Saturday, February 27, 2010

Dissipating Planet

A paper by Shu-lin Li, et al. (2010) entitled “WASP-12b as a Prolate, Inflated and Disrupting Planet from Tidal Dissipation” describes some peculiar properties of an extrasolar planet. WASP-12b is an extrasolar gas giant planet that has 1.4 times the mass of Jupiter and it is located so remarkably close to its parent star that it takes only a little over an Earth day to orbit the star. In fact, WASP-12b orbits its parent star at a distance of just 2 stellar radii from the star’s surface and the planet is distorted by the star’s gravity into a prolate shape, similar to that of a rugby ball.

The nonzero orbital eccentricity of WASP-12b makes it very susceptible to the strong tidal effects from its parent star which heats the planet’s interior and causes the planet to expand. WASP-12b has an orbital eccentricity of about 0.05 and this is odd because orbital circularization should have already circularized its orbit into a zero eccentricity orbit. Another planet with a mass of a few Earths is probably responsible for perturbing WASP-12b to maintain its nonzero orbital eccentricity. WASP-12b is extremely “bloated” and it has a diameter that is 80 percent larger than Jupiter’s. The atmospheric temperature of WASP-12b is estimated to be a scorching 2500 degrees Kelvin.

WASP-12b has ballooned so much that its own gravity is unable to retain its mass from the gravitational pull of its parent star. Gas from WASP-12b is flowing towards the parent star through a nozzle that is located at the L1 Lagrangian point, a region that is situated between the planet and the star. WASP-12b is losing mass to its host star at a rate of a few billion metric tons each second. The material that is pulled off from WASP-12b forms an accretion disk around the parent star and gradually spiral inwards into the star.

Wednesday, February 24, 2010

Intergalactic Space

The Local Group contains dozens of galaxies contained within a volume about 10 million light years across and its two largest galaxies are the Andromeda galaxy and the Milky Way galaxy. Both galaxies account for over 80 percent of the visible light of the Local Group. The third most prominent galaxy in the Local Group is the Triangulum galaxy and it has approximately one-fifth the luminosity of the Milky Way Galaxy. The rest of the galaxies in the Local Group are irregular galaxies, dwarf irregular galaxies, dwarf elliptical galaxies and dwarf spherical galaxies. Many of these smaller galaxies are either in orbit around the Andromeda galaxy or the Milky Way galaxy. Within the Local Group, many more tiny galaxies are still probably waiting to be discovered.

The mutual gravitational attraction from matter within the Local Group dominates over the expansion of the universe. The galaxies in the Local Group range in size from the Andromeda and Milky Way galaxies which contain hundreds of billions of stars each to tiny galaxies which hold fewer than a million stars. The Andromeda galaxy and the Milky Way galaxy are separated by a distance of 2.5 million light years and they are approaching each other at a rate which will most probably result in a merger over the next few billion years. Such a merger will lead to the formation of a giant elliptical galaxy. It is very unlikely that stars will collide when both galaxies merge as stars in galaxies are typically spaced extremely far apart from each other.

Located 700 million light years away, in the vicinity of the constellation Boötes as seen from the Earth is one of the largest known voids in the Universe and it is called the Great Void or Boötes Void. This region of empty space is estimated to be 400 million light years across. For comparison, the luminous disk of the Milky Way galaxy is 100000 light years across and you will need 4000 Milky Way galaxies placed end to end to cover the span of the Boötes Void. Within this incredibly vast void, several galaxies have been detected to extend in a rough tube-shape through the middle of the void. Astronomer Greg Aldering once mentioned: “If the Milky Way had been in the center of the Boötes void, we wouldn’t have known there were other galaxies until the 1960s.”

Friday, February 19, 2010

Evolved Binaries

NASA’s Kepler space telescope was launched on 6 March 2009 with the primary objective of detecting Earth-like planets orbiting other stars. Amongst Kepler’s first major discoveries are two small objects which are hotter than the stars they orbit and both objects are likely to be very low mass white dwarfs. KOI-81b is estimated to be 40 percent as luminous as the Sun with a surface temperature of 13000 degrees Kelvin while KOI-74b is estimated to be 3 percent as luminous as the Sun with a surface temperature of 12000 degrees Kelvin. These objects are definitely not planets as they have surface temperatures an order of magnitude higher than would be consistent with stellar heating alone.

A paper by Rosanne Di Stefano (2010) entitled “Transits and Lensing by Compact Objects in the Kepler Field: Disrupted Stars Orbiting Blue Stragglers” describes the prospects of Kepler in discovering a large number of such objects as described above. These hot compact objects are most likely the cores of stars that have evolved to their present state through a process of stable mass transfer with their current stellar companions. Kepler offers a unique opportunity to study a large sample of such white dwarfs and the role of mass transfer in the evolution of binary star systems.

Tuesday, February 16, 2010

Clouds and Extrasolar Planets

A paper by Daniel Kitzmann, et al. (2010) entitled “Clouds in the Atmospheres of Extrasolar Planets. I. Climatic Effects of Multi-layered Clouds for Earth-like Planets and Implications for Habitable Zones” studies the impact of clouds on the surface temperatures of Earth-like planets orbiting different types of stars. Clouds play a vital role in determining the climatic conditions of planetary atmospheres by reflecting incident stellar radiation back into space via the albedo effect and by trapping infrared radiation in the atmosphere via the greenhouse effect. For the Earth, low-level clouds cause cooling via the albedo effect and high-level clouds cause heating via the greenhouse effect. For mid-level clouds, the albedo effect and greenhouse effect generally balance each other.

In this paper, studies were done for Earth-like planets orbiting F, G, K and M-type stars. Note that our Sun is a G-type star and for this range of stellar types, F-type stars are the hottest while M-type stars are the coolest. With respect to a clear sky model, a planet with a higher percentage of high-level clouds will have a higher surface temperature while a planet with a higher percentage of low-level clouds will have a lower surface temperature. This characteristic applies to all Earth-like planets orbiting F, G, K and M-type stars.

For the Earth, the presence of clouds creates a net cooling effect with respect to the clear sky model and the mean value of the Earth’s surface temperature is 288.4 degrees Kelvin. Using just low-level clouds for maximum cooling effect, planets can be located up to 15 percent closer to their parent star compared to a clear sky planet to achieve the same average Earth’s surface temperature of 288.4 degrees Kelvin. On the contrary, using just high-level clouds for maximum heating effect, planets can be located up to 35 percent further from their parent star compared to a clear sky planet to achieve the average Earth’s surface temperature.

Saturday, February 13, 2010

Dark Stars

The first stars that form in the early universe can be powered by dark matter instead of conventional nuclear fusion and these stars are conveniently termed dark stars. These hypothesized dark stars still remain to be discovered and future telescopes such as the James Webb Space Telescope (JWST) may have the sensitivities required to detect such stars. The term ‘dark’ does not mean that these stars appear dark, but rather it means that the energy from the annihilation of dark matter particles is the primary mechanism keeping these stars shining in hydrostatic equilibrium. A paper by Katherine Freese, et al. (2010) entitled “Supermassive Dark Stars - Detectable in JWST” basically describes such stars and their possible detection by the JWST.

Weakly Interacting Massive Particles (WIMPs), an excellent candidate for dark matter, may be their own antiparticles and this allows them to annihilate among themselves. Wherever the density is sufficiently high, the annihilations of WIMPs can generate enough energy to power such dark stars for millions to billions of years. The WIMP annihilation products thermalize within the star which provides the power required to keep the star shining and prevents the star from gravitationally collapsing. Dark stars are composed primarily of hydrogen and helium, with only a fraction of a percent of their mass in the form of dark matter. Due to the high efficiency of energy production via the annihilation of WIMPs, a small proportion of it is sufficient to sustain the star against its own gravity over astronomical timescales.

The much cooler surface temperatures of dark stars is the primary reason which allows darks stars to grow to become so much more massive than ordinary fusion-powered stars, as long as the annihilation of dark matter persists. Ordinary fusion powered stars produce ionizing photons that provide a variety of feedback mechanisms that cut off further accretion of baryonic matter. On the other hand, dark stars with their cooler surface temperatures allow the continued accretion of baryonic matter all the way up to colossal stellar masses. Dark stars can grow up to millions of times the mass of the Sun and exceed billions of times the luminosity of the Sun. Such massive stars can lead to the formation of supermassive black holes when they run out of dark matter fuel to sustain them. When the production of energy from dark matter annihilation dwindles, it causes the star to contract, heat up and undergo a brief phase as a fusion powered star before collapsing into a massive black hole.

Wednesday, February 10, 2010

Runaway Star

A hypervelocity star is a type of star which travels at a sufficiently high velocity with respect to its host galaxy’s rest frame that its velocity exceeds the galaxy’s local escape velocity. This places the star on a trajectory which removes it from its host galaxy. Gravitational interactions and supernova explosions in binary systems are two possible mechanisms that can produce hypervelocity stars. A paper by Andreas Irrgang, et al. (2010) entitled “The Nature of the Hyper-Runaway Candidate HIP 60350” explains the nature of one such hypervelocity star.

Kinematical observations of HIP 60350 show that it has a velocity which exceeds the local escape velocity of the Milky Way galaxy. This places HIP 60350 on a trajectory that will remove it from the Milky Way galaxy. Tracing its origin back to the plane of the Milky Way galaxy, the estimated travel time of HIP 60350 is about 14 million years. Given an estimated age of 45 million years for the star, a shorter travel time is consistent with the runaway nature of the star. So far, neither gravitational interaction nor a supernova explosion in a binary system can be strictly confirmed or rejected for HIP 60350.

In addition, a neutron star formed from an asymmetric supernova explosion can have sufficient ‘kick’ imparted to it for it to become a hypervelocity star. One such neutron star is RX J0822-4300 and it is measured to travel at a remarkable speed of over 1300 kilometres per second.