X-RAY MODELING OF VERY YOUNG EARLY-TYPE STARS IN THE ORION TRAPEZIUM: SIGNATURES OF MAGNETICALLY CONFINED PLASMAS AND EVOLUTIONARY IMPLICATIONS

摘要

The Orion Trapezium is one of the youngest and closest star-forming regions within our Galaxy. With a dynamic age of ～3 x 10~5 yr, it harbors a number of very young hot stars, which likely are on the zero-age main sequence (ZAMS). We analyzed high-resolution X-ray spectra in the wavelength range of 1.5-25 A of three of its X-ray-brightest members (Θ~1 Ori A, C, and E) obtained with the High Energy Transmission Grating Spectrometer (HETGS) on board the Chandra X-Ray Observatory. We measured X-ray emission lines, calculated differential emission measure distributions (DEMs), and fitted broadband models to the spectra. The spectra from all three stars are very rich in emission lines, specifically from highly ionized Fe, which includes emission from Fe XVII to Fe XXV ions. A complete line list is included. This is a mere effect of high temperatures rather than an overabundance of Fe, which in fact turns out to be underabundant in all three Trapezium members. Similarly there is a significant underabundance in Ne and O as well, whereas Mg, Si, S, Ar, and Ca appear close to solar. The DEM derived from over 80 emission lines in the spectrum of Θ~1 Ori C indicates three peaks located at 7.9, 25, and 66 MK. The emission measure varies over the 15.4 day wind period of the star. For the two phases observed, the low-temperature emission remains stable, while the high-temperature emission shows significant differences. The line widths seem to show a similar bifurcation, where we resolve some of the soft X-ray lines with velocities up to 850 km s~(-1) (all widths are stated as half-width at half-maximum), whereas the bulk of the lines remain unresolved with a confidence limit of 110 km s~(-1). The broadband spectra of the other two stars can be fitted with several collisionally ionized plasma model components within a temperature range of 4.3-46.8 MK for Θ~1 Ori E and 4.8-42.7 MK for Θ~1 Ori A. The high-temperature emissivity contributes over 70% to the total X-ray flux. None of the lines are resolved for Θ~1 Ori A and E with a confidence limit of 160 km s~(-1). The influence of the strong UV radiation field on the forbidden line in the He-like triplets allows us to set an upper limit on distance of the line-emitting region from the photosphere. The bulk of the X-ray emission cannot be produced by shock instabilities in a radiation-driven wind and are likely the result of magnetic confinement in all three stars. Although confinement models cannot explain all the results, the resemblance of the unresolved lines and of the DEM with recent observations of active coronae in Ⅱ Peg and AR Lac during flares is quite obvious. Thus we speculate that the X-ray production mechanism in these stars is similar, with the difference that the Orion stars may be in a state of almost continuous flaring driven by the wind. We clearly rule out major effects due to X-rays from a possible companion. The fact that all three stars appear to be magnetic and are near zero age on the main sequence also raises the issue of whether the Orion stars are simply different or whether young massive stars enter the main sequence carrying significant magnetic fields. The ratio log L_X/L_(bol) using the "wind" component of the spectrum is -7 for the Trapezium stars, consistent with the expectation from O stars. This suggests that massive ZAMS stars generate their X-ray luminosities like normal O stars and magnetic confinement provides an additional source of X-rays.