T Tauri stars were the first group of objects recognized as young pre-main sequence stars (Joy 1945). We now believe that classical T Tauri stars are young low-mass stars () surrounded by circumstellar accretion disks. Since the discovery of the first T Tauri stars, about 800 pre-main sequence stars have been optically identified and listed in the Herbig & Bell (1988) Catalog.
T Tauri stars like other young stars tend to form in groups embedded in molecular clouds. Many are typically found in loose clusters known as T associations. Multiple star formation however does not facilitate the study of the star formation process. Observations of the formation of just one or a couple of stars can certainly provide a simpler laboratory for the study of the star formation process. Isolated T Tauri stars exist and are likely to form in the core of the smallest and simplest molecular clouds: the small Bok globules. The physical conditions in Bok globules are similar to those in dark cloud cores where low-mass star formation has been shown to take place (Myers et al. 1987). Moreover, given their small size and isolation (Clemens & Barvainis 1988), Bok globules are the ideal laboratories for studying the formation of single or a couple of low-mass stars.
During the last years, it has been shown that low-mass star formation is a rather common phenomenon in Bok globules (e.g. Yun & Clemens 1990; 1994a), displaying all ordinary signposts of active star formation, such as the presence of high-velocity molecular outflows, (Yun & Clemens 1992, 1994b), optical and/or near-infrared jets (Moreira & Yun 1995), infrared reflection nebulae (Yun et al. 1993 ; Yun & Clemens 1994a), and infrared binary protostars (Yun 1996).
Both Class I and Class II objects (Adams, Lada, & Shu 1987) have been found to have formed in Bok globules (Yun & Clemens 1995). Class II objects are thought to be T Tauri stars with their infrared excess emission due to the presence of circumstellar disks. Their T Tauri nature can only be definitely confirmed spectroscopically. In fact, classical T Tauri stars are emission line objects, with prominent Balmer lines and frequently displaying the Li I line ( 6707 Å , in absorption) typical of young stars.
Another class of phenomena that is thought to be closely related to the star formation process is named "FU Orionis" phenomenon. A FU Orionis (FUor) object typically undergoes a rapid increase in its optical brightness that may reach 5 magnitudes in less than a year (Herbig 1966, 1977, 1989; Reipurth 1990). Paczynski (1976) and Hartmann & Kenyon (1985) have attributed these outbursts to large (up to three orders of magnitude) increases in the accretion rate on a central T Tauri star, probably due to instabilities ocurring in a massive circumstellar disk. The FUor stage could be a short-lived but important aspect of the star formation process significantly affecting the evolution of a star. Few FUor objects are known (e.g. Hartmann et al. 1993; Strom & Strom 1993; Evans et al. 1994). All are associated with reflection nebulae and large infrared excesses (e.g. Goodrich 1987) indicating they are pre-main sequence objects. In addition, they seem to display distinctive spectroscopic properties: G-type supergiant optical spectra; broad absorption lines, deep, blue-shifted absorption components on H and Na I D; CO absorption bands (e.g. Hartmann et al. 1993). However, it has been noted (Herbig 1989) that the spectra of FUors may not necessarily point to the existence of a self-luminous accretion disk. Instead, the spectra may alternatively be interpreted as indicating a peculiar, rapidly-rotating star with a semi-transparent atmosphere plus circumstellar material.
During our survey of T Tauri stars in Bok globules, we confirmed the T Tauri nature of a subset of our Class II sources. In addition, we found an object that is likely to be a new FU Ori star. In this paper, we report these findings. In Sect. 2 we describe observations and data reduction. In Sect. 3 we present and discuss our results, and in Sect. 4 we summarize our conclusions.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998