X‐Ray Modeling of Very Young Early‐Type Stars in the Orion Trapezium: Signatures of Magnetically Confined Plasmas and Evolutionary Implications

Abstract

The Orion Trapezium is one of the youngest and closest star-formingregions within our Galaxy. With a dynamic age of ~3×105 yr, it harborsa number of very young hot stars, which likely are on the zero-agemain sequence (ZAMS). We analyzed high-resolution X-ray spectra inthe wavelength range of 1.5-25 Å of three of its X-ray-brightestmembers (?1 Ori A, C, and E) obtained with the High Energy TransmissionGrating Spectrometer (HETGS) on board the Chandra X-Ray Observatory.We measured X-ray emission lines, calculated differential emissionmeasure distributions (DEMs), and fitted broadband models to thespectra. The spectra from all three stars are very rich in emissionlines, specifically from highly ionized Fe, which includes emissionfrom Fe XVII to Fe XXV ions. A complete line list is included. Thisis a mere effect of high temperatures rather than an overabundanceof Fe, which in fact turns out to be underabundant in all three Trapeziummembers. Similarly there is a significant underabundance in Ne andO as well, whereas Mg, Si, S, Ar, and Ca appear close to solar. TheDEM derived from over 80 emission lines in the spectrum of ?1 OriC indicates three peaks located at 7.9, 25, and 66 MK. The emissionmeasure varies over the 15.4 day wind period of the star. For thetwo 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 resolvesome of the soft X-ray lines with velocities up to 850 km s-1 (allwidths are stated as half-width at half-maximum), whereas the bulkof the lines remain unresolved with a confidence limit of 110 kms-1. The broadband spectra of the other two stars can be fitted withseveral collisionally ionized plasma model components within a temperaturerange of 4.3-46.8 MK for ?1 Ori E and 4.8-42.7 MK for ?1 Ori A. Thehigh-temperature emissivity contributes over 70% to the total X-rayflux. None of the lines are resolved for ?1 Ori A and E with a confidencelimit of 160 km s-1. The influence of the strong UV radiation fieldon the forbidden line in the He-like triplets allows us to set anupper limit on distance of the line-emitting region from the photosphere.The bulk of the X-ray emission cannot be produced by shock instabilitiesin a radiation-driven wind and are likely the result of magneticconfinement in all three stars. Although confinement models cannotexplain all the results, the resemblance of the unresolved linesand of the DEM with recent observations of active coronae in II Pegand AR Lac during flares is quite obvious. Thus we speculate thatthe X-ray production mechanism in these stars is similar, with thedifference that the Orion stars may be in a state of almost continuousflaring driven by the wind. We clearly rule out major effects dueto X-rays from a possible companion. The fact that all three starsappear to be magnetic and are near zero age on the main sequencealso raises the issue of whether the Orion stars are simply differentor whether young massive stars enter the main sequence carrying significantmagnetic fields. The ratio logLX/Lbol using the ``wind'' componentof the spectrum is -7 for the Trapezium stars, consistent with theexpectation from O stars. This suggests that massive ZAMS stars generatetheir X-ray luminosities like normal O stars and magnetic confinementprovides an additional source of X-rays.