* Astronomy

Title: Hot Gas Lines in T Tauri Stars
Authors: David R. Ardila (1), Gregory J. Herczeg (2), Scott G. Gregory (3,4), Laura Ingleby (5), Kevin France (6), Alexander Brown (6), Suzan Edwards (7), Christopher Johns-Krull (8), Jeffrey L. Linsky (9), Hao Yang (10), Jeff A. Valenti (11), Hervé Abgrall (12), Richard D. Alexander (13), Edwin Bergin (5), Thomas Bethell (5), Joanna M. Brown (14), Nuria Calvet (5), Catherine Espaillat (14), Lynne A. Hillenbrand (3), Gaitee Hussain (15), Evelyne Roueff (12), Eric R Schindhelm (16), Frederick M. Walter (17) ((1) NASA Herschel Science Center, USA, (2) Peking University, China, (3) California Institute of Technology, USA, (4) University of St Andrews, UK, (5) University of Michigan, USA, (6) University of Colorado, Boulder, USA, (7) Smith College, USA, (8) Rice University, USA, (9) JILA, USA, (10) Central China Normal University, China, (11) Space Telescope Science Institute, USA, (12) Observatoire de Paris, France, (13) University of Leicester, UK, (14) Harvard-Smithsonian Center for Astrophysics, USA, (15) ESO, Germany, (16) Southwest Research Institute, USA, (17) Stony Brook University, USA)

For Classical T Tauri Stars (CTTSs), the resonance lines of N V, Si IV, and C IV, as well as the He II 1640 A line, act as diagnostics of the accretion process. Here we assemble a large high-resolution dataset of these lines in CTTSs and Weak T Tauri Stars (WTTSs). We present data for 35 stars: one Herbig Ae star, 28 CTTSs, and 6 WTTSs. We decompose the C IV and He II lines into broad and narrow Gaussian components (BC & NC). The most common (50 %) C IV line morphology in CTTSs is that of a low-velocity NC together with a redshifted BC. The velocity centroids of the BCs and NCs are such that V_BC > 4 * V_NC, consistent with the predictions of the accretion shock model, in at most 12 out of 22 CTTSs. We do not find evidence of the post-shock becoming buried in the stellar photosphere due to the pressure of the accretion flow. The He II CTTSs lines are generally symmetric and narrow, less redshifted than the CTTSs C IV lines, by ~10 km/sec. The flux in the BC of the He II line is small compared to that of the C IV line, consistent with models of the pre-shock column emission. The observations are consistent with the presence of multiple accretion columns with different densities or with accretion models that predict a slow-moving, low-density region in the periphery of the accretion column. For HN Tau A and RW Aur A, most of the C IV line is blueshifted suggesting that the C IV emission is produced by shocks within outflow jets. In our sample, the Herbig Ae star DX Cha is the only object for which we find a P-Cygni profile in the C IV line, which argues for the presence of a hot (10^5 K) wind. For the overall sample, the Si IV and N V line luminosities are correlated with the C IV line luminosities, although the relationship between Si IV and C IV shows large scatter about a linear relationship and suggests that TW Hya, V4046 Sgr, AA Tau, DF Tau, GM Aur, and V1190 Sco are silicon-poor.

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