1. The Young Magnetic O Star  1 Ori C: Multi-phase Chandra High-Resolution Grating Spectra Mary Oksala, Marc Gagné (West Chester University), David Cohen, Stephanie Tonnesen (Swarthmore College), Asif ud-Doula (North Caroline State University), Stanley Owocki (Bartol Research Institute), Joseph MacFarlane (Prism Computational Sciences) ABSTRACT Chandra High-Energy Grating spectra obtained at four rotational phases of the oblique magnetic rotator,  1 Ori C (O6 V), corresponding to four different viewing angles with respect to the magnetic axis, are used to constrain the temperature, spatial location, and kinematics of the hot plasma on this very young hot star with a strong (1100 G) dipole field. The plasma is moving, but only at speeds of a few 100 km s -1, much slower than the terminal wind velocity. It is close to the star (within 1.8 R * of the surface) and hot (peak temperature ~30 MK). We analyze these diagnostics in conjunction with new MHD simulations of the magnetically channeled wind shock mechanism on  1 Ori C. This model fits all the data surprisingly well, reproducing the very high temperatures, relatively narrow and unshifted lines, and the near-star source location. Figure 3 - Geometry of the O6 V star  1 Ori C where the magnetic dipole is B and the rotation axis is omega.  1 Ori C has a measured longitudinal magnetic field which varies from 360 G at viewing angle φ=0 (X-ray maximum) to ~0 G at viewing angle φ =90 (X-ray minimum). The corresponding dipole field strength is 1100 G (Donati et al. 2002). The inclination of the system is i=45° and the obliquity of the dipole is β=42°. As a result, the viewing angle between the line of sight and the magnetic pole varies between alpha=0 and alpha=90 during its 15.422 day period (Stahl et al. 1996). Chandra HETG spectra were obtained at viewing angles α=40˚, α=80˚, α=4˚, and α=87˚ (phases 0.84, 0.38, 0.01, and 0.47). X-ray maximum corresponds to phase 0.0 (magnetic pole-on) and x-ray minimum corresponds to phase 0.5 (magnetic equator-on). Figure 1- First-order MEG spectra of  1 Ori C obtained at viewing angles α=4, α=40, α=80, and α=87 (the viewing geometry is shown in Figure 3). The bottom panel shows the combined MEG spectrum from all four observations. HEG and MEG spectra of  1 Ori C show strong, narrow emission lines and a strong 2-15 Å brehmsstrahlung continuum. Variable-abundance, multi- temperature APEC model fits to the MEG spectra indicate that most of the plasma is hotter than 10 MK, with a peak in the emission- measure distribution at log T = 7.5. Figure 6 - Line widths for the strongest lines in the Chandra spectra plotted against the temperature of peak line emissivity, taken from APED. The open circles represent the Doppler width measured by SHERPA. The filled diamonds are the rms velocity as measured by ISIS. The mean rms velocity and standard deviations are indicated by the horizontal lines. Note that two of the OVIII and Fe XVII lines formed in the coolest plasma are significantly broader than the mean and may be formed by shocks in the far wind. The hotter X- ray the lines are narrow: much narrower than the terminal wind speed. The f/i ratios and the hot line widths suggest strongly that the X-rays are produced in a confined plasma close to the photosphere. Introduction The brightest star in the Trapezium and primary source of ionization radiation in the Orion nebula, the magnetic star  1 Ori C is very young (<400,000 yr). It has a relatively strong wind (dM/dt  10 -7 M , v   2500 km s -1 ) in addition to a strong dipole field (Donati et al. 2002). Its strong, hard, variable X-ray emission suggests it may be the prototype of a new class of stellar X-ray source: an oblique magnetic rotator with magnetically confined wind shock (Babel & Montmerle 1997). We have calculated 2-D MHD models for  1 Ori C, extending the work of ud-Doula & Owocki (2002) to include radiative and adiabatic cooling. Conclusions  1 Ori C’s emission lines and f/i ratios show that most of the very hot plasma is located within 1.8 R  of the photosphere. The amount of occultation and its phase dependence suggest this plasma is concentrated near the magnetic equator. The narrow emission lines show that the hottest, densest plasma is moving at only 10 to 15 % of the wind terminal velocity. MHD simulations of magnetically confined wind shocks produce very hot plasma (T  30 MK) and narrow emission lines. The data analysis show, in conjunction with the MHD simulations, that  1 Ori C is consistent with the general picture of the MCWS model. Figure 2 - To examine wavelength-dependent time variability, the four MEG spectra in Fig. 1 were binned in 1A bins and divided by the binned average spectrum. The emission at most wavelengths is strongest at alpha=4 and alpha=40 (see Fig. 3). X-ray emission is weaker by approximately 30% at alpha=87 with an apparent increase in absorption with increasing wavelength. This pattern is consistent with an X-ray emitting torus above the magnetic equator, partially occulted by the star at alpha=80 and 87. The increased absorption from 5-20A at alpha=87 may be due to higher column density along the equator’s line of sight. MHD models predict that the distorted wind and magnetic field will funnel some wind material out the magnetic equator. Figure 5 - First-order HEG He-like line profiles for Ar XVII, S XV, Si XIII, and Mg XI (solid histograms). Show are the resonant (left), intercombination (middle) and forbidden (right) lines. In hot stars, the forbidden-line strength of He-like ions is reduced by photoexcitation of electrons in the 3 S 1 state by photospheric UV radiation. We do not detect the Mg XI f line. The Si XIII f line is blended with H-like Mg XII, and the Ar XVII f line is blended with H- like S XVI. The Ca XIX and Fe XXV r and i lines are blended, but the f lines are resolved. Hence, we limit our discussion to the reliably measured f/i ratio of S XV and the upper limit for Mg XI. For S XV and Mg XI photoexcitation occurs at 743 and 1036 A, respectively. For the 743 A flux, we custom- computed non-LTE models of the excitation kinematics of the relevant ions. For the 1036 A flux, we used archival Copernicus data. This analysis suggests that, on average, the X-ray emitting plasma is located 1.5-1.8 R  above the photosphere. At this radius, the X-ray emitting torus would be 30% occulted around alpha=90deg, consistent with observation. Figure 4 - Phase-folded light curves of  1 Ori C. Open circles represent the excess C IV 1548, 1551 Å equivalent width (left axis) taken from Walborn & Nichols (1994) and phased to the ephemeris of Stahl et al. (1996) with period 15.422 days and MJD 0 = 48832.50. Maximum C IV absorption occurs near phase 0.0 (alpha=0deg). Filled circles represent the longitudinal magnetic field strength (right axis) from Donati et al. (2002) phased to the Stahl et al. (1996) ephemeris. B l is maximum near phase 0.0 when the star is viewed pole-on. The solid line corresponds to the equivalent width of H  from Stahl et al. (1996). Phase 0.0 corresponds to H  -emission maximum. The X-ray emission maximum (see top panel of Fig. 2) occurs near phase 0.0. Since the X-ray emission is not significantly absorbed, X-ray and H-alpha maxima occur when the entire X- torus above the magnetic equator is visible (low viewing angles). X-ray and H-alpha minima occur when part of the X- ray torus is occulted by the star (magnetic equator-on). Pole-on  =0° Equator-on  =90°