Frozen Stars

“As one great furnace flamed; yet from those flames

No light; but rather darkness visible”

- Paradise Lost by John Milton

In 1997, two astrophysicists, Fred Adams and Gregory Laughlin from the University of Michigan published a paper titled “A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects”. The paper outlines the long term fate of the universe based on what is currently known about the universe. In it, the authors investigate the evolution of planets, brown dwarfs, stars, black holes, galaxies and other astrophysical objects on timescales that vastly exceed the current age of the universe.

One particularly interesting type of astrophysical object mentioned in the paper is the idea of “frozen” stars. In the paper, the concept is described as follow: “The forthcoming metallicity increases may also decrease the mass of the minimum mass main sequence star as a result of opacity effects. Other unexpected effects may also occur. For example, when the metallicity reaches several times the solar value, objects with 0.04 solar mass may quite possibly halt their cooling and contraction and land on the main sequence when thick ice clouds form in their atmospheres. Such “frozen stars” would have an effective temperature of around ~273 K, far cooler than the current minimum mass main sequence stars. The luminosity of these frugal objects would be more than a thousand times smaller than the dimmest stars of today, with commensurate increases in longevity”.

In astronomy, the metallicity of an object is the proportion of its material that is comprised of elements heavier than hydrogen and helium. Basically, all elements heavier than hydrogen and helium are termed “metals”, a term that encompasses even elements such as carbon, oxygen, nitrogen and silicon. The Sun for example, has a metallicity of 0.02, implying that 2 percent of its mass is in the form of “metals”. In the beginning, the universe started out with only hydrogen (75 percent) and helium (25 percent). All “metals” found in the Sun were formed through nuclear fusion processes by generations of stars that preceded the Sun. The demise of these stars enriched the interstellar medium with “metals”. Subsequently, the Sun formed from one of these enriched interstellar clouds and acquires a higher metallicity than its predecessors.

Generations of stars come and go, steadily increasing the concentration of “metals” in the interstellar medium. A consequence of this is stars that form in the far future would have a much higher metallicity than stars today. The increase in metallicity lowers the minimum mass required for an object to sustain nuclear fusion in its core and become a star. Today, a star needs to have at least ~8 percent the Sun’s mass. Short of that, and it would be termed a brown dwarf instead of a star.

However, in the far future, objects formed from the interstellar medium can have several times the Sun’s metallicity. Such an object can sustain nuclear fusion in its core and become a star even if its mass is as low as 4 percent the Sun’s mass. The idea behind this is that the high metallicity makes the core an insulator (i.e. less able to radiate energy), allowing a smaller core and hence, a less massive star to support the temperature required to sustain hydrogen fusion. The rate of hydrogen fusion in the core of such a star is believed to be so low that the star can have a surface temperature of around zero degrees Celsius, cold enough for ice clouds to form in the star’s atmosphere. In their paper, Fred Adams and Gregory Laughlin termed them “frozen” stars.

In the present universe, the least massive stars have surface temperatures around 2000 K and lifespans exceeding 10 trillion years. A “frozen” star like is unlike any found in the present universe. The low rate of hydrogen fusion means a “frozen” star would be able to sustain hydrogen fusion for a much longer period of time, giving it a vastly greater longevity. A star’s lifespan is the amount of time it can sustain hydrogen fusion in its core. For comparison, the present age of the universe is 13.8 billion years, while the Sun has a lifespan of 10 billion years.

“Frozen” stars in the far future of the universe would be orders of magnitude dimmer than the dimmest stars known today. Nevertheless, such objects are clearly considered stars, since, like the Sun, they produce energy via nuclear fusion in their cores. A “frozen” star would be roughly the size of Jupiter. Yet, it is by no means like Jupiter. With 4 percent the Sun’s mass, which is approximately 40 times Jupiter’s mass, the gravity on the surface of a “frozen” star would be a crushing ~100g’s. Still, such exotic stars are interesting astrophysical objects to think about.

Reference:

Fred Adams and Gregory Laughlin (1997), “A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects”, arXiv:astro-ph/9701131

Neither Star Nor Brown Dwarf

EF Eridani is an ultra-short period binary system consisting of a white dwarf and a substellar companion of unknown type. In a study by T.E. Harrison et al. (2004), astronomers used the NIRI on Gemini-North (7:21 to 9:00 UT on 24 December 2002) and NIRSPEC on Keck II (6 September 2003) to learn more about this unusual binary system. Estimates indicate that the white dwarf has ~60 percent the Sun’s mass, while the substellar companion has only ~5 percent the Sun’s mass. Both objects are separated by merely ~400,000 km and they whiz around each other in 81 minutes.

Figure 1: Artist’s impression of the EF Eridani system as it might appear today. Image credit: Gemini Observatory.

The white dwarf in EF Eridani is a dense, burnt-out remnant of a Sun-like star. It is roughly the same size as Earth and has a modelled surface temperature of about 9500 K. Circling around the white dwarf is a substellar companion of unknown type. It is thought that ~500 million years ago, the companion was a typical star that began losing mass to the smaller but more massive white dwarf. Over time, the companion lost so much mass to the white dwarf that it has since regressed into a cool ember, believed to be roughly the size of Jupiter. With only ~5 percent the Sun’s mass, the companion is not massive enough to sustain nuclear fusion in its core, making it far too low in mass to be a star. For an object to be a star, it needs to have at least ~8 percent the Sun’s mass to sustain nuclear fusion.

Although an object less than ~8 percent the Sun’s mass is classified as a brown dwarf, observations reveal that the composition of the substellar companion in EF Eridani does not match any known brown dwarf. This is the case even though the object has an estimated temperature of 1700 K, equivalent to a cool brown dwarf. As a result, the substellar companion is neither star nor brown dwarf, and it represents an object that does not match any known category.

Figure 2: Comparison of the spectrum of the substellar companion in EF Eridani to brown dwarf model spectra. The temperature of each model is listed, and below the temperature are the abundances of carbon, nitrogen and oxygen (shown on a log10 scale in comparison to the Sun’s abundance). Source: T.E. Harrison et al. (2004).

Reference:

T.E. Harrison et al. (2004), “Phase-Resolved Infrared H- and K-band Spectroscopy of EF Eridani”, arXiv:astro-ph/0409735

Planet Candidate Circling a Rapidly Rotating Star

Main sequence stars of spectral types earlier than ~ F6 (i.e. stars > 1.5 times the Sun’s mass) are expected to be rapid rotators as their outermost layers are radiative instead of convective. Stars like the Sun have convective outermost layers that drive surface magnetic activity, generating strong stellar winds that with time carry away the star’s angular momentum and spin down the star. Stars of spectral types earlier than ~ F6 retain their high angular momentum (i.e. rapid rotation rates) since their radiative outermost layers do not produce strong stellar winds that sap the star’s angular momentum with time. Some of these stars are known to spin at near their break-up speeds.

The rapid rotation of ~ F6 type and earlier stars can make then significantly oblate - flattened at the poles and bulging around the equator. This causes the star’s surface gravity to exhibit a pole-to-equator gradient, with the poles having a higher surface gravity, and thus higher temperature and brightness. The star’s photosphere can be several thousand Kevin hotter at the poles than at the equator. As a result, the poles are “gravity-brightened” and the equator “gravity-darkened”. This effect is called gravity darkening and it was first predicted by von Zeipel (1924).

Figure 1: Artist’s impression of a gas giant planet. Image credit: Daniel Mallia.

A study by Ahlers et al. (2014) used precise photometric data from NASA’s Kepler space telescope to constrain the spin-orbit alignment of the KOI-368 planetary system. Basically, the spin-orbit alignment is the inclination of a planet’s orbit normal with respect to its star’s spin axis. The planet in this system, designated KOI-368.01, is probably an inflated gas giant 1.83 times the size of Jupiter. The planet’s host star, KOI-368, is a rapidly rotating early spectral type star that is 2.3 times the size of the Sun and has an effective temperature of 9257 K. The star’s period of rotation is a mere 30.73 hours. For comparison, the Sun has a rotation period of 25 days.

Gravity darkening allows the true orientation of a star’s spin axis to be determined. In turn, this allows the true spin-orbit alignment of any planet transiting the star to be measured. The planet KOI-368.01 transits its host star every 110 days. Due to the rapid spin of its host star, the transit light curve is expected to display the effects of gravity darkening. Using a gravity-darkened stellar model to model the observed light curve when KOI-368.01 transits its host star, the planet’s true spin-orbit alignment was found to be 11 ± 3 degrees. The results from this study show that the orbit of KOI-368.01 is well aligned and might suggest that orbits of objects circling more massive stars are not more likely to be misaligned, contrary to Winn et al. (2010).

Figure 2: Photometry and fits for the KOI-368 light curve with the gravity-darkened fit in blue. Source: Ahlers et al. (2014).

Figure 3: Four possible transit geometries of the KOI-368 system. The effects of gravity darkening and limb darkening are illustrated. All four scenarios produce identical transit light curves; therefore, these geometries are perfectly degenerate. Source: Ahlers et al. (2014).

References:

- Ahlers et al. (2014), “Spin-Orbit Alignment for 110 Day Period KOI368.01 from Gravity Darkening”, Astrophysical Journal, Volume 786:131 (5pp)

- von Zeipel (1924), “The radiative equilibrium of a rotating system of gaseous masses”, Monthly Notices of the Royal Astronomical Society, Vol. 84, p.665-683

- Winn et al. (2010), “Hot Stars with Hot Jupiters Have High Obliquities”, Astrophysical Journal Letters, Volume 718, Issue 2, pp. L145-L149

Influence of Planetary Rotation on Habitability

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

A previous study by Yang et al. (2013) showed that tidally-locked terrestrial planets around red dwarf stars can remain habitable at much closer distances to their host stars than previously thought due to a stabilizing cloud feedback mechanism. Strong convection drives the formation of water clouds that can cover most of the planet’s dayside, increasing the planet’s reflectivity and keeping the planet cool. In a more recent study by Yang et al. (2014), the focus shifts towards terrestrial planets around Sun-like stars, especially those near the inner edge of the habitable zone (HZ). The study examines the influence of planetary rotation on habitability.

A planet’s rotation rate determines its atmospheric circulation by establishing the strength of the Coriolis effect and the length of day. For a fast rotation rate, the Coriolis effect is strong and atmospheric circulation is organised into latitudinal bands, like on Earth - Hadley cell, Ferrel cell and Polar cell (Figure 2). Also, the short length of day due to fast rotation results in small surface temperature differences between day and night.

Figure 2: The three-cell model of atmospheric circulation on Earth. From Equator to Pole - Hadley cell, Ferrel cell and Polar cell. Source: Pearson Prentice Hall.

For a slow rotation rate, the Coriolis effect is weak and the Hadley cells can extend globally. If the rotation rate is slow enough, the dayside can become much warmer than the nightside, leading to atmospheric circulation characterised by ascending air masses on the warm dayside and descending air masses on the cold nightside.

Using 3D atmospheric general circulation models (GCMs), Yang et al. (2014) examined, for an Earth-analogue, rotation periods ranging from 12 hours to 365 days. In the 3D GCMs, the stellar flux is increased until the mean surface temperature reaches ~310 K, which is roughly when a runaway greenhouse effect begins to occur and the planet becomes too hot for habitability.

Rotation rate determines the atmospheric circulation, which in turn influences the spatial distribution of clouds on a planet. Clouds can reflect a great deal of incoming stellar flux back into space. In doing so, clouds play a vital role in regulating the planet’s surface temperature and hence, the planet’s habitability. For example, greater cloud coverage increases the planet’s reflectivity and keeps the planet cool.

The 3D GCMs revealed that for a given stellar flux, the surface temperature of rapidly rotating planets is much higher than that of slowly rotating planets. This is especially true for planets near the warm, inner edge of the HZ. The Earth, considered a rapidly rotating planet in this study, has high cloud coverage in the tropics due to the intertropical convergence zone (ITCZ) associated with the ascending air masses of the Hadley cells. These tropical clouds have the largest influence on Earth’s albedo (i.e. Earth’s reflectivity) because the tropics receive the highest amount of stellar flux.

If the stellar flux is increased for a rapidly rotating planet, the equator-to-polar temperature gradient decreases, weakening the Hadley cells, reducing tropical cloud coverage and decreases the planet’s albedo. The end result is more warming of the planet. This drives a positive feedback where the increase in stellar flux eventually leads to a runaway greenhouse effect, creating temperatures too hot for habitability.

For slowly rotating planets, the atmospheric circulation consists of Hadley cells that extend globally. This is unlike fast rotating planets whose atmospheric circulation is organised into latitudinal bands. On a slowly rotating planet, the substellar point (i.e. spot on the planet’s surface where the planet’s host star is “directly overhead”) moves slowly across the planet’s surface. Around the substellar point, convection leads to the formation of clouds that can cover much of the planet’s dayside. These clouds allow the planet to reflect incoming stellar flux, keeping the planet cool.

When stellar flux is increased for a slowly rotating planet, convection becomes stronger due to more heating of the planet’s dayside. This leads to the formation of yet more clouds, allowing the planet to reflect away more incoming stellar flux and creates a negative feedback that stabilizes the climate from overheating. As a result, the inner edge of the HZ for slowly rotating planets can be much closer to the host star than for rapidly rotating planets.

Figure 3: Dependence of planetary climate on rotation period for planets orbiting a Sun-like star. (a) and (b): Global-mean surface temperature (TS) and planetary albedo as a function of rotation period for a given stellar flux. The surface heat capacity (D) is equivalent to 50m of water. (c) and (d): Global-mean surface temperature (TS) and planetary albedo as a function of stellar flux for a given rotation period with D of 50 m. Source: Yang et al. (2014).

Figure 4: Differences in clouds and atmospheric circulation between rapidly (left) and slowly (right) rotating planets. Source: Yang et al. (2014).

Figure 5: Habitable zone boundaries as a function of stellar type and planetary rotation rate for a 1D radiative-convective model and for the 3D GCM. Blue line: outer edge of the HZ (Kopparapu et al. 2013); green line: inner edge of the HZ (Kopparapu et al. 2013); black line: inner edge of the habitable zone for rapidly rotating planets in this study; red line: inner edge of the habitable zone for slowly rotating planets in this study (rotation period of 128 days for G-type and F-type stars, and tidally-locked with an orbit of 60 days for M-type and K-type stars); gray line: the tidal-locking radius. Source: Yang et al. (2014).

Using the 3D GCMs, Yang et al. (2014) also examined a Venus-analogue with Venus’ orbital characteristics, but with Earth’s atmosphere. The results suggest that such a planet could be habitable, even though it receives a higher stellar flux of 2610 W/m^2 versus 1360 W/m^2 that Earth gets from the Sun. It would imply that when Venus went through a runaway greenhouse to become the hellish, inhospitable place it now is, it must have had a higher rotation rate at that time.

Figure 6: The climate of a planet with modern Earth’s atmosphere and continental configuration, but in Venus’ orbit and with Venus’ (slow) rotation rate. (a): Time series of global-mean surface temperature (TS) simulated for Venus’ rotation rate (slowly rotating, black), Earth’s rotation rate (rapidly rotating, green) and Venus’ rotation rate with clouds artificially set to zero (slowly rotating, no clouds, red). The planet quickly tends toward a runaway greenhouse if it is rapidly rotating or has no clouds, but is habitable if it is slowly rotating. (b): Global-mean TS and vertically-integrated ocean temperature (Tocn) in a coupled ocean-atmosphere simulation using Venus’ rotation rate. (c-e): maps of TS, planetary albedo and thermal emission to space averaged over 1 day in the simulation. The black dot in (c-e) is the transient substellar point, which moves eastward around the planet with a period of 117 days. Source: Yang et al. (2014).

References:

- Yang et al. (2013), “Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets”, arXiv:1307.0515 [astro-ph.EP]

- Yang et al. (2014), “Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate”, arXiv:1404.4992 [astro-ph.EP]

- Kopparapu et al. (2013), “Habitable Zones Around Main-Sequence Stars: New Estimates”, arXiv:1301.6674 [astro-ph.EP]

Stabilizing Cloud Feedback on Tidally Locked Planets

The habitable zone (HZ) is a region around a star where it is neither too hot nor too cold for a planet to sustain liquid water on its surface. It is marked by an inner edge (before it starts to get too hot) and an outer edge (before it starts to get too cold). For a given star, many factors influence where the inner and outer edges of the HZ may lie. When evaluating the habitability of a planet, the presence of water clouds can have a huge influence. Water clouds can either reflect incoming stellar radiation back to space (cooling the planet) or absorb and re-radiate thermal emission from the planet’s surface (warming the planet). In most cases, reflection of stellar radiation (cooling) dominates over absorption of planetary infrared radiation (warming), resulting in net cooling.

Figure 1: Artist’s impression of a habitable planet. Image credit: Fernando Rodrigues.

A study by Yang et al. (2013) used 3D general circulation models (GCMs) to understand the impact of water clouds on planets around red dwarf stars. In particular, the study focused on planets near the inner edge of the HZ. Red dwarf stars are by far the most abundant type of star in the galaxy, constituting ~75 percent of all stars. They are cooler, much less luminous and have smaller masses than Sun-like stars. Current estimates also suggest that there is on average ~1 Earth-size planet in the HZ around every red dwarf star.

Due to their low luminosities, the HZ around red dwarf stars is much closer-in compared to the HZ around Sun-like stars. This means planets in the HZ of red dwarf stars are subjected to stronger tidal forces. If these planets are in circular orbits, they are expected to be tidally-locked, with one side experiencing perpetual day and the other side experiencing perpetual night. The results by Yang et al. (2013) show that the presence of water clouds on tidally-locked planets can drive a stabilizing cloud feedback mechanism, allowing such planets to be habitable even at twice the amount of stellar flux previously thought to mark the inner edge of the HZ. It shows that the inner edge of the HZ can lie much closer-in to the star than previously thought. Consequently, planets that were once considered too hot for habitability can actually have clement surface conditions suitable for life.

On a tidally-locked planet, the substellar point is a spot on the planet’s surface where the planet’s host star is “directly overhead”, and it is where insolation is expected to be highest. Around the substellar point, near-surface convergence of air masses and the resulting convection can cause most of the planet’s dayside to be covered by water clouds. High-level and low-level water clouds can cover ~60 percent and ~80 percent of the dayside, respectively. These water clouds significantly increase the planet’s albedo (i.e. the planet’s reflectivity), allowing the planet to more efficiently reflect the incoming flux from its host star.

Figure 2: Cloud behaviour for a tidally-locked planet (left) and a non-tidally-locked planet (right). High-level cloud fraction (top) and low-level cloud fraction (bottom) are displayed in each case. The non-tidally locked case is in a 6:1 spin-orbit resonance (i.e. the planets makes 6 rotations per orbit). The stellar flux is 1400 W/m^2. The black dots in (a) and (c) denote the substellar point. Source: Yang et al. (2013).

At a higher stellar flux, like for a tidally-locked planet that is closer-in to its host star, the convection around the planet’s substellar point is stronger, producing greater water cloud coverage and further increases the planet’s albedo. The cooling effect associated with the higher reflectivity leads to much lower temperatures on the planet than if water clouds were not present, thereby creating a stabilizing cloud feedback mechanism where the planet’s albedo increases with stellar flux. The inner edge of the HZ around a red dwarf star is generally thought to be the distance from the star where the insolation reaches ~1200 W/m^2. However, with stabilizing cloud feedback, the inner edge of the HZ can shift much closer-in to the star. Even with an insolation of 2200 W/m^2, a tidally-locked planet around a red dwarf star can still have surface temperatures cool enough for the planet to remain habitable.

Figure 3: Climates of tidally locked (1:1) and non-tidally locked (2:1 and 6:1) terrestrial planets - (a) global-mean surface temperature (K), (b) stratospheric H2O volume mixing ratio at the substellar point, (c) planetary albedo and (d) global-mean greenhouse effect (K). 1:1 denotes a tidally-locked state, and 2:1 and 6:1 denote 2 or 6 rotations per orbit, respectively. The stellar spectrum is for an M-star (i.e. red dwarf star) or a K-star (i.e. stars that cover the ranged between red dwarf stars and Sun-like stars). Results for HD85512 b, a super-earth-size planet orbiting a K-star, are represented by a blue pentagram. The gray area denotes the HZ around an M-star with an inner edge of ~1200 W/m^2 and an outer edge of ~270 W/m^2 (not shown). Source: Yang et al. (2013).

The presence of substellar water clouds, indicative of a stabilizing cloud feedback mechanism, can significantly modify the thermal phase curves of tidally-locked planets. Basically, a planet’s thermal phase curve is the change in thermal radiation given off by the planet when different parts of the planet come into view as the planet circles its host star. Thermal phase curve features corresponding to the presence of a stabilizing cloud feedback mechanism would be detectable by the James Webb Space Telescope (JWST) in the near future, especially for super-Earth-size planets orbiting the nearest red dwarf stars. By allowing the inner edge of the HZ to be closer-in to the host star, the stabilizing cloud feedback mechanism enlarges the HZ around red dwarf stars. In turn, this can potentially increase the frequency of habitable, tidally-locked Earth-size planets around red dwarf stars by 50 to 100 percent.

Figure 4: Thermal phase curves of tidally-locked planets for different atmospheres with stellar flux fixed at 1200 W/m^2 - airless, dry-air, water vapour and water vapour plus clouds. Source: Yang et al. (2013).

Figure 5: Thermal phase curves of tidally-locked planets for a full atmosphere including water vapour and clouds for different stellar fluxes: 1400, 1600, 2000 and 2200 W/m^2. Source: Yang et al. (2013).

Recently, the first Earth-sized planet in the HZ of another star was discovered in the Kepler-186 planetary system. This planet, dubbed Kepler-186f, is the 5th planet in the system which consists of a total of 5 known planets circling a red dwarf star. Much of the attention goes to Kepler-186f, since the 4 inner planets are too close to the star and do not lie in the HZ. These 4 inner planets are also expected to be tidally-locked. Nevertheless, with the stabilizing cloud feedback mechanism proposed by Yang et al. (2013), the 4th planet, dubbed Kepler-186e, might be marginally habitable if it has a significant water cloud cover on its dayside.

References:

- Yang et al. (2013), “Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets”, arXiv:1307.0515 [astro-ph.EP]

- Bolmont et al. (2014), “Formation, tidal evolution and habitability of the Kepler-186 system”, arXiv:1404.4368 [astro-ph.EP]

Possible Exotic Object around a Pulsar

Pulsars are highly magnetised, spinning compact stars that emit intense bipolar beams of electromagnetic radiation. A pulse is observed when such a beam happens to sweep pass the Earth, hence the term ‘pulsars’. These objects are the superdense and compact remnant cores of massive stars that have gone supernova. A typical compact star packs as much mass as the Sun within an object just several kilometres in size.

Figure 1: Schematic view of a pulsar. The sphere in the middle represents the compact star; the curves indicate the magnetic field lines and the protruding bipolar cones represent the beam emission zones. Credit: Roy Smits.

A paper by M. Bailes et al. (2011) reported on the detection of a Jupiter-mass object in a very close-in orbit around the pulsar PSR J1719-1438. This Jupiter-mass object is so close to the pulsar that its density has to be greater than 23 grams per cubic centimetre for it to not get torn apart by the pulsar’s titanic gravity. Such a high density is not typical for Jupiter-mass objects. In comparison, Jupiter has a mean density of only 1.3 grams per cubic centimetre. It is theorised that the Jupiter-mass object around PSR J1719-1438 is the dense remnant core of a carbon white dwarf that had its outer layers stripped away and accreted by the pulsar. An object like this would easily have the high density necessary to avoid being tidally torn apart by the pulsar’s gravity.

Figure 2: Artist’s impression of the orbit of the Jupiter-mass object around PSR J1719-1438.

Nevertheless, a paper by J.E. Horvath (2012) suggests that the Jupiter-mass object around PSR J1719-1438 might not be the dense remnant core of a white dwarf. Instead, it could be something more exotic. The proposed solution is that the object is an exotic Jupiter-mass clump of superdense strange quark matter. An object like this is so compact; it would measure a mere ~1 km in radius.

There are a number of mechanisms that can explain the origin of a Jupiter-mass quark matter object. In one such process, the pulsar itself is already comprised of quark matter (i.e. the pulsar is a quark star) and its formation involved the conversion of neutrons to quarks in an explosive and turbulent manner. The turbulence can eject sufficient quantities of quark matter to form a Jupiter-mass quark matter object in a very close-in orbit around the pulsar. Another process involves the merger of two low-mass quark stars. A. Bauswein et al. (2009) show that a merger event like this can easily eject enough quark matter to form a Jupiter-mass object.

Future observations might reveal whether or not the Jupiter-mass object around PSR J1719-1438 is indeed an exotic object. For example, a Jupiter-mass quark matter object measuring ~1 km in radius would be too small to generate a detectable photometric signal (i.e. reflected light from the pulsar). Moreover, an exotic quark matter object is not expected to produce any detectable evaporation signature since there would be no “normal matter” for the pulsar’s energetic radiation to strip off into space. For this reason, the non-detection of circumstellar material around PSR J1719-1438 can be evidence that is consistent with the pulsar’s companion being an exotic quark matter object.

References:

- M. Bailes et al. (2011), “Transformation of a Star into a Planet in a Millisecond Pulsar Binary”, arXiv:1108.5201 [astro-ph.SR]

- J.E. Horvath (2012), “The nature of the companion of PSR J1719-1438: a white dwarf or an exotic object?”, arXiv:1205.1410 [astro-ph.HE]

- A. Bauswein et al. (2009), “Mass ejection by strange star mergers and observational implications”, arXiv:0812.4248 [astro-ph]

Lightning in Exotic Superheated Atmospheres

In the Solar System, lightning activity is present on all planets that have clouds in their atmospheres. With the sheer number and diversity of exoplanets discovered in recent years, it is worth considering if lightning activity is also present on these worlds. A study by Christiane Helling et al. (2012) examined the possibility of lightning being present in the exotic superheated atmospheres of hot-Jupiters and brown dwarfs. Hot-Jupiters are giant gaseous planets that orbit very close to their host stars, while brown dwarfs are objects that bridge the gap between the most massive planets and the least massive stars.

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

Both hot-Jupiters and brown dwarfs (provided the brown dwarf has not cooled sufficiently) have atmospheric temperatures that are much higher than on Earth or on Jupiter. Temperatures in these atmospheres can reach thousands of degrees Kelvin. Such temperatures are too hot for water to exist outside the vapour phase, thereby ruling out the moist thundercloud charging process that is responsible for generating lightning here on Earth, and on the gas giants Jupiter and Saturn. Nevertheless, the superheated atmospheres of hot-Jupiters and brown dwarfs are dominated by mineral clouds, consisting of a number of mineral species in the form of liquid mineral droplets and/or mineral dust particles. For example, at sufficiently high temperatures, droplets of liquid iron (Fe) or droplets of liquid titanium oxide (TiO2) can fall as rain.

Figure 2: Dust cloud material composition in volume fractions in the atmosphere of a hot-Jupiter. The compositional changes with atmospheric height are indicated by the local temperature. Source: Christiane Helling et al. (2012).

In order for lightning to be present, the mineral clouds need to undergo some form of ionisation. A number of mechanisms can lead to sufficient ionisation of the mineral clouds. These mechanisms include energetic radiation from the host star, triboelectric charging and cosmic rays. Of particular interest is triboelectric charging, involving dust-dust collisions. Although dust-dust collisions from gravitational settling (i.e. mineral dust particles raining out) are not energetic enough to induce triboelectric charging, atmospheric turbulence can enhance the dust-dust collisions such that triboelectric charging becomes possible.

Nevertheless, gravitational setting enables large-scale charge separation in the mineral clouds and establishes the condition necessary for lightning to occur. The large-scale charge separation develops as particles of different sizes fall at different rates. Smaller, less negatively charged particles fall slower and remain longer in the upper cloud layers. Conditions for lightning to occur are created when large-scale charge separation eventually causes the electric potential between two regions of the mineral cloud to exceed the breakdown field.

The study by Christiane Helling et al. (2012) concludes that mineral clouds in the atmospheres of hot-Jupiters and brown dwarfs should generate lightning since the breakdown fields are lower than on Earth or on Jupiter. Hot-Jupiters and brown dwarfs do not have solid surfaces like on terrestrial planets such as the Earth. As a result, lightning activity occurs in the form of intra-cloud discharges.

References:

- Christiane Helling et al. (2012), “Dust cloud lightning in extraterrestrial atmospheres”, arXiv:1207.1907 [astro-ph.EP]

- Christiane Helling et al. (2008), “A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres”, arXiv:0809.3657 [astro-ph]

A Self-Lensing Binary Star System

Gravitational lensing is a prediction from Einstein’s general theory of relativity. It states that gravity can bend light and, consequently, massive foreground objects can distort and magnify the light from background sources. A. Maeder (1973) predicted that for edge-on binary star systems in which one star is a compact object (i.e. a white dwarf, neutron star or black hole), the gravity of the compact object can repeatedly magnify the light of its companion star each time the compact object is observed to pass in front of its companion star. Using data of high photometric precision from NASA’s Kepler space telescope, E. Krusel & E. Agol (2014) report on the detection of such a “self-lensing” system in a paper published in the April 18 issue of the journal Science.

This is the first detection of a “self-lensing” system. Dubbed KOI-3278, the system consists of a Sun-like star and a white dwarf - the compact object. The white dwarf crosses in front of its companion Sun-like star once every 88.18 days. Each time it does so, the white dwarf’s gravity acts as a magnifying glass and slightly boosts the brightness of its companion star. The brightness boost created by such a “self-lensing” system is small, typically with amplitudes of a part in one thousand or less. In the case for KOI-3278, each passage of the white dwarf in front of its companion star creates a 5 hour pulse with 0.11 percent amplitude.

Figure 1: Schematic of the KOI-3278 binary star system. Credit: Eric Agol.

Figure 2: Light curve of the KOI-3278 binary star system showing the pulse when the white dwarf passes in front of its companion Sun-like star. Source: E. Krusel & E. Agol (2014).

In the Kepler dataset, 16 gravitational lensing pulses were found for KOI-3278, in addition to 16 occultations. The occultations happen when the white dwarf passes behind its companion star. By modelling the gravitational lensing along with orbital and stellar models, the white dwarf’s mass is estimated to be 63 percent the Sun’s mass, and its size, 1.1 percent the Sun’s radius - nearly the same size as Earth. In addition, the white dwarf’s companion star has nearly the same mass and size as the Sun. The companion star would eventually burn out and leave behind a second white dwarf in the system, although not for another several billion years. Further analysis of the Kepler dataset could turn up more “self-lensing” white dwarf/Sun-like star binaries that are similar to KOI-3278.

References:

- A. Maeder, “Light Curves of the Gravitational Lens-like Action for Binaries with Degenerate Stars”, Astronomy and Astrophysics, Vol. 26, p. 215-223 (1973)

- E. Krusel & E. Agol, “KOI-3278: A Self-Lensing Binary Star System”, Science, Vol. 344 no. 6181 pp. 275-277 (2014)

Evolution of Iron-Core White Dwarfs

Figure 1: A typical white dwarf packs as much mass as the Sun into a volume the size of Earth. With so much mass packed into such a small object, white dwarfs are incredibly dense. A teaspoon if its material would weight many tons.

George C. Jordan et al. (2012) and Isern J. et al. (1991) have proposed ways that can lead to the formation of iron-core white dwarfs (WDs). A study by J. A. Panei et al. (2000) examined the structure and evolution of iron-core WDs, and found that iron-core WDs are markedly different from carbon-oxygen WDs which make up the majority of WDs. The study modelled the evolution of iron-core WDs with various masses and compositions. Also included for comparison in the study are standard carbon-oxygen WD models.

Iron-core WDs with masses of 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 solar mass are adopted. Firstly, the models examined pure iron cores comprising 99 (pure iron model), 75, 50 and 25 per cent of the WD’s mass, with a carbon-oxygen envelop for the last 3 cases. The models then examined WDs with a homogeneous composition of iron and carbon-oxygen by adopting a mass fraction for iron of 0.25, 0.50 and 0.75. In all cases, an outer helium envelope comprising 1 per cent of the WD’s mass is included.

The models show the evolution of iron-core WDs to be very different from carbon-oxygen WDs. Compared with carbon-oxygen WDs of the same mass; iron-core WDs have smaller radii and greater surface gravities because of their denser interiors. In particular, iron-core WDs cool a lot faster than carbon-oxygen WDs. This is because iron nuclei are much heavier than carbon or oxygen, resulting in iron having a much lower specific heat per gram. As a consequence, iron-core WDs are poor at storing heat and given the same amount of time, can cool to much lower luminosities than carbon-oxygen WDs. For iron-core WDs, crystallization of iron also occurs much earlier and at much higher luminosities than carbon-oxygen WDs of the same mass.

Figure 2: Radii in terms of effective temperature corresponding to pure iron (full lines) and carbon-oxygen (dot-dashed lines) WD models, and to WD models with a pure iron core containing 50 per cent of the total WD’s mass (dashed lines) for WD masses of 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 solar mass (the higher the mass the smaller the radius). Source: J. A. Panei et al. (2000).

Figure 3: Same as Figure 2, but now dashed lines show the results for homogeneous WD models with an iron abundance by mass of 0.5. Source: J. A. Panei et al. (2000).

Figure 4: Age versus luminosity relation corresponding to pure iron (full lines) and carbon-oxygen (dotted lines) WD models, and to WD models with a pure iron core containing 50 per cent of the total WD’s mass (dashed lines) for WD masses of 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 solar mass. Note that pure iron models cool down ~5 times faster than carbon-oxygen WDs. At high luminosities in this figure, the discontinuity in the slope of pure iron WD models is a result of crystallisation. Source: J. A. Panei et al. (2000).

Figure 5: Same as Figure 4, but now dashed lines show the results for homogeneous WD models with an iron abundance by mass of 0.5. Source: J. A. Panei et al. (2000).

References:

- J. A. Panei, L. G. Althaus & O. G. Benvenuto, The evolution of iron-core white dwarfs”, MNRAS (2000) 312 (3): 531-539

- George C. Jordan et al. (2012), “Failed-Detonation Supernovae: Sub-Luminous Low-Velocity Ia Supernovae and Their Kicked Remnant White Dwarfs with Iron-Rich Cores”, arXiv:1208.5069 [astro-ph.HE]

- Isern J., Canal R., & Labay J. (1991), “The outcome of explosive ignition of ONeMg cores - Supernovae, neutron stars, or ‘iron’ white dwarfs?”, ApJ, 372, L83

Formation of Iron-Core White Dwarfs

Type Ia supernovae are among the most energetic explosions in the universe. They originate from the thermonuclear explosions of carbon-oxygen white dwarfs (WDs). Models of normal Type Ia supernovae involve a sufficiently massive WD accreting material from a companion star. This eventually triggers unstable thermonuclear burning within the WD which transitions to a detonation, subsequently consuming and completely unbinding the WD in a violent supernova explosion. The explosive thermonuclear burning fuses a large fraction of the original carbon-oxygen WD into intermediate-mass elements (IME’s) and iron group elements (IGE’s).

Simulation of a thermonuclear flame plume bursting through the surface of a white dwarf. Credit: Flash Centre for Computational Science, University of Chicago.

In a study by George C. Jordan et al. (2012), a variant of the Type Ia supernovae is proposed. Here, the thermonuclear burning is too weak to transition to a detonation and does not completely unbind the WD in a supernova explosion. Such an event is known as a failed-detonation supernova and it leaves behind a bound remnant of the original WD. A failed-detonation supernova is characterised by low ejecta expansion velocities, low luminosities and low ejecta-mass. Although the explosive thermonuclear burning process during a failed-detonation supernova can generate more than 100 per cent of the WD’s binding energy, the WD does not become completely unbound because the energy produced is partitioned in such a way that a large bound remnant of the original WD remains. For example, a fair amount of the energy goes into launching just a small portion of the original WD’s mass as high velocity ejecta.

A failed-detonation supernova asymmetrically ejects material from the WD. This “kicks” the WD to velocities of a few hundred km/s. Such a velocity is high enough to fling the WD from its binary system, leading to a hypervelocity WD. Although some ejecta are produced, much of the burned and partially-burned material from the thermonuclear burning falls back, enriching the remnant WD with IME’s and IGE’s. These heavy elements segregate to the core, forming a WD with a heavy/iron-rich core. A number of studies have termed such peculiar objects as iron-core WDs. In particular, a study by Isern J. et al. (1991) proposes yet another mechanism for forming iron-core white dwarfs. It involves oxygen-neon-magnesium (ONeMg) cores that result from the evolution of stars with 8 to 12 times the Sun’s mass. Depending on a number of conditions, the ONeMg cores can undergo explosive thermonuclear burning to form iron-core WDs.

References:

- George C. Jordan et al. (2012), “Failed-Detonation Supernovae: Sub-Luminous Low-Velocity Ia Supernovae and Their Kicked Remnant White Dwarfs with Iron-Rich Cores”, arXiv:1208.5069 [astro-ph.HE]

- Isern J., Canal R., & Labay J. (1991), “The outcome of explosive ignition of ONeMg cores - Supernovae, neutron stars, or ‘iron’ white dwarfs?”, ApJ, 372, L83