X-ray Study of Low-mass Young Stellar Objects in the ρ Ophiuchi ...

8.6. EVOLUTION OF YSOS AND THEIR FLARE ACTIVITY 1158.6 Evolution of YSOs and Their Flare ActivityCombining all the results discussed in the previous sections, we propose a simple view of theevolution of flare activity on low-mass objects as follows. In their earlier stage (class I), sources haverelatively strong magnetic field (≈500 G) and show frequent X-ray flares with higher temperature(≈5 keV). As stars evolve (class II and IIIs), the magnetic field gradually decreases (200–300 G)and makes moderate temperature plasma (2–4 keV) via X-ray flares. During these phases (classI–III), the length of the flare loop does not change significantly (10 10 –10 11 cm). As approachingto the main-sequence stage, the magnetic field and length of the flare loop are becoming weak(50–150 G for the sun) and short (10 8 –10 9 cm), which causes lower plasma temperature (0.1–1keV) and shorter flare timescales (10–100 s) as seen in the sun.B(G)PMSMSlog[L(cm)]1000binary?RS CVn1110single9100sun84 5 6 7 8log[Age(yr)]Fig. 8.6.— Schematic view of the evolution of X-ray flares of low-mass objects. Thick solid anddashed lines represent the predicted evolution of the magnetic field strength (B) and loop length(L), respectively.We should note that the flare from the class 0 candidate A-29 (§6.5) has relatively short rise anddecay timescales (1–2 ks). Although we could not plot the data of the A-29 flare in the < kT >–τ rplane because < kT > is not determined due to the limited statistics, the small τ r value predictsthat A-29 has a large magnetic field; if < kT > is larger than 10 keV, the expected B value is1000 G, which may be the initial value of magnetic field in the YSO evolution.Generally, RS CVn systems show smaller flare timescales than YSOs (∼100 s), while < kT >is comparable (∼5 keV). Based on the above idea, this may be due to larger B value. In fact,