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

104 CHAPTER 7. OVERALL FEATURE OF X-RAY EMISSION FROM YSOSstar-forming region IC348 (< L X >∝ M 2 : Preibisch & Zinnecker, 2002). Since the stellar mass isroughly as a function of L bol (Figure 2.1), this is basically the same as the constant < L X > /L bolrelation (§7.4.2).Fig. 7.8.— Relation between the stellar mass and X-ray luminosity (< L X >) for (left) quiescentand (right) flare phases. Symbols are the same as Figure 7.6. Dashed lines are the < L X >–massrelation derived for X-ray sources in IC 348 (Preibisch & Zinnecker, 2002). The position of themagnetic B star S1 (= A-41) is indicated in the figure.7.4.4 < L X > vs L RFigure 7.9 shows the relation between < L X > and monochromatic radio luminosity at 1.5 GHz(L R ergs s −1 Hz −1 ) for sources detected at this wavelengths (André et al., 1987; Stine et al., 1988;Leous et al., 1991, see column 3 in Table 5.3). Possible log-linear correlation can be seen for boththe quiescent and flare X-rays except for A-75/BF-11; most sources have log(< L X > /L R ) of 13.5–15.5, which is almost comparable to those of MS stars (Güdel & Benz, 1993). Although it is notclear that cm emission of the ρ Oph sources is thermal or non-thermal, the < L X > vs L R relationindicates that the majority of its origin is basically the same as that of X-rays. One plausible ideais that accelerated non-thermal electrons (see §2.3.1) emit synchrotron radio continuum, then heatambient matter which gives rise to the thermal X-ray flux. We also note that the background AGNcandidate LFAM21 (A-75 = BF-11, §6.6.3) has much lower log(< L X > /L R ) of ∼11 than theothers. This is the typical value of AGNs (Smith et al., 1999), hence further supports its AGNnature. On the other hand, other unidentified sources detected at the cm wavelengths (A-4, A-31,A-37, and BF-37 2 ) have large < L X > /L R comparable to YSOs and MSs. We therefore expectthat these are possible candidates of new YSOs, as already proposed for A-31 (§6.5).2 BF-37 is not plotted in Figure 7.9 because we can not determine < L X > due to the limited statistics. However,BF-37 = LFAM32 has relatively small L R of ∼1.5×10 14 ergs s −1 Hz −1 , hence would be in this class.