INTRODUCTION

The formation and early evolution of low-mass stars is generally discussed in terms of gravitational and hydrodynamical processes. A molecular cloud core collapses, a protostar emerges at the center while high angular momentum material forms a circumstellar accretion disk, bipolar outflows are produced, the star becomes visible, contracting quasi-statically along the Hayashi and radiative tracks in the T Tauri phase to the Zero Age Main Sequence. Magnetic fields are believed to play a central role in regulating collapse through ambipolar diffusion and transferring disk orbital motion to collimated outflows. Coupling between the neutral material and magnetic fields is thought to be provided by a 2#2 fractional ionization produced by low-energy galactic cosmic ray interaction with the molecular gas. Ancient meteorites show indications that additional high energy processes affected the solar nebula, but their origins are uncertain and they are often omitted in astrophysical discussions of star formation and YSOs.

However, empirical studies of YSOs in the 1980s and 1990s provide direct and ample evidence for keV-energy radiation and MeV particles produced within protostellar and T Tauri systems. The X-ray emission is understood to be thermal emission from gas rapidly heated to temperatures around 10⁷ K by violent magnetohydrodynamical reconnection events, analogous to solar magnetic flaring but elevated by factors of 10¹-10⁶ above levels seen on the contemporary Sun. Other indicators of strong magnetic activity include large starspots revealed by optical photometry, an enhanced chromosphere and Zeeman effects seen in optical and ultraviolet spectroscopy, and powerful nonthermal radio continuum flares.

These results reveal a deep relationship between high-energy phenomena observed in YSOs, the Sun, and other magnetically active late-type stars such as dMe flare stars and RS CVn binary systems. The Sun has been known to be an X-ray emitter for 50 years, and the X-ray emission of the solar surface is now monitored in great spatial and spectral detail. Images from solar X-ray satellites show hot plasma largely confined within magnetic loops, which can be suddenly heated by magnetic reconnection events (flares), often accompanied by ejection of substantial amounts of matter (coronal mass ejections and solar energetic particles). Circularly polarized gyrosynchrotron emission at radio wavelengths indicate the acceleration of electrons to mildly relativistic energies, and nuclear gamma-ray lines are produced by spallation reactions of energetic protons in the photosphere. All of these phenomena are observed or inferred to be present, in YSOs at elevated levels.

These observations raise many astrophysical issues. The magnetic activity in YSOs may arise, as in the Sun, from a magnetic dynamo generated in the deep convection zones of the stellar interior. But the magnetic field topologies cannot be easily predicted or deduced from activity tracers. The circumstellar material of YSOs may allow unusual magnetic configurations such as star-disk, star-envelope or disk-disk fields which are not seen in older cool stars. Furthermore, magnetic activity may provide a crucial astrophysical link between the central star, circumstellar envelope, disk, and bipolar ejecta in YSOs.

X-ray ionization of circumstellar material will typically dominate galactic cosmic ray ionization on scales up to 3#3 AU, and thus should be a major player in the dynamics and evolution of protoplanetary disks. The integrated effects of particle irradiation from YSO flaring may account for a variety of characteristics in the meteoritic record of the solar nebula. Finally, YSO magnetic activity also leads to insights regarding the population of young stars. X-ray surveys of large areas in the sky considerably enlarge the census of YSOs both in star-forming regions and away from them, addressing such diverse problems as the star formation histories of molecular clouds, the longevity of circumstellar disks, and the kinematic dispersal of young stars into the Galaxy.

After covering some background material on YSOs (§2), this review describes the evidence for YSO magnetic activity revealed in multiwavelength studies (§3). Astrophysical models of YSO magnetic fields are outlined in §4, followed by discussion of high energy irradiation of interstellar and circumstellar material (§5). Section 6 reviews magnetic activity as a tracer of YSO populations, and section 7 gives concluding remarks.