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

astro.swarthmore.edu/~cohen/presentations/zori_AAS2006/ Presented at the AAS meeting, Washington DC, January 2006

X-ray emission from OB starsGeneral 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?

The massive, radiation-driven winds of OB stars are inherently unstable

Phenomenology of Chandra grating observations

Pup (O4 If) and

Ori (O9.7 Ib)

Owocki & Cohen (2001) developed a simple, phenomenological model of x-ray line profiles

from hot plasma embedded in an expanding emitting and absorbing medium

Chandra data for Ori Fits to individual lines in the Chandra spectrum of Ori

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.

As in the Chandra spectrum of Pup, the lines are broad (typical FWHM ~ 1500 km/s), but what

are the profile shapes telling us?

Pup: lines are resolved.

Capella: lines are unresolved.

As with UV spectra of OB stars (Si IV lines of Ori seen with Copernicus, left; from Snow & Morton, 1976), are the resolved line profiles telling us something about wind kinematics?

Simulations (below) show that this instability leads to shock-heating, and x-ray emission.

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.

The key model parameters are:

* : 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).

First, we fit a Gaussian line profile, with its centroid fixed at the lab rest wavelength: rejected at the 94% conf. level.

Fe XVII line at 15.014 Å

Next, we fit a Gaussian, but with the centroid a free parameter. Fit is now marginal (rejected at the 74% conf. level).

Finally, we fit a wind-profile model. The fit is good (29% rejection prob.) – Rmin=1.4, *=0.6, q=-0.5.

Confidence limits on model parameters

68% and 90% contours in 2-D slices of the 3-D model parameter space. umax = R

*/Rmin.

Fits to 2 other lines – O VIII Ly

and Ne X Ly.

Summary of fits to seven line complexes

•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, * 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)?

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.

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: Rmin increases from top to bottom. * increases in each frame, from black to red to blue.