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Phil Armitage

Content About: Phil Armitage

Galaxy mergers routinely occur in our Universe.  And, when they take place, it takes years for the supermassive black holes at their centers to merge into a new, bigger supermassive black hole.  However, a very interesting thing can happen when two black holes get close enough to orbit each other every 3–4 months, something that happens just before the two black holes begin their final desperate...

Graduate student Greg Salvesen, JILA Collaborator Jake Simon (Southwest Research Institute), and Fellows Phil Armitage and Mitch Begelman decided they wanted to figure out why swirling disks of gas (accretion disks) around black holes often appear strongly magnetized.  They also wanted to figure out the mechanism that allowed this magnetization to persist over time.  In the process, they hoped...

Fellow Phil Armitage and group collaborator Jacob Simon of the Southwest Research Institute are leading work to answer a central question about planet formation: How do pea- and pebble-sized objects orbiting within a protoplanetary disk evolve into asteroid-sized objects tens to hundreds of kilometers in size?  This is an important question to answer because the eventual formation of planets...

Fellow Phil Armitage and his collaborator Jake Simon of the Southwest Research Institute recently conducted a theoretical study of turbulence in the outer reaches of an accretion disk around HD 163296, a nearby young star.  Meanwhile, the Atacama large Millimeter/submillimeter Array (ALMA) in northern Chile observed the same accretion disk.  There were intriguing and unexpected differences...

Ever wondered how magnetic pressure alone might be able to maintain the structure of an accretion disk around a black hole in an x-ray binary system?  Fellow Mitch Begelman recently gave the idea a lot of thought.  And, in the process of working on the idea with Fellow Phil Armitage and Chris Reynolds of the University of Maryland, Begelman came up with a new model for accretion disks around...

A Be star is a luminous, blue B-type star with distinctive spectral lines that can provide two types of feasts (tasty snacks or full-scale banquets) for a former companion star in a binary system.  The feasting begins when the companion star goes supernova and becomes a neutron star or, more rarely, a black hole.  Typically, the companion blows up with enough force to kick itself into an...

Fellows Mitch Begelman and Phil Armitage have just solved the 40-year old mystery of what causes the gas of stellar debris surrounding black holes in binaries to flip back and forth cyclically between a spherical cloud and a luminous disk. 

When stellar-sized black holes orbit around another star, the black holes feed themselves by pulling material off their companion stars, funneling it...

To search for answers to this question, research associate Jake Simon and his colleagues are performing a series of high-level computer simulations of the outer disks around young stars such as TW Hydrae, shown here.  Simon’s daunting task is being facilitated with new information that has just started to come in from the Atacama Large Millimeter...

Giant planets form inside a disk of gas and dust orbiting a new star.  At first, gravitational interactions between the disk and the planets will keep planetary orbits circular, according to Fellow Phil Armitage.  But, once the disk begins to disperse, things get very interesting. 

Over millions of years, X-rays emitted by the central star evaporate the protoplanetary disk until it...

The Solar System has a remarkable number of planets.  It includes four rocky planets (Mercury, Venus, Earth, and Mars), four giant gaseous planets, and countless smaller worlds.  Early on, there may even have been a fifth rocky planet that collided with the Earth, forming the Moon.  We owe the survival of so many terrestrial planets (and our own evolution as a species) to the...

Fellow Phil Armitage and colleagues from the Université de Bordeaux and Google, Inc. are key players in the quest to understand the secrets of planet formation.  Current theory posits that there are three zones of planet formation around a star (as shown in the figure).  In Zone One, the hot innermost zone, small rocky planets form over a period of hundreds of millions of years.  The...

Fellow Phil Armitage studies the migration of gas giant planets through evolving protoplanetary disks.  He and former JILA postdoc Richard Alexander (Universiteit Leiden) have designed relatively simple models that reproduce the observed frequency and distribution of extra-solar giant planets, many of which orbit very close to their stars.  The models also replicate the masses, lifetimes, and...

The merger of supermassive black holes is a hot topic in astrophysics.  Such mergers may occur after the formation of black hole binaries during galaxy collisions.  The mergers are predicted to emit gravitational waves, whose detection is the mission of the Laser Interferometer Space Antenna (LISA).  In preparation for the LISA mission, which is scheduled for launch in 2018, Fellow Peter Bender...

Most known extrasolar planetary systems comprise planets whose orbits vary wildly from the nearly circular ellipses found in our solar system.  This wide variation in eccentricity is thought to occur when large gas planets interact with each other, causing gyrations in planetary orbits, planet migrations toward and away from the central star, and even the ejections of planets out of the star...

Astrophysicists know that the centers of galaxies have supermassive black holes whose size correlates with the size of the galaxy surrounding them.  They’ve also observed that galaxies collide and merge.  In fact, galactic mergers were even more common billions of years ago in the Universe when today’s galaxies were still being assembled. 

Like people, planets can migrate far from where they were born.  In the case of planets, they usually travel toward their parent star, but some may also move away.  Some wind up in blistering proximity to their Sun-like parents, orbiting them in 1.2 to 8 days.  Such orbits are well inside the magnetic-field-induced cavities that typically separate such stars from their planet-forming accretion...

Fellow Phil Armitage is excited about the discovery of several new galaxies in which a disk of water masers is orbiting within half a light year of the central massive black hole.  Like their counterpart M106 (NGC 4258) discovered in 1995, these hot (600 K) water molecules mase, i.e., emit coherent radio wavelength photons when they return to lower energy states after being excited by...

Two egg-shaped necklaces of magnificent stars orbit the enormous black hole known as Sagittarius A* (Sgr A*) at the center of the Milky Way Galaxy.  Sgr A* (shown right) has long been thought to be well past promoting new star formation; until the necklaces were discovered, the black hole was considered to be just an aging, depleted relic of its glory days of organizing the Galaxy. 

If you wanted to see a new planet forming, the first thing you'd think of is looking in an accretion disk with a star forming in its center.  The problem is it's virtually impossible to actually see a nascent planet inside a swirling, turbulent, and opaque disk of gas and dust.  It's much easier to identify planets orbiting stars a billion years old (or older) whose accretion disks...

For astrophysicists working to discover the origins of stars and planets, a small clue can go a long way.  They can't get a close look at distant stars and planets, so they only know the barest details about other planetary systems.  One such detail is that some extra-solar planets revolve around their stars in elliptical orbits rather than the nearly circular orbits that are the norm in...

Gamma-ray bursts signal the birth of a new black hole, whether it's created during the collapse of a massive star or via a merger between two compact objects such as neutron stars.  Astrophysicists have determined that long gamma-ray bursts are associated with collapsing stars and short bursts are associated with binary mergers.  In both cases, however, black-hole accretion powers the...

Scientists believe that planetary systems coalesce from disks of gas and dust orbiting a star.  Similarly, stars can form within massive accretion disks orbiting a black hole.  Determining the mechanisms that create stars and planets from these orbiting disks is a hot topic among astrophysicists, according to JILA Fellow Phil Armitage and colleagues W. K. M. Rice of the University of...

Giant gas planets don't often stay in orbit where they're formed.  They often move closer to their star or, occasionally, further away.  Seldom do they remain in almost circular orbits such as those of Jupiter and Saturn.  In fact, all but one of the giant gas planets discovered around other stars are closer to their star than Jupiter is to the Sun.  A fraction of these planets are even...

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