Kepler-101 is a Planetary System in Reverse

Kepler-101 is a planetary system with a hot super-Neptune (Kepler-101b) and an Earth-sized planet (Kepler-101c) in orbit around a metal-rich G-type star. Kepler-101b and Kpler-101c were first discovered by NASA’s Kepler space telescope thanks to the transit method. Both planets orbit a host star with ~1.17 times the mass and ~1.56 times the radius of the Sun. The star’s effective surface temperature is ~5667 K and it has approximately twice the metallicity of the Sun.

Figure 1: Artist’s impression of an exoplanet.

The radius of Kepler-101b is ~5.77 Rᴇ, where Rᴇ denotes the Earth’s radius. Subsequent radial velocity measurements obtained with the HARPS-N spectrograph indicate that the mass of Kepler-101b is ~51.1 Mᴇ, where Mᴇ denotes the Earth’s mass. With the planet’s size and mass known, its density is found to be ~1.45 g/cm³. Interior models of Kepler-101b suggest that a significant fraction of its interior is made up of heavy elements; more than 60 percent of the planet’s total mass. Kepler-101b is 0.047 AU from its host star and its orbital period is 3.488 days. As a result of its close-in orbit, the planet is hot, and its estimated equilibrium temperature is ~1500 K.

Kepler-101c is an Earth-sized planet whose radius is ~1.25 Rᴇ. Although its mass could not be determined, a 1σ upper limit of less than 3.8 Mᴇ can be imposed. Interior models of Kepler-101c exclude a pure iron composition with a 68.3 percent probability. Kepler-101c orbits its host star at 0.068 AU, taking 6.030 days to complete an orbit. It is further from its host star than Kepler-101b. The estimated equilibrium temperature on Kepler-101c is ~1400 K, not as hot as Kepler-101b.

Kepler-101 is an interesting planetary system because it does not follow the trend whereby in compact two-planet systems with at least one Neptune-sized or larger planet, the larger of the two planets usually has the longer orbital period. Kepler-101b is a relatively uncommon planet because in the mass-radius diagram of known transiting planets with radius less than 12 Rᴇ and mass less than 500 Mᴇ, the position of the planet is in the underpopulated transition region between Neptune-like and Saturn-like planets.

Figure 2: Transit light curve indicating the presence of the hot super-Neptune Kepler-101b. Bonomo et al. (2014)

Figure 3: Radial velocity curve indicating the presence of the hot super-Neptune Kepler-101b. Bonomo et al. (2014)

Figure 4: Transit light curve indicating the presence of the Earth-sized planet Kepler-101c. Bonomo et al. (2014)

Figure 5: The locations of Kepler-101b and Kepler-101c on the mass-radius diagram of known transiting planets with radius less than 12 Rᴇ and mass less than 500 Mᴇ. Bonomo et al. (2014)

Reference:
Bonomo et al. (2014), “Characterization of the Kepler-101 planetary system with HARPS-N. A hot super-Neptune with an Earth-sized low-mass companion”, arXiv:1409.4592 [astro-ph.EP]

The Hottest White Dwarfs in the Galaxy

White dwarfs are the dense leftover cores of stars that were not massive enough to end their lives in supernova explosions. After a white dwarf forms, it can be extremely hot, with surface temperatures exceeding ~100,000 K. H1504+65 and RXJ0439.8-6809 are two of the hottest white dwarfs known in the galaxy. Both white dwarfs are estimated to have ~0.015 ± 0.01 times the radius of the Sun, which translates to a radius of roughly 10,000 km.

Figure 1: Artist’s impression of a white dwarf.

Observations of H1504+65 and RXJ0439.8-6809 show that both white dwarfs have extreme surface compositions comprised of carbon-oxygen dominated atmospheres that are devoid of hydrogen and helium. It remains unknown as to how the hydrogen-helium envelopes of both white dwarfs can be eroded away to expose their hot carbon-oxygen interiors.

H1504+65 is estimated to have 0.68 to 1.02 times the mass of the Sun and its progenitor was probably a massive main-sequence star with 8 to 10 times the mass of the Sun. The surface temperature of H1504+65 is estimated to be 200,000 ± 20,000 K. H1504+65 is also relatively nearby, located ~2,000 light years away.

RXJ0439.8-6809 is record holder for the hottest white dwarf discovered to date and its surface temperature is estimated to be 250,000 ± 30,000 K. It has 0.73 to 1.02 times the mass of the Sun and it is the leftover core of a relatively massive star that contained several times the mass of the Sun. RXJ0439.8-6809 is located ~30,000 light years away, in the outskirts of the Milky Way galaxy.

The location of RXJ0439.8-6809 is puzzling because its progenitor star is probably too massive to have formed in the outskirts of the galaxy. As a result, the progenitor star of RXJ0439.8-6809 may once have been part of a binary system comprised of two massive stars located within the main disk of the galaxy. The progenitor star of RXJ0439.8-6809 was ejected into the outskirts of the galaxy when its companion star exploded in a supernova.

Figure 2: Positions of H1504+65 and RXJ0439.8-6809 in comparison with other white dwarfs. Werner & Rauch (2015)

Reference:
Werner & Rauch (2015), “Analysis of HST/COS spectra of the bare C-O stellar core H1504+65 and a high-velocity twin in the Galactic halo”, arXiv:1509.08942 [astro-ph.SR]

WASP-103b is a Hot-Jupiter Stretched by Tidal Forces

WASP-103b is a transiting hot-Jupiter in an ultra-short period orbit around a F8V star with 1.4 times the diameter and 1.2 times the mass of the Sun. Transit and radial velocity measurements show that WASP-103b has 1.6 times the diameter and 1.5 times the mass of Jupiter. Because WASP-103b orbits so close to its host star, it is expected to raise significant tides on its host star and experience tidally-induced orbital decay. Over a time interval of 10 years, the orbital period of WASP-103b could decrease by ~100 seconds.

Tidally-induced orbital decay may be detectable by precisely measuring when WASP-103b transits its host star to look for any slight deviations in periodicity. At present, this is not detectable as it requires many years of observations with high quality transit timing data. Nevertheless, more precise transit observations of WASP-103b have improved the time of mid-transit to an accuracy of 4.8 seconds. For comparison, the time of mid-transit was only accurate to 67.4 seconds at the time of the planet’s discovery. A more accurate time of mid-transit would help in future searches for tidally-induced orbital decay.

The extreme closeness of WASP-103b to its host star causes it to be tidally stretched into an ellipsoid with its longest axis oriented towards its host star. Assuming Rᴊᴜᴘ denotes the equatorial radius of Jupiter (i.e. 71,492 km); the dimensions of WASP-103b are 1.721 ± 0.075 Rᴊᴜᴘ at the substellar point, 1.710 ± 0.072 Rᴊᴜᴘ at the antistellar point, 1.537 ± 0.043 Rᴊᴜᴘ at its poles, and 1.571 ± 0.047 Rᴊᴜᴘ at its sides. WASP-103b is significantly distorted from a spherical shape, with its longest axis ~10 percent longer than its shortest axis.

Reference:
Southworth et al. (2014), “High-precision photometry by telescope defocussing. VII. The ultra-short period planet WASP-103”, arXiv:1411.2767 [astro-ph.EP]

Two Hot-Jupiters in a Twin Star System

WASP-94 is a wide binary system comprised of two stars with a projected separation of approximately 2700 AU. This system hosts two hot-Jupiters, one for each star. The primary and secondary stars in this system are identified as WASP-94A and WASP-94B, respectively. Observations have shown that hot-Jupiters are very rare objects. As a result, it is very unlikely to find a binary system with each star hosting a hot-Jupiter.

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

WASP-94A is a F8V star with 1.29 ± 0.10 times the mass and 1.36 ± 0.13 times the radius of the Sun, and its effective surface temperature is 6170 ± 80 K. It hosts a transiting hot-Jupiter, identified as WASP-94Ab. WASP-94Ab has 0.445 ± 0.026 times the mass and 1.72 ± 0.06 times the radius of Jupiter, and its orbital period is 3.95 days. The mass of WASP-94Ab was determined using the radial velocity method which measures how much the planet’s host star wobbles due to gravitational perturbations from the planet itself. Additionally, the Rossiter-McLaughlin effect is clearly observable each time WASP-94Ab transits its host star.

The Rossiter-McLaughlin effect occurs when a planet transits across the face of its host star. Since the star is rotating, half of its observable hemisphere will be rotating towards the observer (i.e. approaching quadrant) and the other half of its observable hemisphere will be rotating away from the observer (i.e. receding quadrant). Light from the star is blue-shifted on the approaching quadrant and red-shifted on the receding quadrant. Since the approaching and receding quadrants are symmetrical, a net redshift is generated when the planet is in front of the approaching quadrant and a net blueshift is generated when the planet is in front of the receding quadrant.

A net redshift to blueshift change indicates the planet is in a prograde orbit (i.e. planet orbits in the same direction as the star’s spin); while a net blueshift to redshift change indicates the planet is in a retrograde orbit (i.e. planet orbits in the opposite direction to the star’s spin). Measuring the Rossiter-McLaughlin effect allows the spin-orbit angle (i.e. angle of the planet’s orbital plane with respect to the spin axis of its host star) of WASP-94Ab to be determined and the measurements indicate that WASP-94Ab is in a retrograde orbit.

WASP-94B is a F9V star with 1.24 ± 0.09 times the mass and 1.35 ± 0.12 times the radius of the Sun, and its effective surface temperature is 6040 ± 90 K. It hosts a non-transiting hot-Jupiter identified as WASP-94Bb. WASP-94Bb was detected using the radial velocity method. The amplitude of the radial velocity measurements indicates that WASP-94Bb has a mass of at least 0.617 ± 0.028 times the mass of Jupiter, and the periodicity of the radial velocity measurements show that the orbital period of WASP-94Bb is 2.008 days.

Figure 2: Top: radial velocity measurements indicating the presence of WASP-94Ab. Middle: residuals of the best-fit curve to the radial velocity measurements. Bottom: zoom-in on the radial velocities measurements taken during the transit over-plotted with the best solution for the Rossiter-McLaughlin effect. M. Neveu-VanMalle et al. (2014)

Figure 3: Top: radial velocity measurements indicating the presence of WASP-94Bb. Bottom: residuals of the best-fit curve to the radial velocity measurements. M. Neveu-VanMalle et al. (2014)

Reference:
M. Neveu-VanMalle et al. (2014), “WASP-94 A and B planets: hot-Jupiter cousins in a twin-star system”, arXiv:1409.7566 [astro-ph.EP]

OGLE-2011-BLG-0265Lb is a Cold Jupiter-Mass Planet

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

OGLE-2011-BLG-0265Lb is a Jupiter-mass gas giant planet in orbit around a red dwarf star. It was detected using a technique known as gravitational microlensing. When a foreground star crosses the line of sight to a background star, the gravitational field of the foreground star can act as a lens and magnify the brightness of the background star. This phenomenon can be observed as a light curve where the brightness of the background star changes with time. If the foreground star has a planet around it, the presence of the planet can induce perturbations in the light curve.

The gravitational microlensing light curve indicating the presence of OGLE-2011-BLG-0265Lb yields two solutions. For the first solution, the planet has 1.0 ± 0.3 times the mass of Jupiter and it orbits a star with 0.23 ± 0.07 times the mass of the Sun. For the second solution, the planet has 0.6 ± 0.2 times the mass of Jupiter and it orbits a star with 0.15 ± 0.06 times the mass of the Sun. In both cases, the planet is ~2 AU from its host star - a red dwarf star. At that distance, the planet is well beyond the "snow line" of its host star and it can be considered a "cold Jupiter".

The discovery of OGLE-2011-BLG-0265Lb is an important one because gas giant planets are very rare around red dwarf stars. The core accretion mechanism of planet formation predicts that gas giant planets rarely form around red dwarf stars, while the disk instability mechanism of planet formation predicts that gas giant planets can form around red dwarf stars. Detecting more planets like OGLE-2011-BLG-0265Lb around red dwarf stars can provide more insight on the formation scenario of such planets.

Figure 2: Gravitational microlensing light curve indicating the presence of OGLE-2011-BLG-0265Lb. The planet’s perturbations to the light curve are marked with arrows. Skowron et al. (2014)

Reference:

Skowron et al. (2014), "OGLE-2011-BLG-0265Lb: a Jovian Microlensing Planet Orbiting an M Dwarf", arXiv:1410.8252 [astro-ph.EP]

Ongoing Star Formation in the Dwarf Galaxy Leo P

Leo P is a faint, gas-rich dwarf galaxy with ongoing star formation. It is located 5.28 ± 0.49 million light years away, at the edge of the Local Group, far from the influence of any massive galaxy. The total mass of stars in Leo P is estimated to be ~600,000 times the Sun’s mass. Leo P is also home to a massive star with at least 25 times the mass of the Sun. The presence of this star shows that massive stars can form even with star formation rates as low as ~0.00001 solar-mass per year. Observations of the different populations of stars in Leo P show a constant rate of star formation over the lifetime of the dwarf galaxy.

The luminosity of Leo P appears similar to the dwarf spheroidal (dSph) galaxies around the Milky Way. However, unlike Leo P, the dSph galaxies around the Milky Way contain little to no gas, and have no ongoing stars formation. This indicates that Leo P is what a dSph galaxy would look like if it evolved in an isolated environment and held on to its gas content. It also shows that the environment around the Milky Way has the effect of quenching star formation in its satellite dwarf galaxies.

Reference:
McQuinn et al. (2015), “Leo P: An Unquenched Very Low-Mass Galaxy”, arXiv:1506.05495 [astro-ph.GA]

2013 AZ60 is a Potential Super-Comet

2013 AZ60 is an extreme object in a highly eccentric orbit around the Sun. It comes as close as 7.9 AU from the Sun (i.e. between the orbits of Jupiter and Saturn) and swings out to a whopping ~1950 AU from the Sun. Spending most of its time far from the Sun, the orbital period of 2013 AZ60 is estimated to be ~30,000 years. 2013 AZ60 is a trans-Neptunian object (TNO), and it may be classified either as a Centaur based on its closest distance from the Sun or as a scattered disk object based on its large average distance from the Sun.

Figure 1: Artist’s impression of an icy object far from the Sun. Image credit: ESA.

Optical measurements of 2013 AZ60 show that it has a rotation period of roughly 9.4 hours and a change in brightness of only 4.5 percent during each rotation. Thermal measurements of 2013 AZ60 were also done to estimate the object’s size and reflectivity. The benefit of thermal measurements is that it can distinguish whether an object is “large but dim” or “small but bright”. From the thermal measurements, 2013 AZ60 is estimated to have a diameter of 62.3 ± 5.3 km and a remarkably low geometric albedo of only 2.9 percent. The low albedo indicates that the surface of 2013 AZ60 is extremely dark.

Simulations of the orbit of 2013 AZ60 show that its present orbit is highly unstable. There is a 50 percent chance 2013 AZ60 will be ejected from the Solar System within the next ~700,000 years and a ~4 percent chance it will be perturbed into an Earth-crossing orbit. Given its relatively large size, 2013 AZ60 will be a super-comet if it ever gets perturbed into the inner Solar System. The highly unstable orbit of 2013 AZ60 indicates that the object was only recently perturbed into its current orbit and it is likely a pristine object that came in from the Oort cloud.

Figure 2: Slope parameter versus albedo relations for 111 TNOs, including 2013 AZ60 and 2012 DR30 (an object with a similar orbit as 2013 AZ60). The purple square at the very left side of the diagram represents 2013 AZ60 and the other purple square represents 2012 DR30. A. Pál et al. (2015)

Reference:
A. Pál et al. (2015), “Physical properties of the extreme centaur and super-comet candidate 2013 AZ60”, arXiv:1507.05468 [astro-ph.EP]

Pluto with an Iron Core

Figure 1: Image of Pluto taken by NASA’s New Horizons spacecraft. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Observations of Pluto and Charon by NASA’s New Horizons spacecraft show the presence of numerous geologically young surface features which serve as evidence for recent geological activity. For example, Pluto has enormous ice mountains ~3 km in hight, and a large, craterless icy plain named Sputnik Planum. It is worth considering whether the presence of an iron core in Pluto and in Charon may have given rise to the geological features seen on their surfaces.

Since all inner planets in the Solar System, including the Moon, have iron-rich cores, it is not improbable for Pluto and Charon to also possess iron cores. Interior models of Pluto and Charon typically have two components - a rocky core and an overlying shell of ice. The presence of an iron core will mean a three component interior structure consisting of an iron core, a rocky mantle and an outer shell of ice. Just after formation, the interior of Pluto was supposedly much hotter than it currently is, and may have supported a molten iron core.

As Pluto cools, its iron core solidifies inwards, beginning from the core-rock boundary. Since molten iron is denser than solid iron, the iron core contracts and rock from the mantle above descends to occupy the space that becomes available. To fill the space created by the descending rock, some of the ice at the rock-ice boundary descends as well. Such a process occurring on Pluto can also occur, possibly to a lesser extend, on Charon. The descending ice at the rock-ice boundary can potentially create widespread surface deformations on Pluto and Charon, thereby explaining the many geologically young surface features observed by the New Horizons spacecraft.

Reference:
A. Aitta et al. (2015), “Internal structure of Pluto and Charon with an iron core”, arXiv:1510.06604 [astro-ph.EP]

Supernovae Enrichment in the Globular Cluster NGC 6273

Massive stars end their lives in powerful supernovae explosions. These explosions eject massive quantities of heavy elements into space and these heavy elements can get incorporated into subsequent generations of stars. As a result, these stars become more enriched with heavy elements. A number of massive globular clusters such as Omega Centauri and Terzan 5 show evidence for supernova enrichment. These massive globular clusters are believed to be the leftover nuclei of disrupted dwarf galaxies, and unlike typical globular clusters, they are massive enough to retain supernovae ejecta.

Observations of the globular cluster NGC 6273 show that it too has evidence for supernovae enrichment. The red giant stars in NGC 6273 appear to fall into two distinct populations with different calcium abundances. Other observations of the red giant stars in NGC 6237 also show two distinct populations - a metal-poor group with a lower abundance of iron and a metal-rich group with a higher abundance of iron. These observations suggest that NGC 6273 was massive enough to retain supernovae ejecta, allowing it to have subsequent generations of stars that are more enriched with heavy elements.

Figure 1: Artist’s impression of the view from a planet near a globular cluster.

References:
- Han et al. (2015), “Evidence for Enrichment by Supernovae in the Globular Cluster NGC 6273”, arXiv:1510.06044 [astro-ph.GA]
- Johnson et al. (2015), “A Spectroscopic Analysis of the Galactic Globular Cluster NGC 6273 (M19)”, arXiv:1507.00756 [astro-ph.SR]

The Leftover Nucleus of a Dwarf Galaxy

NGC 3628-UCD1 is an ultra-compact dwarf that is embedded in a stream of stars around the spiral galaxy NGC 3628. UCD1 is made up of a compact cluster of stars with a total luminosity of approximately 1.4 million times the Sun’s luminosity and a half-light radius of roughly 30 light years. UCD1 is likely to be associated with the stream of stars it is embedded in. This is because UCD1 is located in the brightest region of the stream of stars and the spatial density of stars in the stream appears to fall off gradually in all directions away from UCD1.

The size and luminosity of UCD1 is remarkably similar to Omega Centauri, the most luminous Milky Way globular cluster. Omega Centauri is believed to be the leftover nucleus of a dwarf galaxy that was tidally stripped as it got accreted by the Milky Way. UCD1 and the stream of stars it is embedded in were probably once a dwarf galaxy before it got tidally stripped and accreted by the much larger spiral galaxy NGC 3628. Measurements of the total brightness of the stream of stars UCD1 is embedded in suggest it was once a dwarf galaxy with approximately 40 million times the Sun’s luminosity.

Reference:
Jennings et al. (2015), “NGC 3628-UCD1: A possible ω Cen Analog Embedded in a Stellar Stream”, arXiv:1509.04710 [astro-ph.GA]