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Astron. Astrophys. 324, 155-160 (1997)

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1. Introduction

T Tauri stars (TTS) are known to exhibit X-ray emission of considerable and variable intensity from the pioneering observations with the Einstein Observatory X-ray mission (see for instance Gahm 1981, Montmerle et al. 1983, Feigelson et al. 1988, Walter et al. 1988, Feigelson & Kriss 1989). With the launch of the ROSAT satellite, observations with higher sensitivity in both flux and spatial resolution could be achieved. Investigations of ROSAT surveys of the Chameleon I, Taurus Auriga and [FORMULA] Oph star forming regions have revealed an increased number of X-ray emitting young stellar objects (Feigelson et al. 1993, Casanova et al. 1995, Neuhäuser et al. 1995a and Neuhäuser et al. 1995b). The origin of the X-ray emission is not fully understood but is usually interpreted as arising from coronae and/or magnetic flare-like events. If the magnetic fields of TTS are generated by a dynamo process a relation between the rotation and X-ray emission would be expected. From ROSAT data of the Taurus Auriga star forming region, Neuhäuser et al. (1995a) found a correlation between the stellar rotational period and the X-ray surface flux. Feigelson et al. (1993) found no relation between the apparent rotational velocity ([FORMULA]) and the X-ray luminosity of TTS in the Chameleon I star forming region. Both authors found, however, strong relations between the stellar mass, radius, bolometric luminosity and the X-ray luminosity (see also Krautter et al. 1994).

TTS show pronounced activity in the optical with brightness variations ranging from less than one hour (flare like) to years. There are, however, differences in the properties of the optical flares between the two types of TTS (classified according to their equivalent width, [FORMULA], of H [FORMULA] ; the weak lined T Tauri stars (WTTS) with [FORMULA] Å and the classical T Tauri stars (CTTS) with [FORMULA] Å) in that the events on the WTTS have shorter rise times and are hotter than those on the CTTS (Gahm 1990, Gahm et al. 1995). The infrared (IR) excesses found for the CTTS indicate the presence of circumstellar disks around these stars. The distinction that CTTS have disks while WTTS do not is, however, spurious since there are a number of WTTS with low [FORMULA] but with IR-excesses. The increasing evidence that the CTTS are surrounded by circumstellar disks strongly indicates that the activity on the CTTS is related to accretion of material from such disks (see Guenther & Hessman 1993, Gullbring 1994, Gullbring et al. 1996 hereafter Paper I). It is still controversial whether the X-ray properties of WTTS and CTTS are similar or not. Feigelson et al. (1993) found no correlation between the X-ray flux and the strength of the H [FORMULA] emission for different TTS while Neuhäuser et al. (1995a) found that WTTS have larger X-ray luminosities than CTTS and indications that the X-ray emission is harder for CTTS than for WTTS.

Simultaneous observations in X-ray and optical of events on TTS can provide constraints on the underlying mechanisms responsible for the energy production. In particular, if the flare events are produced in magnetic loops as is believed to be the case for UV Ceti flare stars, a correlation between the optical and X-ray brightness variations should be expected. Only a very limited number of X-ray observations of TTS have been carried out simultaneously with other wavelength regions. Recently Feigelson et al. (1994) observed V773 Tauri (a WTTS) in X-ray (ROSAT), ultraviolet, optical and radio simultaneously during [FORMULA] 8 hours. During what could be the decline of a radio-flare the star was constant in all other observed spectral regions. Since TTS rarely show fast variability in the optical (less than [FORMULA] 5% of the observed time of these objects; Gahm 1994), simultaneous measurements should be performed over at least several nights to put constraints on the relation in the behaviour between different wavelength regions when the star is active. There is also only a limited number of patrol observations of single T Tauri stars solely in X-rays. Such measurements are important in order to trace the nature of the X-ray variability and to answer the question of whether the quiescent X-ray emission of T Tauri stars is due to the superposition of many flare-like events or to a more steady source like a coronae containing long lived structures. Time resolved observations have been carried out with the Einstein Observatory by for instance Feigelson & De Campli (1981) of DG Tau and SU Aur, Walter & Kuhi (1984) of AS 205 and AA Tau and with EXOSAT by Tagliaferri et al. (1988) of HD 560B (a Post-T Tauri star candidate). A recent investigation is the ROSAT observation of the CTTS LkH [FORMULA] 92 that was observed during 23 675 seconds (Preibisch et al. 1993). A huge X-ray flare was recorded, but no simultaneous observations at other wavelengths were made.

In this article we present five nights of simultaneous optical UBVRI and ROSAT PSPC monitoring of the CTTS BP Tauri. BP Tauri is typical for the CTTS class of TTS with IR-excess (Rucinski 1985). The optical short term variability of BP Tauri was extensively investigated in Paper I where it was found that the star normally exhibits smooth brightness changes on time scales of hours. Occasionally, short term events on time scales [FORMULA] 1 hour occurred. EINSTEIN observations of BP Tau showed an X-ray luminosity of 1.15 1030 erg/s (Walter & Kuhi 1981) and from the ROSAT All-Sky Survey the luminosity was recorded as 0.7 1030 erg/s (Neuhäuser et al. 1995a). The optical variability raises the question of a possible short term variation in X-ray and its relation to the optical activity. The main goal of this investigation is to determine if the mechanisms behind the optical and X-ray activity are related or are of different origin.

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998