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

6.2. X-RAY BRIGHTEST T TAURI STARS (DOAR21, ROXS21, AND ROXS31) 67We investigate the Na overabundance seen in ROXs21 in further detail (Figure 6.6b), becauseNa is a relatively rare element compared with the major 4N nuclei ( 12 C, 16 O, etc) and because theNa Lyman-α line lies near the Lyman-β line of the more abundant element Ne. Figure 6.7a showsthe best-fit result when the Na abundance is collectively varied with the other elements. There issignificant data excess at ≈1.2 keV, near the Lyman-β line of Ne (1.211 keV). The intensity ratioof Lyman-β to -α lines of Ne should increase as a function of kT for kT < 1 keV and stay almostconstant at ≈ 7 for kT > 1 keV (Mewe et al., 1985), hence 1/7 of the Lyman-α flux should be anupper-limit to the Lyman-β line flux. Since the flux of the Ne Lyman-α line is fairly strong, theLyman-β line flux is well constrained as seen in Fig 6.7a. Thus the excess flux should originatefrom another element; the Lyman-α line of Na (1.236 keV) is the best candidate. Allowing the Naabundance to be a free parameter improves the fit as demonstrated in Figure 6.7b. The reductionof χ 2 (d.o.f.) from 114 (97) to 108 (96) is significant with ∼98 % confidence level for the F -test 2(Bevington & Robinson, 1992).NeHe-like Kα Ne Lyα Ne Lyβ Na LyαFig. 6.7.— Close-up view of the fitting results of ROXs21 in phase Q, in which the Na abundance(a) is varied collectively with the “other” elements and (b) is treated as a free parameter. Arrowsand dashed squares show the positions of K-shell lines from (a)Ne/(b)Na and the prominent featurenear the Lyman-α line of Na, respectively.2 A statistical test to check the difference between the fitted and parent functions. See §11.4 in Bevington &Robinson (1992).