Spring 2002 -- Vol. 19, No. 1

X-Ray Emissions Detected from Elusive Cosmic Objects

A type of celestial object that has long stumped astronomers has been found to emit X-rays, thus proving a theory of how the objects form.

A team of astronomers including Steven Pravdo of NASA's Jet Propulsion Laboratory in Pasadena, California; Eric D. Feigelson, Gordon Garmire, Yoshitomo Maeda, and Yohko Tsuboi at Penn State; and John Bally at the University of Colorado, have concluded that these objects, called Herbig Haro objects, are produced by high velocity shocks. Pravdo is the lead author of a paper published in the Oct. 18 issue of the journal Nature.

Herbig Haro objects are found in regions where new stars are forming. They are nebulae, or dust and gas clouds. They form when high-velocity gas emitted from young stars collides with clouds of interstellar material. The collision heats the gas in the surrounding nebula to sufficiently high temperatures to produce X-rays.

Observations for the past 20 years showed no evidence of X-ray emission from these objects, which are named for astronomers George Herbig and Guillermo Haro. Previous instruments lacked the resolution and sensitivity necessary to 'see' these X rays. The discovery of the X-ray emissions was possible through Chandra's very powerful Advanced CCD Imaging Spectrometer on NASA's Chandra X-ray Observatory, which has been in orbit since 1999. Garmire, the Evan Pugh Professor of Astronomy and Astrophysics at Penn State, is the principal investigator for the CCD Imaging Spectrometer, which was conceived and developed for NASA by Penn State and Massachusetts Institute of Technology under his leadership.

On Oct. 8, 2000, astrophysicists used the instrument to study HH 2, one of the brightest and closest Herbig Haro objects in the Orion Nebula. They determined that HH 2 contains shock-heated material with a temperature of about 1 million degrees Kelvin (about 1.8 million degrees Fahrenheit). Pravdo and his team used three criteria to rule out the possibility that the emissions came from any other source. First, Chandra's high spatial resolution pinpointed the location of the X-rays at HH 2. Second, the X-rays appeared to be covering a region bigger than a star. Third, the temperature of the X-rays was about 1 million degrees, cooler than nearby X-ray stars. One million degrees is about the temperature expected if material moving at about 300 kilometers per second (about 600,000 miles per hour) collides. At this speed, you could go from Los Angeles to San Diego and back in one second.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. TRW, Inc.,in Redondo Beach, California, is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Massachusetts.

The Chandra X-ray Observatory is managed for NASA by the Smithsonian Astrophysical Observatory, Cambridge, Massachusetts. JPL is managed for NASA by the California Institute of Technology in Pasadena.

~~ Colleen M. Sharkey, Jet Propulsion Laboratory

 

The image on the left is a Palomar Digital Sky Survey image of the region of the Orion Nebula that contains Herbig-Haro objects known as HH1 and HH2. The inset (right) presents a zoom that shows the position of the X-ray source (green circle) detected by Chandra in HH2, superimposed on a false-color optical image from the Hubble Space Telescope. Herbig-Haro objects (HH) are clouds of dust and gas that are either part of high-speed jets of gas streaming away from very young stars, or clouds of gas that have been hit by such jets. The detection of X-rays from HH2 implies that a 600,000 miles per hour jet is plowing into a slower moving cloud. The resulting shock wave heats gas to a million degrees Celsius. The young star producing the jet is heavily obscured and detectable only with infrared and radio telescopes. In the image on the left, It lies about halfway between HH2, and HH1, the small bright cloud above and to the right of HH2.

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