The Application of Forbidden Line X-Ray Diagnostics to the

Hot Star Tau ScoAuthor: Geneviève de Messières

Swarthmore College ‘04

Advised by: David CohenSwarthmore College

In Collaboration with: Joseph MacFarlane, Prism Computational Sciences

Carolin Cardamone, Wellesley College ‘02Stanley Owocki, University of DelawareAsif Ud-Doula, University of Delaware

Presented at the Keck Northeast Astronomy Consortium, November 3, 2001

• 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.

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.

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.

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.

The relationship of velocity and density in the previous time-height

simulation.

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 Chandra telescope yields unprecedented

spectral resolution.

ROSAT (1993) spectrum of tau Sco

Chandra (2000) spectrum.

How can we study the processes occurring

on tau Sco?

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.

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.

In the presence of enough UV radiation, the forbidden line can disappear.

The oxygen VII helium-like triplet.

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.

The silicon XIII helium-like triplet.

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.

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

Assuming reasonable densities, the effects of the UV field dominate and indicate a radius from

the star at which emission is taking place.

1E10 1E11 1E12 1E13 1E14 1E151E-3

0.01

0.1

1

10

T = 6 x 106 K

He-like Ne Ratios for Sco

r / RS = 20

r / RS = 10

r / RS = 5

r / RS = 3

r / RS = 2

r / RS = 1.5

r / RS = 1.2

r / RS = 1.1

r / RS = 1

Ra

tio (

fo

rbid

de

n /

inte

rco

mb

ina

tion

)

Electron Density (cm-3)

F line destruction simulations of neon IX

Observed range for tau Sco Ion Range of radii (r/R)

oxygen VII ~5 - 10neon IX 2.2 - 3magnesium XI 1.8 - 2.5silicon XIII 1.1 - 1.5

Results of the simulations

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.

Y- Velocity

-1000 vy (km/s) 1000

Density

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.