disk heats up one hemisphere of the companion star, resulting in a modulation in the light curve of the LMXB as the heated hemisphere of the companion star rotates in and out of view.

Both the black hole and the companion star in V1408 Aquilae circle around each other every 9.3 hours. Models indicate that the black hole most likely has 3 times the Sun’s mass. However, the mass of the black hole is not well constrained and can be as high as 6 times the Sun’s mass, with a 90 percent probability of not exceeding this value. Nevertheless, the black hole in V1408 Aquilae is still a good candidate for a compact object that lies within the mass gap of 2 to 4 times the Sun’s mass.

Reference:
Gomez, Mason & Robinson (2015), “The Case for a Low Mass Black Hole in the LMXB V1408 Aquilae (4U 1957+115)”, American Astronomical Society, AAS Meeting #221, #142.25

Thursday, July 2, 2015

Vanishing Behind a Red Dwarf Star

Derekas et al. (2015) present the discovery of a totally-eclipsing binary system identified as J0640+3856. This binary system consists of a hot subdwarf O (sdO) star and a red dwarf star. An sdO star is basically a very hot low mass star that is burning helium in its core and a thin envelope of hydrogen surrounds the star’s core. Such a star can be created when a red giant star sheds its outer layers. J0640+3856 has an orbital period of 4.5 hours and the orbital plane of the system is orientated edge-on such that the sdO star disappears entirely behind the red dwarf star every orbit.


Light curve of J0640+3856. The large dip in relative flux occurs when the more luminous sdO star disappears behind the red dwarf star, while the smaller dip in relative flux occurs when the sdO star passes in front of the less luminous red dwarf star. Derekas et al. (2015).

Spectroscopic and photometric observations of J0640+3856 show that the sdO star has 0.567 ± 0.138 times the Sun’s mass, 0.0955 ± 0.0077 times the Sun’s radius, 73.692 ± 11.819 times the Sun’s luminosity and a very hot surface temperature of 55,000 ± 3,000 K. As for the companion red dwarf star, it has 0.177 ± 0.051 times the Sun’s mass, 0.1985 ± 0.0159 times the Sun’s radius, 0.016 ± 0.004 times the Sun’s luminosity and a surface temperature of roughly 4,000 K. Both the sdO star and the red dwarf star are separated by a distance of only 1.25 times the Sun’s radius (i.e. a distance of 870,000 km).

Since the sdO star is orders of magnitude more luminous than its companion red dwarf star, the total brightness of the binary system falls nearly to zero when the sdO star vanishes from view behind the companion red dwarf star. The hot sdO irradiates the nearby companion red dwarf star so intensely that the “dayside” of the red dwarf star is heated to a temperature of about 22,500 K. The irradiation is so strong that the total brightness of the binary system actually raises and falls as the irradiated “dayside” of the red dwarf star rotates in and out of view. J0640+3856 appears to be a good analogue to planetary systems with hot-Jupiters in close-in orbits around Sun-like star.

Reference:
Derekas et al. (2015), “A new sdO+dM binary with extreme eclipses and reflection effect”, arXiv:1505.06487 [astro-ph.SR]

Wednesday, July 1, 2015

Discovery of a Highly Inflated Hot Jupiter

Fulton et al. (2015) report the discovery of a highly inflated hot Jupiter identified as KELT-8b. Orbiting at a distance of 7 million km from a mildly evolved Sun-like star, KELT-8b has an orbital period of only 3.24 days and its dayside is intensely irradiated. KELT-8b was first detected as it periodically transits its host star. Follow-up observations show that KELT-8b has 0.87 times the mass and 1.86 times the radius of Jupiter. The large radius of KELT-8b makes it the second largest transiting exoplanet known; only WASP-17b has a larger radius. The size of KELT-8b places it well above the theoretical mass-radius curve for pure hydrogen planets, indicating that it is extremely inflated (Figure 3).

Figure 1: Artist’s impression of a transiting exoplanet. Image credit: ESO/L. Cal├žada.

Figure 2: Transit light curve of KELT-8b. Fulton et al. (2015)

Figure 3: Mass radius diagram of all confirmed transiting exoplanets (red circles). KELT-8b is the annotated black square. Solar System planets are represented by green triangles. Fulton et al. (2015)

Being so close to its host star, the equilibrium temperature on KELT-8b is estimated to be 1675 K. Also, given the planet’s extremely inflated size, the surface gravity on KELT-8b is only 3/5th of Earth’s. For comparison, the surface gravity of Jupiter is 2.5 times the surface gravity on Earth. It is important to note that KELT-8b is a gaseous planet like Jupiter, with no physical surface. For such planets, the “surface” is typically defined as the level where the atmospheric pressure is one bar.

Given the high temperature and low surface gravity, the scale height of a hydrogen-dominated atmosphere on KELT-8b is estimated to be 1113 km. The scale height is defined as the increase in altitude over which the atmospheric pressure decreases by a factor of 2.718. For comparison, the scale height of Earth’s atmosphere is 8.5 km and of Jupiter’s atmosphere is 27 km. The remarkably large scale height of KELT-8b implies that its atmosphere is extremely puffy. KELT-8b joins a small but intriguing class of highly inflated hot Jupiters. The host star of KELT-8b is currently expanding into a red giant. The star’s surface is encroaching on the planet and in the next few hundred million years, the planet is expected to be destroyed by its host star.

Reference:
Fulton et al. (2015), “KELT-8b: A highly inflated transiting hot Jupiter and a new technique for extracting high-precision radial velocities from noisy spectra”, arXiv:1505.06738 [astro-ph.EP]

Tuesday, May 26, 2015

Antipholus & Antipholus

Figure 1: Artist’s impression of how a Kuiper Belt Object might look like.

2001 QW322 is a binary Kuiper Belt Object (KBO). It lies beyond the orbit of Neptune, roughly 6 billion kilometres from the Sun. What makes 2001 QW322 extreme is the remarkably huge separation between its two roughly equal-sized components - nicknamed Antipholus and Antipholus. Both objects are estimated to have diameters of roughly 100 kilometres, and are separated by a whopping ~125,000 kilometres (about one-third the Earth-Moon separation distance), roughly 10 times the separation of other near-equal-mass binaries. Antipholus and Antipholus are so far apart that both objects take ~25 to 30 years to complete one orbit around each other.

This system is the most tenuously bound pair of objects known in the Solar System. Such a tenuously held binary is difficult to create and maintain. Since this system is prone to disruption by massive interlopers, it is estimated to remain bound for no more than ~1 billion years. Antipholus and Antipholus circle around each other with an average orbital speed of just ~0.85 m/s or ~3 km/h, comparable to a slow human walking pace. On any one of the two Antipholuses, the surface gravity is predicted to be ~600 times weaker than on Earth. From the surface of one component, the existence of the other component would be visually perceptible since the other component would appear as large as a pinhead held at arm’s length and can appear as bright as Saturn seen from Earth.

Figure 2: Secondary-to-primary mass ratio versus average separation (in units of the primary’s radius). The dashed box represents the known binary asteroids; all on the left side of the plot (the largest separation barely exceeds 100). Also shown are the most extreme, outer-planet irregular satellites and several other binary KBOs. 2001 QW322 clearly stands out in the top-right corner of the diagram as the widest near-equal-mass binary.

Reference:
Petit et al. (2008), “The Extreme Kuiper Belt Binary 2001 QW322”, Science 322 (5900): 432-434

Monday, May 25, 2015

Quadruple Star System with a “Planetary” Architecture


HD 91962 is an unusual quadruple star system that consists of a Sun-like star with 3 lower-mass companion stars revolving around it. The central Sun-like star is estimated to have 1.14 times the Sun’s mass. Around it, the 3 companion stars have orbital periods of 170.3 days, 8.84 years and 205 years; and they are also estimated to have 0.31, 0.64 and 0.64 times the Sun’s mass, respectively. The orbits of the inner and middle companion stars are probably locked in a 1:19 orbital resonance. Each time the middle companion star completes one orbit around the central star, the inner companion star would have completed 19 orbits. With 3 stars revolving around a central star, HD 91962 appears to have a “planetary” architecture.

In the long run, HD 91962 is dynamically stable. This unique quadruple star system probably formed from the collapse of a rotating core of gas and dust. The rotation prevented the core from collapsing directly, causing the gas and dust to form a massive and unstable disk around the central star. The disk fragmented into a number of companion stars that migrated in towards the central star. The first few companion stars probably merged with the central star, while the companion stars that formed further out stopped their inward migration when the disk of gas and dust eventually dissipated. Hence, the 3 companion stars observed today are the ones that have survived.

Reference:
Andrei Tokovinin, David W. Latham & Brian D. Mason (2015), “The unusual quadruple system HD 91962 with a "planetary" architecture”, arXiv:1504.06535 [astro-ph.SR]

Sunday, May 24, 2015

2007 RW10 - A Large Quasi-Satellite of Neptune


Quasi-satellites have been found around Venus, Earth, Jupiter and Saturn. Basically, a quasi-satellite is an object that co-orbits the Sun together with a planet in what is known as a 1:1 orbital resonance. This means that the orbit of a quasi-satellite has the same orbital period as the planet, although the orbital eccentricity of a quasi-satellite is usually greater. From the perspective of the planet, a quasi-satellite will appear to revolve around the planet even though a quasi-satellite is technically in orbit around the Sun.

2007 RW10 is a quasi-satellite of the planet Neptune. It has been a quasi-satellite of Neptune for ~12,500 years and will continue to do so for another ~12,000 years. 2007 RW10 co-orbits the Sun together with Neptune in a 1:1 orbital resonance. Although it orbits the Sun and not Neptune, 2007 RW10 appears to go around Neptune every 164.8 years (i.e. the duration of one Neptunian year) from the perspective of Neptune. 2007 RW10 can come as close as 0.86 AU to Neptune. That is close to Neptune’s Hill radius of 0.775 AU. To become a satellite of a planet, an object must have an orbit that lies within the planet’s Hill radius. With an estimated diameter of roughly 250 km, 2007 RW10 is probably the largest known object in a 1:1 orbital resonance with a planet.

The relative motion of 2007 RW10 with respect to Neptune over the next 3,000 years appears to trace a kidney-shaped path when viewed from Neptune. C. de la Fuente Marcos & R. de la Fuente Marcos (2012).

Reference:
C. de la Fuente Marcos & R. de la Fuente Marcos (2012), “(309239) 2007 RW10: a large temporary quasi-satellite of Neptune”, arXiv:1209.1577 [astro-ph.EP]

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