Simulated HETGS Spectroscopy of the
Orion Trapezium Region

At the core of the Orion Nebula lies the Trapezium cluster, a small group of young OB stars responsible for the bulk of the excitation of the visible nebulosity (Figure 1). Like most OB stars, the Trapezium stars are known to be luminous X-ray sources, though the X-ray emission mechanism of such hot stars is poorly understood.

Spectroscopy of the Trapezium region with AXAF and HETG should help astronomers understand the origin of the X-ray emission in early type (OB) stars. To investigate whether such observations are feasible, and what results they might yield, we carried out simulations of AXAF/HETG observations of the Trapezium region with the Model of AXAF Response to X-rays (MARX), developed at the AXAF Science Center at MIT.

Figure 1 - The Orion Nebula as imaged by the Hubble Space Telescope. The Trapezium is the set of four optically visible stars at the core of the nebula.

The Trapezium lies at the front surface of a massive, dusty molecular cloud that is teeming with newborn stars (Figure 2). Most young, Sun-like (T Tauri) stars are also X-ray sources; hence the potential for confusion in the identification of a dispersed photon with a unique source makes the Trapezium region a challenging target for HETG.

Since the opacity of a typical molecular cloud to 1 keV photons is approximately the same as the dust opacity of such a cloud at 2 microns, the near-infrared image in Figure 2 may resemble an AXAF image obtained at energies of 1 keV and higher. Hence this image makes an appropriate starting point for our simulations, and we used it as a template for the X-ray field as detected by AXAF with its ACIS-S CCD array. We assume that the total X-ray luminosity from the Trapezium region is about $3\times10^{-11}$ ergs cm$^{-2}$ s$^{-1}$ and, for simplicity, we assume that the emission from each star is characteristic of plasma at a temperature of 1 keV.

Figure 2 - A near-infrared (2.2 micron, K band) image of the Trapezium region obtained with the 50 inch telescope at Kitt Peak (Weintraub & Kastner 1995). Near-infrared photons escape the dusty Orion molecular cloud much more easily than optical photons, enabling detection of hundreds of young stars embedded within the cloud core that lies just behind the Trapezium. Most prominent of these optically obscured, newborn stars is the Becklin-Neugebauer object, which also illuminates an extensive near-infrared reflection nebula (seen to the northwest of the Trapezium).

Figure 3 shows a 150 ksec simulated pointed observation centered on the Trapezium with the HETG instrument. The cluster is located in the middle of the field of view centered near the left edge of CCD S3. This 0th-order image of the Trapezium region (which is, of course, just a renormalization of the near-infrared image) has an extend of roughly 5.5 x 5.5 arcmin and thus does not entirely fill the 8 x 8 arcmin extend of the center CCD. The dispersed emission extends to either side along the ACIS-S CCD array. Due to the nature of the X-ray spectra, the traces of the MEG (from bottom left to top right) and HEG (from top left to bottom right) are nicely separated.
The dispersed spectra of the Trapezium stars dominate; within these spectra one can discern, in emission lines, the distinctive spatial pattern of the bright stars seen in 0th order (Figure 4).

Figure 5 shows the extracted spectrum of the brightestTrapezium star. This Figure illustrates how the unprecedented combination of spatial and spectral resolution achieved by AXAF and HETG will allow us to identify and extract spectra from individual stars, despite the complexity of the Trapezium region. Furthermore, we can readily detect and identify features (lines) in the extracted spectra, including lines that are diagnostic of plasma physical conditions. Hence, AXAF/HETG observations of this intriguing region of recent star formation should greatly enhance our understanding of how and why hot, young stars emit X-rays.

Figure 3 - Simulated HETG/ACIS-S image obtained with MARX, using the near-infrared image of Figure 2 as a template for the Trapezium X-ray field. The pointing was set to last about 150 ksec. The view does not quite capture the full ACIS-S array, starting with S1 to the left and the first 2 arcmin of S5 to the right. The whole shown array then spans about a 33 arcmin FOV from left to right.

Figure 4 - Close-up view of Figure 3 showing the center CCD S3 (left) and the adjacent CCD S4 (right). Most of the dispersed X-rays in this view are 1st order X-rays of the MEG. At the center one can nicely see the patterns of the four main Trapezium stars due to dispersed bright lines.

Figure 5 - HETG Spectrum of an individual Trapezium star, as extracted from the dataset used to construct the images presented in the preceeding two figures.

Simulation courtesy of Norbert S. Schulz and Joel Kastner (MIT/ASC)

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Michael Wise - wise@space,
Last Updated: July 24, 1997