Wednesday, April 30, 2014

HD 141399’s Retinue of Four Giant Planets

A planetary system consisting of 4 giant planets has been detected around HD 141399, a nearby, slightly evolved, K-type star. This discovery was made by Vogt et al. (2014) using precise radial velocity data from the HIRES (High Resolution Echelle Spectrometer) instrument on the Keck-I telescope and the Levy spectrometer on Lick Observatory’s Automated Planet Finder (APF). 91 observations spread over 10.5 years indicate the presence of 4 planets with orbital periods of 94.35, 202.08, 1070.35 and 3717.35 days, and minimum masses of 0.46, 1.36, 1.22 and 0.69 Jupiter-mass. In order of increasing distance from the host star, the 4 planets are dubbed HD 141399 b, c, d and e.

Figure 1: Artist’s impression of a ringed giant planet with a moon in the foreground. Image credit: Nick Stevens.

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

Compared to the Sun, HD 141399 has 1.59 ± 0.39 times its luminosity, 1.46 ± 0.15 times its radius and 1.14 ± 0.08 times its mass. HD 141399 is a rare example of a star with a planetary system consisting of multiple giant planets. Other examples of such planetary systems include Upsilon Andromedae - a system of 4 giant planets, and 55 Cancri A - a system with one close-in super-Earth and 4 giant planets. An unusual aspect of HD 141399 is that its inner 3 giant planets lie at distances normally associated with the terrestrial planets (i.e. Mercury, Venus, Earth and Mars) in our Solar System. The fourth planet, labelled HD 141399 e, is a Jupiter-like planet with a Jupiter-like orbital period of roughly a decade.

The two inner planets - HD 141399 b and c, have estimated equilibrium temperatures of 500 K and 390 K, respectively. At these temperatures, the atmospheres of both planets are expected to be dominated by water clouds and complex chemistry. Transits by HD 141399 b or c, if they do occur, would be of great scientific value since it would allow their atmospheres to be studied via transmission spectroscopy by the Hubble Space Telescope (HST) or James Webb Space Telescope (JWST). HD 141399 b and c lie in a relatively unexplored temperature regime between the well-studied hot-Jupiters at one extreme and the giant planets (i.e. Jupiter and Saturn) in our Solar System. Nevertheless, the odds that HD 141399 b or c can be observed in transit are 1.3 and 0.8 percent, respectively.

Vogt et al. (2014), “A 4-Planet System Orbiting the K0V Star HD 141399”, arXiv:1404.7462 [astro-ph.EP]

Saturday, April 26, 2014

A Cold Brown Dwarf in the Sun’s Neighbourhood

By analysing data from NASA’s Wide-field Infrared Survey Explorer (WISE) and Spitzer Space Telescope, Kevin Luhman, an astronomer at Pennsylvania State University recently announced the discovery of the coldest brown dwarf found to date in a paper published on 21 April 2014 in the Astrophysical Journal Letters. The brown dwarf, identified as WISE J085510.83-071442.5 (hereafter WISE J0855-0714) is reported to have an estimated temperature of 225 to 260 K (-48 to -13 °C) and a mass of 3 to 10 times Jupiter’s mass. This makes WISE J0855-0714 literally as cold as ice, about as chilly as summer at the South Pole. Before this discovery, the coldest known brown dwarfs, also found by WISE and Spitzer, were about room temperature.

Figure 1: Artist’s impression of a cold brown dwarf. Image credit: NASA/JPL-Caltech.

Brown dwarfs are objects that form as stars do, but they lack the mass to sustain nuclear fusion in their cores to shine like stars. They cool with time and emit nearly all of their energy in the form of infrared radiation. WISE was able to detect WISE J0855-0714 because it surveyed the entire sky in infrared twice, with some areas up to three times. The area of sky where WISE J0855-0714 is situated was imaged by WISE on 4 May 2010 and 11 November 2010. Between these two epochs of WISE images, WISE J0855-0714 was found to have moved by an amount which indicates a proper motion that is unusually high among the known stars. In fact, the proper motion of WISE J0855-0714 is the 3rd highest for any known object outside the Solar System, behind only Barnard’s star and Kapteyn’s star. “This object appeared to move really fast in the WISE data,” said Luhman. “That told us it was something special.”

The high proper motion of WISE J0855-0714 indicates that it must be a relatively nearby object. An analogy for this is the view looking out the window of a moving train where nearby foreground objects would appear to move by much more rapidly than distant mountains. Combined with data from Spitzer, WISE J0855-0714 is estimated to lie at a distance of only 7.2 light-years, making it the 4th closest system to the Sun. The three closest systems are Alpha Centauri AB (including Proxima Centauri), Barnard’s star and Luhman 16AB. The discovery of WISE J0855-0714 demonstrates just how little is known about the population of objects in the Sun’s neighbourhood. “It is remarkable that even after many decades of studying the sky, we still do not have a complete inventory of the Sun’s nearest neighbours,” said Michael Werner, the project scientist for Spitzer at NASA’s Jet Propulsion Laboratory in Pasadena, California. In fact, the discovery of Luhman 16AB - a pair of warmer brown dwarfs at a distance of 6.5 light-years, was made by Luhman only in March of 2013.

Figure 2: This diagram illustrates the locations of the star systems closest to the Sun. The year when the distance to each system was determined is listed after the system’s name. Data from WISE led to the discovery of two of the four closest systems: the binary brown dwarf Luhman 16AB and the brown dwarf WISE J0855-0714. Image credit: NASA/Penn State University.

WISE J0855-0714 could either be a brown dwarf or a gas giant planet that was ejected from its planetary system. The latter is probably unlikely since planetary-mass brown dwarfs are known to exist, while the frequency of ejected gas giant planets is still unknown. Without the overwhelming glare from a nearby star, WISE 0855-0714 presents a good opportunity to study atmospheric models in an unexplored temperature regime, offering more insights about the atmospheres of planets. In depth observations of the atmosphere of WISE 0855-0714 can be done with the deployment of the James Webb Space Telescope (JWST). WISE J0855-0714 is most probably a Y-dwarf and its discovery means that the 4 closest known systems to the Sun consist of at least one object of each spectral type from G through Y. This lettering is a classification scheme for stars and brown dwarfs using the letters O, B, A, F, G, K, M, L, T and Y - a sequence from the hottest (O-type) to the coolest (Y-type).

K. L. Luhman (2014), “Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun”, ApJ 786 L18

Thursday, April 24, 2014

Occurrence Rates of Circumbinary Planets

Figure 1: Artist’s impression of circumbinary planets. Image credit: NASA/JPL-Caltech/T. Pyle.

Circumbinary planets are a subset of planets that orbit two stars instead of one. In recent years, precise photometric data from NASA’s Kepler space telescope has lead to the detection of several circumbinary planets. Kepler is a planet detection telescope that measures tiny dips in a star’s brightness when a planet happens to transit in front of the star. Using publicly available Kepler data, Armstrong et al. (2014) present the first ever estimations for the rate of occurrence of circumbinary planets. The study examines binary stars with orbital periods < 60 days, and planets > 4 Earth-radius with orbital periods < 300 days.

The rate of occurrence of circumbinary planets largely depend on the inclination distribution of their orbits with respect to the orbits of their host binaries. Results from the study show that if circumbinary planets have orbits that are preferentially coplanar with their host binaries, their rate of occurrence will be ~10 percent. Instead, if the orbits of circumbinary planets have an isotropic distribution, their rate of occurrence increases dramatically to at least ~50 percent. This is expected because in the isotropic distribution case, many more circumbinary planets must exist in order to produce the few detected transits.

Figure 2: Probability density functions for the rate of occurrence of circumbinary planets following a Gaussian inclination distribution with one-sigma inclination spread of 5 degrees. The distributions are shown for (from left to right) planets with >10, 8-10, 6-10 and 4-10 Earth-radius. The >10 Earth-radius density function has been scaled down by a factor of three for clarity, and takes a different form to the others due to the zero detections of planets within this group. Source: Armstrong et al. (2014).

Another result from the study is that Jupiter-like circumbinary planets, > 10 Earth-radius, in coplanar orbits, are much less common than Saturn-like or smaller equivalents. This is in line with the suggestion by Pierens & Nelson (2008) that higher mass planets in coplanar circumbinary orbits have increased chances of being ejected. So far, this study is merely a first rough estimate on the rate of occurrence of circumbinary planets. A larger sample of such planets is required to better understand their occurrence rates.

- Armstrong et al. (2014), “On the Abundance of Circumbinary Planets”, arXiv:1404.5617 [astro-ph.EP]
- Pierens & Nelson (2008), “On the evolution of multiple low mass planets embedded in a circumbinary disc”, Astronomy and Astrophysics, 483, 633

Monday, April 21, 2014

Habitability of Kepler-186f

The Kepler-186 system consists of 5 known planets circling a red dwarf star. The 5 planets have sizes ranging from 1.0 - 1.5 Earth-radius and orbital periods of 3.9 - 130 days. All 5 planets are probably rocky, since planets with large hydrogen-helium gas envelops tend to be larger than 1.5 - 2.0 Earth-radius. Of particular interest is Kepler-186f, the fifth planet in the system. Kepler-186f is the first confirmed Earth-sized planet in the habitable zone around another star. Its detection was reported by Quintana et al. (2014) in a paper published in the April 18 issue of the journal Science. Another paper by Bolmont et al. (2014) evaluates the habitability of the Kepler-186 system and, in particular, the habitability of Kepler-186f. Additionally, the paper also investigates the formation and tidal evolution of the Kepler-186 planetary system.

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

Figure 2: Orbital configuration of the Kepler-186 planetary system. The shaded regions denote the habitable zone. The bottom part of the plot shows a comparison between 4 different planetary systems that contain planets in the habitable zone: the Solar System, Kepler-62, Kepler-186 and GJ 581. Source: Bolmont et al. (2014).

Being situated in the habitable zone does not necessarily imply than Kepler-186f is habitable. Habitability also depends on the planet’s atmospheric characteristics. In the study, simple climate models are used to assess the habitability of Kepler-186f. The model atmospheres are assumed to be composed of carbon dioxide (CO2), nitrogen (N2) and water (H2O) only. A key criterion for habitability is the ability for a planet to sustain liquid water on its surface. In the case for Kepler-186f, to keep mean surface temperatures above 273 K - the freezing point of water, the models show that modest amount of CO2 are needed in most of the cases. For large amounts of atmospheric N2 (~10 bars), 200 - 500 mbar of CO2 is all that is required to keep surface temperatures above 273 K.

Figure 3: Surface temperature as a function of CO2 partial pressure, for different N2 partial pressures. Water triple point temperature of 273 K is indicated by the horizontal dashed line. Top to bottom rows: decreasing insolation - 0.32, 0.29 and 0.27 (Earth = 1.0). Left to right columns: increasing gravity. Source: Bolmont et al. (2014).

Given such favourable prospects for habitability, it is worth considering the possibility of photosynthesis occurring on Kepler-186f. The amount of insolation Kepler-186f receives from its host star is estimated to be 32 percent the intensity of insolation Earth receives from the Sun. An atmosphere containing 5 bar of CO2 and 1 bar of N2 would support a surface temperature of 285 K on Kepler-186f, close to Earth’s current mean surface temperature. Due to atmospheric absorption, such an atmosphere would further depress the amount of insolation that reaches the surface of Kepler-186f to a factor of 7 times less insolation than what the Earth’s surface gets. At wavelengths of 500 - 700 nm, corresponding to plant chlorophyll, the difference becomes even larger with Earth getting 10 - 20 times more flux than Kepler-186f. Although such a low level of insolation does not preclude photosynthesis, it does suggest that photosynthesis on Kepler-186f would occur at a much slower rate than on Earth.

Figure 4: Net stellar insolation received at the top of atmosphere (TOA) and at the surface for Kepler-186f, assuming an atmosphere containing 5 bar of CO2 and 1 bar of N2. This is shown in comparison to modern Earth. Source: Bolmont et al. (2014).

- Quintana et al., “An Earth-Sized Planet in the Habitable Zone of a Cool Star”, Science 18 April 2014: Vol. 344 no. 6181 pp. 277-280.
- Bolmont et al. (2014), “Formation, tidal evolution and habitability of the Kepler-186 system”, arXiv:1404.4368 [astro-ph.EP]

Friday, April 18, 2014

Kepler-186f: An Earth-Sized Planet in the Habitable Zone

“In space there are countless constellations, suns and planets; we see only the suns because they give light; the planets remain invisible, for they are small and dark. There are also numberless earths circling around their suns, no worse and no less than this globe of ours.”
- Giordano Bruno, 1584

In recent years, many planets have been found within the habitable zone - the region around a star where the temperatures are just right for a planet to sustain liquid water on its surface. By analysing data from NASA’s Kepler space telescope, a team of astronomers have announced the discovery of the most Earth-like planet yet detected. Kepler is a planet-hunting telescope that measures subtle changes in the brightness of stars to see if an orbiting planet is crossing in front of a star. This newfound planet, dubbed Kepler-186f, is the first confirmed Earth-sized planet in the habitable zone of another star. “This is the first definitive Earth-sized planet found in the habitable zone around another star,” said Elisa Quintana, research scientist at the SETI Institute at NASA’s Ames Research Center, and lead author of the paper published on April 18 in the journal Science.

Figure 1: Artist’s depiction of Kepler-186f, the first validated Earth-sized planet to orbit in the habitable zone of another star. Image credit: NASA Ames/SETI Institute/JPL-Caltech.

Before the announcement of Kepler-186f, the smallest habitable zone planets known are all somewhat larger than the Earth, placing them in the super-Earth-sized rather than Earth-sized regime. Although these planets are still potentially habitable, their environments are likely to be quite different compared to the Earth. Examples of these habitable zone super-Earths include Kepler-62e, Kepler-62f and Kepler-22b. Furthermore, Kepler has also detected a number of planets the size of Earth or smaller around other stars. However, these planets all orbit too close to their host stars and are therefore too hot to be habitable. Examples of these close-in planets include Kepler-20e, Kepler-20f, Kepler-78b and Kepler-37c. Unlike these planets which are either ‘at the right distance but too large’ or ‘at the right size but too close’, Kepler-186f is the first confirmed planet that has both the right size and the right distance.

Kepler-186f is part of a planetary system with 4 other known planets. The 4 companion planets, Kepler-186b, Kepler-186c, Kepler-186d and Kepler-186e, circle around their host star every 3.89, 7.27, 13.3 and 22.4 days. All 4 planets orbit much closer-in than Kepler-186f and are therefore too hot to be habitable. These 4 planets range in size from 1.0 to 1.5 Earth-radius and were confirmed using the first 2 years of data collected by Kepler. The detection of Kepler-186f, the fifth planet, required an additional year of data.

Kepler-186f measures only 10 percent larger than Earth (1.11 ± 0.14 Earth-radius), making it a truly Earth-sized planet. Its host star is a red dwarf star about half the size of Earth’s Sun and located about 500 light years from Earth. Kepler-186f orbits within the habitable zone of its host star - the “Goldilocks” zone that is neither too hot nor too cold for liquid water to exist if Kepler-186f has an Earth-like atmosphere. Being a red dwarf star, the host star of Kepler-186f is cooler and dimmer than the Sun. It means that the habitable zone around the host star of Kepler-186f is located much closer-in compared to the habitable zone around the Sun.

Figure 2: The diagram compares the planets of our inner solar system to the planetary system hosting Kepler-186f. The parent star of Kepler-186f is a red dwarf star with half the size and mass of the Sun. Image credit: NASA Ames/SETI Institute/JPL-Caltech.

 Figure 3: The 5 planets of the Kepler-186 planetary system compared to the inner planets of our solar system. Image credit: NASA.

Kepler-186f circles its host star in the habitable zone once every 130 days. The orbit of Kepler-186f around its host star is actually slightly smaller than the orbit of Mercury around the Sun in our own solar system. Kepler-186f’s orbit places it near the cooler, outer edge of the habitable zone. Despite receiving only 32 percent of the intensity of stellar radiation that Earth receives from the Sun, Kepler-186f is in fact comfortably within the habitable zone. This is because light from a red dwarf star is “redder” than light from stars like the Sun and it changes how an Earth-like planet would interact with the star’s light.

Compared to the Sun, whose dominant form of radiation is in the visible wavelength, red dwarf stars are cooler and a larger proportion of their energy output is in the form of infrared radiation. For Earth-like planets around red dwarf stars, infrared radiation is absorbed by ice instead of being reflected. Additionally, water vapour and carbon dioxide also absorb and trap infrared radiation. These characteristics make Kepler-186f more efficient at absorbing energy from its host star to avoid freezing over. As a result, Kepler-186f is still considered habitable even though it receives less light from its host star than Mars receives from the Sun.

Figure 4: Position of Kepler-186f within the habitable zone, show in comparison with a number of other known planets. Image credit: NASA/Chester Harman.

Although the size of Kepler-186f is known, its mass is not known since data from Kepler is unable to yield such measurements. In spite of this, Kepler-186f is small enough for its composition and mass to be well constrained using existing planetary models. These models predict that planets smaller than 1.5 times the size of Earth are unlikely to be dominated by hydrogen-helium gas envelopes like Jupiter or Neptune in our own solar system. Mass estimates for Kepler-186f range from 0.32 Earth-mass for a pure water/ice composition to 3.77 Earth-mass for a pure iron composition. The composition of Kepler-186f is highly unlikely to be anywhere close to these two extremes. Instead, Kepler-186f probably lies somewhere in the middle, most likely with a rocky composition similar to Earth. For an Earth-like composition, Kepler-186f would have a mass of 1.44 Earth-mass.

Red dwarf stars, like the host star of Kepler-186f, are by far the most common type of star in the galaxy. Such stars make up over 80 percent of the closest stars to the Sun. They are less massive, smaller, cooler and dimmer than the Sun, and they range in size from around 10 to 50 percent the size of the Sun. Besides being the most abundant type of stars, red dwarfs stars are good targets in the search for transiting habitable Earth-sized planets. This is because a red dwarf star is smaller than a Sun-like star, so an Earth-sized planet around a red dwarf star would have a larger planet-to-star size ratio. As a consequence, an Earth-sized planet transiting a red dwarf star would create a deeper transit than if the same planet were to transit a larger Sun-like star. Deeper transits are easier to detect than shallower ones.

Figure 5: Diagram showing the deeper transit depth of an Earth-sized planet transiting a red dwarf star. Image credit: NASA.

Furthermore, transiting planets in the habitable zone of red dwarf stars would undergo more frequent transits than those in the habitable zone of Sun-like stars. This is because the habitable zone of red dwarf stars is much closer-in, resulting in shorter orbital periods than those around Sun-like stars. Given the abundance of red dwarf stars, planets such as Kepler-186f are almost certainly the most abundant type of habitable planet. Unfortunately, Kepler-186f itself is just too far away for even future NASA missions, like the Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope (JWST) to determine its composition and atmosphere. Nevertheless, TESS and JWST will be able to detect and characterize Earth-sized planets around the nearest stars. Most of these planets would be around red dwarf stars, much like Kepler-186f.

The discovery of Kepler-186f supports the emerging view that Earth may not be such a unique place, and that the galaxy is home to billions and billions of habitable worlds. “The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth,” said Paul Hertz, NASA’s Astrophysics Division director at the agency’s headquarters in Washington. “Future NASA missions, like the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope, will discover the nearest rocky exoplanets and determine their composition and atmospheric conditions, continuing humankind’s quest to find truly Earth-like worlds.”

Quintana et al., “An Earth-Sized Planet in the Habitable Zone of a Cool Star”, Science 18 April 2014: Vol. 344 no. 6181 pp. 277-280.

Sunday, April 13, 2014

An Isolated Dwarf Galaxy GHOSTS I

Using data from the Hubble Space Telescope, A. Monachesi et al. (2013) report the discovery of a faint dwarf galaxy, GHOSTS I. Based on observations of some of the stars in that galaxy, the estimated distance of GHOSTS I is ~40 million light years. GHOSTS I appears to be a very isolated dwarf galaxy as there are no large galaxies anywhere within ~13 million lights years from it. Also, the process of star-formation seems to be evident within GHOSTS I, resulting in it being tentatively classified as a dwarf irregular (dIrr) galaxy. Nevertheless, more observations are probably needed for a more confident classification.

The Small Magellanic Cloud, a dwarf irregular galaxy located only 200,000 light years from the Milky Way. Credit: Stéphane Guisard.

In general, most dwarf galaxies that are far from large galaxies tend to be dIrr galaxies, while those closer to large host galaxies tend to be dwarf spheroidal (dSph) galaxies. There is a dichotomy between dIrr and dSph galaxies - dIrr galaxies have ongoing star-formation, while dSph galaxies do not. It is believed that tidal and ram pressure effects from large host galaxies probably transformed many star-forming dIrr galaxies into non-star-forming dSph galaxies. Work by Slater & Bell (2013) show that even a single close passage by a large galaxy can extinguish star-formation, transforming a dIrr galaxy to a dSph galaxy.

GHOSTS I is tens of thousands of times fainter than a large galaxy like the Milky Way or Andromeda. Together with just a few other dwarf galaxies such as Leo T and Leo P, GHOSTS I is one of the faintest and least-massive star-forming dwarf galaxies known. Future observations of GHOSTS I can reveal how such tiny galaxies can retain gas and form stars.

- A. Monachesi et al. (2013), “GHOSTS I: A New Faint very Isolated Dwarf Galaxy at D = 12 +/- 2 Mpc”, arXiv:1312.0602 [astro-ph.GA]
- Slater & Bell (2013), “Confronting Models of Dwarf Galaxy Quenching with Observations of the Local Group”, arXiv:1306.1829 [astro-ph.CO]

Saturday, April 12, 2014

Origin of the Equatorial Ridge on Iapetus

Iapetus is the 3rd largest moon around Saturn and like many of Saturn's moons; it is locked in synchronous rotation where the same hemisphere faces Saturn all the time. A striking two-tone colouration exists over the leading and trailing hemispheres of Iapetus. The leading hemisphere and sides are dark (albedo 0.03 - 0.05), while most of the trailing hemisphere and poles are bright (albedo 0.5 - 0.6). This two-tone colouration has a pattern analogous to a spherical yin-yang symbol.

Figure 1: A mosaic showing an entire hemisphere of Iapetus. Credit: NASA/JPL/Space Science Institute.

Iapetus has a remarkable equatorial ridge system extending over 110 degrees in longitude. Parts of the ridge have peaks that rise up to 20 km above the surrounding landscape, making these mountains amongst the highest in the Solar System. The prominent equatorial ridge system gives Iapetus an overall walnut-like appearance. A number of endogenic (i.e. processes such as tectonism or volcanism arising from the interior of Iapetus) and exogenic (i.e. processes such as debris in-fall that originate from beyond the surface of Iapetus) mechanisms have been proposed to explain the origin of the equatorial ridge system.

A recent study by Lopez Garcia et al. (2014) suggests an exogenic formation mechanism for the equatorial ridge on Iapetus. Using a total of 506 topographic profiles of the ridge system obtained by NASA’s Cassini spacecraft, a topographic analysis was performed where 6 types of ridge morphologies were identified - triangular (33% of profiles), trapezoidal (21% of profiles), crowned (8% of profiles), twinned (14% of profiles), dissimilar (7% of profiles) and saddle (17% of profiles). The triangular peaks form the most common morphology and have the steepest slopes, with some slopes reaching ~40 degrees. Also, the triangular peaks are probably the least impact-modified ridge morphology on Iapetus.

Figure 2: Representative examples of the six ridge morphological types observed in the topographic profiles. Vertical exaggeration ~10 times. Source: Lopez Garcia et al. (2014).

As proposed by Ip (2006), an exogenic origin for the ridge system would most probably occur via debris in-fall, whereby the ridge is the remains of an early ring system that collapsed onto the equator of Iapetus. Depending on the material, the resulting ridge would have slopes with angles close to the angle of repose. For rounded icy grains, the angle of repose is ~25 degrees; and for snow mixed with particles of hail, the angle of repose is ~45 degrees. Indeed, as shown in this study, the presence of slope angles close to the angle of repose favours an exogenic origin for the equatorial ridge on Iapetus.

- Lopez Garcia et al. (2014), “Topographic Constraints on the Origin of the Equatorial Ridge on Iapetus”, arXiv:1404.2337 [astro-ph.EP]
- Ip (2006), “On a ring origin of the equatorial ridge of Iapetus”, Geophysical Research Letters 33, L16203.

Wednesday, April 9, 2014

Influence of Hot Jupiters on their Host Stars

A star’s magnetic activity allows it to extend its influence far beyond its surface. The Sun’s magnetic field extends billions of kilometres into space, far beyond the orbit of Pluto. Phenomena such as stellar flares and coronal mass ejections are magnetically induced processes that can profoundly influence the atmospheres of close-in exoplanets. It is well established that the magnetic activity of a star depends on its rate of rotation. A star’s rotation gradually slows as angular momentum is carried away by the stellar wind. As a result, the magnetic activity of stars like the Sun decreases over billions of years.

Nevertheless, stars hosting hot-Jupiters (i.e. massive planets in close-in orbits) can maintain fast rotation rates and hence high magnetic activity. In a study by K. Poppenhaeger and S.J. Wolk (2014), 5 binary star systems with widely-spaced stars (over 100 AU apart) were observed in X-rays. For each binary system, one star is known to host a close-in massive planet, while the other star does not have a detected planet and acts as a negative control. Using X-ray emission as an observational proxy for stellar magnetic activity, the stars in each binary system were observed with Chandra and XMM-Newton.

The 5 binary star systems involved in the study are HD189733 AB, CoRoT-2 AB, Tau Boötis AB, Upsilon Andromedae AB and 55 Cancri AB. For 2 of the systems - HD189733 AB and CoRoT-2 AB, where the strongest tidal interaction is expected between planet and host star, the X-ray emission of the planet-hosting star is stronger than expected when compared to the companion star. It implies that when compared to its companion star, the planet-hosting star is over-rotating.

This study shows that hot-Jupiters may inhibit the spin-down of their host stars. Two possible mechanisms can drive this process. One involves the transfer of angular momentum from the planet’s orbit to the star’s rotation through tidal interaction, spinning-up the star as a result. The second mechanism involves the hot-Jupiter opening a gap in the protoplanetary disk during the early evolution of the star, resulting in a weaker star-disk coupling and causing a smaller rate of spin-down.

K. Poppenhaeger and S.J. Wolk (2014), “Indications for an influence of Hot Jupiters on the rotation and activity of their host stars”, arXiv:1404.1073 [astro-ph.SR]

Tuesday, April 8, 2014

Supermassive Stars around Supermassive Black Holes

Quasars are among the most energetic and luminous objects known in the universe. These objects are powered by supermassive black holes (SMBHs) accreting matter at prodigious rates in the centres of massive galaxies. SMBHs have masses ranging from ~100 million to billions of times the Sun’s mass. A quasar consists of a SMBH surrounded by a massive accretion disk. The accretion disk feeds matter to the black hole, thereby powering the quasar. A number of studies have proposed that exotic supermassive stars may form in such an accretion disk, somewhat like how planets form in protoplanetary disks around young stars.

An artist’s impression of ULAS J1120+0641, a distant quasar powered by a black hole with 2 billion times the Sun’s mass. Credit: ESO/M. Kornmesser.

Beyond a few hundred to a few thousand Schwarzschild radii from the SMBH, a quasar’s accretion disk starts to become self-gravitating. The term “Schwarzschild radius”, is a unit of measurement, where a value of one Schwarzschild radius is the distance from a black hole where the escape speed would equal the speed of light. For a SMBH with 100 million times the Sun’s mass, its Schwarzschild radius would be 300 million km.

When a quasar’s accretion disk becomes self-gravitating, it can fragment into gravitationally bound clumps and form very massive stars with masses easily exceeding a few hundred times the Sun’s mass. The term “supermassive” is truly justified here since such stars can attain masses of up to perhaps ~10,000 times the Sun’s mass, making them far more massive than the most massive stars currently known.

Once a supermassive star forms, it can clear a gap in the quasar’s accretion disk and start migrating towards the central SMBH. The migration timescale to the SMBH is comparable to the lifespan of the supermassive star. During this period, the supermassive star can interrupt the flow of gas to the SMBH and temporary dim the quasar. The final merger of the supermassive star, or what is left of it, with the SMBH could be a strong source of low frequency gravitational waves.

Detecting the presence of a supermassive star can be challenging. Although supermassive stars are very luminous, they would still be overwhelmed by the glare of the nearby quasar. Nevertheless, the presence of a supermassive star around the SMBH of a quasar can show up as a periodic milli-magnitude amplitude modulation of the quasar’s brightness, with a period ranging from a few days to a few years. The physical conditions found in a quasar’s accretion disk are much more extreme than in a conventional star-forming region. This can favour the rapid formation of truly supermassive stars.

- Jeremy Goodman and Jonathan C. Tan (2004), “Supermassive Stars in Quasar Disks”, arXiv:astro-ph/0307361
- Yanfei Jiang and Jeremy Goodman (2010), “Star Formation in Quasar Disk”, arXiv:1011.3541 [astro-ph.HE]

Saturday, April 5, 2014

Enceladus’ Subsurface Ocean of Liquid Water

With a diameter of 500 km, Enceladus is a small icy moon of Saturn. Images taken by NASA’s Cassini spacecraft show large plumes of water vapour and ice erupting from the south-polar region on Enceladus. The source of these plumes, or geysers, is believed to be an ocean of liquid water beneath Enceladus’ icy crust. Tidal interactions between Enceladus and Dione (another moon of Saturn) generate the heat necessary to keep this body of water in a liquid state. A study done in 2011 found that Enceladus’ south-polar region pumps out an estimated 15.8 gigawatts of endogenic heat. This amount of heat is sufficient to maintain an ocean of liquid water under a thermally conductive icy crust.

Figure 1: Saturn’s moon Enceladus, covered in snow and ice, resembles a perfectly packed snowball in this image from Cassini. Credit: NASA/JPL-Caltech/Space Science Institute.

From 2010 to 2012, Cassini performed 3 close flybys of Enceladus that allowed for ultra-precise radio tracking of the spacecraft from Earth using the giant ground antennas of NASA’s Deep Space Network. For these close flybys, Cassini flew within 100 km of Enceladus’ surface, twice above the southern hemisphere and once over the northern hemisphere. During each flyby, the spacecraft’s velocity is perturbed by small but measurable amounts that depend on variations in the gravity field of Enceladus.

In a new study published in the April 4 issue of the journal Science, a team of researches used the Doppler data from the ultra-precise tracking measurements to map out Enceladus’ gravity field. “The way we deduce gravity variations is a concept in physics called the Doppler Effect, the same principle used with a speed-measuring radar gun,” said Sami Asmar of NASA’s Jet Propulsion Laboratory in Pasadena, California, a co-author of the paper. “As the spacecraft flies by Enceladus, its velocity is perturbed by an amount that depends on variations in the gravity field that we’re trying to measure. We see the change in velocity as a change in radio frequency, received at our ground stations here all the way across the Solar System.”

What the team found is the presence of a negative mass anomaly at Enceladus’ south-polar region. A negative mass anomaly means the area contains less mass than would be expected for a perfectly spherical body. Although a negative mass anomaly makes sense since Enceladus’ south-polar region is depressed by a depth of ~1 km, the observed negative mass anomaly turned out to the significantly smaller than expected. As a result, there must be “extra” mass beneath the surface to account for the smaller than expected negative mass anomaly.

The team’s calculations suggest that the presence of a subsurface ocean of liquid water, which is 8 percent denser than the surrounding ice, is the only reasonable explanation. In the model, the ocean is ~10 km think and lies beneath a shell of ice 30 to 40 km thick. The ocean extends from the pole to roughly 50° south latitude and its thickness diminishes toward the lower southern latitudes. Nevertheless, the current data does not rule out the possibility of a global ocean. Furthermore, the ocean is believed to be in direct contact with a rocky seafloor.

Figure 2: Enceladus’s gravity disturbances mapped onto a reference ellipsoid. The negative mass anomaly at the south-polar region is clearly indicated. Credit: L. Iess et al. (2014).

Figure 3: This diagram illustrates the possible interior of Saturn’s moon Enceladus based on a gravity investigation by NASA’s Cassini spacecraft and NASA’s Deep Space Network, reported in April 2014. The gravity measurements suggest an icy outer shell and a low density, rocky core with a regional water ocean sandwiched in between at high southern latitudes. Credit: NASA/JPL-Caltech.

This study is the first time gravity measurements were used to infer the presence of an ocean on another world. Enceladus’ subsurface ocean is probably the source of its geysers. Along with water vapour and ice, these geysers also spill out organic molecules. In the Solar System, other worlds such as Europa and Ganymede also harbour subsurface oceans of liquid water. However, Enceladus and Europa are the only ones with subsurface oceans that are in direct contact with rocky seafloors, allowing their oceans to play host to a wide range of complex chemical reactions that are conducive for life, just as in Earth's oceans. This makes Enceladus and Europa amongst the best destinations in the Solar System to search for the presence of life.

L. Iess et al., “The Gravity Field and Interior Structure of Enceladus”, Science 344, 78 (2014)