Observations of Comet Hyakutake with the National Science Foundation’s millimeter-wave radio telescope in Arizona have revealed new information about our Solar System’s original material, including the first detection of the Carbonyl Sulfide (OCS) molecule in a comet. Since mid-March, the 12-Meter telescope, part of the National Radio Astronomy Observatory (NRAO) and located on Kitt Peak in Arizona, has been used by four separate research teams to measure the emission from primordial molecular species within the bright Comet Hyakutake (C/1996 B2). Results obtained near the comet’s closest approach on March 25th have yielded a wealth of information about the primordial composition of this comet.
Comets are thought to be remnants from the formation of our Solar System, and as such, can provide valuable information about the early stages of our Solar System’s chemical and physical development. Especially important is the study of cometary “parent molecules” — molecules which have been present since the comet’s birth, but have not been disturbed by chemical processing. Studying these molecules gives radio astronomers a very accurate “snapshot” of the material from which our Solar System, including Earth, was formed.
The 12-Meter Telescope allows astronomers to observe what scientists call “millimeter waves,” electromagnetic waves with wavelengths of just a few millimeters. These waves are shorter than radio microwaves and longer than infrared waves. Numerous molecules emit radiation at these wavelengths and thus can be detected by the 12-Meter telescope. The 12-Meter telescope, more than 25 years old, inaugurated the science of millimeter-wavelength molecular astronomy and is the instrument responsible for the discovery of dozens of molecules in space.
Observations by Maria Womack (Pennsylvania State University), Michel C. Festou (Observatoire Midi Pyrenees, Toulouse), and Alan Stern (Southwest Research Institute) have measured the abundances of a number of suspected “parent molecules” within Comet Hyakutake. Measurements of H2CO, Hydrogen Cynaide (HCN), and Carbon Monoxide (CO) emission toward the comet were made on March 16 and 17. The derived production rates for these molecules indicate that the HCN production rate is similar to that measured for CN. Current theory holds that the similar production rates for these two molecules suggest a direct link between them.
The observing team found dissimilar production rates when they observed CO and H2CO in Comet Hyakutake. According to prevalent theory, this dissimilarity suggests that these could be parent molecules, produced by different chemical processes. Comparison to other CO measurements done prior to these March measurements indicates very rapid changes in the CO production rate over time scales of weeks.
A second research team has made the first detection of Carbonyl Sulfide in a comet by measuring its characteristic emission at 146 GHz. Laura Woodney, Michael A’Hearn (University of Maryland), and J. McMullin (University of Arizona) made their discovery on March 19, when the comet was 20 million miles from the Earth. By comparing the molecular production rates of OCS and OH and comparing them to theoretical predictions, they find that the OCS molecule is likely a parent molecule in Comet Hyakutake.
On March 21st, Jeff Mangum of the National Radio Astronomy Observatory made an image of the CO emission toward Hyakutake. This image indicates that the “cloud” of CO surrounding the comet’s nucleus is quite extended, with a size of about 2,500 miles. Stronger emissions indicate increased sublimation and studies of the changes in CO distribution will continue as the comet approaches perihelion.
Comet Hyakutake will not be visible to northern hemisphere optical observers during the second leg of its journey because it will be in our daylight skies. But, since radio telescopes can operate 24 hours a day, the 12-Meter radio telescope will continue its observations until the comet exits the solar neighborhood early this summer.