A non-LTE modeling of narrow emission components of He and Ca lines in optical spectra of CTTS

Abstract

A spectrum of a hot spot, produced by radiation of accretion shock at T Tauri star's surface, has been calculated taking into account non-LTE effects for HeI, HeII, CaI and CaII, using LTE-calculations of spot's atmospheric structure, calculated by Dodin & Lamzin (2012). Assuming that pre-shock gas number density N_0 and its velocity V_0 are the same across the accretion column, we calculated spectra of a system "star + round spot" for a set of N_0, V_0 values and parameters, which characterized the star and the spot. It has been shown that theoretical spectra with an appropriate choice of the parameters reproduce well observed veiling of photospheric absorption lines in optical band as well as profiles and intensities of so-called narrow components of HeII and CaI emission lines in spectra of 9 stars. We found that the accreted gas density N_0>10^{12} cm^{-3} for all considered stars except DK Tau. Observed spectra of 8 stars were succesfully fitted, asuming solar abundance of calcium, but it appeared possible to fit TW Hya spectrum only under assumption that calcium abundance in accreted gas was three times less than solar. We derive spot's parameters by comparison of theoretical and observed spectra, normalised to continuum level, so our results are independent on unknown value of interstellar extinction. We have found that the predicted flux in CaII lines is less than observed one, but this discrepancy can be resolved if not only high-density but also lower density gas falls onto the star. Theoretical equivalent widths as well as relative intensities of He\,I subordinate lines disagree significantly with observations, presumbly due to a number of reasons: necessity to take into account non-LTE thermal structure of upper layers of a hot spot, poorly known collisional atomic data for HeI upper levels and inhomogeneity of the hot spot.