A Trio of Inflated Hot Jupiters

Figure 1: Artist’s impression of a hot Jupiter.

As part of the SuperWASP (Wide Angle Search for Planets) program, an international team of researches have reported the discovery of three highly irradiated and bloated hot Jupiters. Like Jupiter, these planets are gas giants and are comprised almost entirely of hydrogen and helium. However, unlike Jupiter, all three planets circle in very tight orbits around stars that are hotter and larger than the Sun - F5 to F7-type stars with effective surface temperatures of 6,250 K to 6,500 K. In comparison, the Sun is a cooler G2-type star with an effective surface temperature of 5,780 K. The three planets, identified as WASP-76 b, WASP-82 b and WASP-90 b, take just 1.81, 2.71 and 3.92 days respectively to circle their parent stars.

All three planets are bloated, with diameters of 1.6 to 1.8 times the diameter of Jupiter, and their intensely irradiated day-sides are scorched to temperatures of ~2,000 K. WASP-76 b, WASP-82 b and WASP-90 b belong to a class of inflated hot Jupiters. Planets of this nature tend to orbit in very close proximity to stars that are somewhat hotter than the Sun. In fact, planets with as much as ~2 times the diameter of Jupiter have been found. An example is the hyper-inflated WASP-17 b. It appears that stellar irradiation plays a key role in determining where a hot Jupiter is inflated, because all known inflated hot Jupiters receive more than 150 times the amount of stellar irradiation Earth gets from the Sun (or 4,000 times the stellar irradiation Jupiter gets from the Sun). There is an extensive literature regarding the mechanisms for inflating hot Jupiters. Such mechanisms include tidal heating and Ohmic dissipation.

Figure 2: Parameters for WASP-76 b, WASP-82 b and WASP-90 b.

 Figure 3: A number of planets with measured mass, radius and stellar incident flux. The black points are planets with less than 150 times the mass of Earth and the red points are planets with more than 150 times the mass of Earth (i.e. giant planets). Giant planets with inflated radii are shown at the top-right corner of the plot. (Weiss et al., 2013)

References:

- West et al. (2013), “Three irradiated and bloated hot Jupiters: WASP-76b, WASP-82b & WASP-90b”, arXiv:1310.5607 [astro-ph.EP]

- Weiss et al., “The Mass of KOI-94d and a Relation for Planet Radius, Mass, and Incident Flux”, 2013 ApJ 768: 14

Pan and Daphnis

Figure 1: An image of Saturn and its rings taken by the Cassini spacecraft on 15 April 2008. Credit: NASA/JPL/Space Science Institute.

 Figure 2: A mosaic of a portion of Saturn’s rings, showing the A Ring, and the positions of the Encke Gap and Keeler Gap. Credit: NASA/JPL/Space Science Institute.

Pan and Daphnis are two inner moons of Saturn that orbit within gaps in Saturn’s A Ring. Pan orbits within the 325 km wide Encke Gap and Daphnis orbits within the 42 km wide Keeler Gap. Pan is a small walnut-shaped moon measuring 34.4 by 31.4 by 20.8 km in size. Daphnis is smaller than Pan and it measures 8.6 by 8.2 by 6.4 km in size. Pan was discovered in 1990 from images taken by the Voyager 2 spacecraft and Daphnis was discovered in 2005 from images taken by the Cassini spacecraft.

Pan’s walnut-shape is due to the presence of an equatorial ridge that was formed when the moon swept up ring material from the Encke Gap. Daphnis probably has an equatorial ridge as well. Pan takes 13.8 hours to circle Saturn while Daphnis takes 14.3 hours. The orbits of both moons have slight inclinations which cause then to move above and below Saturn’s ring plane. Pan speeds around Saturn at 19.9 km/s and from its surface, Saturn would span a whopping 53.6°.

Figure 3: Saturn’s moon Pan casting a slender shadow onto the A Ring. This image was taken by the Cassini spacecraft as Saturn was approaching its August 2009 equinox. Credit: NASA/JPL/Space Science Institute.

 Figure 4: The gravitational pull from Saturn’s moon Daphnis perturbs the orbits of the ring particles along the Keeler Gap and sculpts them into vertical structures. These structures cast long shadows across the A Ring in this image taken by the Cassini spacecraft as Saturn was approaching its August 2009 equinox. Credit: NASA/JPL/Space Science Institute.

References:

- Thomas, P. C. (July 2010), “Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission”, Icarus 208 (1): 395-401

- Porco, C.C.; Thomas, P.C.; Weiss, J.W.; Richardson, D.C. (2007), “Saturn’s Small Inner Satellites: Clues to Their Origins”, Science 318: 1602-1607

A Very Rare Type of Wolf-Rayet Star

Massive stars with more than 20 times the Sun’s mass are exceedingly rare. These stars are estimated to be as rare as ~1 in 1,000,000, possibly rarer. Nature does not like to make massive stars. Those that are formed, burn fast, shine bright and have brief lives of just a few million years. Despite their rarity, massive stars come in a very diverse range of stellar types. Some of these stellar types are so uncommon that they are represented by only a few known individual stars in a galaxy containing many billions of stars.

The Large Magellanic Cloud (LMC) is an irregular galaxy and also a satellite galaxy of the Milky Way. It is estimated to contain about 10 billion times the mass of the Sun. A region of the LMC known as Lucke-Hodge 41 (LH41, also known as NGC 1910) has a rich population of massive stars. This region is home to a high concentration of very rare stars including a few luminous blue variables (LBV), a yellow supergiant, a few Wolf-Rayet (WR) stars, as well as a number of other stellar oddities.  These stars are all massive stars at various stages of evolution.

WR stars represent advanced evolutionary stages in the evolution of massive stars. These stars shed mass rapidly by means of a very powerful stellar wind. WR stars are extremely hot and can have surface temperatures as high as ~200,000 K. They are also extremely luminous, ranging from tens of thousands to a few million times the luminosity of the Sun. Due to the extreme mass loss, WR stars are basically stripped-down versions of highly evolved massive stars. These stars have lost the bulk of their outer envelopes to reveal the products of nucleosynthesis in their interiors.

All WR stars show helium emission lines in their spectra. Depending on their evolutionary stage, the spectra of WR stars are dominated by emission lines of nitrogen, carbon or oxygen. As such, WR stars are classified accordingly as WN, WC or WO types. WO stars are believed to be WR stars that have evolved past the WC stage. They represent the most advanced and short-lived evolutionary stage in the life of a massive star before it explodes as a supernova.

While surveying the population of massive stars in LH41, a team of astronomers discovered a new type of WR star with strong enough emission lines from high ionization stages of oxygen and carbon to be classified as a WO star. This newly discovered star is identified as LH41-1042 and further classified as a WO4 star. LH41-1042 is the second known WO star in the LMC and its first known WO4 star, the other being a WO3 star.

Although most of the WR stars in the LMC have already been discovered, a number of WR stars might still await discovery in the most crowded regions of the LMC. Since so few WO stars are currently known, the discovery of another would provide a good opportunity to better understand these short-lived and thus exceedingly rare stars.

Reference:

Kathryn F. Neugent et al. (2012), “The Discovery of a Rare WO-type Wolf-Rayet Star in the Large Magellanic Cloud”, The Astronomical Journal 144: 162 (4pp)

Patchy Clouds on an Exotic World

Figure 1: Artist’s impression of Kepler-7b (left), a gas giant planet 1.6 times the radius of Jupiter (right). Kepler-7b is the first exoplanet to have its clouds mapped. The cloud map was produced using data from NASA’s Kepler and Spitzer space telescopes. Credit: NASA/JPL-Caltech/MIT.

Using data from NASA’s Kepler and Spitzer space telescopes, a team of astronomers have created a cloud map of a scorchingly hot gas giant planet known as Kepler-7b. This planet has a mass of 0.44 ± 0.04 Jupiter mass and a size of 1.61 ± 0.02 Jupiter radii. The low mass and large size gives Kepler-7b an exceptionally low density of just 14 percent the density of liquid water. Kepler-7b circles its host star in a tight 4.89-day orbit. By detecting infrared light from Kepler-7b, Spitzer was able to measure the planet’s temperature, estimating it to be between 1,100 K and 1,300 K. This is somewhat too cool for a planet that orbits so close to its host star. However, this can be explained by the high reflectivity observed for Kepler-7b, where the planet reflects a larger faction of light coming from its host star and so does not heat up as much. Such a high reflectivity is believed to be due to the presence of reflective high altitude clouds in the planet’s atmosphere.

In fact, Kepler-7b’s measure temperature places it within an exceptionally rich region of condensation phase space where silicate clouds can potentially form in the upper, observable portion of the planet’s atmosphere. The same would not be true for a warmer planet (temperatures on the dayside would be too hot for silicate clouds to condense) or a cooler planet (silicate clouds would only be present in the deep unobservable layers of the atmosphere). Hence, Kepler-7b is neither too hot nor too cold for silicate clouds to form in its observable atmosphere.

Observations of Kepler-7b’s dayside by Kepler and Spitzer reveal the presence of reflective high altitude clouds located west of the planet’s substellar point. “By observing this planet with Spitzer and Kepler [telescopes] for more than three years, we were able to produce a very low-resolution ‘map’ of this giant, gaseous planet,” study co-author Brice-Olivier Demory of the Massachusetts Institute of Technology in Cambridge said in a statement. “We wouldn't expect to see oceans or continents on this type of world, but we detected a clear, reflective signature that we interpreted as clouds,” he said.

Figure 2: Models of the dayside temperature structure of Kepler-7b. Both a cloud-free model (orange) and cloudy model (blue) are shown. (Demory et al., 2013)

Figure 3: Models of the dayside planet/star flux ratio for Kepler-7b. Compared to the cloudy model (blue), the cloud-free model (orange) is fainter in the optical but brighter in the mid-infrared. The cloudy model is brighter in the optical due to the scattering of light by clouds. Dashed curves represent the thermal emission component (i.e. heat from the planet) and solid curves represent the total flux. The optical detection in the Kepler band (red) is shown, along with the Spitzer 1-σ (cyan) and 3-σ (red) upper limits. (Demory et al., 2013)

The tell-tale sign for clouds first came from a westward shift seen in the Kepler visible light curve of Kepler-7b. This corresponds to a bright region on Kepler-7b that is centred 41 ± 12° west of the substellar point. Previously, this bright region was thought to be a more intensely heated part on the planet. However, observations by Spitzer show a lack of thermal emission from Kepler-7b and this suggests that the bright region is largely caused by reflected light rather than heat. The reflected light is believed to be light from the planet’s host star being scattered back into space by reflective high altitude clouds, with the most likely candidate being silicate clouds. East of the substellar point, the skies appear to be relatively cloud-free. Being so close to its host star, Kepler-7b is tidally-locked where the same side of the planet always faces its host star, resulting in a permanent dayside and a permanent night side. As a result, the cloud patterns on Kepler-7b are not expected to change much over time, unlike those on Earth.

Reference:

Demory et al. (2013), “Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere”, arXiv:1309.7894 [astro-ph.EP]

An Unequal Pair of ‘Identical Twin’ Stars

Figure 1: Artist’s impression of an eclipsing binary system.

In 2008, a team of astronomers announced the discovery of a pair of newborn stellar twins residing in the Orion Nebula. The stellar twins exist as a binary star system, identified as Par 1802. Both stars formed at around the same time from the same natal material, and each star in the binary has a mass of 0.41 ± 0.01 solar masses, identical to within 2 percent. As such, they are expected to possess identical physical attributes and are virtually ‘identical twins’. However, in the study, the team reported that these twin stars have surface temperatures differing b ~300 K and luminosities differing by ~50 percent. Furthermore, the sizes of both stars differ by 5 to 10 percent.

Physical parameters of Par 1802:
	Primary Component			Secondary Component
Mass (Sun = 1)	0.414 ± 0.015			0.406 ± 0.014
Surface Temperature (K)	3,945 ± 15			3,655 ± 15
Luminosity (Sun = 1)	0.72 ± 0.11			0.46 ± 0.12
Radius (Sun = 1)	1.82 ± 0.05			1.69 ± 0.05

Figure 2: Light curve of Par 1802 - an eclipsing binary system with a period of 4.67 days. The ratio of eclipse depths provides a direct measure of the ratio of surface temperatures, with the deeper eclipse corresponding to the eclipse of the hotter component (primary) by the cooler component (secondary). (K.G. Stassun et al., 2008)

Par 1802 is a very young equal-mass eclipsing binary system and its estimated age is ~1 million years. In an eclipsing binary system, both stars periodically eclipse each other as they circle around their common centre of mass. The variation in combined brightness when one component eclipses the other can reveal a lot about Par 1802. Here, the equal-mass stars of Par 1802 clearly show unequal surface temperatures, luminosities and sizes. This can be explained by stellar evolution models for young stars with ~0.4 times the Sun’s mass. The models predict that such stars undergo a brief period of rapid evolution at an age of ~1 million years. As a result, the warmer, more luminous and larger star (primary component) can be interpreted as being slightly younger than its companion. An age gap of only a few hundred thousand years is sufficient.

Figure 3: Comparison of the observed physical properties of Par 1802 with theoretical predictions. The measured properties of the primary and secondary components of Par 1802 are shown as green and red symbols, respectively. (K.G. Stassun et al., 2008)

Considering that stars with ~0.4 times the Sun’s mass evolve most rapidly during the first few million years after formation and that such stars can live for many billions of years, an age difference of only a few hundred thousand years in an equal-mass binary system is observationally detectable only during the first few million years of its evolution. At later times, the physical signs of unequal ages become less observable. Par 1802 is an example of how birth order in ‘identical twin’ stars, with a lag of only a few hundred thousand years, can manifest itself as observable physical differences between the two stars - at least when they are very young.

Reference:

K.G. Stassun et al., “Surprising dissimilarities in a newly formed pair of ‘identical twin’ stars”, Nature, 450, 1979-1082 (19 June 2008)

Possible Belt of Objects between Uranus and Neptune

Numerical simulations have revealed a region of space between the orbits of Uranus and Neptune where it is stable enough for a belt of small objects residing there to survive over the age of the Solar System without being ejected by gravitational interactions with the giant planets - Jupiter, Saturn, Uranus and Neptune. In the study, test particles were used to identify possible regions of stability. The results show that in the region between 24 to 27 AU from the Sun, about 0.3 percent of an initial population of test particles moving on low-eccentricity, low-inclination orbits could survive over the 4.5 billion year age of the Solar System.

Figure 1: A size comparison of Neptune and Earth.

 Figure 2: Orbits of Mars, Jupiter, Saturn, Uranus and Neptune shown to scale. Credit: Lunar and Planetary Institute.

Figure 3: Simulation results of test particles in the region between Uranus and Neptune. The test particles were initially on low-eccentricity, low-inclination orbits. The red points indicate test particles that survived the full 4.5 billion year integration, while the green points indicate test particles that did not survive. (Holman M.J., 1997)

Although members of this hypothetical belt of objects between Uranus and Neptune have yet to be detected, observations that have been carried out so far do not preclude its existence. Instead, the non-detections help constrain the size distribution of objects within this belt. At a very high level of confidence, there can no more than 7 objects larger than 640 km and 2,900 objects larger than 50 km. The total mass of this hypothetical belt is estimated to be comparable to the mass of the asteroid belt between Mars and Jupiter. Survey telescopes such as the proposed Large Synoptic Survey Telescope (LSST) may be able to detect member objects of this belt.

The discovery of objects in the 24 to 27 AU region between Uranus and Neptune would imply that processes happening during planet formation left material in this region undisturbed or that some subsequent process repopulated the region. Nevertheless, an absence of detectable objects would place more significant limits on the population of objects in this region and would suggest that processes such as planet migration have disrupted this fragile population.

Reference:

Holman M.J., “A possible long-lived belt of objects between Uranus and Neptune”, Nature, 387, 785-788 (19 June 1997)

An Isolated Giant

Very massive stars with ~100 times the Sun’s mass are exceedingly rare. It is still not known if such massive stars can form in isolation or only in star clusters. The centre region of the Milky Way galaxy hosts a unique environment where the process of star formation may differ from elsewhere in the galaxy. This makes the Galactic Centre an important test-bed for determining if very massive stars can indeed form in isolation. In the Galactic Centre are 3 very massive star clusters. The central Cluster surrounds the galaxy’s supermassive black hole, while 2 other massive star clusters, the Arches cluster and the Quintuplet cluster, are located no more than ~100 light years away.

Figure 1: Artist’s impression of the Arches cluster.

Images of the Galactic Centre acquired by the Hubble Space Telescope (HST) reveal a number of isolated massive stars located outside of the 3 known massive star clusters. Four scenarios that may explain the origin of these isolated massive stars are that these stars (1) were formed in isolation; (2) were formed within clusters that have already dispersed; (3) were ejected from one of the 3 known massive star clusters; (4) belong to clusters that have yet to be discovered.

WR 102ka is one of the most massive and most luminous stars known in the galaxy. It also happens to be one of those isolated massive stars in the Galactic Centre. The current mass of WR 102ka is estimated to be ~110 times the Sun’s mass. Since a massive star like WR 102ka has a very high mass loss rate, the initial mass of WR 102ka is believed to be ~150 times the Sun’s mass. WR 102ka blazes with ~3 million times the Sun’s luminosity. Its estimated age is ~2 million years. Very massive stars like WR 102ka live fast and die young.

Figure 2: Artist impression of a massive star.

 Figure 3: A massive star with ~20 times the Sun’s mass is shown next to a stack of lighter, Sun-like stars. For every one such massive star, there could be 500 to as many as 2000 smaller stars. Such a proportion of small to big stars is also expected for WR 102ka if it formed in a massive star cluster where it would be accompanied by a large number of less massive counterparts. Credit: NASA/JPL-Caltech.

In 2009, a team of astronomers used the European Southern Observatory’s (ESO) Very large Telescope (VLT) in Chile to study WR 102ka and its surroundings. If WR 102ka belongs in a star cluster that has yet to be discovered, simulations predict that in such a massive star cluster initially containing a single star with the mass of WR 102ka, there should be ~300 stars exceeding 20 times the Sun’s mass and ~10 of these stars may even exceed 100 times the Sun’s mass. Nevertheless, observations reveal no massive star cluster is associated with WR 102ka, even though such a cluster would have been clearly detectable in the observations.

It is also unlikely that WR 102ka formed within such a massive star cluster ~2 million years ago and that the cluster has since dispersed. This is because ~2 million years is too short a time for such a massive star cluster to disperse. For example, the Quintuplet cluster is an older massive star cluster with an estimated age of 3 to 5 million years and it is still very much intact.

It may be possible that WR 102ka was ejected from one of the 3 massive star clusters. There are a number of mechanisms that can eject a star from it natal cluster. Measurements show that WR 102ka has a radial velocity of 60 km/s. Even with a slow runaway velocity, say equal to its radial velocity of 60 km/s, WR 102ka could easily have arrived at its current location, far from any of the 3 massive star clusters in the Galactic Centre within its ~2 million year lifetime.

However, a number of observational evidences do not support an ejection scenario for WR 102ka. Firstly, it is unlikely that a very massive star like WR 102ka (possible the most massive star were it part of a cluster) can be ejected from a cluster while many less massive stars remain bound. Secondly, infrared observations using the Spitzer space telescope show that a bow shock in the direction of motion is absent around WR 102ka, while the same observations detected bow shocks around two other isolated massive stars in the Galactic Centre. For these two stars, one was ejected from the Central Cluster while the other was ejected from the Quintuplet cluster. Each star has a bow shock in its direction of motion as it ploughs through the interstellar medium.

Thirdly, images from the Spitzer space telescope and from the Wide-field Infrared Survey Explorer (WISE) show the presence of a dusty circumstellar nebula around WR 102ka. The nebula is probably stellar material thrown out by WR 102ka during previous evolutionary stages. The central position of WR 102ka within its nebula suggests that the star has remained at the same location.

All these observations show that WR 102ka, one of the most massive and most luminous stars in the galaxy, may have formed in isolation. In fact, the majority of isolated massive stars in the Galactic Centre do not display obvious bow shocks, suggesting that the massive star population in the Galactic Centre consists of stars formed in clusters, stars ejected from clusters and stars that formed in relative isolation.

Reference:

L. M. Oskinova et al. (2013), “One of the most massive stars in the Galaxy may have formed in isolation”, arXiv:1309.7651 [astro-ph.SR]

Spiral Structure around R Sculptoris

R Sculptoris is a dying red giant star located ~1,000 light-years from Earth. The star is surrounded by a detached shell of dust and gas that was created during a thermal pulse event where the star underwent a brief period of increased mass loss. A team of astronomers using the Atacama Large Millimeter/Submillimeter Array (ALMA) in the Atacama Desert of northern Chile discovered the presence of a spiral structure within the shell of material around R Sculptoris. The spiral structure extends from the central star outwards to the shell. “We’ve seen shells around this kind of star before, but this is the first time we’ve ever seen a spiral of material coming out from a star, together with a surrounding shell,” says Matthias Maercker, the lead author on the paper presenting the results.

Figure 1: A visualization of the spiral structure around R Sculptoris. Credit: ALMA (ESO/NAOJ/NRAO)

Towards the end of their lives, low-mass and intermediate-mass stars, such as the Sun, become red giant stars and start to lose a large amount of their mass. During the red giant phase of a star, it also periodically experiences thermal pulses. A thermal pulse occurs when a thin shell of helium surrounding the star’s core fuses the helium into carbon in an explosive fashion. The thermonuclear convulsion dumps a huge amount of energy into the star’s interior which causes the star to blast off a significant amount of material. For this reason, red giant stars like R Sculptoris are major contributors to the bulk of raw materials such as carbon and oxygen that are incorporated into the formation of future generations of stars and planets.

Figure 2: The detached shell and spiral structure observed in different velocity channels. Numbers in the top right corners indicate the velocity in km/s with respect to R Sculptoris. The detached shell is most pronounced at lower velocities while the spiral structure can be traced through all velocity channels. (M. Maercker et al., 2012)

 Figure 3: The green curve outlines the spiral structure around R Sculptoris. (M. Maercker et al., 2012)

Observational data combined with hydrodynamic simulations suggests that the shell of dust and gas around R Sculptoris was created when the star underwent a thermal pulse ~1,800 years ago, lasting for ~200 years. The spiral structure observed around R Sculptoris is most likely caused by the presence of a companion star that is shaping the stellar wind into a spiral pattern, like a rotating garden sprinkler. Extending from the central star out to the shell, the spiral structure can be followed over about 5 windings. Based on the spacing of the windings and the present-day expansion velocity of the stellar wind streaming from R Sculptoris, the companion star around R Sculptoris is estimated to have an orbital period of 350 years.

The amount of material present in the shell of dust and gas around R Sculptoris is estimated to have a mass of ~0.003 times the Sun’s mass (~1000 times the Earth’s mass). All these material was blasted outward at 50,000 kilometres per hour. Since the thermal pulse lasted for ~200 years, the mass-loss rate of R Sculptoris during the thermal pulse is ~5 Earth masses per year. This is about 30 times lower than the present-day mass-loss rate of ~0.15 Earth masses per year. “It’s a real challenge to describe theoretically all the observed details coming from ALMA, but our computer models show that we really are on the right track. ALMA is giving us new insight into what’s happening in these stars and what might happen to the Sun in a few billion years from now,” says Shazrene Mohamed, a co-author of the study.

Reference:

M. Maercker et al., “Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris”, Nature 490, 232-234 (11 October 2012)

Moonlets in Saturn’s A Ring

Figure 1: An image of Saturn’s main rings taken by the Cassini spacecraft on 21 June 2004, a few days before the spacecraft entered orbit around Saturn. Credit: NASA/JPL/Space Science Institute.

Figure 2: An annotated version of an image of Saturn’s A Ring. This image was taken by the Cassini spacecraft on 9 May 2007, at a distance of approximately 1.1 million km from Saturn. Credit: NASA/JPL/Space Science Institute.

Saturn’s main rings are comprised primarily of water-ice particles ranging between ~1 cm and ~10 m in radius. In 2006, four ‘propeller’-shaped features were discovered in Saturn’s A Ring from images previous taken by the Cassini spacecraft. Simulations have show that these ‘propeller’-shaped features are formed by gravitational perturbations of ring particles from moonlets measuring tens to hundreds of metres in size.

Figure 3: Images of 4 ‘propeller’-shaped features discovered in Saturn’s A Ring. These images were taken by the Cassini spacecraft in 2004 and the nominal image resolution is 52 m per pixel. (M. S. Tiscareno et al., 2004)

These moonlets are embedded within the A Ring and are too small to be seen directly by the Cassini spacecraft. Instead, the Cassini spacecraft sees the ‘propeller’-shaped disturbances created by these moonlets. By 2008, ~150 features have been found in Saturn’s A Ring that are associated with moonlets embedded within the ring. About half of these features are sufficiently well resolved to reveal a characteristic ‘propeller’ shape.

As the moonlets orbit Saturn within the A Ring, they partially sweep up the ring particles around them to create the observed ‘propeller’-shaped features. However, they are not large enough to sweep clean their entire orbit around Saturn, unlike the moons Pan and Daphnis. Both these moons reside within gaps in Saturn’s A Ring. Pan orbits inside the 325 km wide Encke Gap, while Daphnis orbits inside the 42 km wide Keeler Gap.

The ‘propeller’-shaped features associated with the moonlets lie primarily in three ~1000 km wide belts in the middle section of Saturn’s A Ring, between 126,750 km and 132,000 km from Saturn’s centre. Also, relatively larger moonlets do exist and they can create bigger ‘propeller’-shaped features that can be tracked for years. On such feature is even nicknamed Bleriot, after a French aviator named Louis Bleriot. Bleriot is believed to be associated with a moonlet measuring ~1 km in size, significantly larger than many other moonlets.

Figure 4: In this image taken by the Cassini spacecraft, the central ‘propeller’ structure of Bleriot is estimated to be ~100 km in length. Credit: NASA/JPL-Caltech/Space Science Institute.

References:

- M. S. Tiscareno et al., “100-metre-diameter moonlets in Saturn’s A ring from observations of ‘propeller’ structures”, Nature 440, 648-650 (30 March 2006)

- M. Seiß, F. Spahn, M. Sremčević and H. Salo, “Structures induced by small moonlets in Saturn’s rings: Implications for the Cassini Mission”, Geophysical Research Letters Vol. 32 L11205 (2005)

- M. Sremčević et al., “A belt of moonlets in Saturn’s A ring”, Nature 449, 1019-1021 (25 October 2007)

- M. S. Tiscareno et al., “The Population of Propellers in Saturn’s A Ring”, Astronomical Journal, Volume 135, Page 1083-1091 (2008)

Ancient Nova Sell around Z Camelopardalis

Z Camelopardalis is a dwarf nova located at a distance of about 530 light years. It consists of a white dwarf accreting hydrogen-rich matter from a companion red dwarf star. The accreted matter forms an accretion disk around the white dwarf where instability in the accretion disk causes it to episodically dump much of itself onto the white dwarf. The hydrogen-rich matter gradually accumulates on the white dwarf’s surface. Each dump liberates a large amount of gravitational potential energy and causes Z Camelopardalis to brighten by up to a factor of 40. This process repeats itself every 20 days or so.

Figure 1: Artist’s concept of Z Camelopardalis, a stellar system featuring a white dwarf accreting hydrogen-rich matter from a companion star. Credit: NASA/JPL-Caltech.

It is predicted that white dwarfs like Z Camelopardalis will eventually accumulate sufficient hydrogen-rich matter to undergo classical nova eruptions. During a classical nova, the accumulated hydrogen-rich matter on the white dwarf ignites and fuses hydrogen into other heavier elements in a thermonuclear runaway process. The surface of the white dwarf explodes and creates clearly visible shells of ejected material in the aftermath. A classical nova is thousands of times more luminous than a dwarf nova.

In January 2004, an image of Z Camelopardalis and its surroundings was acquired by NASA’s Galaxy Evolution Explorer (GALEX) satellite, a space-based ultraviolet (UV) telescope. The image shows an arc of UV emitting material centred on, and located southwest (SW) of Z Camelopardalis. Linear nebulosities to the northeast (NE) and southeast (SE) are also visible. These features appear to extend out to a distance of up to ~2 light years around Z Camelopardalis and are believed to be part of a shell of ejected material that was formed when Z Camelopardalis underwent a classical nova eruption in the past.

Figure 2: Image of the dwarf nova Z Camelopardalis and its surroundings. Z Camelopardalis is circled. (M. Shara et al., 2012)

Observations of other classical novae show ejection velocities in the range 300 to 3000 km/s. Given the physical size of the classical nova ejecta around Z Camelopardalis, it allows the age range of the ejecta to be set at 2,400 to 240 years. A larger age is favoured due to the “snowplough effect” observed with other classical novae where the ejecta sweeps up the interstellar medium and decelerates. In fact, an age of more than 1,300 years is most likely for the ejecta around Z Camelopardalis. This means Z Camelopardalis underwent a classical nova eruption over 1,300 years ago. During such an event, Z Camelopardalis would have been one of the brightest stars in the sky for at least a few days. It is possible that historical observations of such an event might exist.

References:

- M. Shara et al., “An ancient nova shell around the dwarf nova Z Camelopardalis”, Nature 446, 159-162 (8 March 2007)

- M. Shara et al., “The Inter-Eruption Timescale of Classical Novae from Expansion of the Z Camelopardalis Shell”, ApJ 756 (2012) 107 arXiv:1205.3531 [astro-ph.SR]