Christopher M. Johns-Krull, Ph.D. Department of Physics & Astronomy - MS 108 Rice University, 6100 Main Street Houston, TX 77005 U.S.A.
Phone: (713) 348-3531 Fax: (713) 348-5143 email: cmj@rice.edu Office: Herman Brown 352
My research interests are quite varied, but all are related to the
astrophysics of lower mass stars, including the Sun.
Much of my work has been centered on trying to understand the (sometimes very extreme) variability found in Classical T Tauri stars. One particular Classical T Tauri star, SU Aur shows dramatic periodic variations in its H-alpha and H-beta emission lines, indicating that large amounts of material a few stellar radii away from this star are, nevertheless, tied to the stellar surface. Another Classical T Tauri star, Sz 68, shows weaker, but similar periodic variations in its H-alpha emission line. Unfortunately, another star we have studied in detail, DR Tau, does not show such clear cut periodic variations. The best guess for what ties the material there is very strong, large scale magnetic fields.
Along with studying the variability of the emission lines on T Tauri stars, I study the photospheric absorption lines to learn a variety of things about these stars. One of the most interesting avenues of this research is Doppler imaging the surface of the star to discover the location of the starspots that might be present on them. Sunspots were noticed by ancient Chinese astronomers, as early as 28 BC. Telescopic observations of sunspots have been performed on the solar surface since the time of Galileo. Sunspots are regions of the Sun's surface where very strong magnetic fields are emerging from the solar interior. It is believed that starspots trace the location of strong magnetic fields on the surface of other stars, so identifying their location on the surface of T Tauri stars provides clues to the nature of the magnetic fields believed to be responsible for controlling the winds and accretion flows around these stars. Recently, we have Doppler imaged the Classical T Tauri star Sz 68, and have found that the starspots on its surface are concentrated near the rotation pole. This fact indicates that most of the magnetic field emerging through the surface of Sz 68 is also concentrated at the rotation poles, which suggests that the overall magnetic field geometry on this star is Dipolar, such as that produced by a simple bar magnet. Current theories which attempt to explain the process by which disk material accretes onto Classical T Tauri stars all require the star to possess a strong, dipolar magnetic field and the Doppler image of Sz 68 suggests that this might in fact be true.
Strong stellar magnetic fields are now thought to play a crucial role in the interaction of Classical T Tauri stars with their surrounding accretion disks. The stellar magnetic field is believed to truncate the inner accretion disk, forcing material which accretes onto the star to follow the magnetic field lines to the stellar surface. It has even been suggested that the stellar magnetic field halts the migration of gas giant planets as they spiral through the disk toward the central star. This might be the reason that so many massive planets have been recently discovered very close to their central stars. Evidence for this picture is quite varied including the variability and Doppler imaging studies described above. However, if such strong magnetic fields do exist on T Tauri star, shouldn't we be able to measure them in some way? In fact, my advisor, Gibor Basri and his collaborators made the first measurements of a magnetic field on any T Tauri stars: the Naked (diskless) T Tauri star Tap 35. They measured an average field of somewhere between 500 and 1500 Gauss covering the surface of Tap 35, which is quite remarkable in contrast to the Sun where the average field is typically 1-4 Gauss (on the Earth, the average magnetic field is about 0.5 Gauss). To be fair to the Sun, the field is quite inhomogeneous, with the field strength ranging up to around 2500 Gauss in sunspots (which only cover a small fraction of the solar surface). Additionally, the Sun is not regarded as a very magnetically active star.
The large uncertainty of the magnetic field measurement for Tap 35 results from the fact that the measurements were made using optical spectra, and it turns out that the effect of the magnetic field on spectral lines in the optical is quite small, and hence difficult to measure precisely. There are two main ways to improve the measurement sensitivity to look for magnetic fields on T Tauri stars. One is to use spectra taken at infrared (IR) wavelengths where the effect of the magnetic field is stronger, or to look in circularly polarized light since magnetic fields are one of only a few physical processes which can produce circular polarization. Most of my recent research efforts have followed along these two lines, resulting in the first measurements of Zeeman broadening in a T Tauri star due to magnetic fields, as well as the first detection of circular polarization in the emission lines of Classical T Tauri stars. To find out more about this work, you can follow these links to the IR Zeeman broadening and the circular polarization observations.
I originally got started working on magnetic field by trying to measure them on low mass M class main sequence stars. Several years ago, Saar and Linsky (1985) used a medium spectral resolution (by today's standards) IR spectrometer on the KPNO 4-m Mayall telescope to measure a magnetic field of about 3800 Gauss covering about 73% of the surface of the flare star AD Leo (dM3e). If such strong pervasive fields are common among M stars, they should be relatively easy to measure at optical wavelengths using very high resolution spectrometers.
There is some theoretical support for the idea that M stars should have very strong magnetic fields. It turns out on the Sun that the magnetic field is in pressure equilibrium with the surrounding photospheric gas. Even though M stars are cooler than the Sun (about 3500 K compared to 5800 K), their atmospheres are more dense so that the atmospheric pressure is about a factor of 2 - 3 higher in dMe star than it is on the Sun. Thus, it is expected that the equilibrium field strength should be quite large on these cool, low mass stars. My collaborator, Jeff Valenti and I have made several observations of the magnetic field on a number of M dwarf flare stars. More information about this research can be found on my M dwarf page.
Perhaps the most spectacular manifestation of magnetic fields in the solar system is the production of solar flares. In the solar photosphere, most of the magnetic fields that emerge from the interior are too weak to have any affect on the motions of the gas around them (the exception is sunspots - that is related to why they appear dark). Therefore, the boiling motions of the Sun's convection zone pushes these magnetic fields around. As the magnetic fields in the corona get stretched and pushed around by the motions in the photosphere, they build up energy. Occasionally, this energy is released in a violent event called a solar flare.
Click here to see a solar flare in the H-alpha line.
Part of my work involves trying to understand exactly what happens during a solar flare. Flares produce a lot of very high energy X-rays, which appear to come from very energetic electrons that are accelerated during the flare. I am working with the software team on a new satellite, HESSI, which will take pictures of these X-rays in unprecedented detail in order to try and understand how flares occur. Go to my HESSI Software directory to see documentation on using the HESSI Imaging software as well as some documents related to software tests I have been working on.
For more information on this topic read the scientific articles referenced in publication list, or contact me, at cmj@rice.edu.
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