In resolving the hot core of the Orion Trapezium Cluster, one of the Earth's closest and most massive star-forming regions, the Chandra X-ray Observatory showed that almost all the young stars' temperatures are more extreme than expected.
Chandra has also detected, for the first time in X-rays, a stellar fingerprint known as a P Cygni profile -- the distinctive spectral signature of a powerful wind produced by an object in space. Both results were presented at a meeting of the High-Energy Astrophysics Division of the American Astronomical Society in Honolulu earlier this month.
The Orion Trapezium Cluster, only a few hundred thousand years old, offers a prime view into a stellar nursery. Its X-ray sources detected by Chandra include several externally illuminated protoplanetary disks ("proplyds") and several very massive stars, which burn so fast that they will die before the low-mass stars even fully mature.
One of the major highlights of the Chandra observations are identification of proplyds as X-ray point sources in the near vicinity of the most massive star in the Trapezium. Previous observations did not have the ability to separate the contributions of the different objects.
"We've seen high temperatures in stars before, but what clearly surprised us was that nearly all the stars we see appear at rather extreme temperatures in X-rays, independent of their type," said MIT Research Scientist Norbert S. Schulz, who leads the Orion Project. "And by extreme, we mean temperatures that are in some cases well above 60 million degrees." The hottest massive star known so far has been around 25 million degrees.
The great Orion Nebula harbors the Orion Nebula Cluster (ONC), a loose association of around 2,000 mostly very young stars of a wide range of mass confined within a radius of less than 10 light years. The Orion Trapezium Cluster is a younger subgroup of stars at the core of the ONC confined within a radius of about 1.5 light years. Its median age is around 300,000 years.
The constant bright light of the Trapezium and its surrounding stars at the heart of the Orion nebula (M42) are visible to the naked eye on clear nights.
In X-rays, these young stars are constantly active and changing in brightness, sometimes within half a day, sometimes over weeks. "Never before Chandra have we seen images of stellar activity with such brilliance," said Professor Joel Kastner of the Chester F. Carlson Center for Imaging Science at the Rochester Institute of Technology. "Here the combination of very high angular resolution, with high quality spectra that Chandra offers, clearly pays off."
The observation was performed using the High Energy Transmission Grating Spectrometer (HETGS) and the X-ray spectra were recorded with the spectroscopic array of the Advanced CCD Imaging Spectrometer (ACIS). The ACIS detector is a sophisticated version of the CCD detectors commonly used in video cameras or digital cameras.
It is generally assumed that low-mass stars like our sun, when they are young, are more than 1,000 times more luminous in X-rays. The X-ray emission here is thought to arise from magnetic activity in connection with stellar rotation. Consequently, high temperatures would be observed in very violent and giant flares. Here, temperatures as high as 60 million degrees have been observed in very few cases. The absence of many strong flares in the light curves, as well as temperatures in the Chandra ACIS spectra that exceed the ones in the giant flares, could mean that they are either young protostars (i.e., stars in the making), or a special class of more evolved hot young stars.
Dr. Schulz conceded that although astronomers have gathered many clues in recent years about the X-ray behavior of very young stellar objects, "we are far from being able to uniquely classify evolutionary stages of their X-ray emission."
The five main young and massive Trapezium stars are responsible for the illumination of the entire Orion Nebula. These stars are born with masses 15 to 30 times larger than the mass of our sun. X-rays in such stars are thought to be produced by shocks that occur when high-velocity stellar winds ram into slower dense material.
The Chandra spectra show a temperature component of about 5 million to 10 million degrees, which is consistent with this model. However, four of these five stars also show additional components between 30 million and 60 million degrees.
"The fact that some of these massive stars show such a hot component and some not, and that a hot component seems to be more common than previously assumed, is an important new aspect in the spectral behavior of these stars," said David Huenemoerder, a research physicist at the MIT Center for Space Research.
Standard shock models cannot explain such high temperatures, which may be caused by magnetically confined plasmas, which generally are only attributed to stars like the sun. Such an effect would support the suspicion that some aspects in the X-ray emission of massive stars may not be different from our sun, which also has a hot corona. More study is needed to confirm this conclusion.
The HETGS was built by MIT with Bruno Rossi Professor Claude Canizares as principal investigator. The ACIS X-ray camera was conceived and developed for NASA by Pennsylvania State University and MIT. The Orion observation was part of Professor Canizares's guaranteed observing time during the first round of Chandra observations.
P CYGNI PROFILE
The discovery of the P Cygni profile reveals a 4.5-million-mile-per-hour wind coming from a highly compact pair of stars in our galaxy, reported Dr. Schulz and Professor Niel Brandt of Penn State. The team made the discovery during their first observation of a binary-star system using Chandra. The system, known as Circinus X-1, is located about 20,000 light years from Earth in the constellation Circinus near the Southern Cross. It contains a super-dense neutron star in orbit around a normal fusion-burning star like our sun.
The P Cygni spectral profile, previously detected primarily at ultraviolet and optical wavelengths but never before in X-rays, is the textbook tool astronomers rely on for probing stellar winds. It is named after the famous star P Cygni, in which such profiles have been observed for more than 100 years.
"We were hoping to detect some kind of X-ray line emission from the accreting neutron star in Circinus X-1, but it caught us totally by surprise to observe a complex emission structure like a P Cygni profile in high-energy X-rays," Dr. Schulz said. "This detection clearly marks a new area in X-ray astrophysics, where we will be able to study dynamical structures in the universe like we currently do at ultraviolet or optical wavelengths."
P Cygni profiles carry much diagnostic information that is hard to obtain in other ways -- such as how fast the wind is moving, how much material it contains, how dense it is, and its chemical composition.
This research was supported by the Chandra X-ray Center, the Alfred P. Sloan Foundation, and the Smithsonian Astrophysical Observatory.
To follow Chandra's progress, see http://chandra.harvard.edu and http://chandra.nasa.gov.
A version of this article appeared in MIT Tech Talk on November 15, 2000.
