A confluence of astronomical techniques - imaging X-ray astronomy, polarization-sensitive radio interferometry, highly accurate optical spectroscopy and photometry, and laboratory analysis of meteoritic materials - persuasively show that high energy processes are prevalent in low-mass YSOs. The rapid heating and cooling of plasma to 107 -108 K and acceleration of particles to MeV energies is almost certainly not the product of hydrodynamical phenomena such as gravitational collapse and accretion. These violent phenomena must arise from efficient MHD processes such as solar-type magnetic reconnection flares. The X-ray manifestation of this enhanced magnetic activity is a ubiquitous characteristic of low mass YSOs, present from the Class I protostellar phase with ages of approximately 105 yr to stars approaching the ZAMS with ages of 107 yr. X-ray tracers of YSOs has led to substantial increases in YSO samples, which in turn provide new insights into the history of star formation in the solar neighborhood.
These findings show that the traditional hydrodynamic paradigm for understanding the earliest stages of stellar evolution is not complete. The most important astrophysical effect may be the ionization produced by flare X-rays which, unlike the HII regions of early-type stars, produces extended regions of partially ionized molecular gas. If X-ray emission begins in the earliest Class 0 phase, then YSO ionization may crucially affect the gravitational collapse of star formation. It is not clear today whether this occurs, as there is only one tentative X-ray detection of a Class 0/I YSO (CrA IRS7). Because X-ray emission is definitely prevalent in the Class I-II phases, X-ray ionization is quite likely to play a central role in the astrophysics and evolution of the circumstellar disk. One important effect is the induction of MHD turbulence and viscosity, thereby regulating accretion onto the star. X-ray ionization may also be necessary for the magnetic coupling of the central star to the inner disk, and coupling of the disk to magnetically accelerated and collimated outflows.
The energetic photons and particles produced in YSO flares may furthermore account for long-standing findings in meteorites which are quite difficult to explain in the quiet hydrodynamic model of the solar nebula. MeV protons, presumed to accompany the MeV electrons detected in YSO gyrosynchrotron radio flares, may account for excess particle tracks and spallogenic isotopes in meteorites. Flare shocks may melt chondrules.
In the near future, empirical investigations of `hot' energetic processes in `cold' YSOs are likely to thrive. The Chandra X-ray Observatory (CXO) and the X-ray Multimirror Mission (XMM), to be launched in 1999 and 2000, respectively, represent a considerable improvement in X-ray observational capability, particularly in the harder bands that are much less attenuated by foreground material. A number of problems raised in this review may be solved with these new observatories. For instance, CXO, with its subarcsecond resolution, will be able to detect thousands of T Tauri stars in the Orion giant molecular cloud. Combined with optical studies placing the stars on the Hertzsprung-Russell diagram (Hillenbrand 1997), one can disentangle Lx-mass and Lx-age correlations to study the causes of YSO magnetic activity (§3.2). The excellent high throughput of XMM will allow the detection of very faint and low-mass YSOs deeply embedded in molecular cores, including young brown dwarfs, and will provide direct measurements of extinction needed to obtain accurate X-ray luminosities. The search for X-rays from Class 0 protostars has particularly important astrophysical implications, such as deciding whether infall is braked by the onset of YSO ionizing radiation or whether Class 0 outflows have a gravitational or magnetic origin.
Proposed radio astronomical facilities like the Atacama Large Millimeter Array, Very Large Array upgrade and Square Kilometer Array would significantly boost the study of MeV particles in YSOs, as YSO fluxes are close to the sensitivity limits of current telescopes. Insight should emerge on the magnetic structures, disk gas chemistry and dust properties in magnetically active YSOs. The proliferation of 8-meter-class optical/infrared telescopes, with techniques such as Doppler imaging, high-resolution IR spectroscopy and interferometry, as well as planned infrared space missions such as SIRTF, will permit more accurate characterization of the magnetic properties of the central stars and the accretion process in YSO systems.
Finally, there are urgent needs for more theoretical and laboratory development of the issues discussed here. The concept of a magnetically-active but thermodynamically cold molecular disk linked to other structures within YSO systems must be investigated in greater detail. This is a challenging problem requiring time-dependent three-dimensional MHD calculations with reconnection, ionization and reheating. The effects of X-rays on circumstellar gas and disk, which so far have been studied primarily for disks, should be extended to protostellar envelopes. These efforts are closely related to the study of high energy effects on cold material around active galactic nuclei. There has also been inadequate study of the effects of flare shocks and energetic particles on solids. Further accelerator experiments with X-rays and particles should be actively pursued. The final evaluation of the astrophysical importance of energetic processes in YSOs awaits these future investigations.
ACKNOWLEDGEMENTS: The authors thank Philippe André, Claude Bertout, George Herbig, Eugene Levy, Roberto Pallavicini, Jean-Claude Pecker and Frank Shu for valuable comments on the manuscript. Lee Carkner and Nicholas Grosso kindly provided figures. We also benefited from discussions with many other colleagues, particularly at the 1998 Protostars and Planets IV conference. EDF was supported by NASA grants NAS8-38252 and NAG5-8422.