1. Post Processing of ZEUS MHD Simulations of Young, Hot Stars Stephen V. St.Vincent and David H. Cohen Swarthmore College Department of Physics & Astronomy Magnetic field lines quickly become stretched from their original configuration (above) to a new and constantly changing configuration (below). Introduction/Background Visualization and Dynamics Post-Processing Young, hot stars are very luminous and are strong sources of X-rays (very short- wavelength, or very blue, light). Recent detections of strong magnetic fields on luminous hot stars such as Θ 1 Ori C may help to explain the source of some of these X-rays. Schematic of θ 1 Ori C. From Earth, we get many different views of this star as the star rotates. This is due to the fact that the magnetic and rotational axes are offset from one another. This diagram shows the relative position of the observer (at α) relative to the magnetic axis (solid vertical line). The values signify the angle of the axis to our line- of-sight. The dashed line represents the magnetic equator, while the curved solid lines represent magnetic field lines that have been warped due to the stellar wind. Hot stars have strong stellar winds (very fast outflows of material from the star’s surface). Our group studies how the magnetic fields of the stars interact with these strong stellar winds. Namely, this interaction occurs via a mechanism called Magnetically-Channeled Wind-Shock, or MCWS. In this model, the strong magnetic fields channel the high-velocity winds to the equator from each pole, where the two fronts of material collide. This causes shock-heating of the material, which can bring it to temperatures high enough to produce X-rays. As an example, typical speeds of channeled winds are around 2000 km/s. We can relate this kinetic energy to thermal energy with the following equation: Using v=2000 km/s, the Boltzmann constant k, and the mass of a hydrogen atom for m H and solving for T, we get a temperature in the area of 10 8 K. As a general rule, any material hotter than 10 6 K is capable of emitting X-rays. Our colleagues have created computer simulations to model θ 1 Ori C and other stars. The simulations run through time-evolution of the star, and often represent thousands of kiloseconds of the star’s simulated life. One important aspect of this project was creating a way to visualize the results of these simulations. Below are color contour plots of temperature, material density, speed, and a representation of the magnetic field, all of which are data that are included in the output files of the simulations. In all of these images, the small white circle is the star itself, while the remainder of the image is the surrounding stellar wind. These contour plots show how the temperature at different areas around the star change with time. Below, large amounts of material are being ejected from of the star. Θ 1 Ori C is the brightest star in the Trapezium (the four stars shown on the right) which make up the middle star in Orion’s sword (above). The right-hand picture is of the famous Orion Nebula, an active star-forming region also known as M42. The images directly above and below are density contour plots that correspond to the same times as the temperature contours above. The material may be hot, but its density is low. The images above and below show the speed of the material in the stellar wind, with times corresponding to the temperature contour plots. As you can see, the hottest material is moving the most slowly. We produced some diagnostics by manipulating the data output from the simulations. Many of these synthesized observables can be compared to actual data. Below are examples of a few of these synthesized observables that we created. Conclusions We have found the synthetic data from the simulations that use the Magnetically- Channeled Wind-Shock Model (MCWS) to be in good agreement with what is actually observed with respect to θ 1 Ori C. It is important to remember that this simulation is still incomplete since it is only a two-dimensional simulation; however, that being said, the extrapolation into three dimensions performed by this project appears to indicate that the two-dimensional simulation is in fact a good approximation for a fully-three-dimensional model, and that it matches the data well. Line-of-Sight Velocity. These contour plots show the speed at which the material is moving towards or away from the observer, who is positioned in the top center of the image. Blue corresponds to material moving towards the observer (which would be blue- shifted), while red corresponds to material moving away from the observer (red-shifted). Black contours indicate material hotter than 10 6 K. Line Profiles. This diagnostic shows the relative blue- or red-shift of an emission line the stellar spectrum. These lines are very narrow, suggesting that most of the hottest material is moving very slowly, as is shown in the contours above. The observed lines are about as narrow as these models predict. Emission Measure. This is proportional to the amount of light emitted from a region. The equation for EM is: Here, n e is the number of electrons and n H is the number of hydrogen atoms in a given volume element. EM vs. Temperature Histogram. This diagnostic bins the emission measure as a function of temperature. Since plasma of a certain T tends to give off a particular wavelength of light, this helps show how much X-ray radiation will be emitted. The bulk of the material (at log(T)=4.66) has been omitted from this graph.