PowerPoint Presentation

X-ray Line Profiles of Magnetically Confined Hot-Star Winds

Stephanie K. Tonnesen1, David H. Cohen1, Stanley P. Owocki2, Asif ud-Doula2,3, Marc Gagne4, Mary Oksala4

(1) Swarthmore College, (2) Bartol Research Institute, University of Delaware, (3) North Carolina State, (4) West Chester University

Magnetic Hot Stars

Hot Star X-rays

The standard model of hot star wind emission explains many X-ray observations of O stars (see Roban Kramer’s poster (113:05)): Broad and asymmetric lines, due to Doppler-shifted emission from shock zones embedded in the radiation-driven winds, affected by continuum absorption in the cooler component of the wind.

But observations of some hot stars, including q1 Ori C, cannot be understood in the context of the standard wind shock model:

1.Lines are quite narrow.

2.They are relatively unshifted and symmetric.

3.The X-ray emission is hard.

4.And modulated on the rotation period (such that occultation by the star seems to be the cause of reduced X-ray emission at certain phases).

Line Profile Simulations

X-ray Lightcurve

An 1100 G magnetic field has been detected on the O7 V star, q1 Ori C (Donati et al. 2002).

This star is a strong X-ray source with very hard X-rays (Schulz et al. 2000) modulated on the 16 day rotation period (Gagne et al. 1997).

Based on the Zeeman magnetic field detection, UV and optical line strength variability, and X-ray variability, the following picture has emerged of the geometry of this star’s circumstellar matter:

Babel and Montmerle proposed the Magnetically Confined Wind Shock model (1997) to explain the hard X-rays seen in q1 Ori C. In this model, a radiation-driven wind is forced to flow along magnetic field lines, leading to a collision of oppositely directed flows at the magnetic equator, and the associated shock heating of the confined gas. This leads to an X-ray magnetosphere of relatively slow moving hot gas.

In these calculations, the field is assumed to be completely rigid.

Note also, a post-shock cooling disk forms in this steady-state model. This disk may be a significant source of X-ray opacity.

Note that due to the tilt of the field and the viewing inclination angle, there is a phase dependence of the view with respect to the magnetosphere. We indicate the four phases for which we have Chandra observations.

Modeling Line Profiles

New MHD simulations of magnetically confined winds (ud-Doula and Owocki 2002) go beyond the Babel and Montmerle model in that they allow for the relaxation of the magnetic field structure based on the kinetic energy of the wind flow.

Crucial parameter for magnetic confinement is h*, proportional to the ratio of the magnetic energy density to wind kinetic energy density.

Note the the equator-ward flow in the confined loops

The shock velocity corresponds to temperatures above 50 million K

These recently calculated models are specific to q1 Ori C and include post-shock radiative cooling. We post-process these simulations to produce line profiles, as seen from arbitrary viewing angles (corresponding to different rotation phases), in the next columns to the right.

We calculate the emissivity at each grid zone based on the simulation temperature and density. The line-of-sight velocity determines the wavelength of the emission.

We take occultation by the star into account but not (currently) X-ray absorption by the cold wind component.

Note that these simulations show that cooler post-shock gas periodically falls back onto the star, and so a dense, opaque cooling disk does not readily form. Furthermore, not all of the hot gas is in the closed magnetic structures, but rather in the interface between the confined equatorial gas and the wind.

Below we show synthetic line profiles post-processed from the MHD simulation snapshot shown at the bottom of the previous column.

We show profiles as seen from 0˚ (pole-on), 45˚ and 90˚ (equator-on). For each view we show two profiles: including occultation by the star and not including occultation. We also show the corresponding line-of-sight velocity contours (color plots on right), with the contours of hot (T>106 K) plasma superimposed.

Line of Sight Velocities

Scale in terms of UV-based vterm=2500 km s-1

Density

Temperature

MHD Simulations of q1 Ori C with energy equation (radiative as well as adiabatic cooling)

Initial MHD simulations: isothermal, but large shock at equator

We first explore line profiles from non-spherical winds by considering a disk (opening angle 20˚) with a purely radial outflow.

Pole-on view,

0°

45° view

Equator-on view, 90°

NOTE: expect strong viewing-angle dependence of line-profile.

Speed

Strong magnetic confinement of the wind, but geometrically thick shock zone and relatively little ‘cooling disk’

Emission Line Profiles

The magnetosphere periodically empties

as material falls back onto the star, leading to more irregular structure in the X-ray emission region

A different snapshot in the MHD simulation shows a dense ‘disk infall’ back onto the star

The X-ray and Ha emission maxima, magnetic field maximum, and UV absorption minimum occur at phase=0.0.

Overall X-ray flux synthesized from the same MHD simulation snapshot. The dip at oblique viewing angles is due to stellar occultation.

Data from four different Chandra observations is superimposed: green squares represent the relative fluxes in the strong lines near 5 Å while purple circles represent the relative fluxes in the strong lines between 10 and 15 Å, where absorption is likely to be more important.