Introduction
Stars form in cold molecular clouds (T~10K) which collapse into protostars (surface T~10³ K) surrounded by protoplanetary disks (T~10²). Eric Feigelson and his colleagues were leaders in the unexpected discoveriy that yount stellar systems also produce gas with T~10⁶-10⁸ K and energetic particles with MeV energies.  Today, the phenomenology of X-ray and radio emission from pre-main sequence stars is very rich. But the fraction of energy entering kev and MeV processes is still very small, so it is unclear whether they are incidental or important to the astrophysics of star formation and early stellar evolution.  Our achievements during the 1980-90s include:

The early years
We had a number of notable achievements during the 1980-90s.  We were the first to:  detect their variable X-ray emission (1981), report X-ray-discovered weak-lined T Tauri (WTT) stars (1981), encounter a powerful WTT radio continuum flare (1985), uncover VLBI-scale redio structure in active WTT stars (1991), organize a multiwavelength campaign on a flaring WTT star (1994 , detect X-rays from protostars (1996), detect circularly polarized radio emission from a protostar (1997), discover the nearest young stellar cluster found in a century using X-ray imagery (1999).  Several Penn State graduate and undergraduate students participated in these efforts, including first-authored papers by Pete Stine, Doug O'Neal, Jim Leous, Lee Carkner and Eric Mamajek. These findings, and those of many other researchers in the field, are brought together in a 1999 Annual Reviews of Astronomy & Astrophysics article entitled High energy processes in young stellar objects by Feigelson & Montmerle.

The Chandra years
In mid-1999, NASA Great Observatory for X-ray astronomy was launched with the Advanced CCD Imaging Spectrometer developed by Penn State and MIT.  Evan Pugh Professor Gordon Garmire is PI of the ACIS instrument, and Eric Feigelson serves on the ACIS Team specializing in issues relating to young stars.  A Chandra image of an active star formation region will often show hundreds of X-ray emitting stars ranging from OB stars to pre-main sequence brown dwarfs, from Class I protostars (age ~10⁵ yrs) to post-T Tauri stars (age ~ 10⁷ yrs).  The X-ray populations are comparable to those in the latest infrared images, and can  penetrate extraordinarily deeply into the cloud (A_V ~ 500 mags).   

Eric has particularly emphasized study of the Orion Nebula where ~2000 X-ray stars appear in a single field, including both the full initial mass function of ~1 Myr stars and deeply embedded populations associated with the BN/KL and other molecular cores (2000, 2002a, 2002b, 2003)  He is leading the Chandra Orion Ultradeep Project (COUP) which is analyzing and interpreting a 10-day near-continuous Chandra ACIS observation made in January 2003.  Kosta Getman led the huge data analysis effort which uncovered 1616 X-ray sources (2004).  A Special Issue of the Astrophysical Journal Supplement devoted to the initial COUP studies is planned for early 2005.  He also studies older, dispersed pre-main sequence stellar populations in collaboration with Warrick Lawson of UNSW.  Here are some of the issues addressed by these efforts:
 

• Origins of pre-main sequence magnetic activity  The enhanced magnetic activity of T Tauri stars is largely consistent with stellar activity seen in the Sun and other late-type stars where magnetic fields are generated in a dynamo driven by the differential rotation in the stellar interior.  However, our Chandra data show that the expected link between activity and surface rotation, which is the principal correlate of X-ray emission in older main sequence stars, is completely absent in pre-main sequence stars (2003).  Instead, correlations between X-ray emission and stellar mass, size/volume and bolometric luminosity are present.  The reasons are not clear: either the solar-type alpha-Omega magnetic dynamo is fully saturated, or a new distributed turbulent dynamo (widelly predicted to be present in fully-convective stars) is operative.

• Evolution of stellar magnetic activity  From two complementary long Chandra observations, the COUP study of the Orion Nebula Cluster and the CDF-N study of a high-Galactic latitude field, we have improved our knowledge of the decay of magnetic activity in late-type stars (2005).  A monotonic decay is now traced from 10⁶ to 10¹⁰ yrs, although the simple link between activity and rotational decay may not be the only cause.  
  

• X-ray effects on the circumstellar environment     Theoretical calculations show that young stellar X-rays are likely to be the dominant source of ionization in the vicinity of pre main sequence stars.  See our two reviews on this fascinating topic (2000, 2005).  Ionization of the circumstellar disk may have critical effects on disk dynamics (promoting MHD turbulence and viscosity) and on coupling between the disk and bipolar outflows.   Less clearly, the X-ray Dissociation Regions around embedded stars may be sufficiently extended to affect ambipolar diffuse and future star formation in molecular clouds.  The violent magnetic reconnection events producing X-ray flares will also accelerate particles to MeV energies.  These may impact the disk, produce radioactive nuclides in disk solids via spallation reactions, and thereby account for the anomalous abundances of short-lived radionuclides in CAI inclusions of carbonaceous chondritic meteorities which give direct information about condition in the protoplanetary disk that gave rise to our solar system (2002a).

• The fate of OB stellar winds    For 25 years, theorists have discussed whether the massive high-velocity winds from the highest mass stars would slam against surrounding molecular material producing an X-ray emitting shock.  The issue is critical for understanding HII regions around young OB clusters, as the standard theory of photoionized nebulae omits the effects of wind kinetic energy.  Our Chandra studies of high-mass star forming regions, led by Leisa Townsley, have clearly discovered the predicted shock plasma.  We find that most of the HII region volume is filled with 10 MK gas, but most of the wind energy and mass flows unimpeded away from the cloud into the Galactic interstellar medium (2003).  These X-ray flows are small-scale analogies of the Galactic winds of starburst galaxies.   A major Chandra survey of high-mass star forming regions across the Galaxy is underway.  

• X-rays from Herbig-Haro jets    Collimated bipolar outflows are a ubiquitous feature of protostars, probably arising from magnetic interactions between the accreting star and the inner disk.  Material is quickly accelerated to 100-500 km/s velocities, producing the well-known low-excitation emission line Herbig-Haro objects, and then entrains surrounding materials to produce the slower bipolar outflows seen in mm molecular lines.  We have found weak X-ray emission from the terminal shock of one HH object (2001) and from shocks at the base of another HH object (2003).

• Discovering 10 Myr old stars and disks    While the earliest stages of stellar evolution are now readily studied in molecular clouds, it has proved difficult to establish quality samples of somewhat older pre-main sequence stars that have dispersed into the Galaxy.   X-ray surveys are the most effective tool for finding these post-T Tauri stars.  Using the ROSAT and Chandra satellites, we found two nearby, poor but compact groups of pre-main sequence stars:  the eta Cha cluster (1999) and the quintet around HD 104237 (2003b).  Both of these clusters,  like the TW Hya Association and beta Pic moving group, are kinematically consistent with membership in the large Sco-Cen OB association (2000, 2001).   The existence of young stars dispersed over such a wide area is explained by a model in which stars inherit velocities from supersonic turbulent gas motions seen in giant  molecular clouds (1996).  The eta Cha cluster is now the most complete sample of coeval 9+/-1 Myr stars ranging from spectral types B8 to M5 (2001).  Infrared and optical study shows that 2/3 of them still possess infrared excess disks and 1/3 are still accreting at a low level (2003a) .  We are now in the process of identifying other ~10 Myr-old stars for studies of planet formation.