The Application of Forbidden Line X-Ray Diagnostics to the Hot Star Tau Sco

1. The Application of Forbidden Line X-Ray Diagnostics to the Hot Star Tau Sco Author: Geneviève de Messières Swarthmore College ‘04 Advised by: David Cohen Swarthmore College In Collaboration with: Joseph MacFarlane, Prism Computational Sciences Carolin Cardamone, Wellesley College ‘02 Stanley Owocki, University of Delaware Asif Ud-Doula, University of Delaware Presented at the Keck Northeast Astronomy Consortium, November 3, 2001

2. The processes by which hot stars emit X-rays are not yet fully understood. While dimmer stars like the Sun generate X-rays through magnetic confinement of the corona, it is generally thought that the X-rays from hot stars are created in radiatively driven stellar wind shocks. • Using a high-resolution spectrum of the B-type star tau Scorpii from the telescope Chandra, we have studied the strength of the ultraviolet field at the location of the X-ray-emitting plasma by examining the forbidden and intercombination lines of helium-like elements in the plasma. • A stronger UV field, close to the surface of the star, destroys the forbidden line in favor of the intercombination line, so the diagnostic can indicate whether the generation of X-rays is occuring close to the star or far away.

3. Magnetic confinement in the corona causes regions of hot, dense material. This is one way to generate X-rays. However, hot stars are typically thought to not have magnetic fields. Coronal loops on the surface of the Sun.

4. The radiation pressure from luminous stars accelerates the stellar wind to high speeds. Eta Carina is hidden by the nebula created by its stellar wind.

5. This acceleration is not uniform. Fast shells of the wind crash into slower regions in a typical shock-driven wind. Time-height simulation of an O-type star.

6. The collision of fast and slow shells of the stellar wind results in dense, hot X-ray emitting regions in the radiatively driven wind shock model. The relationship of velocity and density in the previous time-height simulation.

7. The Chandra telescope yields unprecedented spectral resolution. How can we study the processes occurring on tau Sco? ROSAT (1993) spectrum of tau Sco Chandra (2000) spectrum.

8. The sizes and shapes of the lines can be resolved, distinguishing even closely spaced groups. The magnesium XI helium-like triplet, fitted with gaussian models using the CIAO software package.

9. The strength ratio of the forbidden to intercombination line indicates the strength of the UV field.* In a strong UV field, electrons are often excited out of the long-lived upper level of the forbidden line before they spontaneously de-excite, weakening the forbidden line. * If electron densities are high enough, collisional excitation will destroy the forbidden line in the same manner. However, the effects of the UV field are likely to dominate.

10. In the presence of enough UV radiation, the forbidden line can disappear. The oxygen VII helium-like triplet.

11. My work has primarily been to identify the F/I ratio for each helium-like element present in the spectrum by fitting models to the spectral data. The silicon XIII helium-like triplet.

12. The silicon XIII helium-like triplet.

13. The neon IX helium-like triplet. Nearby iron lines interfered with the data and had to be fitted separately to be eliminated from the fit of the neon lines.

14. Basic properties of tau Sco: • B0 V • Teff = 31,400 K • L = 4.69 LSun • Mass loss = 3.1 x 10-8 MSun yr-1 • v∞ = 2400 km s-1

15. Assuming reasonable densities, the effects of the UV field dominate and indicate a radius from the star at which emission is taking place. Results of the simulations Observed range for tau Sco F line destruction simulations of neon IX

16. Results of the Diagnostic • The radius of X-ray emission appears to be at about 1.5 - 3 R*. This is closer to the surface than expected for a normal stellar wind but too far for normal coronal activity. • From Carolin Cardamone’s research, we see that the lines are slightly broadened, but indicate a velocity no greater than 200-300 km s-1. This is much less than the wind’s terminal velocity. • How can we interpret this? Tau Sco is an unusually young star, and it could retain a primordial magnetic field.

17. Density Y- Velocity -1000 vy (km/s) 1000 A large-scale magnetic field might channel ionized wind material toward the magnetic equator, where it would crash into other material, generating X-rays. This would explain both the moderate distance from the star seen in this research and the slow wind velocities discussed by Carolin in her talk.