Hot Gas Lines in T Tauri Stars

Abstract

For Classical T Tauri Stars (CTTSs), the resonance doublets of N v, Si iv, and C iv, as well as the He ii 1640Å line, trace hot gas flows and act as diagnostics of the accretion process. In this paper we assemble a large high-resolution, high-sensitivity dataset of these lines in CTTSs and Weak T Tauri Stars (WTTSs). The sample comprises 35 stars: one Herbig Ae star, 28 CTTSs, and 6 WTTSs. We find that the C iv, Si iv, and N v lines in CTTSs all have similar shapes. 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. For CTTSs, a strong BC is the result of the accretion process. The contribution fraction of the NC to the C iv line flux in CTTSs increases with accretion rate, from ∼20% to up to ∼80%. The velocity centroids of the BCs and NCs are such that V BC 4 V N C , 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, with FWHM and redshifts comparable to those of WTTSs. They are less redshifted than the CTTSs C iv lines, by ∼10 km s −1 . The amount of flux in the BC of the He ii line is small compared to that of the C iv line, and we show that this is consistent with models of the pre-shock column emission. Overall, the observations are consistent with the presence of multiple accretion columns with different densities or with accretion models that predict a slow-moving, lowdensity 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, while CV Cha, DX Cha, RU Lup, and RW Aur may be silicon-rich.

Accretion dynamics of EX Lupi in quiescence

“…If the NC lines are produced in a localised accretion spot structure, stellar rotation would affect their RVs imprinting a sinusoidal modulation. A similar scenario has been proposed for UV lines, for which some stars are also consistent with accretion-related spots getting in and out of view as the star rotates, and showing different lines-of-sight depending on whether they form in the accretion column or at the associated spot (Romanova et al 2004;Ardila et al 2013). Simulations predict that the coverage of the spot decreases as the density of the accretion column increases and at small misaligment angles between the stellar rotation and the magnetic moment (Romanova et al 2004).…”

“…The post-shock region at the bottom of the accretion column would be similar to a plage as seen in active stars (Ardila et al 2013;Dumusque et al 2014), although in this case, our plage would be 3D and stratified in density, velocity, and temperature. If a plage (or accretion structure) is at a low latitude, it would only be visible during about half of the rotational period, producing an incomplete sinusoidal modulation (from blueshifted to redshifted; the part from redshifted to blueshifted would not be visible).…”

“…The differences in profile/velocity between high-and low-temperature lines would be associated with different parts of the accretion structure. Detailed models (Romanova et al 2004(Romanova et al , 2011Ardila et al 2013) have suggested various possibilities, ranging from vertical stratification (e.g. in an accretion structure with an aspect ratio where photons can easily escape through the walls), to horizontal stratification across the accretion structure (where the low-density, cooler material at the edges of the accretion column would also show a slowerthan-free-fall motion).…”

“…Fe I and Fe II), line strength and opacity differences make this stratification difficult to identify. We explore two options for stratification: along the extended spot vs. across the spot (Ardila et al 2013). In the first case, material closer to the shock region would be expected to be hotter and denser compared to material closer to the stellar photosphere, and the relative velocities would depend on a mixture of infall and magnetic/gas pressure.…”

Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars

“…7 and 8 for the timespace plots of temperature evolution for runs B50-D11-Dip1 and B50-D11-Dip2, respectively: the slab appears less buried in the chromosphere (or even above the chromosphere in run B50-D11-Dip1) than in the uniform field case (see the dotted lines in the figures, representing the minimum amount of sinking of the slab into the chromosphere in cases with uniform B). Interestingly, Ardila et al (2013) found no evidence of the postshock becoming buried in the stellar chromosphere from the analysis of C IV line profiles for a sample of CTTSs. Although our result largely depends on the level of complexity of the impact region and on the location of the plasma component from which the emission arises, our model suggests that the bending of magnetic field lines at the base of the accretion column might also be considered in the interpretation of the observations.…”