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

98 CHAPTER 7. OVERALL FEATURE OF X-RAY EMISSION FROM YSOSsamples (Stelzer et al., 2000), for which there was a significant difference between the distributionof flare < L X > of class II (CTTS) and class III+III c (WTTS) sources.Also, we see no significant difference of < L X > in the quiescent phase among all the classes.This is consistent with the previous estimation for this region with ROSAT derived by Grosso etal. (2000). They tried to estimate the significances rather strictly by considering the upper limitof undetected sources. Our samples give more severe constraint because the detection thresholdis largely reduced to give the mean luminosities of 10 29.5 –10 29.8 ergs s −1 , which is about 10 timeslower than that of ROSAT.7.3 Distribution of X-ray ParametersIn Figures 7.3–7.5, histograms of N H , < kT >, τ r , and τ d are shown for each class separately,together with the mean values and standard deviations. We exclude the samples whose best-fitparameters and/or those errors are not determined due to the limited statistics. For sources withmulti-temperature plasma (BF-46 and BF-96), we regard < kT > of the soft and hard componentsas the quiescent and flare values, respectively, following the discussion that the former wouldbe steady coronal emission while the latter is flare activity (§6.2). Like the case of L X , we estimatethe significance level for the differences of these parameters among all the classes with twononparametric two-sample tests in ASURV. The results are also given in Table 7.2.Absorption (N H )N H is distributed from 10 21.5 to 10 23.5 cm −2 (Figure 7.3). The mean value of N H ∼ 10 22.5 cm −2is roughly consistent with the average value through this cloud derived by the C 18 O radio observations(Tachihara et al., 2000). Also, the distribution of N H of class I sources is larger than thatof the others. Using ASURV, we estimate that the significance level of this tendency is rather highof 97 %. This supports the idea that X-rays from class I sources come near the central sourcedeeply embedded in denser envelopes and suffer larger intrinsic absorption.Plasma temperature (< kT >)Most of the < kT > values in the quiescent phase are in the range of 0.2–5 keV, while itbecomes systematically higher (1–10 keV) in the flare phase, indicating the plasma heating duringthe flare. Although the flare temperature of all classes is distributed around 3–4 keV, some flares ofclass I and II sources show higher temperature than 5 keV, while all flares of class III+III c sourceshave < kT > less than 5 keV. The mean values of < kT > during flares for class I and II (4–5 keV)is therefore larger than that of class III+III c (2.7 keV).From Table 7.2, we see that both the GW and logrank tests for the difference of < kT >