Pauline McGinnis
During the formation of a low mass star, it goes through a phase of evolution known as the T Tauri phase, characterized by a forming star, which is contracting due to its own gravity, surrounded by a circumstellar disk. Stars in this phase have strong magnetic fields which interact with the inner part of the disk, guiding material that is falling from the disk onto the star (a process known as accretion). This magnetic field may be misaligned from the axis of the star’s rotation by an angle called the magnetic obliquity. The magnetic obliquity has strong implications on the geometry of the accretion flow, the formation of warps in the inner accretion disk, and likely on the formation and migration of planets in this part of the disk. In this study, we infer magnetic obliquities for a sample of 11 T Tauri stars, by measuring Doppler shifts in the HeI emission line of these stars’ spectra. This line originates in a region on the stellar surface where the accreting material falls onto the star, heating the gas, creating what is known as an accretion shock (where HeI and several other emission lines are formed). This region closely coincides with the poles of the stellar magnetic field, therefore by measuring emission from this region we can infer the location of the magnetic poles on the stellar surface. For our sample, we find relatively small values of magnetic obliquity, between 5 and 23 degrees, meaning these stars’ magnetic fields are generally close to being aligned with their rotation axes. Combining our sample with other measurements from the literature, we find a tentative correlation between the magnetic obliquity and the structure of the star’s interior, suggesting that the stellar structure may be partly responsible for the geometry of a star’s magnetic field.
The magnetic obliquity is the angle of misalignment between the axis of the stellar magnetic field and the stellar rotation axis. This misalignment strongly affects the geometry of the accretion flow and can lead to the formation of warps in the inner accretion disk. To better understand the role of the magnetic obliquity in star formation, we need to measure it for a large number of T Tauri stars, then perform a statistical study in search of correlations between it and different parameters of the star/disk/accretion.
One way to measure the magnetic obliquity is from the Helium I emission line, which is formed in the accretion shock — the region where the accreting material impacts the stellar surface. These shocks are formed very close to the star’s magnetic poles, so by measuring their location on the star we can infer the magnetic obliquities. As the star rotates, the accretion shock at times moves towards our line-of-sight and at times moves away from our line-of-sight, leading to Doppler shifts in the radial velocity of the He I line. The amplitude of this shift is related to the latitude of the accretion shock and the star’s rotational velocity projected in our line-of-sight (the latter can be measured directly in a star’s spectrum).
For this work, 11 T Tauri stars were observed over approximately 8 nights with the SOPHIE high-resolution spectrograph (in France). Their Helium I line profiles show strong variability in intensity, as well as in radial velocity.
We show 4 examples of plots of the radial velocity of the Helium I line versus the stellar rotation phase. We take the amplitudes of these radial velocity plots to infer the magnetic obliquities. This sample of 11 T Tauri stars presents relatively small magnetic obliquities (at most 23 degrees), meaning that these stars' magnetic fields are not strongly misaligned with the stellar rotation axis.
Other studies in the literature show that large magnetic obliquities (>40 degrees) are not uncommon among accreting T Tauri stars. Joining our sample with the literature, we plot an HR diagram (a plot of a star’s luminosity versus its temperature) using a color gradient to represent the magnetic obliquity measured for each star. Stellar evolutionary models from Tognelli et al. (2011) are also plotted, showing the theoretical temporal evolution for stars of different masses. We also plot a line which represents the point at which a star will begin to develop a radiative core and no longer be fully convective.
We can see a tentative trend between the magnetic obliquity and the position of a star on the HR diagram, indicating that there may be a significant difference between the magnetic obliquities of stars that are fully convective and those that have developed a radiative core. We conclude that our sample may be biased towards small obliquities because most of our stars are still fully convective.