SPECTROSCOPY - Better detector gives new insight into comets

When comet Hale-Bopp streamed across the night sky in 1997, eyes and binoculars and backyard telescopes turned to watch its passage. And thanks to advances in spectroscopy, scientists who study the origin of comets and the development of the solar system were able to glean a deeper insight into the history of this dirty snowball.

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When comet Hale-Bopp streamed across the night sky in 1997, eyes and binoculars and backyard telescopes turned to watch its passage. And thanks to advances in spectroscopy, scientists who study the origin of comets and the development of the solar system were able to glean a deeper insight into the history of this dirty snowball.

Using the Infrared Telescope Facility (IRTF) 3-m telescope at Mauna Kea Observatory in Hawaii, scientists from NASA's Goddard Space Flight Center (Greenbelt, MD), the Catholic University of America (Washington, DC), and the University of Notre Dame (Notre Dame, IN) observed Hale-Bopp on several nights as it moved closer to the sun from beyond Jupiter (see figure on p. 46). According to a paper published in the journal Nature in June, they were able to determine the ratio of carbon monoxide (CO) to water.

Most comets lie in a halo well beyond the orbit of Pluto, known as the Oort cloud, where they exist unchanged for eons. "Whether they formed there or actually migrated there is a matter of debate," said Michael DiSanti of Catholic University and Goddard.

DiSanti and his colleagues found that the amount of CO ice was 12% relative to the amount of water ice in the comet. That fact suggests that Hale-Bopp spent a substantial amount of its life somewhere between the orbits of Jupiter and Neptune. Astronomers have found that the enormous molecular clouds that condense to form solar systems are as much as 50% CO ice in regions far from the central star. That percentage drops closer in, where temperatures are higher. At a distance equivalent to the distance of Jupiter from the sun, the percentage of CO ice in such clouds is about 10%. Hale-Bopp may therefore have formed somewhere beyond the orbit of Jupiter or spent time there gathering ice before being flung out into the Oort cloud by the giant planet's gravity.


Fig 1. A two-dimensional spectral-spatial image of Hale-Bopp (top) shows emission lines of carbonyl sulfide (OCS), with the spread from top to bottom representing distance from the comet's nucleus and each horizontal point showing a different wavelength. Peaks (bottom) show the level of OCS over that expected in Earth's atmosphere (dotted line).
Click here to enlarge image

The scientists made their observations with the IRTF's cryogenic echelle grating spectrometer (CSHELL), a long-slit spectrometer for the 1- to 5.5-µm range, with a spectral resolving power of about 20,000. It has a 256 x 256 indium antimonide (InSb) detector array. The arrival of such a bright comet now that good-quality spectrometers are available was lucky, DiSanti said.

"The instrumentation has gotten a lot better in the last several years in the infrared region," he said. "Mainly the semiconducting array detectors have gotten a lot more sensitive to light."

Arrays made of InSb have been built since the late 1980s and are about 10 years behind the development of charge-coupled-device (CCD) detectors, DiSanti said. The earliest InSb detectors had a lot of blemishes and suffered from an inefficient transfer of electrical charge. Now engineers have made bigger, cleaner InSb arrays with faster readout and a much lower noise level.

"I think the next several years will continue to march this area forward," DiSanti said. Even the CSHELL is not as powerful as some of the instruments now being developed.

Now that engineers have achieved greater sensitivity with InSb detectors, the next step will be to improve the optics, he said. "The range in frequencies that you sample at any given time is fairly limited, just because of the optical path that is laid out."

The instruments he uses produce only one grating order at a time, and he and his colleagues could only look at two emission lines of CO per setting. But astronomers at the Keck telescope on Mauna Kea are testing the new Near Infrared Spectrometer, an InSb array that will produce five grating orders and has an area of 1024 x 1024 pixels, so that each order covers a wider frequency range.

"You can take one exposure and get a really large range of frequencies, so you can get a lot more emission lines in one exposure," DiSanti said.

The improvements mean astron omers will be able to learn a lot, even with smaller, less spectacular comets. "I wouldn't expect another comet like Hale-Bopp for awhile, but we don't need one that bright," DiSanti said.

Neil Savage

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