Wednesday, May 25, 2016

Black Hole Tore Apart and Swallowed a Star

Sun-like stars, white dwarfs and giant stars can be wholly swallowed by black holes with masses greater than ~100 million, ~100 thousand and ~10 billion times the mass of the Sun, respectively. When a star is wholly swallowed by a black hole, no flares will be observed. In contrast, for a black hole that is not massive enough to swallow a star whole, a tidal disruption flare can be generated. A star approaching such a black hole on a low angular momentum orbit can be torn apart by tidal forces and a fraction of the star's mass can form an accretion disk around the black hole, powering a tidal disruption flare.

XMMSL1J063045.9603110 is a candidate tidal disruption event. This event was first detected in X-rays with an underlying soft X-ray thermal emission. Twenty days later, XMMSL1J063045.9603110 was again detected with soft X-ray thermal emission, this time ~10 times dimmer than when it was first detected. The X-ray emission over time appears to be consistent with the presence of an accretion disk around a black hole. In this case, the accretion disk represents a fraction of the material left over from a tidally disrupted star that got swallowed by the black hole.

Depending on assumptions, the black hole responsible for this tidal disruption flare is estimated to have between ~10 thousand to ~100 thousand times the mass of the Sun. Optical observations suggest that the black hole associated with XMMSL1J063045.9-603110 likely resides within either a very faint dwarf galaxy or a very bright globular cluster. If the black hole resides within a globular cluster, then XMMSL1J063045.9-603110 could be the first tidal disruption flare observed in a globular cluster.

Mainetti et al. (2016), "XMMSL1J063045.9-603110: a tidal disruption event fallen into the back burner", arXiv:1605.06133 [astro-ph.HE]

Tuesday, May 24, 2016

Discovery of the First Transiting Jupiter Analog

Kipping et al. (2016) present the discovery of the first transiting Jupiter analog. This planet is identified as Kepler-167e and it is part of a planetary system which contains three known transiting super-Earths that are in relatively close-in orbits around their host star. Kepler-167 is the host star of this planetary system. It is a K3V star with ~0.77 times the Sun's mass, ~0.73 times the Sun's radius and an effective temperature estimated to be ~4890 K.

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

The three super-Earths in this planetary system are Kepler-167b, c and d. Kepler-167b has ~1.615 times the radius of Earth, a 4.393 day orbital period, an estimated equilibrium temperature of about 914 K and it receives 116 times the amount of flux Earth gets from the Sun. Kepler-167c has ~1.548 times the radius of Earth, a 7.406 day orbital period, an estimated equilibrium temperature of about 758 K and it receives 57.7 times the amount of flux Earth gets from the Sun. Kepler-167d has ~1.194 times the radius of Earth, a 21.804 day orbital period, an estimated equilibrium temperature of about 536 K and it receives 13.7 times the amount of flux Earth gets from the Sun.

Kepler-167e orbits much further out compared to the three inner super-Earths. Kepler-167e measures ~10.15 times the size of Earth, which is roughly 90 percent the radius of Jupiter. It orbits its host star every 1071 days, its estimated equilibrium temperature is about 130 K and it receives only ~7 percent the amount of flux Earth gets from the Sun. Kepler-167e is about twice as far from its host star as Earth is from the Sun and its orbit around its host star is close to being perfectly circular. With these properties, Kepler-167e bears a great deal of resemblance to Jupiter and hence, it is termed a Jupiter analog.

The compact trio of super-Earths around Kepler-167 raises the possibility that many of the currently known compact multi-planetary systems may host Jupiter analogs on distant orbits. Kepler-167e is a rare find because it transits its host star once every 2.9 years. Since one would need 2 transits to confirm its planetary nature and given that the primary Kepler mission ran for 4.3 years, the detection of Kepler-167e was indeed a fortunate one. Because Kepler-167e is a transiting planet, it also offers a unique opportunity for the characterisation of the atmosphere of a Jupiter analog.

Figure 2: Folded transit light curves of Kepler-167b, Kepler-167c, Kepler-167d and Kepler-167e. For the upper three, data (gray points) are binned to a 10 minute cadence. Light curve of Kepler-167e uses 30 minute binning and uses circles to denote the first transit (Q4) and squares to denote the second transit (Q16). Note that all of the transits were fitted using the original unbinned data. Kipping et al. (2016)

Figure 3: Catalogue of known transiting exoplanets with colour depicting the peak wavelength colour of the parent star. Solar system planets are shown with black symbols, and the Kepler-167 planets with squares. The blue box depicts Jovian-sized planets beyond the snow-line, with Kepler-167e being the first transiting planet to be in this space. Kipping et al. (2016)

Figure 4: Schematic illustrating the scale of the Kepler-167 system. Planet sizes are scaled relative to the key, rather than the orbital distances in order to make them visible. The four known planets display remarkable coplanarity and near-circular orbits with the habitable-zone notably devoid of transiting planets. Kipping et al. (2016)

Kipping et al. (2016), "A Transiting Jupiter Analog", arXiv:1603.00042 [astro-ph.EP]

Monday, May 23, 2016

A Red and Rough Kuiper Belt Object

1994 JR1 is a Kuiper Belt Object in a 3:2 orbital resonance with Neptune. That means for every three times Neptune goes around the Sun, 1994 JR1 will go around the Sun twice. 1994 JR1 was observed by the Long Range Reconnaissance Imager (LORRI) on NASA's New Horizons spacecraft on 2 November 2015 and on 7 April 2016. This represents the first close observations of a small KBO. Due to Earth's proximity to the Sun, only the dayside of 1994 JR1 is observable from Earth. As a result, the New Horizons spacecraft, which is beyond the orbit of Pluto, is the only means of observing 1994 JR1 at large solar phase angles.

Combining observations from the New Horizons spacecraft with observations from the Hubble Space Telescope and ground based observatories, 1994 JR1 appears to be a very red KBO with a high surface roughness (i.e. it is probably heavily cratered). 1994 JR1 has a relatively fast rotation period of 5.47 ± 0.33 hours and its diameter is assumed to be ~250 km. Simulations also indicate that the orbit of 1994 JR1 brings it close to Pluto every 2.4 million years. Each close encounter causes the orbit of 1994 JR1 to be gravitationally perturbed by Pluto.

Porter et al. (2016), "Red, Rough, Fast, and Perturbed: New Horizons Observations of KBO (15810) 1994 JR1 from the Kuiper Belt", arXiv:1605.05376 [astro-ph.EP]

Sunday, May 22, 2016

A Giant Planet in the Light Curve

Figure 1: Artist's impression of a giant planet.

Gould et al. (2016) present the discovery of a massive planet with 4.4 ± 1.6 times the mass of Jupiter in orbit around a red dwarf star with 0.37 ± 0.14 times the mass of the Sun. This system was discovered via gravitational microlensing, whereby the planet-star system crosses the line-of-sight to a background star and the gravitational field of the planet-star system acts as a lens, magnifying light from the background star. The projected separation between the planet and star is estimated to be ~1.2 AU.

This gravitational microlensing event is identified as OGLE-2015-BLG-0954 and it was observed by the Korea Microlensing Telescope Network (KMTNet), a system of three 1.6 m telescopes located in Chile, South Africa and Australia. The wide field of view and the high cadence (6 measurements per hour) of KMTNet allowed for this gravitational microlensing event to be measured despite the short line-of-sight crossing time of only 16 minutes.

Figure 2: Light curve and best-fit model for KMTNet observations of OGLE-2015-BLG-0954 with data from Chile (red), South Africa (blue) and Australia (magenta). Insets show the caustic which extends from 7164.62 to 7165.15. Error bars are omitted from the main figure to avoid clutter but are shown in the residuals. Gould et al. (2016)

Gould et al. (2016), "A Super-Jupiter Microlens Planet Characterized by High-Cadence KMTNet Microlensing Survey Observations", arXiv:1603.00020 [astro-ph.EP]

Saturday, May 21, 2016

Aquarius 2 is a Difficult to Detect Dwarf Galaxy

Torrealba et al. (2016) present the discovery of Aquarius 2, a dwarf galaxy that is a distant satellite galaxy of the Milky Way. Aquarius 2 was identified based on an overdensity of Red Giant Branch (RGB) stars and an overpopulation of Blue Horizontal Branch (BHB) stars. RGB and BHB stars are stars that have evolved off the main sequence and they are many times more luminous than their main sequence progenitors. These luminous evolved stars can indicate the position of what would otherwise be an invisible galaxy.

The estimated distance of Aquarius 2 from the Milky Way is ~360,000 light years and its estimated half-light radius is ~500 light years. The half-light radius of a galaxy is the size of the volume around the center of the galaxy which accounts for half the galaxy's brightness. The half-light radius of Aquarius 2 is estimated to enclose ~3 million times the Sun's mass.

Aquarius 2 has a low surface brightness of only 30.4 mag/arcsec², and the total luminosity of this dwarf galaxy is only ~4000 times the Sun's luminosity. Aquarius 2 has a large mass-to-light ratio of ~1300, indicating it is a dark matter dominated galaxy. The low surface brightness and low overall luminosity of Aquarius 2 makes it a particularly difficult galaxy to detect. The detection of Aquarius 2 suggests the presence of more dwarf galaxies lurking out there.

The red solid line denotes the elliptical half-light contour of the best fit model for Aquarius 2. The orange, red and blue circles/squares mark the locations of stars confirmed as foreground, RGB and BHB stars, respectively. Torrealba et al. (2016)

Absolute magnitude versus half-light radius for Milky Way satellite galaxies (red open circles), M31 satellite galaxies (black unfilled triangles), globular clusters (black dots), extended objects larger than ~300 light years in size (grey dots) and Local Group/nearby galaxies (grey crosses). Torrealba et al. (2016)

Torrealba et al. (2016), "At the survey limits: discovery of the Aquarius 2 dwarf galaxy in the VST ATLAS and the SDSS data", arXiv:1605.05338 [astro-ph.GA]

Friday, May 20, 2016

Clouds of Water on a Cold Brown Dwarf

Figure 1: Artist's rendering of Jupiter.

WISE 0855 is the coldest brown dwarf known. It has an effective temperature of only ~250 K. This is comparable to the atmospheric temperatures found on Earth, Mars, Jupiter and Saturn. WISE 0855 is estimated to have between 3 to 10 times the mass of Jupiter, and it is located ~7.5 light years away. WISE 0855 provides a good opportunity for the direct study of an object with similar physical characteristics as Jupiter. Observations of WISE 0855 with the Gemini-North telescope located near the summit of Mauna Kea in Hawaii revealed the presence of atmospheric water vapour and clouds on WISE 0855. The spectrum of WISE 0855 appears remarkably similar to Jupiter's. 

Figure 2: Temperature-pressure profiles of Jupiter, WISE 0855 and Gl 570D. Two dashed lines show the boundaries where H2O gas and NH3 gas begin to condense into clouds composed of H2O ice and NH3 ice. Skemer et al. (2016)

Skemer et al. (2016), "The First Spectrum of the Coldest Brown Dwarf", arXiv:1605.04902 [astro-ph.EP]

Thursday, May 19, 2016

Two Giant Planets on Very Dissimilar Orbits

Raetz et al. (2016) present the first ever detection of a directly imaged planet in a wide-separation orbit around a star that also hosts a short-period transiting planet candidate. The directly imaged planet is identified as CVSO 30c and the short-period transiting planet candidate is identified as CVSO 30b. Both CVSO 30b and CVSO 30c are gas giant planets, each estimated to contain a few times the mass of Jupiter. The host star of CVSO 30b and CVSO 30c is a relatively young star estimated to be only a couple or so million years old. Its effective surface temperature is ~3470 K, its mass is 0.34 to 0.44 times the Sun's mass and its luminosity is about a quarter the Sun's luminosity.

CVSO 30b is short-period transiting planet candidate with an extremely short orbital period of only 10.8 hours. In contrast, the directly imaged planet CVSO 30c has an orbital period of about 27,000 years. The orbits of both planets could not have been more different. Both planets may have gotten into their current orbits following a violent planet-planet scattering event. CVSO 30c is 662 ± 96 AU from its host star, and this is far enough that the planet is not lost in the glare of its host star, allowing it to be directly detected. The direct observations indicate that the equilibrium temperature of CVSO 30c is ~1600 K.

Raetz et al. (2016), "YETI observations of the young transiting planet candidate CVSO 30 b", arXiv:1605.05091 [astro-ph.EP]

Wednesday, May 18, 2016

Powerful White-Light Flare from a Red Dwarf Star

Schmidt et al. (2016) present the detection of a powerful white-light flare on a diminutive L0 spectral type red dwarf star. The event is given the designation ASASSN-16AE and the red dwarf star is identified as SDSS0533, estimated to be 315 ± 75 light years away. SDSS0533 appears to be near the stellar-substellar boundary. Nevertheless, SDSS0533 is most likely a red dwarf star rather than a brown dwarf because it is an old object.

Such a conclusion can be made because spectral observations indicate that SDSS0533 does not have a low surface gravity. This implies that SDSS0533 had sufficient time to contract to its current size, and the same mass in a smaller volume gives a higher surface gravity. Since SDSS0533 is a red dwarf star, it is hot enough to have an L0 spectral type despite its old age. If SDSS0533 is a brown dwarf, it would have already cooled too much to maintain an L0 spectral type.

The white-light flare that erupted on SDSS0533 is one of the strongest detected so far. During the flare, a significant area on the red dwarf star is predicted to have reached temperatures exceeding 10,000 K. For comparison, the normal temperature on such a red dwarf star is only ~2000 K or so. Based on the best-fit model, the flare's luminosity appears to have a half life of approximately 180 seconds. The detection of ASASSN-16AE shows that violent stellar-type activity can occur for objects belonging to the L spectral class. Objects in this spectral class include the least massive stars, and the youngest and/or most massive brown dwarfs.

Schmidt et al. (2016), "ASASSN-16ae: A Powerful White-Light Flare on an Early-L Dwarf", arXiv:1605.04313 [astro-ph.SR]

Tuesday, May 17, 2016

Discovery of a Super-Sized Rocky Planet

Osborn et al. (2016) present the discovery of a super-sized rocky planet identified as EPIC212521166 b. This planet orbits an old metal-poor K3 host star every 13.86 days. A combination of high-precision transit and radial velocity observations indicate that EPIC212521166 b has 2.6 ± 0.1 times the radius and 18.3 ± 2.8 times the mass of Earth. This gives EPIC212521166 b a similar density as Earth, making it the most massive known planet with a sub-Neptune radius. The surface gravity on EPIC212521166 b is nearly 3 times the surface gravity on Earth.

Figure 1: Artist's impression of a rocky planet.

Interior models of EPIC212521166 b show that the planet can be comprised of an Earth-like core with 9 times the mass of Earth containing ~70 percent rock and ~30 percent iron, and an overlying layer of water amounting to 9 times the mass of Earth. Alternatively, EPIC212521166 b can be comprised of an iron core and rocky mantle totalling 18.1 times the mass of Earth, with a ~2500 km thick hydrogen-helium atmosphere measuring just 0.2 times the mass of Earth.

The remarkably high mean density of EPIC212521166 b suggests that rocky planets with 10 to 20 times the mass of Earth can form without accreting significant amounts of hydrogen and helium. EPIC212521166 b is unlikely to be the massive core of a gas-giant planet whose hydrogen and helium envelope was stripped away by stellar radiation because radiation from its host star is incapable of removing any substantial amount of hydrogen from the planet. The proximity of EPIC212521166 b from its host star gives it an equilibrium temperature of 640 ± 20 K.

Figure 2: EPIC212521166 b (solid cross, right) compared to other super-Earth, sub-Neptune and Neptunian planets. The mass-radius curves shown here are for planets with 100 percent iron, Earth-like, 50 percent water and 100 percent water compositions (dashed lines from bottom to top). Osborn et al. (2016)

Osborn et al. (2016), "EPIC212521166 b: a Neptune-mass planet with Earth-like density", arXiv:1605.04291 [astro-ph.EP]

Saturday, May 14, 2016

Detecting the Presence of an Unseen Hot-Jupiter

Hot-Jupiters are a class of Jupiter-like planets that orbit very close to their host stars. They have orbital periods of only a few days. The most commonly accepted mechanism regarding the formation of hot-Jupiters is that they formed at larger distances from their host stars before migrating inwards. Alternatively, hot-Jupiters may also form in situ via gas accretion onto massive cores with 10 to 20 times the mass of Earth. Hot-Jupiters that formed in situ are expected to be accompanied by low-mass companion planets with orbital periods of less than ~100 days.

Millholland et al. (2016) present the possible detection of a non-transiting hot-Jupiter in a planetary system consisting of two low-mass transiting planet candidates with longer orbital periods. The technique employed to detect this non-transiting hot-Jupiter is a novel one which combines optical phase curve analysis and astrometric transit timing variations (TTVs). Optical phase curve analysis involves measuring the reflected light from the hot-Jupiter as its dayside rotates in and out of view.

Astrometric TTVs occurs when the transit timings of the outer planets are not perfectly periodic as the star they orbit is a "moving target". This is because the mass of the hot-Jupiter is sufficiently large to be non-negligible in comparison to the mass of the host star. As a result, the gravitational influence of the hot-Jupiter causes the star to wobble as both the star and hot-Jupiter are orbiting their common center-of-mass.

The non-transiting hot-Jupiter in this study was detected around a Sun-like star identified as KOI-1858. Orbiting KOI-1858 are two known transiting planet candidates - KOI-1858.01 and KOI-1858.02. KOI-1858.01 has ~3.53 times the radius of Earth and has a 116.3 day orbital period. KOI-1858.02 has ~2.06 times the radius of Earth and has a 86.0 day orbital period. Both the observed optical phase curve and astrometric TTVs are mutually consistent with the presence of a non-transiting hot-Jupiter with 1.5 ± 0.4 times the mass of Jupiter and a 2.991 day orbital period.

Millholland et al. (2016), "On the Detection of Non-Transiting Hot Jupiters in Multiple-Planet Systems", arXiv:1602.05674 [astro-ph.EP]