Accreting magnetized stars, such as Classical T Tauri stars, cataclysmic variables, and millisecond X-ray pulsars show a wide variety of variability in their light curves and spectra. In some cases, exact periodicity is observed, while in other cases the light curves show stochastic fluctuations or quasi-periodic oscillations. Understanding these magnetized stars requires global 3D magnetohydrodynamic (MHD) modeling. Recent 3D and 2.5D MHD simulations performed by Dr. Romanova and her collaborators have shown a wide variety of possible paths of matter flow around magnetized stars, including the possibility of direct accretion through the Rayleigh-Taylor instability. These investigations have shown that magnetized stars may be either in the stable or unstable regime of accretion, with strikingly different observational properties. This and other interesting phenomena found in their MHD simulations may describe different properties or stages of evolution of different accreting magnetized stars.
However, to compare the results with observations, one needs to calculate continuum and line spectra from the stars and the surrounding matter. This requires high-level radiative transfer modeling, which is the primary goal of this project: to perform full 3D radiative transfer modeling, using the results of the 3D MHD simulations as a base. Results of such 3D (MHD) + 3D (radiative) models will be compared with observations of different Classical T Tauri stars for which spectral and photometric observational data are available. A number of new MHD simulations will be done aimed at understanding different phenomena around magnetized stars, including (1) accretion through instabilities; (2) modeling of outflows; (3) investigation of disk oscillations and warping generated by a misaligned dipole.
It is expected that this research will be an important new step in modeling accreting magnetized stars. Two modern state-of-the-art codes (3D MHD and radiative transfer) will be combined in a major effort to understand the physics of magnetized stars. The results will be valuable for understanding the whole range of accreting magnetized stars, from magnetic brown dwarfs to neutron stars. The results on disk oscillations and warping will be valuable for understanding processes around compact stars. The methods and numerical MHD codes developed here also have general value and are applicable in other areas of science such as planetary science, geophysics, earth magnetospheric science and heliospheric science, and in engineering.
The project is ideal for training young scientists - graduate and undergraduate students - who will learn magnetohydrodynamics and programming, how to parallelize the codes, and to write auxiliary programs. The project will also support a public exhibition at the Ithaca Sciencenter on the birth of stars featuring the roles of accretion and plasma physics.
Many newly born stars have a strong magnetic field, which is about 1000 times stronger than the magnetic field of the Sun. They show variability in their brightness and spectrum because the strong magnetic field governs the flow of matter around these stars, and matter flows to the star in two or multiple funnel streams. The magnetic field can be complex (non-dipolar) as well, leading to a correspondingly complex matter flow. We have developed a powerful numerical code which calculates how matter flows around stars with complex magnetic fields. Figure 1 shows an example of such a flow around a star with a superposition of dipole and quadrupole fields. Our numerical model is like a `numerical telescope', using which we can see young stars on a computer screen. It would be impossible to see young stars as closely even with the most powerful modern telescopes. We were able to model matter flow around some stars whose magnetic field structure has been measured by special telescopes. Figure 2 shows an example of such a calculated model for the star V2129 Oph. This is a very young star (2 to 3 million years old), which is a much younger version of the Sun. It is located in an active star forming cloud near us (about 400 light years away). However, we need to test the model of matter flow and to compare it somehow with observations. Hence, the simulation results must somehow be translated into the light that astronomers can see through their telescopes. We have developed such a tool (a radiative transfer model) which can predict the light emitted not only by hydrogen, but also by helium atoms. The tool is constructed for a general purpose, and it can be used for many different astronomical objects. We calculated the spectrum emitted by hydrogen atoms for different rotational phases of the star V2129 Oph which rotates about once every 6 days. The results have been compared with the observational data obtained by telescopes in Hawaii, Chile, Italy and Uzbekistan. We found that our model qualitatively agrees with the observations in many aspects, and the gas flow geometry predicted by our simulation (shown in Figure 2) is close to what is really happening around the young star, V2129 Oph. Understanding the gas accretion process in these types of stars is important since the Sun should have experienced a similar process when it was young. A number of theorists and observers were involved with this project. Our Research Associate Ryuichi Kurosawa worked hard on developing the new radiative transfer code Torus. Graduate students Akshay Kulkarni and Min Long worked on modeling matter flow around magnetized stars. Mathematicians from Russia and observers from all over the world participated in this project. We presented results at various scientific meetings, both in the US and abroad. We also gave many lectures for undergraduate students. We have a special Plasma Astrophysics seminar where young scientists learn the complex subject of Plasma Astrophysics. The Ithaca Science Museum has an exibit "Mars and Stars" where everyone can learn about young stars and how they shine (or hide) in star-formation clouds. The exhibit has been initiated and supported by our group. Our graduate student Akshay Kulkarni developed a set of tools to create stereoscopic images and animations of the modeled stars. Figures 4-6 show a few images of stars with the dipole and complex magnetic fields. You need red and blue glasses to view these images. We have also made a web page where you can see stereoscopic animations of the modeled stars: www.astro.cornell.edu/us-rus/stereo.htm
