X-ray emission from protostars

Strong and flaring X-ray emission, 2 - 5 orders of magnitude above main sequence levels, is well-established for low-mass T Tauri stars (ages 10(6) - 10(7) yr). This has been attributed to enhanced solar-type magnetic activity. We summarize here recent reports for X-ray emission from earlier phases of star formation, corresponding to Class I/II (ages similar to 10(5) yr) and Class I (ages similar to 10(4) - 10(5) yr) infrared sources. These include XZ Tau in the Taurus L1551 cloud, the Coronet Cluster of infrared sources in the R Corona Australis cloud, and IRS 43 in the Ophiuchi cloud core. Data are obtained from the PSPC and HRI detectors on ROSAT, and the SIS and GIS detectors on ASCA. Two of the sources, R1 in CrA and IRS 43 in Oph, exhibited powerful flares during the observations. The presence of X-rays in protostars may involve non-solar-type magnetic flaring events involving the circumstellar disk. Protostellar X-rays should have a significant effect on circumstellar material, such as producing ionization for magnetic coupling of disks and outflows, and promoting disk accretion through photoionization.