Chandra Observations of Variable Embedded X-Ray Sources in Orion. I. Resolving the Orion Trapezium
This is the pre-print of an article published by the American Astronomical Society. The final, published version is available here: https://doi.org/10.1086/319078
© 2001 The American Astronomical Society.
Also archived in: arXiv: astro-ph/0011366 v1 20 Nov 2000
The authors thank John Bally for generously providing us with the HST PC image of the Trapezium. We also thank all the members of the Chandra team for their enormous efforts. This research is funded in part by the Smithsonian Astrophysical Observatory contract SV-61010 (CXC) and NAS8-39073 (HETG) under the Marshall Space Flight Center.
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Abstract
We used the High Energy Transmission Grating Spectrometer (HETGS) onboard the Chandra X-ray Observatory to perform two observations, separated by three weeks, of the Orion Trapezium region. The zeroth order images on the Advanced CCD Imaging Spectrometer (ACIS) provide spatial resolution of 0.5" and moderate energy resolution. Within a 160"x140" region around the Orion Trapezium we resolve 111 X-ray sources with luminosities between 7×10^28 erg s^-1 and 2×10^32 erg s^-1 . We do not detect any diffuse emission. All but six sources are identified. From spectral fits of the three brightest stars in the Trapezium we determine the line of sight column density to be NH = 1.93±0.29×10^21 cm^-2. Many sources appear much more heavily absorbed, with NH in the range of 10^22 to 10^23 cm^-2. A large fraction of sources also show excursions in luminosity by more than a factor 5 on timescales >50 ks; many are only detected in one of the observations. The main objective of this paper is to study the Orion Trapezium and its close vicinity. All five Trapezium stars are bright in X-rays, with Theta-1 Ori C accounting for about 60% of the total luminosity of the Trapezium. The CCD spectra of the three very early type members can be fit with a two-temperature thermal spectrum with a soft component of kT ~0.8 keV and a hard component of kT~2 to 3 keV. Theta-1 Ori B is an order of magnitude fainter than Theta-1 Ori E and shows only a hard spectrum of kT~3 keV. Theta-1 Ori D is another order of magnitude fainter than Theta-1 Ori B, with only a kT~0.7 keV component. We discuss these results in the context of stellar wind models. We detect eight additional, mostly variable X-ray sources in the close vicinity of the Trapezium. They are identified with thermal and non-thermal radio sources, as well as infrared and optical stars. Five of these X-rays sources are identified with proplyds and we argue that the X-ray emission originates from class I, II and III protostars at the cores of the proplyds (Refer to PDF file for exact formulas).
