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FOR RELEASE: 10:10 a.m. Hawaiian Standard Time, November 10th, 2000

Lifting the Veil: Hot Stars in the Making

The core of the Orion Trapezium cluster shows an unprecedented accumulation of star showing extreme temperatures, some are associated with externally illuminated protoplanetary disks.

Norbert S. Schulz, Claude R. Canizares, Dave Huenemoerder, Sara-anne Taylor

Massachusetts Institute of Technology

Joel Kastner

Rochester Institute of Technology

Article accepted by the ApJ Main Journal


The Chandra X-ray Observatory has resolved the hot core of one of the closest and most massive star forming regions. The Orion Trapezium Cluster is only a few 100 000 years old and offers a prime view into a stellar nursery. Detected X-ray sources included 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.

"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 Norbert S. Schulz, Research Scientist at the Chandra X-ray Center at the Massachusetts Institute of Technology, who leads the Orion Project. "And with extreme we consider temperatures above 20 Million degrees".

Figure 1 (caption, see below)

Figure 2 (caption, see below)

The bright light of the Trapezium and its sourrounding stars at the heart of the Orion Nebula (M42) is visible to the naked eye on clear nights and if one would have a glimpse at it once every night, it would work just the same. Quite the opposite in X-rays. These young stars are constantly active and changing in X-ray brightness, sometimes within half a day, sometimes over weeks. "Never before Chandra have we seen images of stellar activity with such brilliance" said Joel Kastner, Professor at 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). Both instruments were built at MIT.

It is generally assumed that low-mass stars like our sun, when they are much younger, are over 1000 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. The absense of many strong flares in the lightcurves as well as extreme temperatures in the Chandra ACIS spectra could mean that they are either young protostars (i.e stars in the making), or a special class of of more evolved, hot young stars. Here Schulz concedes: "Although we have gathered many clues in recent years about the X-ray behaviour of very young stellar objects, we are far from being able to uniquely classify evolutionary stages of their X-ray emission."

Figure 3 (caption, see below)

Figure 4 (caption, see below)

The five main Trapezium stars are very young and massive, and are reponsible 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 in shocks produced in a high velocity stellar wind raming into slower dense material. The Chandra spectra do show a temperature component of about 5-10 Million degrees, which is quite consistent with this model. However four of these five stars also show additional components between 30 and 60 Million degrees. The hottest massive star known so far, exhibited about 25 Million degrees.

"The fact that some of these 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 behaviour of these stars." adds Dr. Dave Huenemoerder from the MIT Center for Space Research. Standard shock models cannot explain such high temperatures and it may be possible that magnetically confined plasmas are responsible for these temperatures. Such an effect would nourish the suspision that some aspects in the X-ray emission of massive stars may not be different from our sun, which has a hot corona, afterall, though many times more energetic. Clearly many more studies are necessary to underline such a conclusion.

Figure 5 (caption, see below)

One of the major highlights of the Chandra observations are several clear identifications of detected X-ray sources point sources with proplyds in the near vicinity of the most massive star in the Trapezium. These dust cocoons have the size of less than a few solar systems and current models involve a young star at the core with a circumstellar disk embedded in an outer wind shock region. The X-ray emission clearly shows all the signatures we know so far to be from evolved protostars of the size of our sun or smaller. Their appear deeply embedded in absorbing material. They are also unexpectedly hot. One of them, for example, shows a moderately strong X-ray flare showing a temperature of 50 Million degrees, two sources indicate almost 100 Million degrees.

Figure Captions:

Figure 1: The Chandra star field around the Orion Trapezium as observed on October 31st UT 05:47:21 1999. Within a 3'x3' region 111 X-ray sources were resolved. The colors represent energy, where blue and white indicate very high energies and therefore exterme temperatures. This star field is one of the densest clustering of X-ray sources in our galaxy and over 100 are detected within a distance of less than 2 light years. The distance between our sun and the next star is about 4 light years. Note also that many stars are either missing of different brightness when compared to the ones in figure 2.

Figure 2: The same field as in figure 1 but observed on November 24th UT 05:37:54 1999.

Figure 3: The Orion Trapezium as observed on October 31st UT 05:47:21 1999. The colors are the same as is figure 1. The size of the X-ray source in the image also reflects its brightness, i.e. more bright sources appear larger in size. The is an artefact caused by the limiting blur of the telescope optics. The projected diameter of the field of view is about 80 light days.

Figure 4: The same field as in figure 3 but observed on November 24th UT 05:37:54 1999.

X-ray contours of the Chandra observation overlaid onto the optical Hubble image (courtesy of J. Bally, CASA Colorado). The field of view is 30"x30". Besides the bright main Trapezium stars, which were found to be extremely hot massive stars, several externally illuminated objects are also X-ray emitters. Some of them with temperatures up to 100 Million degrees. The ones that do not show X-ray contours are probably too faint to be detected in these particular Chandra observations.

References

The HETGS team studying the Orion Trapezium Cluster are: Prof. Claude Canizares, Dr. Norbert S. Schulz, and Dr. Dave Huenemoerder at the Massachusetts Institute of Technology (MIT);
Prof. Joel Kastner at the Rochester Institute of Technology.
Prof. Claude Canizares is also Principal Investigator of the HETGS onboard Chandra and Co-Director of the Chandra X-ray Center.

The Chandra ACIS detector was built at MIT under the leadership of Prof. Gordon Garmire from Penn State University.

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

Background definitions

Orion Trapezium Cluster

The great Orion Nebula harbors the Orion Nebula Cluster (ONC), a loose association of about then 2000 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.

Stellar Classification

Stars are generally classified in the Hertzsprung-Russell diagram by their relation of bolometric luminosity with surface temperature. Most stars live most of their lives on the main sequence in this diagrams. O and B-stars are so-called early type stars, they are very massive, with large surface temperatures (> 20000 K). They are quite short lived with a few times 10 Million years of age and therefore reach the main sequence very early in their lifes. Our sun belongs to the class of late type stars (A- to M-types) with surface temperatures < 10000 K on the main sequence. They reach the main sequence late.

Protostars

Protostars are also called pre-main sequence (PMS) stars which have not yet reached the main sequence. In general they are divided into four classes based on their infra-red light; class 0 objects are young protostars which are still deeply embedded and in their gravitational collapse phase; class I objects are evolved protostars, which are accreting from a proto-stellar cloud; in Class II objects are also called classical T Tauri stars, which are optically bright and have strong signatures of a circumstaller disk; Class III objects are so-called weak line T Tauri stars and basically no disk. The typical durations of these phases are 10000, 100000, 1 Million, and 10 Million years, respectively. These phases mainly apply for late type stars, although they could be somewhat similar for early types, but on different time scales.

X-ray Astronomy

The study of objects in the universe using X-rays rather than visible light of 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 ACIS detector 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 X-rays from objects in the sky. An X-ray telescope cannot work on ground because the X-rays are absorbed by the Earth's atmosphere.

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