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

74 CHAPTER 6. INDIVIDUAL SOURCESdespite of large positional uncertainty. S2 therefore has the large variation of < L X > = 10 30 –10 31 ergs s −1 together with that of the abundance = 0.2–0.7 solar, although its mechanism is anopen question.Interestingly, both members of a candidate wide binary system, GY21 and S2 (Haisch et al.2002; see also Figure 6.18) show overabundance. This may indicate anomaly of global chemicalcomposition of this region, although this interpretation is insufficient to explain the abundancevariations of GY21 and S2.6.3.4 A-34 – GY51The offset between A-34 and the nearest class II source GY51 is ∼0. ′′ 72, which is slightly larger thanthe positional uncertainty of the Chandra observation (∼0. ′′ 5) at its small off-axis angle (∼1. ′ 1, seeFigure 5.3). However, the N H /A V and < L X >/L bol ratios between A-34 and GY51 are respectively∼1.7×10 21 cm −2 mag −1 and ∼10 −3 , which are typical of YSOs (Figure 7.6 and 7.7). We henceregard that A-34 is surely associated with GY51. A detection of a solar-like flare (Figure A.1) isalso consistent with out idea.6.3.5 A-41 – S1A class III S1 is among the brightest and hottest sources in ρ Oph (L bol ∼ 1000 L ⊙ , T eff ∼20000 K), which is classified as “Herbig Ae/Be stars” (HAeBes = intermediate-mass YSOs: Andréet al., 1988). André et al. (1988) detected polarized non-thermal radio emission, which probablycomes from gyro-synchrotron particles. This indicates that S1 has relatively strong magnetic field.In obs-A, we detect significant X-rays (A-41) with a small-amplitude flare near the end of theobservation (Figure A.3). S1 was also detected at the ACIS-S2 chip in obs-BF (Hamaguchi et al.,2002). The light curve of S1 in obs-BF shows no apparent flare, although it seems to decreasethroughout the observation with the e-folding time of ∼40 ks.Figure 6.13 shows time-averaged spectra of S1 in (left) obs-A and (right) obs-BF. Except for amarginal hump at ∼6.5 keV in obs-BF, clear emission lines can not be seen, indicating either lowmetal abundance or non-thermal nature. Also, there is possible edge absorption feature around4 keV in obs-A. We hence fit them by the power-law model, as well as by the MEKAL model withabundance to be free. For obs-A, we separately make and fit the spectra in the flare and quiescentphases. Table 6.4 summarizes the fitting results. Both the thermal and non-thermal models wellreproduce the spectra. The derived abundance of the thermal model is relatively low of 0–0.2 solar.For obs-A, we additionally apply the fitting with an edge absorption, which slightly reduces the