ALMA Spies Baby Stars' Planetary Workshops
For the first time, astronomers have found compelling evidence that the dust-free gaps surrounding young stars are caused by massive exoplanets.
Planetary formation remains one of the biggest puzzles in modern astronomy. Although we know that the vast majority of stars possess systems of planets - from tiny Mercury-sized rocky worlds to massive gas giants that would dwarf Jupiter - mysteries remain as to how material accretes to form small planetoids and how long it takes for these planetary embryos to plump-up into what we would consider to be planets.
Now, with the help of the awesome Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have zoomed-in on a selection of very young stars, revealing never-before-seen detail in the planet-forming regions surrounding them. And what they found were monster planets, several times more massive than Jupiter, hiding inside the dusty planetary workshops.
When a star is born, it will often be accompanied by a protoplanetary disk. As the star settles and disk matures, small dusty particles accrete (clump together), eventually creating gravitationally-dominant protoplanets that rapidly vacuum up more and more material, growing bigger and more massive. Of particular interest to astronomers are transitional disks that have a surprising lack of dust in their centers, in the region between the disk and star.
This may not seem surprising; astronomers have explained away these features as either a consequence of stellar radiation pressure (as the star matures, its radiation blasts any nearby dust away), or massive planets could be lurking in this zone, having cleared their orbits of dust through their gravitational dominance.
We've been stuck at this impasse for some time; how can we tell whether this dust gap is caused by radiation pressure or planetary formation?
This is where ALMA comes in. The array of radio antennae are sensitive to emissions from the gas these transitional disks contain and through studies of 4 young stars, astronomers have found that inside these dust gaps, there are also gas gaps, but they are 3 times thinner. Only with ALMA's precision observations could these gas gaps be pinpointed and they can mean only one thing.
"Previous observations already hinted at the presence of gas inside the dust gaps," said astronomer Nienke van der Marel, of Leiden Observatory in the Netherlands. "But as ALMA can image the material in the entire disc in much greater detail than other facilities, we could rule out the alternative scenario. The deep gap points clearly to the presence of planets with several times the mass of Jupiter, creating these caverns as they sweep through the disc."
Although we are looking at very alien star systems, it's studies such as these that will ultimately reveal how the planets in our own solar system formed, likely clearing up many mysteries surrounding our understanding of planetary evolution. And as observatories become more sophisticated answers are likely to come sooner rather than later.
"Direct planetary detection is just within reach of current instruments, and the next generation telescopes currently under construction, such as the European Extremely Large Telescope, will be able to go much further. ALMA is pointing out where they will need to look," added Ewine van Dishoeck, also of Leiden Observatory and the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.
This artist's impression shows the formation of massive planets in the dust gap of a transitional disk surrounding a young star.
The Atacama Large Millimeter/Submillimeter Array, or ALMA, is nearing completion after over 30 years of planning and collaborating among astronomers and engineers from several nations. The completed array will consist of 66 antennas and two supercomputers, called correlators, at the backend to collect the signals and make the array function as one large telescope.
On March 12th and 13th, 2013, over a hundred journalists descended upon the array along with politicians, scientists, engineers, and other VIPs to celebrate the official inauguration of the array and tour its facilities. I was there along with 11 other journalists from around North America as guests of the National Radio Astronomy Observatory.
Overview of the telescope's technical capabilities, science goals, and history were given by (from left to right) Michael Thorburn, Head of the ALMA Department of Engineering, Pierre Cox, incoming ALMA director, Thijs de Graauw, current ALMA director, Al Wootten, ALMA Program Scientist for North America, and Ewine van Dishoeck, Professor at Leiden University and former ALMA board member.
The ALMA control room is the hub of activity at the "low site" or Operations Support Facility. From here, telescope operators manage and control all array operations and conduct observing runs. Astronomers who win time on the telescope through a merit-based proposal process do not actually travel to the site or control the telescope in real time. Instead, as with other radio interferometers, they prepare observing scripts based on what science they want to achieve, and their scripts are then scheduled and run by ALMA staff. The data is then made available to the astronomer for a proprietary period of one year, when it is then released to the public.
Everyone gets into the spirit of ALMA, which in Spanish actually means "soul," as an art contest was held for local children. Featured here are some of the submissions and several winners in the six to nine year old group.
54 of the final 66 antennas were at the "high site" or Array Operations Site, at an altitude of 16,500 feet. The 12-meter dishes are contributed by partners in Europe, North America, and East Asia, whereas several smaller, 7-meter dishes from East Asia make up a compact array in the center. The completed array will eventually have the capability of expansion by moving the antennas to different concrete "pads" spread around the Chajnantor Plateau. With a baseline, or distance between antennas, of up to 14 kilometers, ALMA will be able to obtain resolution ten times better than that of the Hubble Space Telescope.
Reporters were allowed a peek at the massive correlator behind glass in the second highest building in the world. The correlator is the computer that brings together signals from all the antennas so that the astronomer can make an image using the full array. We received a thorough explanation of its working from correlator engineer Alejandro Saez who has spent time constructing in in Charlottesville, Virginia, and at the high site in Chile. The correlator has the processing power of 3 million laptops as it has to make calculations for up to 1,125 antenna pairs billions of times per second.
The antennas actually put on a show for us during our three hour tour of the high site, slewing, or moving, back and forth. These antennas are built for perfection, or as close to it as one can come. These sturdy, yet flexible machines must maintain a dish surface accuracy of the width of a human hair and pivot back and forth between a target source and calibrator source on the sky every ten seconds.
Taking a hundred or so journalists to a site at 16,500 feet (5000 meters) altitude is quite risky business. Everyone must pass a basic fitness exam to be cleared for access, though employees must endure a much more rigorous process involving a stress test and heart monitoring to work on site. A team of diligent paramedics were there to hand out oxygen bottles and check for signs of altitude sickness. Here, I got my pulse checked in an ambulance after running around like an excited child a bit too much.
The "front end" of a radio telescope is the place where radio light that is collected by the dish is received, amplified, and transferred to the next stage for digital processing. The large blue drum is a cryostat and holds the most sensitive electronics in the telescope at a cold 4 Kelvin using liquid helium. The silver circles are the actual windows into the feedhorms, the first stage where radio light passes into the front end. Don't think those metal plates are transparent? They are to the radio light that ALMA receives.
Just one of ten sets of electronics that can fit inside those blue drums are on display here. Various stages of the system are cooled to difference temperatures, and engineers needs to wear special gloves and shirts when handling these so that they do not impart a spark of static electricity that could ruin a sensitive (and expensive) piece of equipment. The superconducting receivers used by ALMA are state-of-the-art and were developed specifically for this purpose.
How do you move a giant antenna? You build a giant antenna mover. The on-site guests were treated to an amazing site when one of the two antenna crawlers rumbled down a huge dirt road to pick up one of the dishes that is still under testing. Otto and Lore, as the crawlers are named, have to pick up each 100-ton antenna and place them down on their pads with millimeter accuracy. They move the antennas from the Operations Support Facility up to the high site and back and will be used to change array configurations to change the telescopes resolving power.