X - R AY D IAGNOSTICS of G RAIN D EPLETION in M ATTER A CCRETING onto T T AURI S TARS

1. Drake & Testa (2005) Chandra / MEG TW Hya Ne X Ne IX O VIII solar “photospheric” X-RAY DIAGNOSTICS of GRAIN DEPLETION in MATTER ACCRETING onto TTAURI STARS Jeremy Drake (SAO), Paola Testa (MIT), Lee Hartman (SAO) Analyses of Chandra X-ray high resolution spectra show that the Ne/O abundance ratio is remarkably constant in stellar coronae, on a wide range of activity levels and evolutionary stages (Drake & Testa, 2005); we show that the Ne/O ratio in accreting classical T Tauri stars (CTTS) can be used to: (1) discriminate accretion-related X-rays from coronal emission, and (2) probe the measure of grain depletion of the accreting material. We apply the Ne/O diagnostic to the CTTS BP Tauri and TW Hydrae – the two stars found to date whose X-ray emission appears to originate, at least in part, from the accretion process. TW Hya appears to be accreting material significantly depleted in O relative to Ne, while BP Tau has a Ne/O abundance ratio consistent with that observed for post-T Tauri stars. We interpret this result in terms of the different ages and evolutionary states of the disks: in BP Tau (~0.6 Myr old) dust is still present near the disk corotation radius and can be ionized and accreted, re-releasing elements depleted onto grains; for TW Hya’s disk (~10 Myr), there is evidence of ongoing coagulation of grains into larger bodies (centimeter size bodies; Wilner et al. 2005) that can resist the drag of inward-migrating gas, and accreting gas is consequently depleted of grain-forming elements. Ne/O DIAGNOSTICS from HIGH RESOLUTION SPECTRA Ne/O ABUNDANCE RATIO DIAGNOSTICS: • diagnostics based on line ratios • the ratio of NeIX He-like resonance line (at ~13.45Å) + NeX Ly (at ~12.13Å) with the O VIII Ly (at ~18.97Å) is a good diagnostics for the Ne/O abundance ratio, since the ratio of the emissivities (shown in the right plot) as a function of temperature is flat • Advantages of this diagnostics: • based on strong lines (reliable atomic physics) • independent on thermal distribution of coronal plasma, which can vary remarkably from star to star TW Hya Chandra Medium Energy Grating (MEG) spectrum and line identification. The lines used for the Ne/O abundance diagnostics are marked by red labels. Ne/O in T TAURI STARS • The analysis for post T Tauri stars (Drake & Testa, 2005) shows that: • Ne/O is essentially constant in stellar coronae for a wide range of activity level, showing no fractionation effects that characterize other elements with lower first ionization potential (FIP) (e.g. Drake 2003, Audard 2005) • the Ne/O abundance ratio in stars (~0.41) is significantly larger than the current assessments of the solar ratio, but it is in line with inference from solar oscillations (Antia & Basu, 2005; Bahcall et al. 2005) Ne/O abundance ratio (by number) for three T Tauri stars, namely TW Hya, BP Tau, and TWA5, compared with the Ne/O ratios found for 21 post TTS (Drake & Testa, 2005). The Ne/O ratios are plotted as a function of the coronal activity index LX/Lbol . The currently assessed solar value (Asplund et al. 2005), together with earlier assessments in common usage are also illustrated. Observed T Tauri stars and derived Ne/O abundance ratio (by number) • In the analyzed classical T Tauri stars X-ray spectra: • for TW Hya Ne/O~1, significantly higher than for more evolved stars • for BP Tau the Ne/O ratio is consistent with the rest of sample (for the TWA5, a multiple system of TTS [showing no evidence of accretion-related X-ray emission] member of the TW Hydrae association, Ne/O is consistent with the post TTS, indicating that the TW Hya anomalous ratio is not a result of peculiar chemical composition of the parent molecular cloud) a Based on Chandra MEG spectrum (this work). It is also consistent with the Ne/O derived by Kastner et al. (2002) based on a differential emission measure analysis. b From Stelzer & Schmitt (2004), based on XMM-Newton RGS spectra. c From Schmitt et al. (2005), based on XMM-Newton RGS spectra. d From Argiroffi et al. (2005), based on XMM-Newton RGS spectra. DISCUSSION CONCLUSIONS REFERENCES • The constancy of the coronal Ne/O ratio in post T Tauri stars provides new diagnostics: • a ratio significantly different from the coronal value provides further evidence that these lines are not formed in “normal” coronal plasma; the Ne/O abundance analysis therefore provides additional X-ray diagnostics of accretion (besides the anomalous behavior of the cool, Ne and O, He-like triplets) • we can determine whether or not the accreting material has Ne/O consistent with that of the underlying star, as represented by the constant coronal ratio • We interpret the high TW Hya’s Ne/O as a result of grain-depletion of the accreting material, already suggested by Herczeg et al. (2002) to explain the lack of Si in the UV spectrum and invoked by Stelzer & Schmitt (2004) to explain the metal-poor X-ray spectrum. Unlike the Ne/Fe abundance ratio, observed to be in active binaries extremely high as in TW Hya (Drake 2003, Audard 2005), the Ne/O diagnostics appears to be robust to fractionation effects. • We interpret the different Ne/O ratio in TW Hya and BP Tau as arising from different depletion of O in the accreting gas. TW Hya’s (~10 Myr) disk evolution appears well advanced (e.g. Calvet et al. 2002) with centimeter size bodies (Wilner et al. 2005) which can resist the drag of inwardly migrating gas that feeds the accretion. In contrast, in BP Tau’s (~0.6 Myr) disk dust is still accreted, re-releasing elements depleted onto grains. • we have shown that TW Hya is characterized by an anomalously high Ne/O abundance ratio reflecting the O depletion into grain in the material accreting from the inner disk. 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