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

90 CHAPTER 6. INDIVIDUAL SOURCESThe average temperature of the soft-band unidentified sources is ∼0.2 keV, which is significantlylower than those of YSOs, although the data excess around 1–2 keV (Figure 6.23a) indicatesthe existence of higher temperature component of < kT > = 1–2 keV. Considering the low temperatureplasma and small absorption column (∼ 2×10 21 cm −2 ), one plausible idea is coronal emissionof MS stars in front of the ρ Oph cloud. However, it would be less possible because there is nooptical counterpart (Chini, 1981). Another idea is that an outflow from YSOs in ρ Oph collides ata dense interstellar matter and makes a low temperature (0.1–0.5 keV) plasma, like X-ray emissionfrom Herbig-Haro objects (HHs: Pravdo et al., 2001; Favata et al., 2002). In particular, A-S1 islocated in the direction of CO outflow from the class 0 source VLA1623 (Figure 6.25), hence is astrong candidate of jet-induced X-rays. The average flux (absorption-corrected) per one source of1.4×10 −15 ergs s −1 cm −2 is converted to < L X > of 3.5×10 27 ergs s −1 assuming the distance of145 pc, which is about 100 times lower than those of HHs (∼ 10 29 ergs s −1 ). This indicates thatthe emitting region of soft-band unidentified sources has smaller volume and/or density than thatof HHs.The hard-band unidentified sources show nearly the same spectral parameters as the totalbandsources, although the average flux is about four times lower, hence are also thought to bebackground AGNs.Fig. 6.24.— Spatial distribution of unidentified sources overlaid on the 1.3 mm continuum map(Motte et al., 1998). Crosses, squares, and diamonds represent total-band, soft-band, and hardbandunidentified sources, respectively.