Tuesday, April 3, 2012

Astronomy: ALMA Observations of Fomalhaut

Over the last few decades, astronomy has leapt forward in leaps and bounds as new world-class facilities have been built. Large, new telescopes on the ground, such as Keck, Gemini, and VLT, or on space, like HST, Spitzer, Herschel, Chandra, and WISE, have revolutionized the way we see the universe. In the extremely arid desert of northern Chile, we are building the greatest astronomical facility on the planet to date: ALMA.

ALMA stands for the "Atacama Large Millimeter Array" and currently consists of an array of about twenty 12-meter antennas that observe the sky at submillimeter and millimeter wavelengths. When completed, it will have fifty 12-meter antennas and a more compact array of twelve 7-m and four 12-m antennas. These can be moved around to provide different baselines that result in greater resolution or greater sensitivity. ALMA observes at wavelengths of 3mm down to 400 microns, hence the 'millimeter' part of its name.
Some of the ALMA antennas already on site. Credit: NRAO/AUI and NRAO/AUI/ESO

ALMA is located at the Chajnantor plateau in Northern Chile, in the Atacama Desert at an altitude of about 5,000 meters (16,500 feet). The Atacama Desert is one of the driest places on Earth, which is important as water vapor absorbs submillimeter and millimeter wavelength radiation very well. All submillimeter facilities on the Earth must then be placed on very dry and very high places in order to avoid all this water. Mauna Kea is another such location on Earth that is suitable for submillimeter astronomy and indeed hosts several such observatories.
Check out this astrobite post for some more information on ALMA and some really neat pictures. Here is a great video introduction to ALMA and what it can do:

Last year, ALMA asked for proposals in order to test its capabilities. This 'Early Science' proposal was extremely competitive, with about a thousand separate proposals submitted for just a few hundred hours of observing time. Chile receives 10% of the time since the facility is located here and I was one of the fortunate few that got ALMA time. My observations haven't yet been carried out, but are scheduled for later this year.

Already a few papers have announced ALMA results, but I want to point out one regarding Fomalhaut, the brightest star in the constellation Piscis Australis/Austrinus (the Southern Fish). You may actually know it best from this figure:
Fomalhaut, its disk, and candidate planet. Credit: NASA, ESA and P. Kalas (University of California, Berkeley, USA)

Fomalhaut is a star about 25-light years away surrounded by a debris disk that has been observed at multiple wavelengths. The image above is from scattered light emission, that is, the dust particles reflect light from the star and we see this light. An alternative way to look at dust particles in disks is through thermal emission. This is the result of dust grains emitting light due to their temperature. The hotter they are, the brighter, and the peak emission will depend on the temperature. Dust grains in circumstellar disks tend to be quite cold, so the peak emission is at hundreds of microns (note that 1000 microns=1 millimeter). Hence, submillimeter observations of disks, like those ALMA can provide, are sampling the thermal (or heat) emission the dust particles give off.

Prior work has already imaged the Fomalhaut disk at these long wavelengths. For example, here is the image from Holland et al. (2003) which used the James Clerk Maxwell Telescope (JCMT) to observe the system at 450 and 850 microns:
Fomalhaut disk seen at 850 microns with SCUBA on JCMT. Credit: Holland et al. (2003)

The star symbol marks the location of the star, which is undetected in the data (stars are really faint at those wavelengths). You can, however, see the thermal emission from the dust very clearly as two lobes at either side of the star. Not bad, but this is what ALMA saw when looking at part of Fomalhaut's disk:
Preliminary data for the Fomalhaut disk presented at AAS 219 by A.M. Hughes.

I, and others in the audience, were amazed at such beautiful data during the presentation at AAS this past January. The paper describing the results is now released. Here are the final images resulting from this work:
ALMA data for Fomalhaut. Credit: Boley et al. (2012)

The red and blue dotted ellipses plotted in the left figure are models for the disk fit to the optical data (the entire disk was not observed with ALMA so that limits how much one can model the system). The plus (+) symbol is actually the center of the ellipses, which is offset from the star (the blob just below it). This was one of the indications that a planet could be lurking in the system. The image on the right has been corrected for the primary beam (PM), since the sensitivity will vary throughout the field. The black ellipse on the lower left corner is a measure of the resolution of the observations.

A planet candidate has been reported and imaged in the Fomalhaut system. This planet is somewhat controversial- while being bright in scattered light, it has not been detected in thermal emission, where a normal Jovian-class planet would be brightest. Hence, the planet, if it is real, must have a substantial ring of material (think: Saturn's rings) to produce so much optical light, but little thermal light. Such a ring might be detectable at submillimeter wavelengths with very sensitive observations; however, it's not clear that ALMA sees any such circumplanetary emission.

However, something must be responsible for such a narrow (~16 AU) dust ring in the system. While an interior planet could certainly be responsible for the sharp inner edge of the disk, it can't be the whole story as you also need a sharp outer edge. The dust could have been produced by a collision of two rocky planets and has not yet had time to disperse (and is therefore narrow), but the velocities of objects at that separation (~140 AU from the star) is too slow to destroy the Earth-mass planets required to account for the dust. The final mechanism presented in their paper, and the one the authors think most likely, is that of shepherding planets. This is very much analogous to shepherding satellites on the rings of our solar system's giant planets. These planets could be of such low masses that they haven't been detected yet, but their gravity effectively corrals the dust into a narrow ring around the star.

The paper itself, led by Dr. Aaron Boley of the Department of Astronomy at the University of Florida, can be accessed and read here. It provides more details on their observations, results, and disk formation scenarios.

No comments:

Post a Comment