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

6.5. NEW YSO CANDIDATES – A-29, A-31, AND BF-36 85A-29 A-31 BF-36Fig. 6.20.— X-ray spectra of (left) A-29, (middle) A-31, and (right) BF-36. Black and gray linesindicate spectra of the flare and quiescent phases, respectively. The upper panels show the datapoints (crosses) and the best-fit MEKAL models (solid line). For A-29 and A-31, we show thebest-fit models with the assumed 5 keV temperature. The lower panels are the data residuals fromthe model.6.5.4 Possible origin of A-29, A-31, and BF-36A-29A-29 is found near the center of the 850 µm core SMM J16264−2424 with the separation angleof ∼11 ′′ . This is larger than the position accuracy of Chandra but is comparable to that of thebeam size of the 850µm observation (∼14 ′′ , SCUBA). Assuming a uniform density core, the columndensity of SMM J16264−2424 is estimated to be ∼10 23 cm −2 from the radio flux (Johnstone et al.,2000), which is consistent with the X-ray-determined N H of A-29 (∼ 2×10 23 cm −2 ) within largeuncertainty for both the values. It is hence highly possible that A-29 is located in or behind SMMJ16264−2424. Using the LogN–logS relation by Mushotzky et al. (2000), we estimate the chanceprobability to detect a background AGN within the ∼11 ′′ radius to be ∼5 %. Although this valueis relatively large, A-29 shows a clear X-ray flare typical of low-mass stars but not seen in usualAGNs (Figure 6.19). No NIR counterpart of K < 15 mag, the association with the sub-millimetercore SMM J16264−2424, and the large absorption lead us to conclude that the origin of A-29 is anembedded star younger than class I.A-31A-31 is located within ∼2 ′′ from the center of the 3 mm clump KSHK-A (Kamazaki et al., 2001),the central source of millimeter/sub-millimeter clump SM1/SMM J16264−2423 (Johnstone et al.,2000). Taking into account of the physical size of KSHK-A (∼4 ′′ ) and position uncertainty ofChandra and the Nobeyama Millimeter Array interferometer observation (∼5 ′′ ), we can conclude