THE ABSORPTION SPECTRUM VARIABILITY OF T TAURI

N.Z. Ismailov, N.Kh. Guliev, A.A. Alieva

Shamakha Astrophysical Observatory, National Academy of Sciences of Azerbaijan,
Shamakha, Azerbaijan
E-mail: Box1955n@yahoo.com

1. INTRODUCTION

T Tau is the well-known prototype of low-mass pre-main-sequence (PMS) stars. Ismailov et al. (2006) presented the results of the analysis of Balmer H-H, H and K CaII, and other emission lines. We demonstrated that the system's spectral activity was different at different times, the variations being possibly of a long-term cyclic character. We also found different emission-spectrum activity at different levels of brightness. During the star's brightest state, the emission-line equivalent widths show variations by a factor of 5 and even more.

Studies of the absorption spectrum for stars with masses and luminosities as low as for р Tauri stars (TTSs) are of a large interest for our understanding of these stars' atmospheres. It is known that the absorption spectrum of classical T Tauri stars (CTTSs) is deformed by superposition of emission continuum or of linear emission of unknown nature in the visual range, known as "veiling" of the absorption spectrum. It can lead to systematic errors in measurements of equivalent widths, radial velocities, and other parameters of absorption spectral lines. To avoid negative effects of veiling, spectroscopy with a high spectral resolution and a high signal-to-noise ratio is needed. For some spectral lines, a veiling coefficient can be introduced, it permits to take into account the degree of distortion of the absorption spectrum compared to standard stars of the same spectral class (see, for example, Basri and Batalha, 1990).

Different studies (e.g., Basri and Batalha, 1990; Alencar and Batalha, 2002) show that usually the correction to equivalent widths for the veiling factor is within 10% for different CTTSs. Thus, an analysis of time changes in the absorption spectrum can provide information on the character of the additional emission spectrum. In this paper, we present the results of our long-term spectroscopy aimed at the analysis of the absorption spectrum of the star T Tau.

2. OBSERVATIONS AND RESULTS

Our spectra of the star were obtained in the Cassegrain focus of the Shamakha Observatory's 2 m telescope using the -prism spectrograph, with a dispersion of 93 Å/mm at H. We observed from 1971 till 1979, virtually each year. We measured the equivalent widths of FeI, TiI, MnI, CaI, and other absorption spectral lines and determined the spectral types and luminosity classes according to criteria specially developed for our equipment (Rustamov, 1987). Table 1 presents night averages of equivalent widths for different absorption lines in the spectrum of the star. In total, we used about 200 spectrograms of the star acquired on 46 observing nights.

To monitor quick variations of the absorption spectrum, time-resolved series of observations were obtained on some nights, spectra taken one after another with exposure times about 25-30 minutes and the total time coverage of 4-10 hours. Table 2 contains the results of our measurements for such series of observations within a night: equivalent widths and central depths of the absorption lines.

JD

FeI 4404

FeI 4383

FeI 4325

TiI, CaI

FeI 4271

FeI 4226

FeI 4045

FeI 4005

4291

244...

0989.143

2.7