X-rays and their implications for

star formation, young stars and protoplanetary disks


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 (AV ~ 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 (20002002a, 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: