Research of X-ray Emission from Young Stellar Objects using ASCA

Hideki Ozawa

To investigate X-ray emission from protostars and T Tauri stars, we observed two star forming regions, IC348 and L1630 with the 4th Japanese X-ray astronomy satellite, ASCA. The young stellar cluster IC348, we detected twenty X-ray sources, which are probably classified as ClassII(CTTS) and ClassIII(WTTS) sources. Among these twenty sources we obtained spectra from 10 sources with reasonably good statistics as well as a high statistics spectrum from the core region. Temperatures of these sources derived from the spectral fits are in the range of 2.0-5.7 keV, which are systematically larger than the average temperatures of ~1 keV measured with the ROSAT ovservations. We investigated this contradiction in details by adopting a two-temperature plasma model and revealed that the X-ray emission from T Tauri stars in IC348 can be interpreted by multi-tmperature plasmas.

During the ASCA observation,we detected five X-ray flares each from different X-ray sources in IC348. The largest flare among them exhibited an interesting feature that the hard X-ray flare in 2.0-7.0 keV band rose faster than the soft X-ray flare in 0.7-2.0 keV band. Combining the flare light curves with the spectral evolution of the flare, we concluded that the temperature reached to the peak earlier than the emission measure along the progress of the flare. This time lag between the temperature peak and the peak lags are often observed in solar flares. From the flare decay time analysis, we estimated the size of this magnetic loop that was responsible to the flare to be of the order of several solar radii.

We detected 7 X-ray sources from the analysis of the HH24-HH26 region of L1630 cloud in Orion observed with ASCA. One of the X-ray sources was identified with the position where two ClassI protostars SSV63E and SSV63W are located, however, the ASCA image resolution does not allow us to resolve either of which is responsible for the X-ray emission (hence we denote this X-ray source as SSV63E+W). The spectrum fo SSV63E+W is well explained by an optically thin thermal plasma model with a high temperature of kT=5.0(3.3-7.9)keV and a heavy absorption of NH=1.5(1.2-1.8)*1023cm-2, suggesting that it is embedded deep in the surrounding dusts. A significantly large X-ray flare was detected from SSV63E+W during the present ASCA observation with a peak flux of about 9 times that of the quiescent flux. The 0.5-10 keV luminosity of SSV63E+W was about 1*1032erg s-1 in the quiescent flux.

These results are compared with the properties of solar flares. Then we interpreted these results in the context of (1) generation and evolution of magnetic loop originated by the differential rotation of a star and the surrounding gas disk, and (2) plasma heating and propagation that follow the reconnection of evolved magnetic loops.