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) 696.2.6 Chemical composition of X-ray emitting plasma – the FIP and IFIP effectsGenerally, the abundances of a stellar corona of MS stars may be different from those in a photosphere,depending on the FIP of the relevant elements. For instance, in the solar corona, elementswith low FIP are overabundant relative to the solar photospheric values (“the FIP effect”, Feldman1992). Recent observations, however, showed that in some of stellar coronae the abundances ofhigh-FIP elements are larger than the others (the Inverse FIP = IFIP effect). The IFIP effectappears in X-ray active stars, while the FIP effect is generally seen in less active stars (Audardet al., 2001; Brinkman et al., 2001; Güdel et al., 2001a,b,c; Huenemoerder et al., 2001; Kastner etal., 2002). The dependence of abundance on X-ray activity is also existent within individual stars;some sources show the enhancements in abundance from quiescent to flare phases, or even withina single flare phase (e.g., HR1099, Audard et al. 2001; V773 Tau, Tsuboi et al. 1998). Due to thelimited statistics and samples, however, it is unclear whether or not abundance variations betweenquiescent and flare phases are common and the FIP/IFIP effects are present in low-mass YSOs.The coronal abundances of DoAr21, ROXs21, and ROXs31 are sub-solar, consistent with theaverage values of low-mass YSOs (0.3 solar: Figure 5.4). From Figure 6.6(a) and (b), we see thatboth high-FIP (Ne and Ar) and low-FIP (Na, Mg, and Ca) elements show higher abundances thanthe other elements (the IFIP and FIP effects).For the abundances in the solar corona, the FIP effect appears in elements with FIPs below10 eV. A possible interpretation for the FIP effect is as follows (Feldman, 1992). The low-FIPelements (FIP < 10 eV) are collisionally ionized in the chromosphere at 6000–7000 K temperatureand electric fields would preferentially transfer them to the upper coronal region. DoAr21 andROXs21 are a K0 star and a binary of K4 with M2.5, respectively, hence have surface temperaturesof 4000–5000 K, which is 0.6–0.8 times of that of the sun (Nürnberger et al., 1998). If we simplyassume that the FIP-effect energy-limit is a linear function of the surface temperature, it shouldbe shifted from 10 eV to 6–8 eV, which is near Mg (FIP = 7.8 eV) and Ca (6.1 eV), but well aboveNa (5.1 keV). The abundance enhancements of Mg, Ca and Na provide independent evidencesupporting the universality of the scenario proposed by Feldman (1992).The X-ray luminosities of our TTS samples are a few × 10 30−31 ergs s −1 , which is significantlyhigher than typical FIP sources of ≤ 10 29 ergs s −1 (such as the sun, π 1 UMa, and χ 1 Ori; Güdel etal. 2001c). The high luminosity may also be responsible for the IFIP effect we observed. A relatedissue would be the dramatic abundance increase in the giant flare of ROXs31 (§6.2.7).Güdel et al. (2001c) proposed a possible scenario for the IFIP effect. During the X-ray activephase, the high-energy non-thermal electrons (§2.3.1) produce a “downward-pointing” electric field,which suppresses the transfer of the ionized low-FIP elements into the corona. The electrons also