The role of episodic overturn in generating the surface geology and heat flow on Enceladus
Craig O’|Neill & Francis Nimmo 2010, Nature Geoscience 3, 88–91
http://www.nature.com/ngeo/journal/v3/n2/full/ngeo731.html
Enceladus, Saturn’s second closest satellite, is well known for its icy and even snowy aspect and structure. It has a diameter of 500km and, even though it is all composed out of ice, it has shown signs of tectonic and volcanic activity, which, interestingly, is concentrated in the south pole of the moon, emanating geyserlike jets, watery plumes that spew outward from a region carved up by unusually warm gashes known as "tiger stripes"
O’Neill & Nimmo 2010 observed abnormally high heat flux at the south pole which can be explained by the existence of a convecting mantle plume under the moon’s tectonic plates. They propose a model through which this dispersion can very probably be due to tidal heating. This effect could be possible in the south pole due to the ice shell being thinner over a region with a warmer interior.
During Enceladus orbit around Saturn, the strong gravity from the massive planet causes the icy moon to “stretch”, thus generating tidal heat waves (similar to the tidal waves on Earth cause by the moon). O'Neill & Nimmo 2010 assume that what is being observed on Enceladus currently is quite an unusual behavior which can be described as a brief period of tectonic activity that resurfaces a limited area of the moon. In the model that they create, they describe how the heat is being stored up inside over billions of years, thus, building up to a maximum point at which it starts “erupting, escaping in a brief volcanic-like activity.
Friday, March 5, 2010
Article Summary #14
Saturn's largest ring
A. J. Verbiscer, M. F. Skrutskie, D. P. Hamilton, 2009, Nature 461, 1098-1100
http://www.nature.com/nature/journal/v461/n7267/full/nature08515.html
By analyzing data from the Spitzer Space Telescope’s MIPS (multi-band imaging photometer), Verbiscer et al. 2009 conducted an investigation on one of Saturn’s satellites, Phoebe, in order to look for a broad debris disk. A debris disk is a circumstellar disk of dust and debris in orbit around a star and can constitute a phase in the formation of a planetary system following the protoplanetary disk phase.
By looking at mid-infrared images of Saturn’s ecliptic plane, Verbiscer et al. 2009 detected a diffused band of light with two peaks which they knew did not come from the planet’s scattered light.
In most cases, the moon of a planet continuously supplies dust to a planetary ring associated with it, but this has been observed for the satellites that are closest to their planets. In this paper, Verbiscer et al. 2009 discuss the discovery of a ring of this type for Phoebe. This phenomenon is very unusual because Phoebe is very far from its hosting planet (a distance hundreds of times greater than the radius of Saturn), and the disk appears to be ~40 times thicker than the same radius and matches the satellite’s orbit and its vertical range.
This ring has not been detected before due to the fact that its composed out of particles smaller than 1cm and the impacts on Phoebe are the main material supplement for the ring. Its dispersion, which for other rings is know to be equatorial, was observed as symmetric about Saturn’s plane orbit.
Lastly, Verbiscer et al. 2009 noticed a similarity between Phoebe and its ring: they have retrograde orbits which are different from other moon-ring systems with a counter-clockwise orbit.
A. J. Verbiscer, M. F. Skrutskie, D. P. Hamilton, 2009, Nature 461, 1098-1100
http://www.nature.com/nature/journal/v461/n7267/full/nature08515.html
By analyzing data from the Spitzer Space Telescope’s MIPS (multi-band imaging photometer), Verbiscer et al. 2009 conducted an investigation on one of Saturn’s satellites, Phoebe, in order to look for a broad debris disk. A debris disk is a circumstellar disk of dust and debris in orbit around a star and can constitute a phase in the formation of a planetary system following the protoplanetary disk phase.
By looking at mid-infrared images of Saturn’s ecliptic plane, Verbiscer et al. 2009 detected a diffused band of light with two peaks which they knew did not come from the planet’s scattered light.
In most cases, the moon of a planet continuously supplies dust to a planetary ring associated with it, but this has been observed for the satellites that are closest to their planets. In this paper, Verbiscer et al. 2009 discuss the discovery of a ring of this type for Phoebe. This phenomenon is very unusual because Phoebe is very far from its hosting planet (a distance hundreds of times greater than the radius of Saturn), and the disk appears to be ~40 times thicker than the same radius and matches the satellite’s orbit and its vertical range.
This ring has not been detected before due to the fact that its composed out of particles smaller than 1cm and the impacts on Phoebe are the main material supplement for the ring. Its dispersion, which for other rings is know to be equatorial, was observed as symmetric about Saturn’s plane orbit.
Lastly, Verbiscer et al. 2009 noticed a similarity between Phoebe and its ring: they have retrograde orbits which are different from other moon-ring systems with a counter-clockwise orbit.
Article Summary #13
A single sub-kilometre Kuiper belt object from a stellar occultation in archival data
H. E. Schlichting, E. O. Ofek, M. Wenz et al. 2009, Nature, 462, 895
http://www.nature.com/nature/journal/v462/n7275/full/nature08608.html
In our Solar System one remnant from its early evolutionary state is the Kuiper belt, a flow of material found beyond Neptune that orbits the Sun. For this system, size distribution is crucial because it can give information about the physical history of the Kuiper belt and the objects that it consists of.
Because the Kuiper belt is located so far from the Earth, the direct detection on small objects (<1km in diameter) is not possible, but they their effects can be observed: they cause occultations which are changes in the light curve due to the object’s obscuration of a star located in the background. The variation cause a diffraction pattern of the radiation emitted by the background star which depends on several factors: the Kuiper belt object’s size and distance, angular size of the star and other additional parameters.
Thus, by looking at the occultations, Schlichting et al. 2009 detected object with a radius of ~500m, located at a distance of ~45AU (astronomical units). The probability of result being a cause of random statistical error is almost zero, which is a strong proof of the accuracy of the detection.
Since out of all the Kuiper belt objects observed so far this event has been unique. Schlichting et al. 2009 believe that there is a deficit of these small Kuiper belt objects in comparison with the ones that have radii bigger than 50km, distribution which can be explained through the fact that the sub-kilometer objects are experiencing erosion through collisions.
H. E. Schlichting, E. O. Ofek, M. Wenz et al. 2009, Nature, 462, 895
http://www.nature.com/nature/journal/v462/n7275/full/nature08608.html
In our Solar System one remnant from its early evolutionary state is the Kuiper belt, a flow of material found beyond Neptune that orbits the Sun. For this system, size distribution is crucial because it can give information about the physical history of the Kuiper belt and the objects that it consists of.
Because the Kuiper belt is located so far from the Earth, the direct detection on small objects (<1km in diameter) is not possible, but they their effects can be observed: they cause occultations which are changes in the light curve due to the object’s obscuration of a star located in the background. The variation cause a diffraction pattern of the radiation emitted by the background star which depends on several factors: the Kuiper belt object’s size and distance, angular size of the star and other additional parameters.
Thus, by looking at the occultations, Schlichting et al. 2009 detected object with a radius of ~500m, located at a distance of ~45AU (astronomical units). The probability of result being a cause of random statistical error is almost zero, which is a strong proof of the accuracy of the detection.
Since out of all the Kuiper belt objects observed so far this event has been unique. Schlichting et al. 2009 believe that there is a deficit of these small Kuiper belt objects in comparison with the ones that have radii bigger than 50km, distribution which can be explained through the fact that the sub-kilometer objects are experiencing erosion through collisions.
Article Summary #12
An Orbital Period of 0.94 days for the Hot Jupiter Planet WASP-18b
C. Hellier, D. R. Anderson, A. Collier Cameron, et al. 2009, Nature, Vol 460, 1098
http://www.nature.com/nature/journal/v460/n7259/full/nature08245.html
“Hot Jupiters” are exoplanets that initially form at the edge of their stellar system, far from their host stars, but due to the interactions with the proto-planetary disk (the disk around a young star consisting of highly dense gas that rotates around it) or by interaction with a massive object (i.e. massive planet), they end up in the star’s proximity (~0.02 astronomical units). Due to this short distance, the radiation from the star increases the planet’s temperature.
Hellier et al. 2009 look at the planet WASP-18b (first exoplanet that was confirmed to have a period smaller than a day) and analyze is tidal interactions with its host star. They found the stellar mass to be close to that of our Sun and the stellar age of 630 Myr, this making it one of the youngest known planet-hosting stars.
Tidal interaction theory states that when a massive planet is close enough to its host star it causes the formation of a “tidal bulge” on that star (similarly to the how the moon causes the formation of water tides on Earth). That bulge exerts a torque on the planet, decreasing the angular momentum of the planet and, thus, the latter starts spiraling inwards towards the star. The time required for the planet to spiral onto the star depends on several characteristics of the planet: mass, orbit, distance, and a tidal dissipation parameter Q (a quality factor that describes the ratio of the available energy to the dissipated energy during each period). This paper discusses how the value of Q affects the planet-star interaction.
The result they found was that if the Q calculated for the Solar System bodies would apply to WASP-18b, then the star would be spiraling inwards onto the host star in a very short time (less than 1% of the lifetime of its host star). Thus, it is assumed that this planet is an exception from what has been seen so far, or that the Q value does not apply to all systems in the same way.s
C. Hellier, D. R. Anderson, A. Collier Cameron, et al. 2009, Nature, Vol 460, 1098
http://www.nature.com/nature/journal/v460/n7259/full/nature08245.html
“Hot Jupiters” are exoplanets that initially form at the edge of their stellar system, far from their host stars, but due to the interactions with the proto-planetary disk (the disk around a young star consisting of highly dense gas that rotates around it) or by interaction with a massive object (i.e. massive planet), they end up in the star’s proximity (~0.02 astronomical units). Due to this short distance, the radiation from the star increases the planet’s temperature.
Hellier et al. 2009 look at the planet WASP-18b (first exoplanet that was confirmed to have a period smaller than a day) and analyze is tidal interactions with its host star. They found the stellar mass to be close to that of our Sun and the stellar age of 630 Myr, this making it one of the youngest known planet-hosting stars.
Tidal interaction theory states that when a massive planet is close enough to its host star it causes the formation of a “tidal bulge” on that star (similarly to the how the moon causes the formation of water tides on Earth). That bulge exerts a torque on the planet, decreasing the angular momentum of the planet and, thus, the latter starts spiraling inwards towards the star. The time required for the planet to spiral onto the star depends on several characteristics of the planet: mass, orbit, distance, and a tidal dissipation parameter Q (a quality factor that describes the ratio of the available energy to the dissipated energy during each period). This paper discusses how the value of Q affects the planet-star interaction.
The result they found was that if the Q calculated for the Solar System bodies would apply to WASP-18b, then the star would be spiraling inwards onto the host star in a very short time (less than 1% of the lifetime of its host star). Thus, it is assumed that this planet is an exception from what has been seen so far, or that the Q value does not apply to all systems in the same way.s
Article Summary #11
Dust emission from a parsec-scale structure in the Seyfert 1 nucleus of NGC 4151
Burtscher, L.; Jaffe, W.; Raban, et al. 2009, D., The Astrophysical Journal Letters, Volume 705, Issue 1, pp. L53-L57
http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:0909.5191
Active Galactic Nuclei (AGN) are galaxies with an actively accreting supermassive black hole at the center. The main mechanism of the black holes consists of strong material accretion that results in the emission of broad emission lines which we can observe in the spectra (the area where the broad lines are emitted from is called the “broad line region”). This region is believed to be surrounded by a dusty torus which is a “doughnut”-shaped structure, consisting of gas and it is coplanar with the galactic plane.
The galaxies that are oriented “edge-on” relative to our line of sight (i.e. we cannot see the center directly, but we see it through this “doughnut”) are named “Seyfert type 2”, and the galaxies for which we have a direct (“top”) view are called “Seyfert type 1”. The unified model of these types of AGN states that for both type 1 and type 2 the dust distribution should be identical, and the differences observed in the detections should be caused only by the orientation relative to our line of sight. So far, though, there has not been enough supporting evidence for this theory.
The data used consists of observations made with MIDI (mid-infrared interferometric instrument) in the N energy band (radiation wavelengths between 8μm and 13μm) on the galaxy NGC 4151 (the closest Seyfert type 1). With the infrared data, Burtscher et al. 2009 were able to measure the thermal emission from the center of the galaxy.
What was found was that the temperature of the thermal emission and the diameter of the dust emission region are similar to what has been observed in the Seyfert type 2 galaxies. This is important because it is one of the first major findings that support the unification model, although numerous details need deeper analysis for a better understanding.
Burtscher, L.; Jaffe, W.; Raban, et al. 2009, D., The Astrophysical Journal Letters, Volume 705, Issue 1, pp. L53-L57
http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:0909.5191
Active Galactic Nuclei (AGN) are galaxies with an actively accreting supermassive black hole at the center. The main mechanism of the black holes consists of strong material accretion that results in the emission of broad emission lines which we can observe in the spectra (the area where the broad lines are emitted from is called the “broad line region”). This region is believed to be surrounded by a dusty torus which is a “doughnut”-shaped structure, consisting of gas and it is coplanar with the galactic plane.
The galaxies that are oriented “edge-on” relative to our line of sight (i.e. we cannot see the center directly, but we see it through this “doughnut”) are named “Seyfert type 2”, and the galaxies for which we have a direct (“top”) view are called “Seyfert type 1”. The unified model of these types of AGN states that for both type 1 and type 2 the dust distribution should be identical, and the differences observed in the detections should be caused only by the orientation relative to our line of sight. So far, though, there has not been enough supporting evidence for this theory.
The data used consists of observations made with MIDI (mid-infrared interferometric instrument) in the N energy band (radiation wavelengths between 8μm and 13μm) on the galaxy NGC 4151 (the closest Seyfert type 1). With the infrared data, Burtscher et al. 2009 were able to measure the thermal emission from the center of the galaxy.
What was found was that the temperature of the thermal emission and the diameter of the dust emission region are similar to what has been observed in the Seyfert type 2 galaxies. This is important because it is one of the first major findings that support the unification model, although numerous details need deeper analysis for a better understanding.
Article Summary #10
Dark matter haloes determine the masses of supermassive black holes
C. M. Booth, Joop Schaye, 2009, eprint arXiv:0911.0935
http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:0911.0935
Suppermassive Black Holes (SMBH) are found in the center of most massive galaxies and the characteristics of the black holes are known to be strongly related to its host galaxy.
Black holes, as described by general relativity, are a region of space from which nothing can escape, not even light. It is a very compact and massive object that is know to deform space and time and they receive their mass through the accretion of matter around them. The material accelerates during its fall into the black hole, becomes hotter and starts radiating in various energy ranges (depending on its temperature). This radiated energy is what causes the correlation between the black hole’s properties and those of the Galaxy.
It has bee discovered that SMBH are always gravitationally dominant and this is thought to be due to its ability to regulate the rate at which it accretes the matter around it (this phenomenon is called “self-regulation”). The luminous center of the galaxy is called the “bulge” and it is not clear whether the self-regulation occurs inside the bulge (i.e. <1 kpc), on the galaxy scale, or in the entire dark matter halo around the galaxy.
Their analysis is done through a simulation of a black hole and its host galaxy. The goal is finding the initial conditions that would result in a black hole-galaxy evolution to the current state and correlations observed. What was found was unexpected: the energy transferred from the black hole into the surrounding medium does not change when the fraction of the accreted rest mass energy is increased. Also, another result from the simulations is that it is not the stellar mass in the galaxy, but rather the mass of the dark matter halo that determines the mass of the black hole.
C. M. Booth, Joop Schaye, 2009, eprint arXiv:0911.0935
http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:0911.0935
Suppermassive Black Holes (SMBH) are found in the center of most massive galaxies and the characteristics of the black holes are known to be strongly related to its host galaxy.
Black holes, as described by general relativity, are a region of space from which nothing can escape, not even light. It is a very compact and massive object that is know to deform space and time and they receive their mass through the accretion of matter around them. The material accelerates during its fall into the black hole, becomes hotter and starts radiating in various energy ranges (depending on its temperature). This radiated energy is what causes the correlation between the black hole’s properties and those of the Galaxy.
It has bee discovered that SMBH are always gravitationally dominant and this is thought to be due to its ability to regulate the rate at which it accretes the matter around it (this phenomenon is called “self-regulation”). The luminous center of the galaxy is called the “bulge” and it is not clear whether the self-regulation occurs inside the bulge (i.e. <1 kpc), on the galaxy scale, or in the entire dark matter halo around the galaxy.
Their analysis is done through a simulation of a black hole and its host galaxy. The goal is finding the initial conditions that would result in a black hole-galaxy evolution to the current state and correlations observed. What was found was unexpected: the energy transferred from the black hole into the surrounding medium does not change when the fraction of the accreted rest mass energy is increased. Also, another result from the simulations is that it is not the stellar mass in the galaxy, but rather the mass of the dark matter halo that determines the mass of the black hole.
Article Summary #9
Measuring the Cosmic-Ray Acceleration Efficiency of a Supernova Remnant
E. A. Helder, J. Vink, C. G. Bassa, A. Bamba, et al. 2009, Science, 325, 719
http://www.sciencemag.org/cgi/content/abstract/325/5941/719
A widely studied topic both by astronomers and physicist today is Cosmic Rays, which are highly energetic particles that originate outside the Earth, travel through the universe, and hit our atmosphere. A known sub-category is that of accelerated cosmic rays and the accelerating process behind it is know to be mostly due to supernova remnants (SNR – expanding plasma shells caused by strong and violent exploding stars into supernovae and its boundary is called a “shock wave”) . This paper discusses one scenario of how a supernova remnant can accelerate the cosmic rays.
It has been found that for the known cosmic ray density to be stable in the Milky Way, every century three supernovae transform a tenth of their kinetic energy in cosmic ray energy. Supernova remnants lose a high amount of energy to the acceleration (i.e. kinetic energy) of the cosmic rays and this changes its kinematics. There are two known proofs of these energy loses: 1) a higher compression factor of the post-shock plasma, 2) a lower post-shock temperature.
The supernova remnant they look at is RCW 86 and for this source the radiation from the ultrarelativistic electrons is what causes the X-ray emission that they analyze. They find that behind this supernova remnant’s northeast shock, the cosmic ray induced pressure is higher than the thermal pressure. This implies that the temperature of the protons in the post-shock is different from the theoretically (i.e. standard) shock heating. This implies that cosmic rays are what produce more than half of the post-shock pressure.
E. A. Helder, J. Vink, C. G. Bassa, A. Bamba, et al. 2009, Science, 325, 719
http://www.sciencemag.org/cgi/content/abstract/325/5941/719
A widely studied topic both by astronomers and physicist today is Cosmic Rays, which are highly energetic particles that originate outside the Earth, travel through the universe, and hit our atmosphere. A known sub-category is that of accelerated cosmic rays and the accelerating process behind it is know to be mostly due to supernova remnants (SNR – expanding plasma shells caused by strong and violent exploding stars into supernovae and its boundary is called a “shock wave”) . This paper discusses one scenario of how a supernova remnant can accelerate the cosmic rays.
It has been found that for the known cosmic ray density to be stable in the Milky Way, every century three supernovae transform a tenth of their kinetic energy in cosmic ray energy. Supernova remnants lose a high amount of energy to the acceleration (i.e. kinetic energy) of the cosmic rays and this changes its kinematics. There are two known proofs of these energy loses: 1) a higher compression factor of the post-shock plasma, 2) a lower post-shock temperature.
The supernova remnant they look at is RCW 86 and for this source the radiation from the ultrarelativistic electrons is what causes the X-ray emission that they analyze. They find that behind this supernova remnant’s northeast shock, the cosmic ray induced pressure is higher than the thermal pressure. This implies that the temperature of the protons in the post-shock is different from the theoretically (i.e. standard) shock heating. This implies that cosmic rays are what produce more than half of the post-shock pressure.
Subscribe to:
Comments (Atom)