X-ray Emission Line Profiles of Hot Stars

David H. Cohen and Stanley P. Owocki

Presented at “Two Years of Chandra Science” Washington, D.C., September 5-7, 2001

In collaboration with

J. Elliot Reed (Swarthmore ‘03)

Geneviève de Messieres (Swarthmore ‘04)

Carolin Cardamone (Wellesley ‘02)

Asif Ud-Doula (University of Delaware)

Marc Gagne (West Chester University)

Joseph MacFarlane (Prism Computational Sciences)

Duane Liedahl (Lawrence Livermore National Lab)

Nathan Miller (University of Wisconsin, Madison)

Joseph Cassinelli (University of Wisconsin, Madison)

(Swarthmore College; University of Delaware)

Solar-type magnetic heating? Or massive stellar wind shock heating?

spectroscopy can provide crucial new insights into the fundamental nature of the X-ray emission from hot stars

Uniform emission from a constant velocity, spherical shell

Sum over series of shells to build up a model of a wind

Occultation by the star blocks the back (red) side of the wind

Continuum opacity in the cold part of the wind

X-ray line profiles for parameterized wind emission models

fx ~ 1/rq

for r > Ro

v ~ (1 R / r)

* Ý M

4 vR*

=1, 3 , 5, 10

Why might we expect a smoothly distributed source of X-rays above some minimum radius and embedded in a cold, opaque stellar wind?

The line-force instability -- caused by the feedback inherent in the Doppler de-shadowing of an optically thick, line-driven flow -- provides a natural explanation

But there are other models too

Shock heating of wind can produce X-rays…but perhaps not enough

So, we can test the models with the new, high-resolution spectra being obtained by Chandra

Zeta Pup, an O supergiant with a massive wind: The strong Ne X Ly alpha line can be fit by a model with reasonable wind-shock parameters (

*=1.5; R0=1.5; q=0.5; =0.8)

2vinf/FWHMinst~ 20

Ne X

Within the spectrum of zeta Pup, there are trends seen in the line shapes

The wind is less optically thick at shorter wavelengths;

The O VIII line’s breadth and relative flat-topped-ness indicates an origin in the outer wind

O VIIIMg XII

Four other hot stars have been observed with the Chandra HETGS:

1 Ori C (O7.5 V); Ori (O9.5 II); Ori (O9.5I); Sco (B0 V)

For none of these stars are the line profiles indicative of a smoothly and broadly distributed source within an optically thick wind

For zeta Ori, the lines are broad, but much more symmetrical and less blueshifted

This is the same model that fit the Ne X line in zeta Pup

There are several possibilities:

1. “Turbulence”

2. Line scattering

3. Non-isotropic clumping

4. Hybrid magnetic/wind-shock models (MCWS)

(1) Random motion of hot plasma, not embedded in a wind might explain Sco (and maybe Ori); but for the other stars, the FWHM exceeds the surface escape speed, and the shock temperatures associated with the line widths are huge; the extent of impulsive flare events required is unreasonable

What can lead to lines that are broad, but relatively unshifted and symmetric?

Thin vs. Thick line-emission

=1, 3 , 5, 10

cf. Ignace & Gayley 2001for=0 case

Line Opacity has a very different effect than continuum opacity

It tends to suppress the bluest and reddest photons; escape probabilities are largest for “sideways” trajectories out of Sobolev zones

This effect is most significant for a constant-velocity flow

Clumping can reduce continuum opacity in the wind

And non-isotropic clumping can also favor “sideways” escape, and thus suppression of the bluest and reddest photons, if the clumps are oblate

The Venetian Blind Model...

Zeta Puppis with Bo=400 G no radiative cooling

Initial after 2 days Zoom on final

Magnetically Confined Wind Shock (MCWS) Model

A dipole field plus a line-driven wind can generate a “magnetosphere” with a standing shock, and associated heating

Pup with Bo=2000 G

Zoom on densityDensityY- Velocity

-1000 vy (km/s) 1000

The Magnetically Confined Wind Shock model can

Redirect and confine the wind flow--if the magnetic field energy density exceeds the wind kinetic energy

But even if the magnetic energy density is less, the flow can be affected

This confinement can lead to strong shocks at the magnetic equator, heating gas that will be relatively stationary in the star’s frame

Conclusions

The prototypical O supergiant with a line-driven wind, z Pup, has X-ray line profiles consistent with optically thin line emission embedded in an optically thick wind -- the standard “wind shock model”

Later type O stars and early B stars have line profiles that are more symmetric and unshifted, and in some cases narrower

For magnetic hot stars, the MCWS model provides an intriguing possibility for explaining these profiles

Clumping may also play an important role