A star is born from a molecular cloud core and evolves to main sequence through the protostellar and T Tauri phases. With the ASCA satellite, we probed 17 fields (totally 24 observations) of nearby dark clouds to search X-rays from protostars. In this survey, we detected X-rays from 40% of protostar candidates with S/N > 5 level in the 2-10 keV band. They include Class 0 protostars, i.e. earliest phase of protostars, which had never been detected before.
Based on the results of the outflow observations, we found that the X-ray detection of a protostar is highly dependent on the orientation of the outflow; the X-rays can only be detected from a pole-on viewing angle, where the X-rays penetrate through lower density of the bipolar flow and are less absorbed by dense circumstellar disks. X-ray non-detected protostars have edge-on geometry, hence the X-rays are completely blocked by an extremely high column along the circumstellar disk.
From two Class I protostars (EL29 and YLW15 in ρ Oph), we detected totally four X-ray flares. Their temperatures and emission measures during the flares lie on a universal correlation from solar micro flares to stellar giant flares. From a Class I protostar WL 6, we obtained sinusoidal modulation with constant temperature, which may be due to a spin of a protostar. The period is apparently shorter than the spin periods of the older stages. However, the relation between the X-ray luminosities and V sin i, which are assumed from the period of WL 6, is similar to those in the older phase. These two results imply that at protostellar phase, X-rays in both the flare and quiescent states are emitted magnetically as well as the older phases of low-mass stars.
The flares from YLW 15 was quasi-periodic, which occurred every ~20 hours. It is consistent with a quasi-periodic reheating of the same plasma loop of ~14 Rsolar with diameter-to-length ratio α ~ 0.07. The inferred large-scale magnetic structure and the observed periodicity imply that the reheating events of the same magnetic loop originate in an interaction between the star and the disk due to the differential rotation. This result gives us a crucial information about structure of the central star and its close vicinity, which cannot be studied by any other wavelengths.
We detected an intermediate-mass protostar EC95, which exhibited time variability like contiguous flares with higher temperature than in typical T Tauri stars. Since the temperature and the emission measure is plotted on the same correlation with solar-type stars, our result would be the first support that even an intermediate-mass star has magnetic activity in the protostellar phase. They may have a convective zone inside the star itself or/and the inner disk, which has not been expected by any theory of stellar evolution.