Circumsubstellar Disk around a Young Brown Dwarf

Brown dwarfs are substellar objects that span the gap between the most massive planets and the least massive stars. These objects are believed to form in the same way stars do, and in their infancy, can possess disks of material from which planets might form. Hannah Broekhoven-Fiene et al. (2014) report on the discovery of a circumsubstellar disk of material around a young brown dwarf identified as KPNO Tau 3. The discovery was based on submillimeter observations of KPNO Tau 3 using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). Submillimeter astronomy is a branch of observational astronomy that involves the part of the electromagnetic spectrum between the far-infrared and microwave wavebands.

Artist’s impression of how a planetary system around a brown dwarf might look like. Image credit: Drew Taylor.

KPNO Tau 3 is situated in the relatively nearby Taurus star-forming region, ~450 light-years away. The circumsubstellar disk detected around KPNO Tau 3 is estimated to contain ~130 Earth-masses worth of material, assuming a gas to dust ratio of 100:1. A planetary system consisting of a few sub-Earth-mass or Earth-mass planets might eventually coalesce out from this circumsubstellar disk. Furthermore, the detection of cold, ~20 K dust grains implies that a significant fraction of dust in the circumsubstellar disk is at a large enough distance from KPNO Tau 3 where the radiated energy from the young brown dwarf is too feeble to have sufficiently warmth the dust grains by much.

The presence of cold dust in the circumsubstellar disk around KPNO Tau 3 is consistent with the belief that brown dwarfs, at least some fraction of them, form in the same manner as low-mass stars. An alternate brown dwarf formation mechanism involves the ejection of a stellar embryo from its place of birth. The ejection process ‘starves’ the stellar embryo such that is no long able to accrete enough matter to form a full-fledge star and instead settles as a brown dwarf. A formation scenario like this would truncated the brown dwarf’s circumsubstellar disk and result in the absence of cold dust grains far from the brown dwarf.

Reference:

Hannah Broekhoven-Fiene et al. (2014), “The Disk around the Brown Dwarf KPNO Tau 3”, arXiv:1407.0700 [astro-ph.SR]

Swinging on Highly Eccentric Orbits

A number of exoplanets are known to orbit their host stars in wildly eccentric orbits. These exoplanets have “comet-like” orbits where they come close to their host stars and then recede far out. One example is HD 80606 b - a Jupiter-like planet which orbits its host star in a highly elongated orbit with eccentricity 0.9336. The planet’s distance from its host star varies from 0.03 to 0.88 AU, where 1 AU is the average Earth-Sun distance. At closest approach, HD 80606 b receives over 800 times more insolation than when it is farthest from its host star. Near closest approach, temperatures on HD 80606 b can rise from 800 K to 1500 K in a mere 6 hours. HD 80606 b attains a maximum orbital velocity of 227 km/s when it is closest to its host star. At that speed, a sufficiently large meteoroid barrelling into HD 80606 b could produce a truly spectacular meteor on the planet’s nightside.

Figure 1: A model of HD 80606 b showing the stormy response of the planet’s atmosphere right after closest approach to its host star. Credit: NASA/JPL-Caltech/G. Laughlin et al.

In the Solar System, all known planets go around the Sun in relatively circular orbits. Nonetheless, besides comets, a number of asteroids are known to orbit the Sun in highly eccentric orbits. These asteroids have “comet-like” orbits that bring them from a far out locale, to a close swing around the Sun, and out again. One notable asteroid in this category is 2006 HY51 - a 1.2 km asteroid in an extremely eccentric orbit around the Sun. 2006 HY51 has a remarkable orbital eccentricity of 0.9688. It comes within 0.081 AU of the Sun (1/4 of Mercury’s closest distance to the Sun) and recedes as far as 5.118 AU from the Sun (grazing Jupiter’s orbit).

Figure 2: Artist’s impression of an asteroid.

2006 HY51 goes around the Sun ever 1530 days, spending the vast majority of its time further than Earth is from the Sun. It is believed to be an asteroid with a stony composition and not an inactive comet. When 2006 HY51 is closest to the Sun, it receives nearly 4000 times more insolation from the Sun compared to when it is at its farthest. The insolation it receives at closest approach is 150 times more intense than the insolation Earth gets from the Sun. From 2006 HY51, the Sun at closest approach would appear over 12 times larger than it would from Earth. The orbital velocity of 2006 HY51 reaches almost 150 km/s when it is swinging by the Sun at closest approach. In comparison, Earth orbits the Sun with an average orbital speed of 29.8 km/s. A few asteroids such as 2005 HC4 and 2008 FF5 have highly eccentric orbits similar to 2006 HY51 that take them very near the Sun. However, both are much smaller than 2006 HY51 and do not swing out as far.

References:

- Moutou C. et al. (2009), “Photometric and spectroscopic detection of the primary transit of the 111-day-period planet HD 80606 b”, Astronomy and Astrophysics 498 (5): L5-L8

- Fossey S.J., Waldman I.P., and Kipping D.M. (2009), “Detection of a transit by the planetary companion of HD 80606”, Monthly Notices of the Royal Astronomical Society: Letters 396: L16-L20

- G. Laughlin et al., “Rapid heating of the atmosphere of an extrasolar planet”, Nature 457, 562-564 (29 January 2009)

- Yoonyoung Kim et al. (2014), “Physical Properties of Asteroids in Comet-like Orbits in Infrared Asteroid Survey Catalogs”, arXiv:1405.2989 [astro-ph.EP]

Refraction of Starlight by Exoplanet Atmospheres

When a planet transits its host star, some of the star’s light passes through the planet’s atmosphere and generates a transmission spectrum which carries information about the planet’s atmosphere. This technique of transmission spectroscopy has been used to characterise the atmospheres of exoplanets ranging from hot-Jupiters to super-Earths. The upcoming James Webb Space Telescope (JWST) and the development of larger ground-based telescopes might make it possible for transmission spectroscopy to characterise the atmospheres of smaller, Earth-size exoplanets.

Light refracts or bends when it passes through a planets’ atmosphere due to the atmosphere’s index of refraction gradient. This is because the index of refraction is altitude-dependent. In the rarefied upper atmosphere of a planet, the index of refraction is lower compared to the dense lower atmosphere. During the events associated with the transit of a planet in front of its host star, the main effect of refraction is that some of the star’s light passing through the planet’s atmosphere can be refracted towards a distant observer prior to a transit and refracted away from a distant observer during a transit.

Taking into account the effect of refraction, Amit Misra et al. (2014) modelled the transmission spectrum of an Earth-analogue (i.e. a planet that is identical to Earth in all respects) prior to and during a transit event. The two cases highlighted in the study are: an Earth-analogue orbiting a Sun-like star and an Earth-analogue orbiting an M5V star (i.e. a red dwarf star). Because of refraction, there is a maximum tangent pressure level that can be probed with transmission spectroscopy during a transit event. In the study, the maximum tangent pressure is defined as the pressure level in the planet’s atmosphere at which 50 percent of the stellar flux is transmitted.

Figure 1: Artist’s impression of a somewhat Earth-like planet in orbit around a red dwarf star. Because a red dwarf star is much cooler and less luminous than the Sun, a planet would need to be much closer-in to receive enough warmth for it to be habitable. At such a close-in distance, the planet would be tidally-locked with one hemisphere permanently pointed towards its host star, likely resulting in an unusual climate system.

Figure 2: Artist’s impression of an Earth-like planet.

The results from the study show that for an Earth-analogue orbiting an M5V star, transmission spectroscopy during a transit event can probe the planet’s atmosphere to pressures of up to ~0.9 bar. This pressure is the maximum tangent pressure level and it corresponds to an altitude of roughly 1 km, indicating that almost the entire atmosphere can be probed. For an Earth-analogue orbiting a Sun-like star, the maximum tangent pressure level during a transit event is ~0.3 bar, corresponding to an altitude of roughly 14 km. This means that transmission spectroscopy is ineffective in probing the lower layers of the planet’s atmosphere.

Different gases in the atmosphere generate the different spectral features that can be identified in the transmission spectrum of a planet’s atmosphere from a transit event. The effect of refraction decreases the signal to noise ratio (SNR) of these spectral features. For an Earth-analogue orbiting an M5V star, the decrease in the SNR is ~10 percent for all spectral features and ~15 percent for H2O features. For an Earth-analogue orbiting a Sun-like star, the decrease in the SNR is much greater, ~60 percent for all spectral features and ~75 percent for H2O features.

As a transit progresses, refraction produces temporal variations in the transmission spectrum of the planet’s atmosphere. The differences in the transmission spectra between each stage of the transit progression can reveal the altitude-dependent abundances of gases, thereby allowing vertical profiling of the planet’s atmosphere. On Earth, the abundance of gases such as oxygen and carbon dioxide are uniform throughout the atmosphere. However, gases such as H2O, ozone and methane have altitude-dependent abundances. For example, H2O is abundant at lower altitudes but becomes rarefied at altitudes above ~10 km.

Figure 3: A model of Earth’s atmospheric temperature profile and gas volume mixing ratios. Amit Misra et al. (2014).

 Figure 4: Maximum amount of transmitted stellar flux at each altitude for an Earth-analogue orbiting a Sun-like star and an Earth-analogue orbiting an M5V star. Amit Misra et al. (2014).

For an Earth-analogue orbiting a Sun-like star, it is possible, prior to ingress, to probe the planet’s atmosphere down to pressures greater than the maximum tangent pressure. This is because the denser lower atmosphere of the planet has a larger index of refraction which allows light to be deflected at large enough angles to a distant observer even though the planet is still some distance away from transiting its host star. However, the transmitted stellar flux prior to ingress is small since most of the planet’s atmosphere is opaque. During the main transit event, particularly during mid-transit, the same large deflection angles corresponding to the denser lower atmosphere of the planet, deflects the stellar flux away from a distant observer. This defines the maximum tangent pressure and prevents the denser lower atmosphere from being probed by transmission spectroscopy during the main transit event.

The upcoming JWST is expected to be able to detect the transmission spectrum of an Earth-analogue orbiting an M5V star. Assuming all transits of the Earth-analogue over a 5 year baseline are observed and co-added, many of the spectral features of the transmission spectrum would have a high enough SNR for the planet’s atmosphere to be characterised. Spectral features associated with H2O and carbon dioxide would have SNRs greater than ~7. Furthermore, spectral features associated with gases such as oxygen and methane, which together constitutes a strong biosignature, would have SNRs of ~3. Nevertheless, the transmission spectrum of an Earth-analogue orbiting a Sun-like star would be beyond the detection capabilities of JWST. Vertical profiling of an Earth-analogue’s atmosphere by observing temporal variations in the transmission spectra of its atmosphere is also beyond JWST’s capabilities regardless of whether the planet’s host star is a Sun-like star or an M5V star.

Figure 5: Diagram showing which altitudes can be probed at different times during a transit for an Earth-analogue orbiting a Sun-like star. The coloured regions correspond to regions of the atmosphere where stellar flux is transmitted and the white regions are portions of the atmosphere that are opaque to a distant observer. During the earliest stage (purple), the transmission of stellar flux occurs at ~2 to 15 km. Subsequently, stellar flux is transmitted through higher portions of the atmosphere: ~5 to 17 km (cyan), ~5 to 30 km (yellow), above ~7 km (blue). As the planet reaches centre of transit (green, then red), most of the stellar flux is transmitted at altitudes above ~14 km. Amit Misra et al. (2014).

Figure 6: Altitude-dependent transmitted flux from pre-transit to centre of transit as deviations from the dotted line. The six stages correspond to the colours in Figure 5. Amit Misra et al. (2014).

Reference:

Amit Misra et al. (2014), “The Effects of Refraction on Transit Transmission Spectroscopy: Application to Earth-like Exoplanets”, arXiv:1407.3265 [astro-ph.EP]

Very High Energy Neutrinos from the Galactic Centre

A supermassive black hole (SMBH) with an estimated ~4 million times the Sun’s mass sits in the galactic centre of the Milky Way, in a particular region of space called Sagittarius A*, pronounced “Sagittarius A-Star”. In addition, the galactic centre also contains a high concentration of astrophysical oddballs. Rare elsewhere but not uncommon in the galactic centre, these objects include massive stars, intensely magnetised neutron stars and even intermediate mass black holes (IMBHs). Due to its unique environment, the galactic centre is a source of frequent highly energetic astrophysical events.

Figure 1: Schematic of the IceCube Neutrino Observatory at the South Pole.

In a recent paper by Peterson et al. (2014), the authors investigate whether a subset of very high energy (VHE) neutrinos observed by the IceCube Neutrino Observatory may have originated from Sagittarius A*. Neutrinos are ghostly subatomic particles that pass through normal matter virtually unimpeded. To be detectable, a neutrino has to interact with normal matter. However, the probability of this happening is vanishingly small. As a result, a neutrino detector needs to be enormously large to detect a significant number of neutrinos. The IceCube Neutrino Observatory is the world’s largest neutrino detector. It detects high energy neutrinos using a densely instrumented cubic kilometre of clear ice within the Antarctic ice sheet under the South Pole.

A three-year data set from the IceCube Neutrino Observatory shows the detection of 36 VHE neutrinos with energies in the 30 TeV to 2 PeV range. Of these 36 VHE neutrino detection events, 7 of them were found to occur within a 30° angular region of the galactic centre. These 7 VHE neutrino detection events exhibit both time and space clustering. Of the 7 events, events #14 and #15 took place within one day of each other. The probability that this would occur randomly is only 1.6 percent. Additionally, event #25 occurred only ~3 hours after the brightest X-ray flare of Sagittarius A* was observed by the Chandra X-ray Observatory and the likelihood of it occurring randomly is only 0.9 percent.

These correlated events indicate that Sagittarius A* might be a source of VHE neutrinos. Asteroids, comets, planets and stars that come too close to the SMBH in Sagittarius A* can become disrupted. Such an event would drive an energetic flare, possibly producing some of the VHE neutrinos detected by the IceCube Neutrino Observatory that appear to originate from the galactic centre. Other sources of high energy astrophysical events in the galactic centre region near Sagittarius A* that can generate VHE neutrinos include exotic objects such as SGR J1745-29, a neutron star with an extraordinarily powerful magnetic field.

Figure 2: Properties of the VHE neutrino detection events consistent with a galactic centre origin. Subsequent probability analysis excludes events #12 and #33. “Pos. Err” refers to the angular resolution of the measurement by the IceCube Neutrino Observatory. Peterson et al. (2014).

Figure 3: Time and positions of the 36 VHE neutrino detection events by the IceCube Neutrino Observatory. The 7 events consistent within 30° of the galactic centre, which fall within the inner blue band, show more time clustering than events away from the galactic centre. Peterson et al. (2014).

Reference:

Peterson et al. (2014), “Neutrino Lighthouse at Sagittarius A*”, arXiv:1407.2243 [astro-ph.HE]

Extreme Outliers of the Milky Way

The Milky Way contains a population of a few hundred billion stars. Like any galaxy, the Milky Way does not have a well defined “edge”. As one moves away from the galaxy, the number of stars per given volume of space simply becomes ever so vanishingly small. Bochanski et al. (2014) report on the discovery of two of the most distant Milky Way stars known to date. The two stars are given the identifiers - ULAS J0015+01 and ULAS J0744+25. Both stars were bright enough to be detectable because they have entered their final stages of stellar evolution and have swelled into red giants, resulting in a considerable increase in their luminosities.

ULAS J0015+01 and ULAS J0744+25 have large estimated distances of 274 ± 74 kpc and 238 ± 64 kpc, respectively, making them the first two Milky Way stars found beyond 200 kpc. For comparison, the disk of the Milky Way, which contains the majority of the galaxy’s stars, is approximately 30 kpc, or 100,000 light-years in diameter. ULAS J0015+01 and ULAS J0744+25 are also moving away from the Milky Way’s centre at 52 ± 10 km/s and 24 ± 10 km/s, respectively.

A number of possible scenarios have been considered to explain the existence of ULAS J0015+01 and ULAS J0744+25. Both stars could not have formed in-situ because the gas density at their location is far too low for star formation. The most likely explanation for the origin of these stars is they are part of a population of stars that have been tidally-stripped from passing dwarf galaxies by the Milky Way’s gravity. Alternative scenarios include hypervelocity ejection (~600 km/s) from the Milky Way, or membership in an undetected dwarf galaxy of extremely low surface brightness.

Reference:

Bochanski et al. (2014), “The Most Distant Stars in the Milky Way”, arXiv:1407.2610 [astro-ph.SR]

Two Very Dissimilar Mass Stars in a Contact Binary

CCD photometric observations of V710 Monocerotis by L. Liu et al. (2014) indicate it is an extreme mass ratio, deep contact binary star system whose primary component (i.e. the more massive star) could be in an expanding phase as the star is entering its post-main-sequence stage of evolution. The primary and secondary components have 1.14 and 0.16 times the Sun’s mass, respectively. This large difference in mass between the primary and secondary components is what makes V710 Monocerotis an “extreme mass ratio binary”. Furthermore, being a contact binary system means that both stars are so close to each other, they actually touch. In fact, both stars are in deep, ~60 percent contact with each other, hence the term “deep contact binary”.

Figure 1: Artist’s impression of a planet circling a binary star system. Such a planet is known as a circumbinary planet.

Figure 2: Geometrical configuration of the deep contact binary V710 Monocerotis at phases 0.00 and 0.50. Only the primary component can be seen at phase 0.50. L. Liu et al. (2014).

V710 Monocerotis is a totally-eclipsing binary system. This is because during each orbit, the primary component completely blocks the secondary component. The primary and secondary components circle around each other every 0.4052 days. Observations reveal that the orbital period of the binary system is increasing at a rate of ~17 seconds per thousand years. For a contact binary system, an increase in orbital period is usually caused by the transfer of mass from the less massive component (i.e. secondary component) to the more massive component (i.e. primary component).

However, for V710 Monocerotis, the mass of the secondary component is so low that it wouldn’t be able to transfer mass to the primary component for much longer. After an estimated ~40,000 years, the secondary component’s mass would have gone down to ~0.08 times the Sun’s mass, placing it within the brown dwarf mass regime. A more plausible explanation for the increase in orbital period is that rather than the transfer of mass, the primary component is entering its post-main-sequence phase of evolution, causing the star’s entire envelope to expand. As it expands, its spin rate slows and the binary system settles into a new equilibrium with a longer orbital period.

Reference:

L. Liu et al., “A possible expanding component in the extreme mass ratio deep contact binary V710 Monocerotis”, New Astronomy, Volume 31, August 2014, Pages 60-64.

A Semi-Detached Binary with a Spotted Primary Star

Figure 1: Artist’s impression of what “twin Suns” might look like from the surface of a circumbinary planet.

Photometric observations of NR Pegasi by A. Erdem et al. (2014) in 2007 and 2008 show that it is a highly active semi-detached binary star system. The primary and secondary stars of the binary system are estimated to be 1.60 ± 0.03 and 0.57 ± 0.02 times the Sun’s mass, 3.35 ± 0.07 and 3.55 ± 0.08 times the Sun’s radius, and 9.10 ± 1.78 and 3.47 ± 0.93 times the Sun’s luminosity, respectively. Both the primary and secondary stars also have respective surface temperatures of 5485 ± 200 K and 4186 ± 241 K. The binary system has an orbital period of 3.3978 days. NR Pegasi is an eclipsing binary system where the primary star blocks part of the secondary star and vice versa during each orbit. As a result, the observed brightness of NR Pegasi varies with time.

Light curves of NR Pegasi obtained in 2007 and 2008 show large asymmetries, and variations could be seen in the light curves from night to night. For example, the depth of the primary minimum (i.e. the secondary star passing in front of and blocking part of the primary star) in the 2007 light curves is shallower than that of the 2008 light curves. These peculiar variations are believed to be caused by the presence of large dark sports on the primary star. The dark spots are not literally dark, but rather, they are spots where the temperatures are somewhat lower than on the rest of the star. Models of NR Pegasi show that the presence of 2 large dark spots (2008 light curves) or 4 large dark spots (2007 light curve) on the primary star could account for the peculiarities in the light curves.

The less massive and cooler secondary star fills its entire Roche lobe, while the more massive and hotter primary star fills 64 percent of its Roche lobe. Basically, the Roche lobe of a star is the region around the star where its own gravity is stronger than that of its companion. NR Pegasi is a semi-detached binary system because one star fills its Roche lobe while the other does not. As a result, gas from the Roche-lobe-filling secondary star (i.e. donor star) is transferred to the primary star (i.e. accreting star). Both stars of NR Pegasi have evolved to the point where they are beginning to exhaust hydrogen in their cores, placing them near the terminal age main sequence phase of their evolution. However, the less massive secondary star appears to be significantly over-sized and over-luminous in comparison to theoretical evolutionary models.

Figure 2: Best fits to the 2007 light curves of the NR Pegasi binary system. A. Erdem et al. (2014).

 Figure 3: Best fits to the 2008 light curves of the NR Pegasi binary system. A. Erdem et al. (2014).

Figure 4: (a) Four spots and (b) two spots for the 2007 and 2008 light curves, respectively. A. Erdem et al. (2014).

Figure 5: Roche lobe geometry of the NR Pegasi binary system. A. Erdem et al. (2014).

Reference:

A. Erdem et al., “NR Peg: A new highly active semi-detached binary”, New Astronomy, Volume 33, November 2014, Pages 38-43.

New Class of Totally Metal Stars

Stars form from the collapse of giant gas clouds comprised almost entirely of hydrogen and helium, with heavy elements (i.e. elements heavier than hydrogen and helium) making up only a tiny percentage of the material. In these clouds, dust grains contain a large fraction of the heavy elements. Astrophysicists refer to elements heavier than hydrogen and helium as metals, and the concentration of heavy elements in a star is referred to as the star’s metallicity. For example, the Sun’s metallicity is 0.02, which means 2 percent of its mass is in the form of elements heavier than hydrogen and helium.

Turbulence is a ubiquitous process in nature. A study by Philip F. Hopkins (2014) suggests that given the right conditions in star-forming clouds, dust grains can behave as aerodynamic particles and decouple from the gaseous hydrogen and helium. The presence of turbulence can preferentially concentrate the dust grains in specific regions such that the local abundance of heavy elements can become so high that stars made almost entirely of metal can form. Such stars would have metallicities approaching 1.00. In a star-forming cloud with preferential concentration of dust grains, perhaps one in 10,000 stars could form as a “totally metal” star.

For the preferential concentration of heavy elements to occur in star-forming gas clouds, the dust grains must be of a certain size range such that they are large enough to decouple from the gas but also small enough to “feel” the gas flow. Eddies created by presence of turbulence in these gas clouds would drive these dust grains into regions of lower vorticity (i.e. gaps between eddies). As a result, heavy elements become concentrated separately from gaseous hydrogen and helium. Stars that form in these regions can acquire abnormally high metallicities and might even be “totally metal”.

The process of how a core of dust grains might collapse to form a “metal” star is not entirely clear. Nevertheless, it is reasonable to think that as a core of dust grains collapses under its own gravity, the dust grains would shatter into smaller grains and melt to form gas-phase metals. Such a process would cool the collapsing core and drive a more rapid collapse until a “metal” star is formed. Since planets seem to be ubiquitous around stars, planetary systems should also form around “metal” stars. Given the unique high metallicity conditions, the formation of truly exotic planets might be possible.

With an exceedingly high metallicity, a “metal” star would have a remarkably long lifespan and would appear very unusual. This brings to mind the concept of “frozen stars” proposed by Fred Adams and Gregory Laughlin (1997) in a paper entitled “A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects”. An exceptionally high metallicity low mass star could sustain internal nuclear fusion at a very slow rate for a thousand trillion years or so, outliving the dimmest red dwarf stars by a factor of a thousand. Such a star can even be cool enough to have water-ice clouds, hence the term “frozen stars”.

References:

- Philip F. Hopkins (2014), “Some Stars are Totally Metal: A New Mechanism Driving Dust Across Star-Forming Clouds, and Consequences for Planets, Stars, and Galaxies”, arXiv:1406.5509 [astro-ph.GA]

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

Sub-Stellar Companion around a Young Star

A handful of sub-stellar companions around young stars have been found by direct imaging. Sub-stellar companions are basically a group of objects comprising giant planets and brown dwarfs, although the dividing line between the two is not clear. Young stars are great targets for direct imaging searches for giant planets and brown dwarfs because these sub-stellar objects have yet to cool sufficiently and would still be glowing intensely hot from all the heat acquired during their formation process. M. Bonavita et al. (2014) present the direct imaging discovery of HD 284149 b, a 18 to 50 Jupiter-mass sub-stellar companion located at a relatively large projected separation distance of ~400 AU from a young F8 star.

Figure 1: Artist’s depiction of a sub-stellar object glowing red hot from heat acquired during its formation.

The discovery of HD 284149 b came about from direct imaging of its host star over six epochs between October 2011 and March 2014 with the adaptive-optics assisted Near Infrared Imager and Spectrometer (NIRI) on the 8.19-metre Gemini North telescope located on Hawaii’s Mauna Kea. The host star of HD 284149 b is somewhat more massive and more luminous than the Sun. It has an estimated surface temperature of 5970 to 6100 K and an assumed age of ~25 million years. HD 284149 b is most certainly the same age as it host star. Given its young age, it is still glowing hot with an estimated surface temperature of ~2500 K.

The mass of HD 284149 b places it in the overlapping mass regime comprising the more massive giant planets and the lower mass brown dwarfs. How a sub-stellar object like HD 284149 b could have formed at such a wide separation from its host star remains unclear. HD 284149 b could have formed by coalescing out from a protoplanetary disk like planets do, or it could have formed from the direct collapse of a clump of gas and dust like stars do. HD 284149 b joins a number of sub-stellar companion objects such as AB Pictoris b and ROXs 42Bb which share rather similar properties.

Figure 2: Theoretical models constraining the age and mass of HD 284149 b. M. Bonavita et al. (2014).

Reference:

M. Bonavita et al. (2014), “A new sub-stellar companion around the young star HD 284149”, arXiv:1406.7298 [astro-ph.SR]

Super-Earth in a Polar Orbit

Figure 1: Artist’s depiction of “sunrise” on 55 Cancri e. Image credit: Ron Miller.

55 Cancri e is a transiting exoplanet orbiting the Sun-like star 55 Cancri A. The suffix “e” indicates 55 Cancri e is the 4th planet discovered around the star. 55 Cancri e has 8 times the mass and twice the radius of Earth, placing it in the super-Earth-mass regime. The planet circles its host star in an unusually close-in orbit, racing around once every 17 hours 41 minutes, at an average star-planet separation distance of only 2.3 million km. 55 Cancri e is so near to its host star that its dayside is incinerated to a temperature of well over 2000 K, hot enough to melt most metals.

Spectroscopic observations using the HARPS-N spectrograph were conducted during the transit of 55 Cancri e across its host star. The observations allow the angle between the spins of the planet’s orbit and the star’s rotation, also known as the sky-projected obliquity, to be measured via the Rossiter-McLaughlin (RM) effect. Basically, the RM effect is a spectroscopic phenomenon that can be observed when a planet passes in front of its host star. As a star rotates on its axis, one half of its visible hemisphere will be seen approaching the observer and the other half will be seen receding away. Light from the approaching side would appear blue-shifted and light from the receding side would appear red-shifted.

When a planet passes in front of the star, it sequentially blocks some of the blue-shifted and red-shifted light, or vice versa. If the planet is in front of the blue-shifted portion, the star’s apparent radial velocity will have a positive value (i.e. the star appears to be receding) and if the planet is in front of the red-shifted portion, the star’s apparent radial velocity will have a negative value (i.e. the star appears to be approaching). The way the planet blocks the star’s blue-shifted and red-shifted light can reveal its sky-projected obliquity.

Figure 2: The solid red line shows the best fit to the observed Rossiter-McLaughlin anomaly of 55 Cancri e. The residuals yield a dispersion of 0.28 m/s. Bourrier & Hebrard (2014).

Figure 3: Schematic showing the view of 55 Cancri e. During transit, 55 Cancri e (shown as a black disk) crosses mainly in front of the blue-shifted half of the stellar disk due to its high sky-projected obliquity of 72.4°. Bourrier & Hebrard (2014).

In the case for 55 Cancri e, the measured RM effect is consistent with a high sky-projected obliquity of 72.4° (+12.7° / -11.5°). This indicates that the planet is in a highly misaligned and nearly polar orbit around its host star. Besides 55 Cancri e, the other 4 known planets around 55 Cancri A are also likely to be highly misaligned with the star’s spin axis. The orbits of the 5 known planets around 55 Cancri A are expected to be coplanar with one another despite being highly misaligned with the star’s spin axis.

55 Cancri A joins Kepler-56 as the two stars known to have highly misaligned multi-planet systems. In most multi-planet systems, including our Solar System, the planets all have orbits that are more or less coplanar with the equatorial planes of their host stars. 55 Cancri A has a companion star, identified as 55 Cancri B, at a distance of 1065 AU. It has been shown that the gravitational influence of 55 Cancri B is sufficient to have altered the alignment of the planetary system around 55 Cancri A to what is presently observed.

Reference:

Bourrier & Hebrard (2014), “Detecting the spin-orbit misalignment of the super-Earth 55 Cnc e”, arXiv:1406.6813 [astro-ph.EP]