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Astron. Astrophys. 341, 553-559 (1999)

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1. Introduction

RY Tau belongs to the classical T Tauri Stars (TTS), with irregular light variability and a moderate emission spectrum. The star has long been a target for photometric and spectroscopic observations, particularly after its brightening in 1983/84 from 11th to 9th magnitude in V (Herbst & Stine 1984, Zajtseva et al. 1985). The characteristic pattern of the photometric variability of RY Tau is the near constancy of its colours at different brightnesses. Therefore, the dependence of the colours on the brightness is not well expressed but somewhat similar to that of the UX Ori-type stars: the star becomes slightly redder when fading from V[FORMULA] to [FORMULA], but then turns to be bluer when even fainter, with a large intrinsic dispersion in the colours (Zajtseva 1986, Gahm et al. 1993, Eaton & Herbst 1995, Kardopolov & Rspaev 1995).

The spectral classification is K1e IV,V(Li) (Herbig 1977, Cohen & Kuhi 1979) though the earlier spectral type G2 was estimated by Cabrit et al. (1990). The star has a low level of veiling, [FORMULA] in the visible region of the spectrum (Basri et al. 1991, Hartigan et al. 1995). No veiling was found in the blue (Valenti et al. 1993). The IUE spectrum shows weak Fe II emission and moderate far-UV excess (Herbig & Goodrich 1986). The equivalent width of the H[FORMULA] emission is about 20 Å (which is not far from the conventional threshold of 10 Å between weak-line TTS and classical TTS), H[FORMULA] is sometimes in emission, sometimes in absorption, while higher Balmer line are always in absorption.

Contrary to other classical TTS, RY Tau is a rapid rotator; its [FORMULA] was determined to about 50 km s-1 (see e.g. Bouvier 1990). It is an X-ray emitter with [FORMULA] (erg s-1) (Damiani et al. 1995), which is an average value for TTS in the Tau-Aur region.

RY Tau has a rather large level of intrinsic linear polarization of a few percent. The variability of the linear polarization was first discovered by Vardanyan (1964) and confirmed by Serkowski (1969). The wavelength dependence of the linear polarization indicates that most of the polarization arises in an external, circumstellar dust envelope which lies outside of the high-temperature, gas-emitting region (Bastien & Landstreet 1979). The dependence of the linear polarization on the brightness of the star is not unambiguous, however in the deep minima of the brightness the polarization was higher (Efimov 1980).

RY Tau has a remarkably flat distribution of energy in the far-infrared region (Bertout et al. 1988); it is also a strong source of millimeter continuum emission (Beckwith et al. 1990), but was not detected at radio wavelengths in a search for molecular outflows (Edwards & Snell 1982, Calvet et al. 1983).

There were many attempts to find a periodicity in the light variations of RY Tau on both short and long timescales. Some periods were reported on timescales from 5 to 66 days, but none was confirmed later on (Herbst et al. 1987, Herbst & Koret 1988, Bouvier et al. 1993, Bouvier et al. 1993, Bouvier et al. 1995). It is fair to say that if there is any periodicity in the light curve of RY Tau, it is hidden in a larger amplitude irregular variation and/or not persistent on a long timescale.

The relative constancy of colours during large amplitude variations of brightness indicates that the photospheric parameters of the star remain unchanged. Indeed, RY Tau does not show variations of the TiO bands with brightness even in rather deep minima, which could be expected if a cool spot were the cause of the brightness variability (Herbst & Lavreault 1990).

The H[FORMULA] line profile is variable on a timescale of days, showing transient blue- and red-shifted absorption components (Zajtseva et al. 1985, Petrov & Vilhu 1991, Johns & Basri 1995). The same transient absorption was observed in the line profiles of the sodium doublet, which was interpreted as trace of stellar prominences (Petrov 1990). The most detailed review on the high-resolution line profiles was given by Hamann & Persson (1992).

The dependence of the H[FORMULA] flux on the brightness of the star is somewhat controversial. Holtzman et al. (1986) found from UBVRI and H[FORMULA] photometry that the H[FORMULA] flux generally decreases as the star fades, but can change independently of brightness by a factor of 3 or more. On the other hand, Vrba et al. (1993) reported that the H[FORMULA] flux increases with decreasing brightness, although with poor correlation.

During the patrol photometric observations of TTS at the Crimean Laboratory of the Sternberg Astronomical Institution, we have discovered a new event of brightness increase of RY Tau at the end of 1996. The star went from V[FORMULA] to V[FORMULA] within about one month (Zajtseva et al. 1996). This offered a new opportunity to study the phenomena connected with the brightening in great detail. In this paper we present photometric, polarimetric and spectroscopic data obtained at different brightness levels of the star, and discuss possible causes of its variability.

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998