The Evolution of Magnetic Field Strengths in Low-mass Young Stars

Abstract

Magnetic fields have a significant impact on the internal structure and atmospheric properties of low-mass stars. In particular, during the T Tauri phase, magnetic fields play a fundamental role in the star-disk interaction, in the stellar mass accretion process, and in regulating the angular momentum evolution of the system. Although magnetic fields have been measured for some T Tauri stars, little is known about the magnetic fields in protostars and older pre-main sequence sources. In this thesis, I present the hitherto largest and most comprehensive study of surface magnetic field strength of low-mass young stellar sources. I used iSHELL, a high-spectral resolution R~50,000 near-infrared spectrograph at IRTF to observe over 100 young stars in the K-band from different star-forming regions and young associations. Combining high-quality observations with a detailed magnetic radiative transfer code, I derive magnetic field strengths, temperatures, gravities, infrared veiling, and projected rotational velocities for 107 young sources with ages of <0.5 Myr to over 100 Myr and with masses between ~0.3 M_sun and ~1.3 M_sun. In this work, I performed the first survey of magnetic field strength in Class I and Flat Spectrum sources. I found that the magnetic field strength of Class I sources ranges from 0.5 kG to 4.1 kG with a median strength of 1.7 kG. The distribution of magnetic fields for the Class I sources is statistically indistinguishable from the magnetic fields of the Class II sources (or Classical T Tauri stars). Thus, no evolution in magnetic field strength is detected between the two classes. I also found that the gravities of Class I and II sources are statistically different, although a significant overlap exists. When combined with stellar evolutionary models, these results mean that about half of the Class I sources have ages of < 0.6 Myr and are likely in the protostellar phase, while the other half of Class I sources have gravities and ages consistent with Class II sources (or T Tauri stars). In a separate study, I discovered that T Tauri North is not in the same evolutionary stage as most T Tauri stars. Instead, its lower gravity, and thus earlier age <0.6 Myr, suggest that the iconic T Tau N source is a protostar ejected from the embedded southern binary companion shortly after its formation. In a series of studies, I discovered that infrared temperatures of Class II sources are almost always lower than their optical temperatures. Moreover, the observed temperature differences correlate with the magnetic field strengths of the stars and increase for hotter sources. I attribute this phenomenon to magnetically induced spots on the surface of the highly magnetic young stars. Since low-mass young stars contract isothermally as they descend the Hayashi track, an almost one-to-one correlation between temperature and stellar mass can be established. The discovery of an optical-infrared temperature difference necessarily implies that masses derived from optical temperatures are almost always higher than masses derived from infrared observations. By using independent mass measurements for a sub-sample of source, I found that K-band infrared temperatures produce more precise and accurate stellar masses than optical temperatures when combined with magnetic stellar evolutionary models. Analyzing the full sample of 107 young sources, I found that the magnetic field strength of sources less massive than M_star ~ 0.9 M_sun remains strong at a ~2 kG level during the first ~100 Myr. However, stars more massive than M_star ~ 0.9 M_sun often have magnetic field strengths below our detection limit of ~0.3 kG by the age of ~100 Myr. This suggests a change in the magnetic dynamo operating in these stars. Furthermore, by placing the stars in the theoretical HR diagram and overplotting stellar interior models, I found that the development of a radiative core has no effect on the measured magnetic field strength of the stars. The only substantial change in the magnetic field strength of the stars occurs when the convective layers in the stars thin to less than ~35 % in radial depth or below ~10% in stellar mass.