Peremennye Zvezdy (Variable Stars) 26, No. 5, 2006 Received 5 May; accepted 5 July.

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The Emission Spectrum of T Tau in 1971-1979

Ismailov, N.Z.; Guliev, N.Kh.; Aliyeva, A.A.

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

1. Introduction

The star T Tau is the prototype of pre-main sequence (PMS) low-mass T Tauri-type stars, which are at an early stage of evolution. Numerous spectral and photometric observations, carried out during many years, revealed strong variability of this unique star's emission spectrum and visual light (Joy 1945, Herbig 1977, Ismailov 1974, Zaitseva 1978, Zaitseva and Kolotilov 1977). Sometimes, the light curve shows quasi-periodic light variations with a period of 28 (Zaitseva 1978, Herbst et al. 1987). Recently, a long-period wave in the star's brightness, with a nearly 6-year period, was detected (Ismailov and Samus 2003, Mel'nikov and Grankin 2005).

Studies of the recent 20 years demonstrate that T Tau is an exceptional member of the class, a triple system, with an infrared southern component (T Tau S) in 0 7 from the northern component (T Tau N) (Dyck et al. 1982). T Tau S is itself a binary, with a projected separation of 0 07 (Koresko 2000). Solf and Böhm (1999) found that both T Tau N and T Tau S possessed bipolar jets, the two jet systems directed perpendicularly. From spectroscopy, Tracy et al. (2001) revealed evidence for accretion of matter on T Tau S. The data from Loinard et al. (2003) demonstrate relative motions of the IR components of T Tau S. They show that the lower-mass component, T Tau Sb, is running away from the system.

During 1971-1988, Z.A. Ismailov carried out long-time homogenous low-resolution spectroscopy of T Tau at Shamakha Observatory. Some of the results based on these observations have already been published by different authors (Ismailov 1972, 1973, Guliev, 1991, 1994, Ismailov and Rustamov 1999). Below we briefly remind the basic results from this spectroscopy and from spectroscopic data by other authors.

Synchronous increase of line intensities of hydrogen H, H, H and of H and K CaII emission lines, nearly threefold in 3 hours, was discovered (Ismailov 1974). During the star's transition from photometric quiescence to a more active state, variations of the H emission line profile from night to night, evidencing for mass loss from the star, were observed. The intensity of the H emission increased with increasing UV excess (Zaytseva 1978). 90-120 min and 18-20-day quasi-cyclic variations of equivalent widths and radial velocities of hydrogen lines and flare activity in the H, K CaII lines were revealed by Guliev (1991, 1994). The star's spectral type can be described as G8-K0V, its radial velocity is 20-22 km/s (Herbig 1977, Rustamov 1999).

In this paper, we present parameters of emission spectral lines determined from our complete spectroscopic material for 1971-1979.

2. Observations and Results

Our spectroscopic observations were carried out in the Cassegrain focus of the 2 m telescope of the Shamakha Observatory (National Academy of Sciences, Azerbaijan) using the prism spectrograph with inverse dispersion 94 Å/mm at H. The spectral range was  Å. The spectra were reduced using standard techniques (Guliev 1994, Ismailov and Rustamov 1999). We used microphotometer tracing with a magnification of 40 to measure equivalent widths of the strongest emission spectral lines: H-H, H and K CaII, [SII] Å. Mean standard errors of the equivalent widths were within 15%, those of the radial velocities, within  km/s.

Table 1 presents all measurements of (in Å) and (in km/s) for the program emission lines.

To compare the star's brightness variations to our spectral parameters, we used -band measurements from the Wesleyan University photometric database for young stars (Herbst et al., 1994). The results of this comparison are presented in Fig. 1. Its top panel shows equivalent widths () of the H and H emissions versus time, the middle panel presents radial velocities of the H emission, and the bottom panel displays the -band light curve for the time interval of our spectroscopy.

It appears from the figure that the star's -band brightness was increasing during the whole time interval and that this brightness rise occurred in two cycles, each approximately 6 years long (Ismailov and Samus 2003). The data from Mel'nikov and Grankin (2005) show the presence, besides the 6-9-year short cycles, also of a very long, 30-40-year grand cycle of light variations. In our opinion, the historical light curve shows that the combined time of the star's brightness increase and decrease is about 60 years, with small cyclic fluctuations some 6 years long. Currently, the flux from the star is decreasing. The earlier light increase was accompanied with a 6-year cycle, its amplitude being about 02, and the star was the brightest in 1983. The results of our spectroscopy demonstrate strong spectral emission during some cycles, especially when the star is bright. During fainter stages, the amplitude of variations of the star's spectral parameters decreases. It should be mentioned that the amplitude of the brightness variations with the 6-year cycle was 02 in the band even during the bright state of the star. However, the variations of the emission-line equivalent widths followed the star's mean brightness and did not follow the 6-year cycle. Unfortunately, our spectroscopic data do not permit to verify the 2.8-day period revealed in photometry, better time and spectral resolution is needed. Nevertheless, our observations make it possible to claim that the star's spectral activity increased with its brightness.

3. Conclusions

According to the summary light curve (Mel'nikov and Grankin 2005), the star started to increase its -band brightness from the average level of 106 on JD 2438000, reached a maximum at 98 on JD 2446000, and then began gradual fading, so that, on JD 2453000, it became as faint as 102. Figure 1 shows that our observations mainly cover the time interval of increasing brightness, when the variation amplitude of the emission-line parameters also was gradually increasing. The brightness increase to maximum was 08, so that the flux from the star increased approximately twofold, and the fall from maximum by 04 is equivalent to the flux drop by a factor of 1.4. The hydrogen-emission equivalent widths varied by a factor of two during the faint state of the star and by a factor of five or more during the star's bright state. This evidences for the variability of emission lines and of brightness having the same source during the faint state. However, additional radiation in emission lines was observed during the active, bright phase. A possible additional source can be a hot spot on the star's surface resulting from disk accretion on the star's surface.

Summarizing we can make the following conclusions.

1. Variability of emission-line equivalent widths by a factor from 2 to 5, respectively during the star's faint and bright states, is observed. The spectral activity of the system differs at different stages of our observations, maybe it is of a long-cycle character.

2. Though the star's light varies with an approximately stable amplitude of in the  band during each 6-year cycle, variations of the emission-spectrum activity occur independently of the brightness variations during such cycles. Increased activity of the emission spectrum with increasing average brightness of the star is observed.

3. The total flux variation does not coincide with the variation range for emission-line equivalent widths. This means the existence of an additional source of emission in spectral lines during the bright condition of the star. Such source can be a hot spot formed in the process of disk accretion.

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