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

Chapter 7Overall Feature of X-ray Emissionfrom YSOs7.1 X-ray Detection RateWe detected 195 X-ray sources from the ∼1 pc × 2 pc region of the ρ Oph main body. About60% (110) of them are identified with NIR sources. Table 7.1 summarizes the number of X-raysources and detection rate for each class.High detection rate (≥ 50%) is obtained for class Isources as well as class II and III sources, while that is slightly lower for class III c , BD, and BD csources (< 40%). Figure 7.1 shows the relation between the bolometric luminosity (L bol ) and visualextinction (A V ) for the ρ Oph sources (Table B.1). We see the tendency that most sources with noX-ray detection (gray symbols in Figure 7.1) are distributed at the upper-left area in this figure,i.e., having relatively lower L bol and higher A V . Considering the almost constant < L X >/L boland N H /A V correlations (§7.4.1 and §7.4.2), this means that the X-ray non-detected sources havelower < L X > and higher N H even if they emit X-rays. It hence would be conceivable that theX-ray non-detected sources may emit X-rays below the current sensitivity limit of Chandra. Lowerdetection rate of class III c , BD, and BD c sources is simply due to lower L bol ( ∼ = lower < L X >) ofthese classes. On the other hand, X-ray non-detected class I sources have relatively high L bol withmoderate A V (e.g., GSS30, G304, and GY378; Figure 7.1). However, this could be partly due toshort time variability. For example, WL21 (BF-8) and GY256 (BF-51) are hardly visible exceptwhen they exhibit X-ray flares (see Figure A.1). Also long-term variability, like the 11-year cycleof the solar X-ray activity, may exist. We therefore expect that virtually all YSOs, at least in ρOph, are potential X-ray emitters regardless of their classes.95