Wind Signatures in the X-ray Emission Line Profiles of the O Supergiant z Orionis

1. Wind Signatures in the X-ray Emission Line Profiles of the O Supergiant z Orionis Kevin Grizzard1, David Cohen2, Maurice Leutenegger3, Casey Reed2, Roban Kramer2,3,4, Stan Owocki5 1St. John’s College, 2Swarthmore College, 3Columbia University, 4Prism Computational Sciences, 5University of Delaware Fits to individual lines in the Chandra spectrum of z Ori • X-ray emission from OB stars • General problem – hot stars don’t have convective envelopes and thus no dynamo and no corona. Is the observed x-ray emission evidence of a different type of magnetic activity? Or is it related to these hot stars’ strong winds? Chandra data for z Ori Fe XVII line at 15.014 Å First, we fit a Gaussian line profile, with its centroid fixed at the lab rest wavelength: rejected at the 94% conf. level. • Phenomenology of Chandra grating observations • Pup (O4 If) and z Ori (O9.7 Ib) As in the Chandra spectrum of z Pup, the lines are broad (typical FWHM ~ 1500 km/s), but what are the profile shapes telling us? Next, we fit a Gaussian, but with the centroid a free parameter. Fit is now marginal (rejected at the 74% conf. level). Owocki & Cohen (2001) developed a simple, phenomenological model of x-ray line profiles from hot plasma embedded in an expanding emitting and absorbing medium These spectra are soft (T ~ few million K) and show strong emission lines of He-like and H-like O, Ne, Mg, Si, S as well as L-shell lines of Fe, with a weak bremsstrahlung continuum. The phenomenology is like that of cool stars’ coronae…except that the lines are broad. Finally, we fit a wind-profile model. The fit is good (29% rejection prob.) – Rmin=1.4, t*=0.6, q=-0.5. z Pup: lines are resolved. Confidence limits on model parameters Capella: lines are unresolved. In the schematic on the left, hue represents Doppler shift of the emitted x-rays; intensity is proportional to the density-squared emission strength. (The observer is assumed to be on the left.) Continuum absorption by the unshocked wind preferentially attenuates photons from the back, red shifted, portion of the wind, leading to the characteristic broadened, skewed profile shown on the right, with a blue shifted centroid. 68% and 90% contours in 2-D slices of the 3-D model parameter space. umax = R*/Rmin. As with UV spectra of OB stars (Si IV lines of z Ori seen with Copernicus, left; from Snow & Morton, 1976), are the resolved line profiles telling us something about wind kinematics? Fits to 2 other lines – O VIII Lya and Ne X Lya. Summary of fits to seven line complexes The massive, radiation-driven winds of OB stars are inherently unstable This line-profile model is not tied to any specific wind-shock or coronal model. It can be used to find the physical properties of the x-ray emitting plasma on a hot star. These values can then be related to the predictions of a specific model. Simulations (below) show that this instability leads to shock-heating, and x-ray emission. The key model parameters are: t* : characterizes the amount of attenuation Rmin : radius above which the wind is hot enough to emit x-rays q : power-law index of assumed radial fall-off of the hot plasma filling factor (q=0 implies a constant filling factor). • Global parameters are consistent with standard instability shock paradigm (see color figures at bottom of left-hand column): Rmin ~ 1.5 R*, q ~ 0. • But, t* values are roughly an order of magnitude too small: more evidence for mass-loss rate overestimates (see 182.23 Bouret, Lanz, & Hillier; also FUSE evidence for lower mass-loss rates in Fullerton, Massa, & Prinja)? Effects of wind inhomogeneities (see Oskinova, Feldmeier, & Hamann)? Velocity map from rad-hydro simulation of self-excited instability (left) and snapshot of temperature and density from same simulation (center). Simulation on right is of the instability seeded by turbulence at the base of the wind. Suite of models above: Rminincreases from top to bottom. t* increases in each frame, from black to red to blue. astro.swarthmore.edu/~cohen/presentations/zori_AAS2006/ Presented at the AAS meeting, Washington DC, January 2006