Chandra Finds X-ray Star Bonanza in the Orion Nebula

NASA's Chandra X-ray Observatory has resolved nearly a thousand faint X-ray-emitting stars in a single observation of young stars in the Orion Nebula. The discovery--the richest field of X-ray sources ever obtained in the history of X-ray astronomy--will be presented on Friday, January 14, at the 195th national meeting of the American Astronomical Society in Atlanta, Georgia.

The Orion region is a dense congregation of about 2,000 very young stars formed during the past few million years. The discovery of such a wealth of X-ray stars in the closest massive star-forming region to Earth (only 1,500 light years away) is expected to have a profound impact on our understanding of star formation and evolution.

"We've detected X-rays from so many fantastic objects, such as very young massive stars and stars so small that they may evolve into brown dwarfs," said Gordon Garmire, Evan Pugh Professor at Penn State University, University Park. "Chandra's superb angular resolution has resolved this dense cluster of stars with arcsecond accuracy and unsurpassed sensitivity."

Garmire leads the team using Chandra's ACIS detector, the Advanced CCD Imaging Spectrometer, conceived and developed for NASA by Penn State University and the Massachusetts Institute of Technology.

The brilliant Orion region has awed humankind for millennia. The most massive and brightest of these nascent stars are in the Orion Trapezium, which illuminates the Orion Nebula, also known as Messier 42. The Trapezium and its luminous gas can be seen with the unaided eye in the winter sky in the "sword" of the Orion constellation.

Young stars, such as those found in Orion, are known to be much brighter in X-rays than middle-aged stars such as the Sun. The elevated X-ray emission is thought to arise from violent flares in strong magnetic fields near the surfaces of young stars. The Sun itself was probably thousands of times brighter in X-rays during its first few million years.

Although the enhanced magnetic activity of young stars has been known for some time, the physical causes and evolution of the activity are poorly understood, according to Dr. Eric Feigelson, professor of astronomy and astrophysics at Penn State.

"With hundreds of stars observed simultaneously, possessing a wide range of properties such as mass and rotation rates, we hope the Orion observation will help unravel the astrophysical principles underlying this phenomenon," Feigelson said. "X-ray astronomy now penetrates as deeply into the clouds as the best infrared and optical telescopes, permitting us to study high-energy processes during the earliest phases of star formation."

"This Chandra image is a milestone in the field of X-ray astronomy and very gratifying to me personally," said Garmire. "Chandra's sensitivity is 20 times better than achieved with the best previous X-ray telescopes."

A number of the ACIS X-ray sources in the Orion observation have special importance. Several are associated with a distinct cluster of higher-mass stars deeply embedded within the murky Orion Molecular Cloud, including the infrared-luminous Becklin-Neugebauer object.

"This is the first time X-ray astronomy has resolved individual massive stars still embedded in their natal cloud," said Dr. Leisa Townsley, research associate in astronomy and astrophysics at Penn State.

At least three ACIS sources are associated with cluster members with masses so small (roughly 1/20th of the Sun's mass), that they will evolve into brown dwarfs rather than true stars.

"They more closely resemble proto-Jupiters than proto-stars," said Dr. Yohko Tsuboi, visiting research scholar in astronomy and astrophysics at Penn State. "Over a dozen X-ray sources have no known counterpart, even in the most sensitive Hubble Space Telescope or infrared studies. These too may be very low-mass stars."

The ACIS team studying the Orion X-ray source includes Profs. Feigelson and Garmire and research scientists Patrick Broos, Leisa Townsley, and Yohko Tsuboi at Penn State; Steven Pravdo at the Jet Propulsion Laboratory; and Lynne Hillenbrand at the California Institute of Technology.

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

FOR FURTHER INFORMATION AND IMAGES:

A Web site associated with this press release, < http://www.astro.psu.edu/xray/orion >, provides high-resolution digital images (JPEG, TIFF and PostScript formats) and hypertext links to images of the Orion Nebula at non-X-ray wavelengths, plus related material. Images were made with the Advanced CCD Imaging Spectrometer (ACIS). The captions for these images are as follows:

Figure 1: The full field of the ACIS Imaging array showing about a thousand X-ray-emitting young stars in the Orion Nebula Cluster. This cluster is the nearest massive star forming region to the Sun, and is well-known in the visible sky because it illuminates the Orion Nebula. The angular size of the field is 18x18 arcminutes, or about half the size of the full Moon. The orientation has North at the top and East to the left. Some instrumental effects can be seen, such as the gaps between the four CCDs comprising the array and the streaks of X-rays from the brightest stars in the Trapezium produced during CCD readout. Credit: NASA/CXC/SAO/PSU (Garmire/Feigelson)

Figure 2: Close-up of the crowded center of the Orion Nebula Cluster, demonstrating the capabilities of Chandra's high resolution telescope. Most cluster stars with magnitude V<20 in the visible band (blue boxes) are detected, along with many embedded stars seen only at infrared wavelengths. A few stars are detected here in the X-ray band for the first time. Credit: NASA/CXC/SAO/PSU (Garmire/Feigelson)

Figure 3: Close-up of the Becklin-Neugebauer object and its vicinity, thought to be the closest region of current high-mass star formation. The X-rays from the stars deeply embedded in the molecular cloud (green circles) are heavily absorbed by intervening molecular material and only a tiny fraction of the emitted X-ray luminosity are detectable. Several embedded stars (blue circles) were missed by infrared surveys, but two appear as radio sources. Radio and X-ray flares are known to be associated in very young stars. Credit: NASA/CXC/SAO/PSU (Garmire/Feigelson)

TO FOLLOW CHANDRA'S PROGRESS:

Visit the Chandra sites at < http://chandra.harvard.edu > and < http://chandra.nasa.gov >.

CONTACTS:

Dr. Gordon Garmire, Penn State University, (+) 814-865-1117, ggarmire@astro.psu.edu

Dr. Eric Feigelson, Penn State University, (+) 814-865-0162, edf@astro.psu.edu

Dr. Wallace Tucker, Chandra X-Ray Observatory Center, Cambridge, MA(+) 617-496-7998, wtucker@cfa.harvard.edu

Penn State PIO: Barbara Kennedy, (+) 814-863-4682, science@psu.edu

BACKGROUND DEFINITIONS:

X-ray Astronomy:

The study of objects in the universe using X-rays rather than visible light or other wavelengths of electromagnetic radiation. The X-rays can be imaged with grazing incidence mirrors which must be polished with extreme accuracy to reflect short-wavelength X-rays. An X-ray detector is placed at the focal plane of the telescope. The detector here is a sophisticated version of the CCD detectors commonly used in video cameras or digital cameras.

Chandra X-ray Observatory:

The latest in NASA's series of Great Observatories. Chandra is the "X-ray Hubble", launched in July 1999 on the Space Shuttle Columbia and then sent into a deep-space orbit around the Earth. Chandra carries a large X-ray telescope to focus the X-rays from objects in the sky. An X-ray telescope cannot work on the ground because the X-rays are absorbed by the Earth's atmosphere.

Orion Nebula:

The brightest and most famous star forming region in the sky. The nebula seen at visible wavelengths is mainly gas heated by the most luminous stars in the Orion Nebula Cluster. The cluster has at least two thousand young stars stars in their "pre-main sequence" phase before they begin to burn hydrogen in their cores, as in main sequence stars like the Sun. The nebula lies at the edge of a cold giant molecular cloud, emitting principally millimeter and infrared wavelengths. At a given moment, certain portions of the cloud have conditions ripe for gravitational collapse of the gas which leads to formation of young star clusters as studied here.